GB2227397A

GB2227397A - Microwave ashing furnace

Info

Publication number: GB2227397A
Application number: GB8929250A
Authority: GB
Inventors: Michael J Collins; Wyatt P Harrgett
Original assignee: CEM Corp
Current assignee: CEM Corp
Priority date: 1989-01-18
Filing date: 1989-12-28
Publication date: 1990-07-25
Anticipated expiration: 2009-12-28
Also published as: ATA8990A; CH684914A5; AT408050B; GB2227397B; GB2259836A; GB2259836B; CH680239A5; AU627819B2; AU4789690A; JPH02264850A; JP3069114B2; DE4000515C2; IT9047534A1; IT1239164B; FR2641855A1; GB9224551D0; GB8929250D0; IT9047534A0; DE4000515A1; FR2641855B1

Description

MICROWAVE ASHING AND ANALYTICAL APPARATUSES, COMPONENTS AND PROCESSES The present invention relates to microwave ashing and analytical apparatuses, components of such apparatuses, and processes. More particularly, the present invention relates to such apparatuses which include a source of microwave radiation, a walled chamber into which microwave radiation is directed, which chamber retains such radiation therein, and an ashing furnace in the chamber, the walls of which furnace are heat resistant, of low thermal conductivity, and transmissive of microwave radiation.

The furnace also includes a microwave absorptive material which is capable of being heated to an ashing temperature, temperature sensing means, useful to regulate the temperature in the furnace cavity, passageways for gas through the furnace cavity and through the walled chamber, and a removable dooi in the front side wall of the furnace. Although other heat resistant holders for the ashable samples may be used in the furnace it is preferred that such a holder be a container of this invention which is heat resistant, light in weight, microwave transmissive and porous, and of such containers those of quartz microfibers are highly preferred. The invention also relates to various components of the described apparatus and to uses of the apparatus in ashing and analytical procedures.

In U.S. patent 4,565,669 there are described apparatuses and processes for ashing an ashable material by heating an ashing means, such as a silicon carbide, by means of microwave radiation, and ashing a sample to be analyzed, which may be resting on a support made of fused quartz fibers, by means of the heat generated in the ashing means. In such apparatuses the silicon carbide rests on a refractory material and the sample to be ashed is placed on a relatively thin quartz pad which is in contact with the silicon carbide.Such apparatus is positioned inside a computer controlled analytical apparatus, such as the MDS-81 microwave drying/digesting system, manufactured by CEM Corporation, which is described in their bulletin entitled CEM Corsoration Microwave Drvina/Diaestion Svstem MDS-81 (laboratorv microwave svstem), published in 1981.

Although the microwave ashing apparatus and process of the mentioned patent are useful in speeding ashing operations and analytical determinations dependent on them, the present invention is a further significant improvement. In the invented apparatuses and processes the sample to be ashed is in a furnace made of microwave transmissive (preferably essentially or completely microwave transparent) material, which is an open celled ceramic foam, preferably an open celled fused quartz foam. Such furnace material and the furnace structure help to maintain the ashing temperature uniform throughout the furnace cavity and additionally, such temperature is maintained at a desired level by a thermocouple control system, the probe of which is in the furnace cavity. More uniform heating of the ashable sample makes the ashing operation more consistent and more accurate.

Furthermore, possible loss oE sample material in air leaving the microwave apparatus is minimized and it has been found that it is usually unnecessary to employ a cover/sheet oE fused quartz fiber pad material to hold the ash in place and to prevent it from being carried off in the exhaust air. Thus, the tare weight may be less when using the invention and therefore weighings can be more accurate. Various other advantages attend the present invention, including ease of use of the apparatus, ready removability and replaceability of the furnace door, improved burning off of solvent from the ashable sample, which solvent accompanies any magnesium acetate ashing aid employed, accurate automatic control of ashing conditions, and quicker ashings.

Although various ashing apparatuses for analytical purposes have been described in detail in the literature, most of them utilize muffle furnaces to generate heat and they employ crucibles to hold the samples to be ashed. So far as applicants know, before the filing of their U.S. patent 4,565,669, no other microwave ashing apparatusos and processes had been described in the literature. In U.S. patent 4,307,277 a microwave heating oven was disclosed for heating materials to high temperatures, as in producing a sintered ceramic. However, the heating ovens of that patent were not thermostatically controlled, did not employ applicants' open celled ceramic material for furnace walls and door, and were different from applicants' apparatus in various other important structural features.The various changes incorporated into the present invention are improvements over the structures and processes of U.S. patents 4,307,277 and 4,565,669 and contribute to the improved analytical results and quicker ashings that are obtained when employing the present invention.

Prior to the present invention quartz fiber discs had been disclosed as supports for samples to be ashed by heat generated by directing microwave energy onto microwave absorptive materials. In U.S. patent 4,565,669 a quartz fiber support pad and a cover of the same material were utilized to confine an ashable analytic sample to be analyzed during the ashing of such sample by heat generated by directing microwave radiation at microwave absorptive silicon carbide under such a support pad.

U.S. patent 4,565,669 represents the closest art known to applicant with respect to the invented containers for holding ashable material but it does not describe or suggest the invented containers and does not-make them obvious (and the ashing process of the patent does not result in the improved ashing that is obtainable with the invented containers).

In accordance with this invention an apparatus for ashing ashable samples comprises a walled microwave retaining chamber, a source of microwave radiation for radiating onto contents of such chamber and an ashing furnace within the chamber having a furnace wall of heat resistant material about an internal furnace cavity, with an opening in said wall for insertion and removal of an ashable sample, a door of heat resistant material for closing and opening the opening in the furnace wall, a microwave absorptive material which is capable of being heated to an ashing temperature by microwave radiation, and a passageway through the furnace for passing gas into the furnace cavity and for venting gas from said cavity, which heat resistant furnace wall and furnace door material is of low thermal conductivity and is essentially transparent to microwave radiation, which microwave absorptive material of the furnace has a surface thereof exposed to the furnace cavity, and which microwave retaining chamber wall has inlet and outlet openings therein for the passage of gas into and out of the chamber, around the furnace. In preferred embodiments of the invention a thermocouple or other suitable temperature sensing means is employed in controlling the temperature in the furnace cavity, air is controllably passed through the furnace and through the walled microwave retaining chamber, a microwave transmissive door with a handle or gripping means on it is used to close a doorway opening in the furnace, silicon carbide is the microwave absorptive material employed and is present as strips and/or slabs in the furnace wall interior, the material of construction of the furnace is a microwave transparent open celled quartz material, and a container which is employed to hold the ashable sample is a heat resistant light weight, porous, walled container of essentially microwave transparent quartz microfibers which allow gas flow there through but prevent passage of ash. Also within the invention are components of the described apparatus, such as the furnace, with a removable and adjustable door, a process for ashing and analyzing ashable material and a container for ashable material, which material may be ashed by heat generated by microwave radiation of microwave absorptive elements in an ashing furnace, which container comprises a heat resistant, walled container which is light in weight, microwave transmissive and porous, and is made of quartz microfibers which are held in walled container form.Also within the invention is a process for manufacturing such a container by shaping of a light weight, microwave transmissive and porous sheet of quartz microfibers to containers form and heating such sheet in such form, preferably after wetting and drying it, whereby a form retaining container results, which is resistant to ashing temperatures and other ashing conditions.

The invented containers are especially useful in conjunction with microwave ashing apparatuses like that described in the present application for patent, which is also described in U.S. S.N. 07/298,554, one of the priority applications for the present application, in which the inventors are Messrs. Collins and Hargett. However, the invented containers also find use in other ashing applications, such as those conducted in conventional muffle furnaces, and in other heating operations, including fusions and dry ashings (wherein ash is produced for subsequent analyses, such as analyses for heavy metals).

The invention will be readily understood by reference to this specifications, including the accompanying drawing, in which: FIG. 1 is a front perspective view of the microwave ashing apparatus of this invention, with the chamber door open, with the furnace door removed and without any ashable sample in the furnace; FIG. 2 is an enlarged front perspective view like that of FIG. 1, with the furnace door in place, in almost closed position, with arrows indicating air flow into the chamber, into the furnace, out of the furnace and out of the chamber; FIG. 3 is an enlarged disassembled view of the ashing furnace assembly, with a base support and a protective screen under such support; FIG. 4 is a front perspective view corresponding to that of FIG. 1 but illustrating two containers of ashable material in the furnace;; FIG. 5 is a rear perspective view of the exterior of the microwave ashing apparatus with a temperature control unit in place thereon; FIG. 6 is a schematic electrical circuit diagram of various elements of the microwave ashing apparatus; FIG. 7 is a front perspective view of a microwave ashing apparatus, with chamber door open and with furnace door removed to illustrate two of the invented containers in the furnace, and is similar to FIG. 4 but identifies additional features of the invention; FIG. 8 is a top front perspective view of the a walled ashing container of the present invention; and FIG. 9 is a top front perspective view of an ashing container of the present invention having the side wall thereof formed about a mandrel.

In FIG. 1 ashing apparatus 11 includes a walled microwave retaining chamber like that of a CEM Corporation MDS-81 Microwave Drying/Digestion System which is defined by a bottom, two sides, a top, a rear, a front, and a door, which chamber wall is represented by numeral 13, shown applied to a side wall of the chamber. Door 15 is shown in open position so that ashing furnace 17 may be seen. Such ashing furnace will be described in greater detail subsequently in the description of FIG. 3. Temperature controller 19 is connected to a thermocouple probe 21 in the furnace cavity by an electrical connector, not illustrated.The flow of air into the chamber, into the furnace cavity and out of the cavity and chamber will he described with reference to FIG. 2, as w.ll be the operating and display panels of the "microwave system" portion of the apparatus, which panels are like those of the CEM Corporation MDS-81, mentioned above.

In FIG. 2 air (or gas) flow through the ashing apparatus is represented by the dotted arrows. Air enters the walled microwave retaining chamber, designated by numeral 23, through grill openings 25 and 27 in chamber side walls 29 and 31, which openings are located near the bottom of the chamber, and passes upwardly and around furnace 17, cooling the exterior thereof, after which it passes out through suction opening or duct 33, from whence it is discharged from the apparatus through an air exhaust duct, as illustrated in FIG. 5, preferably to a fume hood or in other permissible manner.In FIG. 2, furnace door 35, which is substantially trapezoidal in horizontal cross-section, with handle portions or finger grips cut into the base of the trapezoid (the front of the door), is in place in the furnace wall but the door opening is not completely closed, thereby allowing passage of air into the furnace cavity (not shown in FIG. 2), as represented by arrows 37 and 39. Although the arrows indicate the air flow under the door, air also enters the furnace cavity through the side clearances between the door and the furnace wall. Similarly, air may leave the furnace cavity through the top thereof, as represented by arrows 41 and 43, and the upper portions of the side openings.Arrow 45 represents the passage of air and combustion products out of the furnace cavity through vertical bore 47, between thermocouple probe 21 and the wall of said bore in the upper part of the furnace 17. The gas exhausted from the furnace cavity passes out through exhaust duct 33 to a suitable hood or other discharge means. Thus, there are created passageways for the air or other gases through the furnace, and through the chamber and furnace cavity. It should be mentioned that air inlet openings 25 and 27 and exhaust duct opening 33 are shielded by shielding material (not specifically shown) to prevent escape of microwave radiation from the microwave retaining chamber. The chamber walls and door are of a metal or metal alloy, such as aluminum or stainless steel, and may be coated with a radiation transmissive polymer, such as polytetrafluoroethylene.

Alternatively, but not as desirably, the door may be glass lined and shielded to prevent any escape of radiation.

The temperature controller 19 includes three control keys and a display. The keys are marked S, increase and decrease (not so marked in FIG. 2), and the use thereof will be mentioned later in connection with a description of how the controller is programmed. The microwave system" portion of the apparatus includes controls like those of the CEM MDS-81 laboratory microwave system. Such are an on-off switch 49, and control panels 51 and 53. Panel 51 includes program, Reset, Enter, Stop and Start keys and panel 53 includes numerals 1, 2, 3, 4, 5, 6, 7, 8, 9 and 0 (none of which is specifically illustrated). Display 55 is of alphanumeric type.

The ashing furnace 17, illustrated in FIG. 3, includes combinable and separable unitary upper and lower sections. Upper portion 57 is of a material of heat resistant and microwave transmissive properties, which is also of low thermal conductivity, preferably being an open celled fused quartz foam.

A vertical bore or hole 58 allows passage through such upper portion of a thermocouple probe and connector, (neither being shown in this view). Ashing furnace 17 also includes a unitary separable lower portion 59 of the same heat resistant material, which contains a cavity therein which, together with a mating cavity in the upper furnace portion, forms the furnace cavity.

Lower portion 59 includes a plurality of slots or grooves 61 at the bottom thereof and other slots or grooves, such as those illustrated at 63 and 65. Grooves 61 are for positioning floor heating elements 62 and grooves 63 and 65 are for positioning the heating elements 64 and 66, respectively. Similar grooves, not visible in FIG. 3, are provided for positioning front heating elements 67 and back heating elements 68. Ceiling heating elements (not shown) may also be provided in upper portion 57 of the furnace, in suitable sloths, grooves, channels or other holding means therein. The various heating elements are of microwave absorptive material that is capable of being heated to an ashing temperature by microwave radiation.A much preferred such material is silicon carbide, and preferably the heating elements are separated, with surfaces that are flush with the furnace cavity interior walls. Furnace door 35, which is shown as being of trapezoidal horizontal cross-section (but may be of other suitable cross-section) matches in shape a corresponding wall opening in the front of the upper furnace portion, and when it is in place its interior and the upper and lower wall portion interiors define the furnace cavity. The door includes in the front face thereof a pair of grooves 69 which function as parts of a handle or gripping means to permit easy hand removal, closure or adjustment of the door position. The furnace is supported by a refractory block 71 which s under a minor proportion of the furnace bottom surface.Such support allows circulation of air or other gas under much of the furnace bottom, thereby facilitating cooling of it. Under the refractory support there is shown a separator, such as a cloth or screen, which may be of temperature resistant plastic, metal or other suitable material. The function of the cloth or screen is to prevent scratching of the finish of the chamber interior by the refractory support, which often has rough surfaces.

Because FIG. 4 is essentially the same as FIG. 1 except for the presence in the furnace cavity of FIG. 4 of a pair of containers of ashable material (or ash), only that aspect of FIG. 4 will be described further herein. In FIG. 4 ashing furnace 17 is comprised of separable upper and lower portions 57 and 59 respectively, together with the heating elements illustrated in FIG. 3, of which rear heating elements 69 are visible in FIG. 4, and such parts define the ashing cavity when door 35 is in place.

In such cavity are positioned two porous, walled containers 75 of quartz microfiber sheet material. In the containers are suitable charges of material 77 to be ashed (or they may contain the resulting ash). Details of the ashing procedure will be described later in this specification.

In FIG. 5 ashing apparatus 11 is illustrated with temperature controller 19 in position thereon, with a thermocouple (in the furnace cavity) connected to the controller. Numeral 79 identifies the power cord to the ashing apparatus and louvers 81 and 83 are to permit airflow through an air jacket around the chamber to help cool the chamber exterior. Between the outer wall 85 and the chamber there is locatel a magnetron from which microwave radiation is directed into the radiation retaining chamber, the walls of which are of microwave reflective material, such as stainless steel or other suitable metal or alloy, which may be coated with a paint or polymeric protective coating. The magnetron is a standard part in microwave apparatuses of the present type and is concealed within walls thereof.Therefore, it is not illustrated in the present drawing. Neither is there shown any cooling fan for the magnetron, although such a fan is present in the apparatus. Number 87 indicates an opening in the apparatus for exhausting the air that is blown across the magnetron to cool it. A blower (not illustrated) is provided within the apparatus to exhaust air and combustion gases from the furnace and to create an air flow through the chamber and through the furnace. The motor for such blower is designated by numeral 89 and the corresponding exhaust is identified by numeral 91. Openings 112 correspond to air inlets 25 and 27 (in FIG. 2) and are for admission of air to the apparatus chamber (and furnace cavity). A receptacle 93 is provided for connection of the temperature controller cable 95. Electric power cord 97 is connected to controller 19 at 99.A fuse is provided at 101 and a power switch is indicated at 103. Thermocouple connector leads 105 and 107 are connected to a thermocouple connector plug 109 and such leads or connector are/is also connected to a thermocouple (not illustrated in FIG. 5), which is preferably located in the top central portion of the furnace cavity. Such connector enters the ashing apparatus 11 at 110.

In FIG. 6 the relationship between the operator keyboard (and alphanumeric display), the microwave processor, the temperature controller, the thermocouple and the power control to the magnetron is illustrated. The operator keyboard controls the amount of power employed and the time of heating, which are displayed in the alphanumeric display after they are set by keyboard operation. The temperature controller controls the ashing temperature and permissible variation of the temperature (often +2 or +3 C.) from that which is set. Thermocouple 114 inputs the temperature controller with the temperature in the furnace cavity and the controller operates the microwave power control to switch the magnetron off when the temperature is higher than set and to turn the magnetron back on when the temperature falls below the set point.More details about the operation of the apparatus operator keyboard and temperature control will be given subsequently.

In FIG. 7 a microwave ashing apparatus 111 comprises top, bottom, side and rear walls, all designated by numeral 113, applied to a side wall, and door 115, which define a microwave retaining chamber 118. Inside the chamber is a furnace 117 which includes top and bottom portions 119 and 121, and a furnace door, 123. Such furnace parts are made of microwave transmissive open celled quartz, which is of low thermal conductivity and is heat resistant, capable of being employed at very high temperatures without deterioration. Such a type material is ECCOFOAMRQ, preferably ECCOFOAM Q-G, which is described in a bulletin entitled ECCOFOAM Plastic and Ceramic Foams, of Emerson and Cumming, Canton, Massachusetts, dated March, 1980. Inside the furnace is a furnace cavity 125 and microwave absorptive material 127 is located in grooves or slots (not shown) in the upier and lower portions 119 and 121, with surfaces thereof even with the internal surfaces that define the furnace cavity. In the furnace cavity are illustrated two of the containers of the present invention, which are designated by numeral 129. Also shown in FIG. 7 are inlets 131 for air to enter the chamber, part of which air will pass through the furnace cavity, but most of which passes around the chamber 118 and serves to cool the walls thereof. Such air exits the chamber through outlet 133. A thermocouple 115 is located in the furnace cavity and is communicated by means of a connector (not illustrated) to temperature controller 137.Both the main microwave generating unit of apparatus and temperature controller 137 include controls and visual displays, which are readily apparent and therefore are not specifically numbered.

In FIG. 8 there is illustrated one of the containers of the present invention. Such container is of unitary construction, with bottom 139 and side wall 141 being made from the same sheet of porous unwoven quartz microfibers. The container illustrated had been made from a square portion of the fibrous material and includes seam lines like that shown at 143.

In FIG. 9 there is illustrated a step in the manufacture of container 129. As shown, the non-woven microfibrous quartz sheet has been formed about the base of cylindrical mandrel 145 and extra material has been trimmed off along top edge 147. A quartz monofilament 149 or an elastic band or similar restraining means holds the porous microfibrous quartz sheet tightly to the mandrel during the forming operation but is later removed, following normal manufacturing procedure. After shaping of the sheet, it is wetted, formed tightly around the mandrel, trimmed, removed from the mandrel and air dried, after which it is heated (fired) to produce the form-retaining container of this invention.

While air drying is preferred it may sometimes be omitted.

Although the invented container is illustrated as a short cylinder, other container shapes may also be produced, utilizing correspondingly shaped mandrels. Thus, containers of rectangular or square horizontal cross-sections may be produced.

Although various shapes of containers may be made it will be preferred that such containers be relatively flat, usually being of a height/major horizontal dimension ratio less than 1:1 and preferably no more than 1:2. Such ratios, as for height/diameter, may be in the range of 1:10 to 1:2, preferably being in the range of 1:5 to 2:5, e.g., about 1:5 or 3:10. While various sizes of containers may be employed, when such containers are flat and cylindrical it will normally be preferred for them to be from 2 to 10 cm. in diameter, preferably 4 to 6 cm., and 0.5 to 4 cm. high, preferably 1 to 2 cm. high, and flat cylindrical containers are preferred.

The apparatus for the application of enough microwave energy so a sample of material can be ashed may be any such suitable microwave apparatus that can direct microwave radiation onto the heating elements in the furnace. As was indicated previously, a CEM Corporation MDS-81 system is useful but similar systems can also be employed, together with an internal furnace, temperature control and container for the ashable material.

Preferably the system will incorporated a microprocessor, a digital computer and controls for regulating the application of microwave radiation to the elements to be heated. Thus, the microwave radiation may be applied for desired lengths of time and at different levels of radiation, if desired, but often the radiation level will be constant at the maximum design capacity.

Key elements of the microwave system utilized will be proper gas (air) flow through it for cooling of the furnace, and no microwave load in the system except that in the furnace. Also, the furnace should be such as to allow exhaustion of combustion gases and inflow of fresh gas (air or suitable oxidizer).

It is noted that in some of the apparatuses referred to the microwave power range may be from 1 to 100% of full power (500 to 1,500 watts in some instances) in 1% increments. Of course, lesser and greater powers may also be employed, for example up to several kilowatts, e.g., 0.3 to 5 or 04. to 2 kw., but 0.9 or 1 kw. will usually suffice. In the United States the frequency of microwave radiation employed will normally be 2.45 gigahertz, and in Great Britain it is usually 0.896 gigahertz. Such a frequency can be in the range from 0.3 to 50 gigahertz (or higher) and is preferably in the range of 0.8 to 3 gigahertz. The readouts of the described apparatuses have as many as 40 characters in their alphanumeric displays and in some instances may include audible tones for operator feedback. The operator controls include a keyboard of up to 20 keys for input.

One of the advantages of the present invention is that the described microwave apparatus may be employed for ashing or in other operations for which each such apparatus may have been primarily designed, such as moisture determinations, volatiles analyses and for the promotion of chemical reactions. Usually when the apparatuses are employed for ashings of materials they will be used at their highest power supply condition, which is often about 560 to 1,000 watts. Times of ashing may be adjusted, as desired, and usually ashing times will be from 2 to 20 minutes or 5 to 15 minutes, but the furnace may be pre-heated over periods from 5 minutes to 2 hours, usually 20 to 60 minutes.

The main material of construction of the furnace, which is inserted into the previously described microwave system and is a part of the present apparatus, is one which is heat resistant, of low thermal conductivity and transmissive of microwave radiation. it has been found that such materials include ceramic, glass and quartz foams, with the quartz foams being highly preferred because they allow operations at higher temperatures, are of low thermal conductivities and are exceptionally transmissive of microwave radiation, being essentially or completely transparent to such radiation. Thus, it is considered that over 99% of the microwave radiation passes through the walls of the present furnaces unless it is absorbed by the microwave absorptive heating means in the furnace. Of the quartz foams those which are open celled and fused are even more preferable.

Such materials are available from Emerson and Cuming, of Canton, Massachusetts and are marketed under the registered trademark ECCOFOAM Q. Two forms of ECCOFOAM Q are sold, ECCOFOAM Q-G and ECCOFOAM Q-R. The latter is heavier and stronger but for the purposes of the present invention it is preferred to employ the former. The characteristics oE such fused quartz open celled foams are described in Technical Bulletin 6-2-12A, issued by such company. It is considered that fused foam materials of the types mentioned are useful in making the present furnaces, especially if they are of a density in the range of 0.3 to 0.8 g./c. cm., a 2 modulus of rupture in the range of 10 to 50 kg./cm. , and a thermal conductivity in the range of 0.5 to 1.5 BTU/hr./sq.

ft./ F./in. Such materials should also be operative in the ashing applications of the present invention at suitable ashing temperatures, which more preferably are in the range of 800 to 1,000 C. The foam quartz, which is essentially pure silicon dioxide, or foam ceramic should not decompose or deteriorate appreciably on subjection to such temperatures. When higher temperature ashings are to be undertaken an appropriate higher temperature material of construction will be employed, and the mentioned Eccofoams are preferred because they are stable at 1,650"C. for relatively short periods of time and are considered to be more stable at 1,090"C., to which they may be exposed for prolonged periods without adverse effects.The mentioned Eccofoam products are available in sheet form, said sheets measuring 30.5 x 45.7 x 7.-6 cm. for Eccofoam Q-G and 30.5 x 45.7 x 11.4 cm. for Eccofoam Q-R. Such sheets or slabs are machined to desired shape, utilizing abrasive cutting and grinding techniques. Although Eccofoam can be cemented to itself and other materials such cementing will almost always be avoided in making the present furnaces because the cements are usually ineffective at higher temperatures or are degraded at such temperatures.

The ashing means is of a microwave absorptive material which does not have a Curie temperature below desired ashing temperatures and which is capable of being heated by microwave radiation to a temperature in the range of 400 or 500"C. to 1,650 or 1,700"C. Sometimes the ashing range can be even higher, being limited by the melting, sublimation or decomposition points of the equipment materials being employed or of the ashable substance or its oxide(s), but normally the range of 600 to 1,000"C. is adequate and 800 to 950 , 975" or 1,000 C. is often more preferred. The ashing means is one which is stable at the intended use temperatures and is essentially or completely nonoxidizable at such a temperature.It should also be structurally sound at such use temperature, being resistant to disintegration, cracking and powdering. Although various materials are capable of absorbing microwave radiation and of being heated to temperatures in the ranges described, silicon carbide is the most useful and most preferred of such materials. Silicon carbide, in powder, granular or other small particulate form (wherein the effective diameters of the particles are usually up to 0.5 to 1 cm.) can be heated by microwave radiation but generally in such a form it is not sufficiently effective to be employed as an ashing means for a variety of ashable materials such as may be encountered and for the analyses of which the present apparatus is intended.However, silicon carbide which is in continuous sintered or nonparticulate solid form is very satisfactory and has been employed successfully in analyses of various materials for ash contents.

The continuous silicon carbide solid ashing means may be in various shapes or forms to suitably fit in a furnace wall cavity, but regular parallelepipeds are preferred, such as flat prisms of rectangular cross-sections. Suitable materials may be commercial "furnishing sticks", which may be used to true grinding wheels; of these those sold by Norton Co. under the trademark CRYSTOLON, especially their Grade 37 C 220, which is vitrified, bonded silicon carbide, are preferred, but other silicon carbide products may also be employed. Among these are Norton Co's. JKV finishing sticks and silicon nitride bonded silicon carbides, designated CN 137 and CN 233.Even if such products may physically deteriorate after many uses they are relatively inexpensive, so scheduled periodic replacements, such as after about every 1,000 analyses, may be undertaken but so far applicants have not yet had to replace any Crystolon silicon carbide. Other microwave absorptive heating elements that may be used include ferrites, garnets and similar materials known in the art.

The thermocouple that is employed to measure the temperature in the furnace during microwave heating thereof may be any such suitable thermocouple which is capable of withstanding the ashing temperature and is unaffected by any combustion products and any other gases released from the ashable material during ashing thereof. It has been found that a Type K thermocouple (chromel-alumel) is satisfactory in the invented apparatuses. In use such thermocouple includes a solid sheath which is electrically grounded to the chamber wall. It has been found in practice that the thermocouple operation and accuracy are not adversely affected by the microwave radiation. Instead of a thermocouple other temperature sensor devices can be used (with the temperature controls) to turn the magnetron power on and off, and thereby regulate the furnace temperature.Such may be infrared sensors, vapor pressure sensitive switches, bimetallic switches and expansion sensitive gauges, which all may be appropriately located in the apparatus and connected to a responsive temperature controller which can translate any signal received into on-off impulses or instructions to the magnetron switch.

The temperature controller is an electronic instrument of conventional design which opens and closes a magnetron electrical supply line in response to electrical signals from the thermocouple. It will be discussed further when the programming thereof is subsequently described. However other forms of the controller may be utilized with other temperature sensing devices.

The ashable sample should not be placed directly on the heating elements or microwave transmissive wall material of the furnace, as is evident, and therefore a support for the sample is employed. Such support should desirable be light in weight and must resist the high temperature of ashing. Also, it should be microwave transmissive, preferably microwave transparent or essentially microwave transparent (usually transmitting over 95% and preferably over 99% of such radiation), and it should not allow passage through it of the ashable sample or the resulting ash. A suitable support or container material for the ashable sample is a quartz microfiber (micron-sized) light weight filter material. The microfibrous quartz sheet will preferably be one of a thickness in the range of 0.2 to 0.7 mm., of such porosity that the pressure drop across it is 1 to 5 mm. of mercury at 5 cm./sec.

face velocity of air, resistant to high temperatures, such as up to 500"C., without any adverse effects, retentive of micron size particles, transmissive of microwave radiation, and of a weight in the range of 50 to 200 g./m. . Preferably the material will be of a thickness in the range of 0.3 to 0.6 mm., of such porosity that the pressure drop across it is 2 to 4 mm. of mercury at 5 cm./sec.

face velocity of air, resistant, although with some embrittlement, to high temperatures, up to 1,000"C., retentive of over 99% of micron size particles, transparent to microwave radiation, and of 2 a weight in the range of 75 to 125 g./m. . Such a container will normally weigh in the range of 0.2 to 0.6 g., preferably weighing 0.3 to 0.5 g.

A very suitable material of construction for the present containers is that sold by Whatman Laboratory Products, Inc., Clifton, New Jersey, for use as air pollution filters, under the name WhatmanRUltra-Pure QM-A Quartz Filters, which are described in their publication No. 860-QM-AA. According to such publication, the described material is an ultra-pure quartz microfiber filter sheet which contains a small proportion (5%) of conventional borosilicate glass microfibers, which are in the sheet for papermaking purposes. Such publication does not describe or suggest the use of the mentioned material as a container, does not refer to ashing of analytical samples, and does not mention the use of microwave heating for ashing such samples or for ashing other materials. According to the Watman 2.

publication the weight of the QM-A quartz filter is 85 g./m. , its thickness is 0.45 mm., it retains 99.999% of 0.6 micron particles at 5 cm./sec. face velocity of air, it is of a dry tensile strength, for a 1.5 cm. wide strip, of 250 to 300 g., and it is capable of withstanding a maximum temperature of 500"C.

To make the present containers a relatively simple process is employed, in which a non-woven sheet of the described microfibrou; quartz is shaped, wetted, formed, trimmed, removed from mandrel, air dried and fired. If the restraint and mandrel material(s) is/are sufficiently heat resistant the firing may be conducted while the sheet material is held in place on the mandrel. Such heating is to a sufficiently high temperature to result in a form-retaining container, which temperature will normally be at least 400"C. but is preferably in the range of 500 to 1,200"C. Heating time at the desired "curing" temperature will normally be in the range of 1 to 20 minutes, with ranges of 1 to 15 minutes and 5 to 12 minutes being preferred and more preferred.

For example, a 10 minute heating period at about 800-900"C. is often employed. It has been theorized that during the curing operation the borosilicate glass component of the microporous quartz filter material is removed leaving a formed container of quartz fibers which are still porous and which are even more heat resistant than the starting material.

The described heating or firing of the container may be effected in various heating means, including ovens and muffle furnace, but preferably is conducted in a microwave ashing furnace of the type in which the container is primarily intended to be employed. Preferably the heating will be to a temperature at least as high as that to which the container will be subjected during ashing operations, but lower temperatures can also suffice.

Moistening of the sheet material may be effected before shaping, as well as after, and such moistening may be by spraying, roll application or immersion. It will usually be preferable to limit the amount of moisture on the microporous quartz material being shaped to that amount which is effective to facilitate its shaping to desired container form, which amount will usually be that which is sufficient to wet all such material. Drying before firing may be conducted on or off the mandrel, and may be by hot air, radiant heating or other means, in addition to ambient air drying.

When a mandrel or other form for the microporous sheet is not used during firing to form retaining configuration, as when a flaring dish shape is desired, the sheet may be formed to such a shape and during heating the outer edges thereof may be unsupported or may be supported, as by the upper walls of a larger cylinder. Various types of forms may be employed, including sleeves between which the desired container walls are held during heating, but for the manufacture of the preferred relatively short cylindrical containers a corresponding cylindrical mandrel, like that illustrated in FIG. 9, will preferably be utilized. Such mandrel may be of any suitable material, including various glasses, plastics, metals and alloys, such as copper, brass, steel and stainless steel, bit if the mandrel is to be in place during firing it should also be heat resistant.If the heating of the shaped sheet on the form is to be carried out in a microwave ashing apparatus, in which the presence of metals will often be avoided, the form is desirably of a microwave transparent material, such as quartz, although various ceramics and glasses may also be employed under proper circumstances. Whichever firing procedure is followed, it will be satisfactory, providing that the container wall does not collapse or distort objectionably.

The heating or firing is preferably undertaken in a microwave ashing apparatus like that described in this application, which operation is convenient and puts the containers made to a test which almost duplicates actual use conditions.

Heating in such apparatus will normally be to the range of about 800 to 100"C., e.g., 850 or 950"C., but may be in the previously mentioned range of 500 to 1,200"C. and can even be as low as 400"C. or as high as 1,600"C. under some circumstances.

It will be noted that in the foregoing recitation of firing temperatures many are in excess of the maximum temperature listed by the manufacturer of the quartz filters, which is 500"C.

Surprisingly, applicant has found that such containers can be made to be shape-retentive by heating to temperatures close to or in excess of the temperature given by the manufacturer as the maximum temperature to which the filters should be raised. During such heating operation the formerly flat sheet of filter material is converted to a form-retaining container, useful to hold ashable samples for microwave ashing operations. Such permanent shaping of the sheet material takes place at temperatures below the melting point of quartz and the porous sheet does not lose its porosity due to fusion. It appears that the presence of the small proportion of borosilicate glass microfibers in the quartz sheet is helpful in manufacturing the present containers but such is not considered to be essential for obtaining the desired result. It is considered that other glasses may be substituted for the borosilicate glass or that such glasses may be omitted, and still, useful form-retaining containers for microwave ash analyses may be made, but it is preferred to utilize the present starting material, containing a small proportion, usually 1 to 10%, of borosilicate glass microfibers.

After heating is completed the container will be removed from the source of heat and will bs allowed to cool in air to room temperature. Slow cooling is favoured to relieve strains and to avoid excessive embrittlement. Cooling times (to room temperature) from 30 seconds to ten minutes are considered to be useful to produce satisfactory microwave ashing containers.

A relatively minor disadvantage of the quartz filter material mentioned is that it apparently crystallizes and becomes brittle when subjected to elevated temperatures, such as those over about 500 C., for relatively long time. Nevertheless, it may be employed to hold the ashable sample and can be used repeatedly if care is taken. It is estimated that between five and fifty analyses can be run before a new container of quartz filter material should be put in service. Such items are relatively inexpensive and accordingly this "disadvantage" is not considered to be significant. A preferred container for the ashable sample is illustrated in FIG's. 4 and 7-9.

Other containers of non-porous materials may be employed to hold ashable samples during ashings, such as crucibles made of quartz, borosilicate glass, ceramic, porcelain and platinum but uses of these are normally limited to certain fusions and "dry ashings". For reasons which will be mentioned later, such containers are not as useful in normal microwave ashings as are supports and containers made of the described quartz microfiber filter material.

Virtually all materials that can be ashed within the operating temperature range for the present apparatus can be satisfactorily ashed in it. Among such materials there may be mentioned synthetic organic polymers, waste water sludges, activated sludges, industrial wastes, river lake and stream bottom sediments, coals, foods, papers and building materials. Often the ash contents of such materials are as low as less than 1% or 0.1%, but they can be higher, even 10% and more, and the invented apparatus will reproducibly and accurately ash such diverse materials and retain all the ash in the described porous containers.

To set up the illustrated and described apparatus and to operate it the following procedure should be followed: 1. If the thermocouple is not in place it should be inserted into the microwave retaining chamber, as illustrated in FIG's. 1, 2, 4 and 5, and as previously described in the specification. The solid thermocouple sheath should be properly grounded to the chamber wall or other grounding location to prevent possible damage to the temperature controller.

2. Place the screen (73) and refractory block support (71) on the floor of the chamber.

3. Remove the top portion (57) of the ashing furnace and place it in the chamber under the thermocouple.

4. Align the hole in the top portion of the furnace with the thermocouple and raise such top portion upward so that the thermocouple is in the furnace cavity (23), which will be created by installation of the furnace bottom section (59).

5. While holding up the top portion of the furnace, slide the bottom portion into the chamber and align it with the top portion.

6. Lower the top portion of the furnace onto the bottom portion. The thermocouple should extend into the ashing furnace cavity approximately 1 cm. but surh distance may te adjusted as desired, based on evaluations of analytical results, and may be within 0.8 to 5 cm. from the top of the furnace cavity, preferably 0.8 to 3 cm. for the described furnace.

7. Place the door (35) in closed position on the ashing furnace.

8. Install the temperature controller on top of the microwave retaining chamber and insert the thermocouple plug into the back of the controller, and connect the temperature controller cable to the microwave system, as illustrated in FIG. 5.

9. Insert the microwave system and controller power cord plugs (not illustrated) into suitable electrical outlets and turn the controller power switch to ON position.

10. To minimize times required to heat the ashable samples, pre-heat the ashing furnace from room temperature to the desired ashing temperature, which desired ashing temperature is set into the temperature controller as described separately below. Then, program the microwave system for 60 minutes of microwave heating and set the power at 100%. Depress the START key and allow the furnace to pre-heat. The furnace will usually achieve an operating temperature of about 950"C. within 30 minutes or one of about 1,200"C. within an hour. If it is desired to hold the furnace temperature longer than 60 minutes the microwave system controls may be programmed for such longer time. Also, the ashing furnace temperature may be re-programmed according to the controller programming procedure to be described below.

11. Place the amount of sample to be ashed in the container, or if several samples are to be ashed at the same time, place them in a plurality of containers.

12. Depress the STOP key, open the chamber door, remove the furnace door and place container(s) of ashable sample(s) in the ashing furnace cavity, using tongs. Replace the furnace door, closing it or leaving it slightly ajar, if preferred, and then close the chamber door.

13. Push the RESET button and depress the START key, which turns on the magnetron and starts heating of the sample(s).

14. After completion of ashing, which usually takes about 10 minutes at the desired temperature, depress the STOP key, open the chamber door and remove the furnace door (which can easily be done by hand despite the high internal temperature of the furnace). Employ tongs to remove the container(s) of ash and allow it/them to cool to room temperature. Replace the furnace door after removal of the container(s) to prevent heat damage to the chamber door. Then close the chamber door and depress the START key, to maintain the furnace at ashing temperature.

The following is a description of the procedure to be employed to program the temperature controller.

1. Insert the thermocouple plug into the controller.

Depress the S key on the controller and 0 will appear on the controller display. Press the Increase key and hold it until 28 appears on the display. If 28 is overshot, press the Decrease key until 28 is reached.

2. Press the S key and "C or F will appear.

a. If "C appears and "C is the desired readout, proceed to step 3.

b. If C appears and "F is the desired read-out, press the Decrease key and "F will appear, and then proceed to step 3.

c. If F appears and "F is the desired read-out, proceed to step 3.

d. If "F appears and C is the desired read-out, press the Increase key and "C will appear, after which proceed to step 3.

3. Press the S key and SP1H will appear momentarily. Press the Increase or Decrease key until the desired operating temperature set point appears. This sets the upper temperature limit, SP1H. The maximum operating temperature is designed into the controller circuitry. For example, it may be 1,200"C. in some instances or 1,650"C. in others, depending on the construction of the apparatus.

4. Press the S key and SP1L will appear momentarily. Then press the Increase or Decrease key until 0 appears. This sets the lower temperature limit SP1L.

5. Press the S key and SP2H will appear momentarily. Then press the Increase key until 2499, the maximum value, appears.

This sets the upper limit value, SP2H, which is not used in the program but is needed to make the unit operate properly.

6. Press the S key and SP2L will appear momentarily. Then press the Increase or Decrease key until 0 appears. This sets the lower limit value, SP2L, that also is not used in the program but is needed to make the unit operate properly.

7. Press the S key and HYS will appear momentarily. Then press the Increase or Decrease key until 1 appears. This sets the operating deadband for maximum operation precision.

8. Finally, press the S key and RUN will appear momentarily. The actual temperature of the ashing furnace will then appear. Controller programming is now complete. Such programming must be completed within two minutes or the controller will exit the programming mode and it will be necessary to perform steps 1-8 again.

It will be noted that by following the immediately foregoing instructions (steps 1-8) for controller programming, upper and lower temperature limits are set into the controller program. Such set points may be identical, in which case when the measured temperature falls below a predetermined hysteresis value (which is usually 2 or 3 degrees) the magnetron will be turned on again, and it will be turned off when the measured temperature increases to about the same value above the set temperature.

The ashing apparatus of this invention is controlled by a combination of temperature controller and single chip type microprocessor. The microprocessor executes instructions from permanent storage in an internal EPROM. In operation the microprocessor receives commands and time data from an operator through the microwave instrument or system keyboard. The operator may view a response to most of the commands on the accompanying 20 digit alphanumeric display.

When the operator enters the time data on the keyboard, the data is stored in temporary RAM memory. Once time has been entered for Stage 1 the microprocessor will allow entry of a Start command. When Start is pressed the microprocessor changes one of its output lines from high to low and begins to count down the time. This digital low is wired through a set of normally closed contacts in the temperature controller and then to the microwave solid state relay (SSR). This low turns on the SSR, which controls microwave power. The SSR, in turn, then switches on the alternating current (AC) to the microwave high voltage section and the magnetron generates microwave energy.

Microwave energy directed into the furnace cavity heats up the ashing furnace heating elements, which heat the furnace cavity and the sample to be ashed. The thermocouple senses the temperature of the ashing furnace and the output of the thermocouple is communicated to the temperature controller, which continually compares the measured temperature to the set point temperature, which has previously been entered. When the measured temperature equals the set point temperature the temperature controller opens the normally closed contact and interrupts the digital signal that had previously turned on the SSR. Without this signal the microwave energy ceases and the ashing furnace holds at this set point temperature and slowly beings to cool.

When the measured temperature falls below a predetermined hysteresis value (usually 2 to 3 degrees), the controller closes the opened contact and the SSR is turned on. The microwave energy then raises the temperature of the ashing furnace to the set point temperature. Such processes continue until the total heating time, as set by the operator, has counted down to 0. The microprocessor changes the digital signal back to a high state and microwave heating and control cease. At any time during the countdown the operator may press the Stop key to stop the countdown and to halt the heating process, if that should be desired.

In the above description the temperature controller is separate from the microwave instrument (CEM Microwave Drying/ Digestion System MDS-81) because the MDS-81 system was available 'hardware" which could be used in conjunction with a less complex new controller. However, it is within the invention to integrate the temperature controller into the microwave instrument.

To ash an ashable sample in the present apparatus is a simple procedure. All that is required is to place the sample in a suitable container, of the type previously described, and insert it into the furnace cavity, close the furnace door and the chamber door, and press the Start button. After ashing temperature has been reached most samples will be completely ashed within about ten minutes but completion of ashing can be verified by weighing the ashed sample (in the container, after cooling) and re-weighing after additional exposure to the ashing conditions. When the weight ceases to decrease completion of ashing is established, and so is the time needed to effect complete ashings, although one will usually employ additional time, say a 20% excess, to be sure.

In such weighings the ashed sample and container should not be weighed hot but should be conditioned for weighing, as is known in the art, but such conditionings proceed very quickly with the invented support.

Normally the present apparatus and process are employed in analyzing materials for ash content. In such procedures the container is weighed without and with ashable sample content before ashing, and the container, with ash, is weighed after complete ashing. The percentage of ash in the original sample can then be readily calculated by dividing the ash weight by the sample weight and multiplying by 100. However, in some ashing operations it is common to employ a dispersing agent, such as magnesium acetate, which acts to prevent production of a vitreous or glass-like residue in the ashing container, which residue may contain some unashed sample. Without the use of such a dispersing agent false high or low readings for ash content could be obtained.When the dispersing agent is employed a blank run will normally be made to determine how much of the apparent ash weight is actually ashed dispersing agent, and such weight will be subtracted from the apparent ash weight to give the true ash weight.

Although various weights of samples and various numbers of containers of ashable samples may be employed in the ashing apparatuses of this invention, in a typical such apparatus, in which the furnace cavity is approximately 14 cm. x 14 cm. or about 200 sq. cm. in area, one will normally charge up to 4 or 5 porous, heat resistant and microwave transmissive containers of ashable sample, which containers will preferably be in short cylindrical form of base area of about 15 to 25 sq. cm. each, e.g., about 20 sq. cm., and with heights in the range of 0.8 to 2 cm., e.g., about 1 or 1.5 cm. Desirably, the weight of such containers will be as low as possible, usually being in the range of 0.2 to 1 g.

each, preferably 0.3 to 0.6 g., e.g., about 0.4 or 0.5 g. The weight of ashable sample will normally be in the range of 1 to 10 g., preferably being in the range of 1.5 to 6 g., e.g., about 2 or 5. g. Ash contents may be high or low, up to a maximum of about 50% and to a minimum of 0.001% or even less. For materials like unfilled synthetic polymeric plastics and grain flours, such will usually be comparatively low, normally being less than 5 and frequently being less than 1%, such as from 0.01 to 0.8%. For the charges of ashable material mentioned, with ash contents in the ranges recited, magnesium acetate dispersing agent is normally employed, dissolved in ethanol (95%), so that the ethanol solution is of a magnesium acetate concentration of about 15 g./l.About 3 ml. of the solution are dripped onto the ashable sample while it is in the container and, if the container employed is porous and of light weight, heat resistant and microwave transmissive quartz microfibers, the solution will wet the entire ashable sample and will also wet the fibers of the container because of container porosity, microfibrous quartz nature, and container design, and the alcohol flashes off during heating in a "gentle" manner, not carrying ash or sample out of the container, whereas when impermeable or conventional containers are employed, such as platinum crucibles, vaporization and combustion of the alcohol are often more violent, and sometimes quantities of samples are carried out of the container, leading to false wash determinations.

The ashing temperature, as it is set for the furnace cavity, is normally in the range of 400 to 1,600"C but such temperature should be chosen in light of the characteristics of the ashable sample and the microwave ashing apparatus. Many ashings and analyses are conductable below 1,200"C. and a large number are conductable in the range of 600 to 1,000 C., such as 950"C. Thus, ashings of wheat and other grain flours may be effected at about 870 or 950"C. and ashings of polyethylene and polypropylene may take place at about 550"C. Ashing times may be adjusted accordingly, but will normally be in the range of 5 to 20 minutes, preferably 8 to 15 minutes, e.g., about 10 minutes.

In some instances the ashing apparatus will be programmable so that the furnace temperature will be changed during the run. In such a situation sometimes the first heating or ramp temperature may be comparatively low, e.g., about 100tic., to dry the sample, after which it may be increased to full ashing temperature.

The present invention possesses many significant advantages over prior art apparatuses and processes for ashing materials and for analyzing such materials for ash contents. It is automatic and allows a single operator to run a multiplicity of ashing operations and analyses in a plurality of ashing apparatuses, each of which may contain a plurality of ashing specimens. The controllable heating of the ashable sample is very uniform, with little heat being lost from the furnace cavity, because the walls thereof are of very low thermal conductivity (and they are also resistant to chemical reactions with combustion products and decomposition products oE the ashed material).As was previously referred to, when the container for the ashable sample is of a porous sheet or quartz microfibers the removal of ethanol (and the accompanying small proportion of water) from the sample that has been treated with dispersing agent may be effected without the need for external ignition of the ethanol and without the loss of sample in any "explosion", such as may occur when conventional crucibles are employed, as in conventional muffle furnaces. The analyses, in addition to being undertaken under more controllable conditions, are also appreciably faster than conventional muffle furnace analyses and the results are just as accurate (actually it is considered that they are more accurate).

The apparatus is easy to set up and easy to use. It is not necessary to wait long times for parts to cool down so that they can be handled by an operator. For example, the furnace door may be removed from the furnace by hand immediately after completion of an ashing operation because, despite the high internal temperature of the furnace, the door and the exterior walls thereof are not hot enough to burn the fingers of an operator touching them. Such is attributable to the low thermal conductivity of the material of construction of such door and the furnace walls, and to the continuous cooling of such surfaces by the air circulating in the microwave retaining chamber.

Air flow into the furnace is also controllable and contributes to the quicker ashing that has been obtained. The portion of the air that is drawn into the chamber passes through the furnace, in part due to a chimney effect. Thus, gases that are generated during ashing of a sample rise up and exit the furnace through upper portions thereof, drawing replacement air into the furnace through lower portions thereof. The venting passageway in the top of the furnace (through which the thermocouple probe passes) carries combustion gases out of the furnace past the thermocouple, keeping the thermocouple sensor sections in contact with circulating gas rather than with stagnant gas, so that the temperature at the sensor corresponds to the actual furnace temperature (and the temperature being applied to the ashable sample). The flow rate of air through the furnace can be employed to regulate the rate of ashing of the ashable sample.

Such rate is readily adjusted b cracking open the furnace door, which can be effected, even during high temperature heating of the furnace, by hand operation, involving finger contact with the door handle or gripping means. Fan or blower speed can also be changed to change air flow rates through the furnace but such is unnecessary because the furnace door opening gives good control.

It may be considered that in the present apparatus the "microwave system", including chamber, blower, ductwork and associated parts, acts as a fume hood for the contained microwave furnace, and does that without the usual space requirement for such a fume hood, without the undesirable heating of the laboratory due to the use of a muffle furnace, and without danger to operating personnel of burning by contacts with heated parts.

In addition to the advantages of the main apparatus and process aspects of the invention the ashing container also provides special advantages to improve ease of analyses, speed and accuracy. Although the ashing temperature in the microwave ashing apparatus may be in excess of the 500"C, maximum temperature specified by the filter manufacturer, it has been found that the invented container can be satisfactorily employed in high temperature ashing without deterioration sufficient to adversely affect the accuracy of the ash content determination. In fact, the same container can be used for a plurality of microwave ashing analyses, often more than 5 and up to 50, e.g., 10.With continued use the container may become more brittle but if handled carefully it will be employable in the numbers of analyses mentioned without losing desired porosity for such ashing, without breaking and without leaking sample or ash.

In addition to the unexpected advantage of high temperature utility the cortainers oE the present invention possess several other unexpected advantages and characteristics that make them ideal for microwave ashing and microwave ashing analyses. The microfibrous quartz material employed is porous, and allows air to pass through it without resulting in loss of sample or ash. This is important because it promotes ignition and oxidation of the sample (most of the ash being in the form of oxides).When a dispersing agent, such as magnesium acetate in ethanol, is employed to treat the ashable sample before ashing, the porosity of the container material (which is maintained despite the high temperature heating thereof in the forming operation) is believed to contribute to smooth flaming of the solvent rather than what resembles an explosive combustion of the solvent, which could carry away some of the sample. Such smooth flaming is believed to occur because the ethanol of the magnesium acetate solution spreads over the container due to the container's absorptive properties. The smooth flaming or combustion may also be partially attributable to the relatively low height of the container wall, which facilitates access of air to the sample and to the ethanol present.With the present containers such flaming can be effected in the furnace of the microwave apparatus during the automated ashing operations whereas when ordinary non-porous crucibles of quartz, porcelain or platinum are employed in muffle furnaces or in microwave ashing furnaces, when suitable, it is usually desirable to remove the alcohol from the sample by flaming it externally of the furnace before beginning the ashing operation.

In addition to being porous, the present containers are light in weic;ht and are of low thermal conductivity. Because they are light in weight their weights are often significantly less than the sample weights and may even be less than the ash weights, in some instances, which leads to more accurate weighings of the sample and ash. Furthermore, despite low thermal conductivity the lightweight and porous container cools faster when removed from the ashing furnace, so time is saved in cooling the container and ash before weighing, compared to when an ordinary crucible is employed. The invented containers, being thinner than ordinary crucibles and other containers, more readily transfer heat to ashable samples from external heat sources, such as microwave absorptive heating elements and refractory muffle furnace walls.

Because the invented containers have side walls, they are superior to the flat sheet type support pads described in U.S.

Patent 4,565,669, and do not require cover pads to prevent loss of feathery ash into the exit air passing through the furnace and retaining chamber of the microwave ashing apparatus. The wall has the desired effect of allowing access of oxidizing air to the sample while at the same time diminishing its velocity, so as to prevent any loss of ash from the container. However, as a safety measure, if it should be desired, a cover can be employed on the present containers, which may be made of the same material, shaped to suit, or may be of a more open porous material or screening, preferably of quartz filament or fibers.

The following examples illustrate, but do not limit the invention. Unless otherwise indicated, all parts are by weight and all temperatures are in "C.

EXAMPLE 1 An official sample of wheat flour was analyzed for ash, utilizing the microwave ashing apparatus of this invention, as described in the previous specification, and the results obtained were compared to those that had resulted from standard analyses in which muffle furnace heating had been employed. Ten "experimental" runs were made, using either single containers of test sample or a plurality of such containers in the ashing apparatus at a time. The wheat flour employed was the standard, which was a check sample obtained from the American Association of Cereal Chemists. The ashing apparatus employed was a 1,000 watt CEM Corporation MDS-81 Microwave Drying/Digestion System unit, modified as described in the specification and employed in conjunction with a thermocouple, a temperature controller and a furnace of the types described previously herein. The materials of construction of the furnace were ECCOFOAM Q-G for the furnace body and door, and Norton Co. Crystallon Grade 37C220 silicon carbide for the heating elements. The base are of the furnace is about 200 sq. cm., with the furnace cavity measuring about 14 cm.

x 14 cm. in horizontal cross-section, and being about 5 cm. high.

The thermocouple is of chromel-alumel type and the circuitry is that of FIG. 6.

The microwave ashing apparatus is set for a temperature of 950"C. and is pre-heated to such temperature for about 1/2 hour. Then the chamber door and the furnace door are opened and a walled container of the sample is inserted into the furnace, using tongs. The container is made of quartz microfiber sheet designated QM-A by the manufacturer, Whatman Labo-atory Products Inc., and is in the shape of a flat cylinder 5 cm. in diameter with a wall height of about 1.5 cm. It contains 2.1241 g. of wheat flour sample and about 3 cc. of a 15 g./l. solution of magnesium acetate in ethanol (95%), which had been dripped onto the sample so as to wet all of it, and the adjacent container bottom and wall.After insertion of the container of sample the furnace door is replaced, in such position that a passageway about 0.3 cm. wide is left, between the door and the furnace wall.

Shortly after addition of the container and test sample to the furnace the alcohol burns off without incident. Ten minutes after charging of the furnace with the sample, the doors are opened and the container is removed, using tongs, and is allowed to cool in a desiccator, which takes about 60 seconds. The container, with the ash and magnesium oxide residue (from the magnesium acetate) therein, is then weighed. Previously, the container had been weighed empty and the equivalent magnesium oxide residue had been determined for the amount of magnesium acetate solution employed.

The amount of ash was 0.0112 g. and the sample weight was 2.1241 g., so the percentage of ash in the sample was 0.527%.

The above ashing determination was repeated nine times, for a total of ten such determinations. Results for these runs are given in Table 1, below.

TABLE 1 Run Code A B C D E F G H I J Wt. of Container + 0.5015 0.5014 0.5228 0.5890 0.4878 0.4940 0.5173 0.5060 0.5790 0.4753 Ash + MgO (g.) Wt. of Container (g.) 0.4893 0.4808 0.5024 0.5668 0.5656 0.4732 0.4967 0.4856 0.5567 0.4528 Wt. of Ash + MgO (g.) 0.0212 0.0206 0.0204 0.0222 0.0222 0.0208 0.0206 0.0204 0.0223 0.0225 Wt. of MgO (g.) 0.0100 0.0100 0.0100 0.0115 0.0115 0.0100 0.0100 0.0100 0.0115 0.0115 Wt. of Ash (g.) 0.0112 0.0106 0.0104 0.0107 0.0107 0.0108 0.0106 0.0104 0.0108 0.0110 Wt. of Sample (g.) 2.1241 2.0392 2.0142 2.0144 2.0305 2.0659 2.0378 2.0276 2.0529 2.0426 Ash Content 0.527 0.520 0.516 0.531 0.527 0.523 0.520 0.513 0.526 0.539 (%, by weight) As is seen from Table 1, the high determination is 0.5398 and the low determination is 0.513%.The average is 0.5248. According to the American Association of Cereal Chemists, fifty-one analyses by muffle furnace ashing techniques, using an oven temperature of 871"C. for one hour, yielded a high of 0.550% and a low of 0.504%, with an average of 0.530%. Thus, it appears that the microwave ashing apparatus yielded more consistent results and has been proven to be sufficiently accurate to be employed in replacement of the muffle furnace ashing procedure.

EXAMPLE 2 Two additional wheat flour samples, identified as B and C, were subjected to microwave ashing, and ash contents of these samples were determined and compared to results obtained by standard muffle furnace analyses. The procedures followed were the same as those of Example 1. For sample B three test runs were made and the ash content results were 0.508%, 0.512%, and 0.520%, giving an average of 0.513%. The ash content by standard muffle furnace analysis was 0.512%.

In three microwave ash analyses of Sample C the results were 0.724%, 0.724% and 0.739%, giving an average of 0.729%. The standard analysis of the same sample resulted in a finding of an ash content of 0.730%.

A sample of polyethylene was analyzed for ash, using the apparatus and method described in Example 1 but omitting the magnesium acetate. Three samples of the same polyethylene were tested, with the ashing temperature being held at 550"C. +3"C. for ten minute periods. Ash contents of 0.008%, 0.008% and 0.006% were obtained, with the average being 0.007%.The various weighings for the three runs made are given in TABLE 2, as are the ash contents TABLE 2 Run Code g R S Wt. of Container + Ash (g.) 0.6032 0.5972 0.5874 t. of Container (g.) 0.6028 0.5968 0.5869 Wt. of Ash (g.) 0.0004 0.0004 0.0005 Wt. of Sample (g.) 5.0187 5.0082 8.0695 Ash Content (%, by weight) 0.008 0.008 0.006 In a manner described for the ash analysis of polyethylene three samples of polypropylene material were analyzed for ash content, using the apparatus and process of this invention. Ash contents determined were 0.024%, 0.025% and 0.024%, with the average being 0.024%. The various weighings are reported in TABLE 3, as are the ash contents.

TABLE 3 Run Code I U Y Wt. of Container + Ash (g.) 0.5984 0.5939 0.5903 Wt. of Container (g.) 0.5972 0.5926 0.5884 Wt. of Ash (g.) 0.0012 0.0013 0.0019 Wt. of Sample (g.) 5.0103 5.1606 8.0431 Ash Content (%, by weight) 0.024 0.025 0.024 EXAMPLE 4 The described microwave ashing apparatuses and processes are useful for performing microwave ash analyses of various other materials, including other foods and other synthetic organic polymers, various sludges and waterway sediments, papers, coals and building materials. The ash contents found in analyses of such materials often range from less than 0.1% to 10% or more and such analyses are readily performed and yield accurate results, compared to standard muffle furnace analyses. Of course, ashing times are substantially reduced, compared to muffle furnace ashing times.Other ashing temperatures are employable, ranging from 400 to 1,200"C., and temperatures as high as 1,600"C. are feasible, with ashing times ranging from 5 to 20 minutes. To ash materials at such highest temperature some modifications of the apparatus and the microwave instrument may be desirable.

In carrying out these additional analyses the apparatus size may be changed, the wattage may be modified and the ashing procedures may be altered. Thus, in some cases a 600 watt or 900 watt basic unit (CEM MDS-81) is employed or such basic unit is replaced by other suitable microwave instrument of such general type, modified as required. Instead of employing the porous quartz microfiber container to hold the sample being ashed one may use a porcelain or quartz container or one made of other suitable material. In such cases the ashing procedure may be varied by ignition of the alcohol accompanying the magnesium acetate (when such is employed) external to the furnace (to avoid loss of sample due to sometimes violent ignitions in the furnace). Sometimes, even when the porous quartz microfiber container is used, if any alcohol present is not removed first by evaporation by low temperature heating, it may be considered desirable to remove the furnace door during the initial heating of the sample so that the flaming of the alcohol can be more controlled and so any loss of sample can be prevented.

As was previously mentioned, air flow rates through the furnace may be adjusted by opening or closing the furnace door.

Such flow rates depend on the degree of such opening and also depend on the total air flow rate through the chamber, which will usually be in the range of 1 to 5 cu. m./min. The total air flow and the various openings into and out of the furnace, in combination, will continue to supply air to the vicinity of the material being ashed so that as the products of combustion are removed they are replaced with fresh air. Despite the relatively large air flow through the chamber the ashing temperature is maintainable within the furnace because of the good insulating properties of the furnace wall and door and because of the relatively minor proportion of gas that enters and leaves the furnace during ashing, with most of the air passing around the furnace.

Other variations of apparatus and process include employing other radiation absorptive materials instead of silicon carbide, e.g., ferrites, installing a quartz fiber safety screen over the ashing container, employing a radiation transparent turntable to provide even more uniform heating of the ashable sample, and varying the size and shape of the furnace to improve even heating and control of air flow through it. Although a turntable would promote more even heating of the microwave absorbent heating elements it has been found that furnace temperatures are essentially uniform throughout and multiple ashable samples are evenly ashed. This is attributable to the excellent insulating properties of the quartz foam (Eccofoam) furnace material. Accordingly, turntables and special radiation mixers are not needed, but are sometimes employed.

EXAMPLE 5 This example and Examples 6-8 relate to manufacture and analytical uses of the invented ashing containers.

A 9 cm. x 9 cm. square of Batman Ultra-Pure QM-A quartz filter, which is a non-woven sheet of quartz microfibers, is shaped about a substantially cylindrical glass form to a flat cylinder with a base about 6 cm. in diameter, and then the cylinder is wetter with about 3.0 g. of water which is applied by spraying it substantially evenly over the surfaces of the filter material. An elastic band is then applied to the cylinder wall, as illustrated in FIG. 9, to hold such wall in position. The application of water to the filter helps it to retain the cylindrical shape. Subsequently, the filter is trimmed and the elastic band is removed.Then the cylinder is removed, and is air dried and then is heated (or fired) in a muffle furnace for about ten minutes at about 870"C. to cure it, after which it is removed from the muffle furnace and allowed to cool in room temperature air. The result is a form-retaining, heat shaped, short cylindrical container, useful for microwave ashing of ashable materials, such as analytical specimens.

The container looks like that of FIG. 8 and those of FIG. 7. Although the container is form-retaining, even during use at elevated temperatures as a container for ashable material during microwave ashing thereof, it retains its desirable porosity.

Alternatively, the container may be fired in a microwave ashing furnace like that illustrated in FIG. 7, at a higher temperature, 950"C., and the result is the same.

EXAMPLE 6 An ashing container in flat cylindrical form, essentially the same as that of Example 5 and FIG. 8, is made by wetting a 9 cm. x 9 cm. square of the same QM-A filter material with the same amount of water, forming it by means of a quartz mandrel, as shown in FIG. 9, into a flat cylinder, trimming such cylinder to desired 1.5 cm. height, and holding a side wall thereof to the mandrel by means of a quartz thread, also as illustrated in FIG. 9. The shaped cylinder, on the quartz mandrel, is then subjected to a curing heating to a temperature of 950"C. for ten minutes in a microwave furnace, like that of FIG.

7, after which the heating is discontinued and the mandrel and flat cylindrical container are removed from the microwave furnace and allowed to cool in room temperature air. After cooling, the container is removed from the mandrel and is ready for use with the thread in place or after removal thereof.

EXAMPLE 7 The container described in Example 5, which weighs 0.50 g., has added to it 2.01 g. of a check sample of wheat flour (from the American Association of Cereal Chemists) and to the sample in the container there is applied approximately 3 ml. of a 15 g./l.

ethanol (95%) solution of magnesium acetate, in such manner as to wet all the sample (and also to wet part of the container). The container of test sample, wetted with the magnesium acetate solution, is placed in the microwave ashing furnace of FIG. 7 after such apparatus furnace is brought to a temperature of 935"C.

and heating at such temperature is continued for ten minutes.

Such heating is then halted and the container of ash is removed.

The weight of flour ash and magnesium oxide is 0.02 g. and the weight of magnesium oxide (previously obtained experimentally for the volume of solution added) is 0.01% g. Thus, the cereal ash weighed 0.01 g., which corresponds to 0.05% of ash, which checks with results obtained by standard muffle furnace ashings (over a 90 minute period) of the same sample.

In variations of this experiment containers produced by the procedure described in Example 5 as alternative and by the procedure illustrated in Example 6 are substituted and the results are the same. Furthermore, when a plurality of samples is ashed at the same time, in a plurality of such containers in a microwave ashing apparatus, such as illustrated in FIG. 7, accurate results for each are also obtainable.

EXAMPLE 8 Containers within the invention that are made from a microfibrous filter paper that does not contain borosilicate glass (which is present in the QM-A filter material) can also be made by the processes described, with suitable heating temperatures being employed in the range of 50 to 1,0000C., such as 950"C., and will be satisfactory, even when only half the water is applied and when no water is applied beforehand (other suitable liquids, such as ethanol, may be substituted). Such containers are employable in microwave ashing apparatuses like those illustrated in FIG. 7 and accurate analytical results are obtainable, as is verifiable.by comparison with standard muffle furnace analyses of the same test samples.

In addition, ash analyses of other materials, including other grain flours, synthetic organic polymeric plastics, such as polyethylene and polypropylene, stream sediments, waste water sludges, coal, milk powder and many other ashable materials, are successfully performable using the described procedures and apparatuses. In such ashings the ashing temperature is varied within a 500 to 1,000"C. range and the ashing times are also varied, usually from 8 to 20 minutes, which will depend on the type of material being ashed and its ashing temperature. In all such instances satisfactory ashings and analyses are the results, which correspond with determinations made following standard muffle furnace procedures applied to the same test specimens.

Such good results are also obtained when the cylinder is covered by a flat cylindrical cover of the QM-A filter material, but use of such cover is noc necessary (although it may be considered to be a safety measure, to make sure that no ash is lost in the exhaust air).

The invention has been described with respect to illustrations, working embodiments and descriptions thereof but is not to be limited to these because it is evident that one of skill in the art, with the present specification before him or her, would be able to utilize substitutes and equivalents without departing from the invention.

Claims

1. An apparatus For ashing ashable samples which comprises a walled microwave retaining chamber, a source of microwave radiation for radiating onto contents of such chamber and an ashing furnace within the chamber having a furnace wall of heat resistant material about an internal furnace cavity, with an opening in said wall for insertion and removal of an ashable sample, a door of heat resistant material for closing and opening the opening in the furnace wall, a microwave absorptive material which is capable of being heated to an ashing temperature by microwave radiation, and passageway through the furnace for passing gas into the furnace cavity and for venting gas from said cavity, which heat resistant furnace wall and furnace door material is of low thermal conductivity and is essentially transparent to microwave radiation, which microwave absorptive material of the furnace has a surface thereof exposed to the furnace cavity, and which microwave retaining chamber wall has inlet and outlet openings therein for the passage of gas into and out of the chamber, around the furnace.

2. An apparatus according to claim 1 which comprises a temperature sensor, for sensing the temperature in the ashing furnace, and controlling means, responsive to the temperature sensor, for controlling te source of microwave radiation and thereby regulating the temperature in the ashing furnace.

3. An apparatus according to claim 2 wherein the temperature sensor is a thermocouple, and which comprises electrically connecting means for connecting the controlling means and the thermocouple, and an opening in an upper portion of the heat resistant wall of the ashing furnace, through which walled opening the thermocouple passes, with the opening being larger than the thermocouple so as to allow passage of gas from the furnace cavity to the chamber through a clearance between the opening and the thermocouple.

4. An apparatus according to claim 3 wherein the walled microwave retaining chamber has a doorway opening therein and a door to selectively close and open the doorway, with such door and doorway being aligned with the furnace opening and door so that ready access to the furnace cavity is obtainable through the chamber opening and furnace doorway opening when both the chamber door and the furnace door are in open positions.

5. An apparatus according to claim 4 wherein the ashing furnace wall is made of a quartz or ceramic foam which is stable at furnace cavity temperatures up to 1,000 C., the microwave absorptive material is silicon carbide, the source of microwave radiation is a magnetron, and the temperature in the furnace cavity is controllable within 10"C. of a set ashing temperature, up to 1,000"C., by the thermocouple and controlling means, which turn the magnetron on when the temperature in the cavity is less than the set temperature and which turn it off when the temperature in said cavity is more than the set temperature.

6. An apparatus according to claim 5 wherein the wall of the ashing furnace is made of an open celled fused quartz foam which is of a density in the range of 0.3 to 0.8 g./c. cm., the open celled fused quartz foam is stable at a furnace cavity temperature up to l,6500C., and the temperature in the furnace cavity is controllab'e to within 5"C. of â set ashing temperature in the range of 600 to 1,000"C. by the thermocouple and controlling means.

7. An apparatus according to claim 6 wherein the ashing furnace wall that defines the ashing cavity and the wall opening for the door is made of upper and lower unitary pieces of the open celled fused quartz foam and the furnace door is made of a separate piece of the same material.

8. An apparatus according to claim 7 wherein the ashing furnace wall defines an ashing cavity having top, bottom and side portions, with the sides and bottom being at least partially lined with silicon carbide, and the door portion in the furnace wall is substantially of trapezoidal shape in horizontal cross-section, and fits with an opening in the wall, facilitating ready removal of the door, partial opening, and closure thereof, as desired.

9. An apparatus according to claim 8 wherein the bottom portion of the ashing furnace includes grooves for insertion of silicon carbide strips or slabs therein, and the door includes handle or gripping means to facilitate holding the door with an operator's fingers when it is being opened or closed.

10. An apparatus according to claim 9 wherein there is present in the furnace cavity at least one heat resistant container for holding analytical sample(s).

11. An apparatus according to claim 10 wherein the container for the analytical sample is a light weight walled container of essentially microwave transparent quartz microfibers which allow gas flow there through but prevent passage of ash there through.

12. An apparatus according to claim 1 wherein the door for the furnace wall is removable from the wall to facilitate chargings of the furnace with containers of ashable samples and removals of such containers after ashings, when the furnace is still hot, which door has handle or gripping means on an exterior portion thereof, and is of a material of low thermal conductivity so that the handle may be grasped by the fingers of an operator while the interior of the door is at or near ashing temperature, without burning the operator.

13. An apparatus according to claim 1 wherein the wall and door of the ashing furnace are of an open cell ceramic foam which is transparent to microwave radiation and is of low thermal conductivity, the microwave absorptive material is silicon carbide, and the furnace door and wall opening are so shaped that the door may be partially opened to permit controlled air flow into the furnace through the passageway resulting.

14. A process for analyzing an ashable material for ash, which comprises adding a sample of such material to a heat resistant container, weighing the container containing the sample, inserting the container and sample into the internal furnace cavity of an apparatus of claim 1, ashing the sample by passing microwave radiation into said apparatus, while controlling the temperature in the furnace cavity so as to maintain it in the range of 400 to 1,600"C., removing the container, with resulting ash, after completion of the ashing operation, weighing the container, with such ash, and calculating the percent of ash in the sample.

15. A process according to claim 14 wherein the furnace ashing temperature range is 600 to 1,000 C. and the heat resistant container is a light weight walled container of essentially microwave transparent quartz microfibers which allow gas flow there through but prevent passage of ash there through.

16. A container for an ashable material to be ashed by heat in an ashing furnace, which container is suitable for use in an ashing furnace that is heated by microwave radiation of microwave absorptive elements thereof, which is heat resistant, walled, light in weight, microwave transmissive and porous, and is made of quartz microfibers which are held in walled container form.

17. A container according to claim 16 wherein the material of construction thereof is a non-woven thin sheet of quartz microfibers, which sheet has been heat cured to walled container form.

18. A container according to claim 17 wherein the material of construction is of a thickness in the range of 0.2 to 0.7 mm., of such porosity that the pressure drop across it is 1 to 5 mm. of mercury at 5 cm./sec. face velocity of air, resistant to high temperature, up to 500"C., retentive of micron size particles, transmissive of microwave radiation and of a weight in the range of 50 to 200 g./m. .

19. A container according to claim 18 which is of substantially flat cylindrical form, with the height/diameter ratio thereof being in the range of 1:5 to 2:5 and with the weight of the container being in the range of 0.2 to 0.6 g.

20. A process for manufacturing a container that is suitable for use as a container for ashable material to be ashed by heat from microwave radiation of microwave absorptive elements in an ashing furnace, which comprises shaping a light weight, microwave transmissive and porous sheet of quartz microfibers to container form and heating such sheet in such form, whereby a form retaining container results.