EP1913368A2

EP1913368A2 - Method and apparatus for measuring the elementary composition of wastewaters

Info

Publication number: EP1913368A2
Application number: EP06765402A
Authority: EP
Inventors: Sándor Lako; Tamás CSERFALVI; Pál MEZEI
Original assignee: Individual
Current assignee: Individual
Priority date: 2005-07-26
Filing date: 2006-07-26
Publication date: 2008-04-23
Also published as: WO2007012904A2; HU0500715D0; WO2007012904A3; HUP0500715A2

Abstract

The object of the invention is a method and apparatus for measuring the elementary composition of wastewaters. In the method and the instrument based on the method the flowing solution of uniform flow velocity and suitable electrical conductivity is forced to assume a free-surface stationary flow shape by means of a suitable hydrophilic boundary surface. A glow discharge is ignited between a metallic anode disposed at a predetermined distance from the surface of the solution and the solution acting as a cathode, and the intensity of light emitted by the glow discharge is measured at characteristic wavelengths of metals and background emitting components, and the concentration of metals is determined utilizing previously recorded calibration diagrams. The invention can also be utilized for measuring the metal content of communal wastewaters in a monitoring regime.

Description

4

METHOD AND APPARATUS FOR MEASURING THE ELEMENTARY COMPOSITION OF WASTEWATERS

Field of the invention and the description of prior art

The object of the invention is a method and apparatus for measuring the elementary composition of solutions, more particularly a method and apparatus that can be operated as an automatic monitoring device for detecting a specific range of metallic elements in water-based solutions with high contents of oily and greasy pollutants and floating materials, that is, solutions containing high amounts of emulsified/suspended disturbing components (typically wastewaters).

Taking into account environmental and other factors (e.g. aspects of cleaning technology) the order of importance of pollutant elements is the following: toxic metals (heavy metals), non-toxic metals (Fe, Mn, Na, K, Ca, Mg), other toxic elements (e.g. arsenic, selenium, halogens, etc.)

A wide range of known methods is currently in use for analyzing the metallic content of aqueous solutions. Apart from the so-called "classic" titration methods and gravimetric analysis, new instrumental analytic methods have produced significant development in both selectivity and sensitivity.

The historical starting point in the field of electroanalytic methods is polarography (Heyrovsky 1953, Nobel-prize), with derived solid-electrode methods (voltammetry) coming later. In favorable cases the selectivity of these methods may make it possible to detect 4-5 components simultaneously present in the sample, with the detection threshold being around mg/1. With the so-called anodic stripping method electrochemical preconcentration was introduced into the measurement process, and for certain metals detection thresholds reached the absolute detection value of one microgram . Beside the advantage of high sensitivity the above method has serious drawbacks. One of these is that complexing agents can disturb, or in certain cases even block measurements. Thus, organic materials of natural origin and organics introduced with communal-industrial effluents (immission level) cause strong disturbance. These instrumental analytical methods require very thorough sample matrix preparation, which means that the sample has to be measured in a (usually strongly acidic) base solution having high buffer capacity and high base electrolyte concentration. The need for the above measures virtually makes it impossible to utilize the method for monitoring purposes due to its high complexity and high reagent consumption.

Even so, as many as two metallic content-monitoring instruments have been produced in the past decade: a graphite-electrode voltammetric metal content monitor for monitoring industrial wastewaters was put out by ISTRAN Kft. (Bratislava, Slovakia), and an Israeli company was also advertising a similar instrument. These instruments, however, didn't prove to be practically applicable and they are now out of the market. Lately, a Budapest-based firm, Aqua-Control Kft. has complemented the laboratory-grade hanging mercury drop electrode polarographic system by Radiometer (Denmark) with an automated sampling and sample-treatment unit, and the new instrument is functioning as an experimental "monitor" at the water monitoring station on the Szamos river (on the border between Romania and Hungary). The operation of the instrument is, however, very costly due to high reagent consumption and the near-laboratory settings required by the highly complex technology.

Because the long-term reliability of measurements is deteriorated by changes in the background components of the wastewater, these developments were not able to produce a viable product for the world market of instruments.

The technology of ion-selective electrode sensors, introduced in the 1970's, is also a part of the group of electroanalytical methods. This technology is based on measuring the boundary surface potential, which requires a very simple measurement setup with a small-sized measurement cell and low energy demand. However, market success of the technology is seriously hampered by the narrow range of available sensors. Stable and sensitive sensors have been developed only for a few simple anions and a few cations. At the required speed of surface ion-exchange processes selectivity is in most cases greatly limited. It also has to be noted that electrodes can only detect non-complexed or aqua-complex ions, which usually represent a tiny fraction in practically measured samples. Only pH, pCN and pNHo measurements have industrial and environmental- monitor applications. The development of sensors based on this technology has come to a standstill, because, although the range of measurable ions widened with the introduction of the last innovation, liquid ion exchange membranes, the disturbance caused by organic materials contained in the samples increased by multiple orders of magnitude. The latter effect reduces sensor life to 1-5 days, after which the sensor membrane and the electrolyte system has to be replaced. Development possibilities are limited by the fact that a different sensor and different sample conditioning technique should be used for practically every ion type to be measured. Thus, the application of ion-selective sensors for the purposes of wastewater analytics remains limited.

Measurement technologies based on atomic emission and absorption are characterized by the huge selectivity of spectrometry, with capability of the simultaneous detection of 10-50 elements. High-sensitivity laboratory methods (AAS, ICP) can be successfully applied for measuring concentration values under one ppm, but they require thorough sample preparation. In most cases, the sample has to be colloid-filtered and heavily acidified. It has to be underlined that the sample solution can only be introduced into the measurement cell by some sort of liquid spraying technology (pneumatic, electrospray, ultrasonic, etc.) In addition, these instruments require bottled gases (AAS: acetylene and occasionally oxygen) or inert gas supply (ICP: argon, 10 1/min!) for their operation. Due to the applied 1-3 kW high-frequency excitation unit, ICP has very significant power demand.

However outstanding the sensitivity and selectivity values are, these advantages are eclipsed by the complex construction and the complicatedness of sample insertion. High energy consumption and the instrument's dimensions make it practically unfeasible to operate the instrument in a monitoring application.

The deeper analysis of the analytical chemical and physicochemical background of the technological problems arising with conventionally applied measurement methods has lead to the conclusion that known methods either do not meet the requirements of analytic monitoring applications or the range of measurable elements is very narrow. Fundamental research carried out by our team resulted in the discovery of a completely new spectroscopical light source, electrolyte cathode atmospheric pressure glow discharge (ELCAD), which was hitherto unknown in analytic chemistry. [T. Cserfalvi, P. Mezei, P, Apai, Emission studies of atmospheric pressure glow discharge using electrolyte cathode, J.Phys.D (Applied Physics), Vol. 26, 1993, 2184-2188], [T.Cserfalvi, P.Mezei: Process for atomizing electrolytes and a chemical analysis thereof, US Patent 5,760,897].

This measurement method comprises the main steps of adjusting the conductivity of the water sample to a reasonably high value by means of acidifying or the addition of alkali salts, introducing the sample into an 5-ml overflow vessel with a continuous flow rate of approx. 5-10 ml/min , and connecting it through a contact to the negative pole (electrolyte cathode) of a high-voltage DC power supply, with the positive pole of the discharge being provided by a metallic anode arranged above the sample's surface determined by the overflow rim. When a discharge is started between the metallic anode and the liquid surface, the surface layer of the sample undergoes sputtering by the generated water vapor glow discharge plasma, with the components leaving the surface being atomized and excited to emit atomic emission lines.

A condition for ELCAD emission (in case conductivity has been adjusted by means of acidifying) is that the pH of the solution should fall in the interval 1-2, which has to be provided utilizing a mineral acid. Due to the high-energy processes of the discharge

(90-100 eV at sputtering and 6-8000 K in the negative glow) the components dissolved in the sample are completely atomized, which means that complexing agents do not cause disturbances in metal content measurement. The bonded metallic content of floating materials, however, cannot be directly sputtered by ELCAD plasma. This means that it is not necessary to filter out the colloidal components and break down complexing agents of the wastewater sample, only the (dissoluble) metal content of floating materials should be dissolved before sampling by providing a suitable acidic pH.

Making the sample flow continuously through the instrument stabilizes liquid level and thus the discharge can be maintained continuously. The small cathode-side bright spot (approx. 0.5 mm in size) of the 3-5 mm glow discharge, the so-called "negative glow", is directed to the spectrometer measuring the atomic spectral lines by means of a quartz glass optics A necessary condition of the measurement is that the negative glow region should be projected into the entrance slit of the spectrometer in a stable, flicker-free manner. The stability of the surface of the flowing sample heavily affects the signal-to- noise ratio of the light emission detected by the spectrometer, and through it the reliability of the measurement.

An important characteristics of ELCAD emission is that the emitted characteristic radiation is relatively weak. This is an especially crucial parameter for detector design. Other important features of ELCAD emission are the special, disc-like geometry of the light-emitting volume and the constraints posed on possible detection directions, the most significant of which being the shading effect of the sample surface. After numerous attempts at increasing the intensity of the emitted light we have come to the conclusion that known solutions for collecting emitted photons and sending them into the entrance slit of the spectroscope (by a system built from discrete optical elements applied in spectroscopy, such as lenses, spherical, ellipsoid and paraboloid mirrors) are not utilizable if we would like to build a relatively small, portable instrument that can be used for field measurements. The greatest difficulty is posed by the fact that the image of the light-emitting region is disc-like, and therefore, in case high numeric aperture photon collection is utilized, the total collected light power can be sent into the polychromator only through a slit having a width which greatly exceeds the thickness of the emitting disc. Thus, in case we would like to apply a known solution for imaging the light-emitting surface onto the entrance slit of the polychromator using wide-angle photon collection with high transmission efficiency, the dimensions of the necessary optical arrangement would make it difficult to use the equipment outside a laboratory. Therefore the special imaging optics/polychromator system constitutes an important part of the devised small-sized instrument.

The introduction of ELCAD brought significant advances in the field of automatic water analytics by the combination of the following features:

- continuous flow-through cell multi-element measurement is provided utilizing atomic emission spectrometry (single measurement system for a whole range of metals from alkali to heavy metals)

- small-sized plasma (10 mm ), small-sized measurement cell (equipment size significantly reduced)

- no need for special gas supply, operated with surrounding air (expensive bottled gases not needed)

-no need for special reagents (only mineral acid is required for acidifying the samples) - power demand less than 100 W for plasma operation, simple DC power supply

With ELCAD spectrometry a narrower range of elements can be determined than with the application of ICP. For instance, non-metallic elements (among them As and Se, which are increasingly important from an environmental aspect) cannot be measured. The measuring range is also smaller, spanning at most two orders of magnitude. As it is indicated by research results, the detection limit for heavy metals, for samples free from floating material and organics (with a so-called capillary discharge cell), can reach the concentration value of 10-30 ppb [T.Cserfalvi, P.Mezei, Subnanogram sensitive multimetal detector with atmospheric electrolyte cathode discharge, JAAS, 2003,18(6) 596-602 ].

Based on the insights leading to ELCAD spectrometry, the development of solutions utilizing the emission of atmospheric pressure water vapor plasma has turned towards half-micro and micro-detectors, meeting the requirements set by laboratory-scale capillary separation methods [G. Jenkins, A. Manz, Optical emission detection of liquid analytes using a mikro-machined DC glow discharge at atmospheric pressure, μ TAS, October, 2001, 349-350] , [G. Y. Gianchandani et al.: Method and apparatus for glow discharges with liquid microelectrodes, US Patent Application No. 20030103205], [K. R. Marcus: Atmospheric pressure, glow discharge, optical emission source for the direct sampling of liquid media, US Patent 6,852,969] .

These analyzers uitlilize capillary-sized flow systems and are exclusively capable of analyzing clean samples that are free from floating materials.

It is important to note that the filtering out of floating materials from wastewaters may solve flow disturbance and blocking problems of the different measurement cells, but for the pH values commonly found in wastewaters the larger portion of metallic contaminants are present precisely in these materials. At first approximation the ELCAD method is less sensitive to floating materials, as only materials floating on the surface or in the thin surface layer are "visible" to the discharge plasma glowing at the surface of the electrolyte cathode. Despite this advantageous feature, with the utilization of the overflow-type measurement cell applied in the known solution the instrument is capable of analyzing wastewater samples only for a short time, because oily and floating contaminants getting deposited on the overflow rim obstruct the flow path of the liquid, cause disturbances in the flow profile, the position of the sample surface and the plasma is changed and thus instead of the negative glow the spectrometer will "see" other regions of the plasma. Deposits on the overflow rim will sooner or later short circuit the discharge.

Thus, it can be maintained that, with the exception of the ELCAD method, the instrumental analytical methods detailed above are not capable of being utilized for wastewater monitoring applications because they require the complete removal of floating materials (or other disturbing components) from the sample. The discharge cell of the original ELCAD method, which comprises an overflow-type cathode vessel, meets most of the requirements set by monitoring applications but its long-term stability is insufficient.

Considering water quality monitoring equipment currently in operation worldwide (taking into account the USA, the EU and Japan), there is no operating metal-content monitor with multi-element analytic capability. After a quick survey of instruments offered by international manufacturers (International Environmental Technology Journal, Vol. 14 May/June 2004) it can be stated that there is currently no wastewater monitoring instrument on the market for the measurement of metal content. This is basically the result of the lack of a method capable of averting the difficulties of huge disturbances caused by the complex composition of wastewaters.

SUMMARY OF THE INVENTION

Our invention will be presented with reference to the following drawings:

Fig. 1 illustrates the general flow shape applied in the invention. Fig. 2 illustrates the simplest implementation of the invention in the form of a gravity- flow discharge cell in plan view (Fig. 2/a) and side view (Fig. 2/b).

Fig. 3 shows a possible embodiment of the rotating-body flow guide cathode. Fig. 4 shows an embodiment of the rotating-body flow guide cathode applying axial sample infeed.

Fig. 5 shows details of the ionically conducting cathode contact implemented with the application of electrolyte (Fig. 5/a) and without electrolyte (Fig. 5/b).

Fig. 6 illustrates the configuration of the cathode applying a hydrophilic membrane, showing the arrangement in side view (Fig. 6/a) and plan view (Fig. 6/b).

Fig. 7 shows the top plan view (Fig. 7/a) and the side view (Fig. 7/b) of a preferred optical arrangement for the detection device.

DISCLOSURE OF THE INVENTION

We have recognized that a wastewater analysis method based on electrolyte cathode DC glow discharge (ELCAD) can be adapted for long-term monitoring of metal content in highly contaminated wastewaters by such an arrangement of the free-surface flow shape, flow route and the cathodic contact of the sample in the discharge cell where emulsified and suspended components are deposited at the operating area of the ELCAD plasma to a minimal extent, and the formed deposits do not affect flow shape for an extended period of time and can be removed in a fast, simple and easily automatizable manner from the ELCAD plasma area. In case of highly contaminated samples, one or both of two additional anti-deposit measures need to be applied: additives should be used to increase the coagulation stability of components, and/or ultrasonic energy should be introduced into the sample with a specific intensity chosen such that the introduced ultrasound increases the degree of dispersion and thereby works against coagulation processes.

According to our recognition the position stability of the light-emitting spatial area of he glow discharge can be most preferably provided by forcing the solution that is flowing with uniform velocity to assume a free-surface stationary flow shape by means of a suitable hydrophilic boundary surface. By igniting a glow discharge between a metallic anode disposed at a predetermined distance from the surface of the flowing solution and the solution acting as a cathode, and measuring the intensity of glow discharge emission at characteristic wavelengths of metals and background emitting components, the concentration of metals can be determined within a reasonable margin of error utilizing previously recorded calibration diagrams.

In the simplest monitoring appliance based on this method the stationary flow shape of the solution is produced on a surface that is disposed at an angle of 10-90 degrees with respect to the horizontal plane utilizing the balance of gravity and surface tension. The slanting surface is preferably of 0.5-10 cm in size, with the liquid being introduced from an upper and/or lateral direction.

In this appliance the stationary flow shape of the solution can also be produced on a rotating surface, utilizing the balance of centrifugal force and surface tension, where the liquid is introduced into the appliance at an arbitrary point, preferably at the axis of rotation, of the surface, the liquid spreading over the rotating surface and leaving the surface at the rim thereof. The rotating surface itself may be any surface of rotation, the only requirement being stable film formation. Thus, conical, funneled or negative cylindrical surfaces are also suitable, and the negative paraboloid shape can also be utilized. The size of the rotating parts is typically a few centimeters.

The cathodic contact may be located below the base point of the plasma, but it can contact the solution at another point. The contact itself can be ionically or electron- conducting. In case of an ionically conducting contact a closure comprising a hydrophilic membrane is preferred. In case of the appliance utilizing centrifugal force the contact can be realized in a geometrically advantageous manner by means of a circular electrode or at the solution inlet portion.

An important feature of our invention is the solution provided for the cleansing of the cathode surface. Mechanical cleansing of the cathode surface is made possible by the arrangement where the cathodic contact necessary for the glow discharge is produced at the base point of the plasma through a mechanically slideable separator layer and thus contaminants can be removed from the liquid-contacting portion of the discharge cell, located under the plasma region, as frequently as is necessary. The movable ionically conducting layer can for instance be a porous ceramic element with a thickness of 0.5-2 mm that can be cleansed after removing it by sliding from the discharge location. According to a more complex but still feasible arrangement the movable ionically conducting layer is a hydrophilic film applied in a tensioned state between two storage rolls. The film can be provided in an endless-loop arrangement. In order to delay the depositing of greasy and oily pollutants characteristic of communal wastewater a dispersing effect of a specific magnitude of 1-100 W/cm is applied, for instance by means of ultrasonic energy.

We have recognized that sample preparedness detection is a crucial step in the wastewater monitoring process that involves samples of highly different composition. Sample preparedness detection is carried out in the instrument by detecting that the specific conductivity of the sample has reached a given value, a significant increase of the concentration of free charge carriers appearing after the addition of mineral acid to the sample acts as an indicator of the fact that the sample has been saturated with the acid and the mobilizable contaminants have been dissolved. On the other hand, by adjusting the specific conductivity of the samples it can be achieved that cathode-side resistance remains the same (within the margin of error of detection) for different samples, which enhances the stability of the discharge. To prevent the contamination of the detector surface and thus ensure long-tem continuous operability of the instrument, the specific conductivity of the sample is measured by detecting inductivity changes of the sample flowing through a suitably formed flow pipe.

Detecting sample preparedness in the above described manner provides that the instrument is capable of reliable automatic analysis of the widely varying samples taken from mixed industrial/communal wastewaters.

According to our recognition it is an important feature of the imaging system that the optical axis of the photon-collecting optical element, or, in case a complex imaging system is applied, the optical axis of an optical element of the imaging system, should be arranged at an angle other than 90 degrees with respect to the surface normal of the cathode surface. It can thus be achieved that the images of such spatial portions of the light-emitting "disc" which have different positions when projected onto the plane spanned by the optical axis and the surface normal of the sample surface will be at different distances from the optical axis. This characteristics of the image makes it possible to treat separately these image point groups located at different positions, and that way transform them into a form suitable for being processed by the polychromator. The new image adapted for being processed by the polychromator can be a plane where the image spots belonging to given object points emit light approximately into the direction of the surface normal with a numerical aperture corresponding to the transformation. Such a transformation can be realized by means of a prism-like optical element/elements preferably having antireflective coating and being most preferably disposed in the light path in the the vicinity of the image generated by the imaging system. This prismatic element or elements disposed in the above manner are capable of efficiently changing the real or virtual focal length of image spots. Because the loss-free transmission of this transformed image still requires a slit width greater than the width of the emitting disc, a further transformation corresponding to the aperture of the dispersing element may be applied. This further transformation can also provide a projectable focal length changing effect for the image spots at different positions. It should be noted that the numeric aperture of focusing is usually smaller for directions in the plane spanned by the optical axis and the surface normal of the sample than for directions in the plane perpendicular to it. Thereby, the application of a cylindrical lens is preferable in order to utilize the numerical aperture of the dispersing element in both directions. In this case the prism and the transforming element can be a single optical element that can be implemented by the axial-direction truncation of a cylindrical body made of glass. The resolution can be further increased by applying a further focusing element in the polychromator before the position of the tested wavelength. In a preferred arrangement this focusing element is a short-focus cylindrical lens that is preferably disposed, for instance in the Seya-Namioka mounting, in the vicinity of the Rowland circle.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS WITH REFERENCE TO THE DRAWINGS

To ensure long-term flow stability of the electrolyte cathode of the discharge cell, according to our invention the solution is forced to flow in a film along a suitable solid substrate surface, where the flow shape is determined by the balance of liquid-solid and liquid-air interfacial tension and the driving force of the flow.

Interfacial tension values are dependent on the chosen materials. According to our recognition for analysing wastewater samples the substrate should have a wettable (hydrophilic) surface to provide stable operation. That way flow shape and flow shape stability are determined by the shape of the substrate surface and the magnitude and spatial distribution of the flow driving force.

We have recognized that from the aspect of stability the most beneficial flow shape is the film-type (two-dimensional) flow. Fig. 1 shows the general flow shape of the invention near the ELCAD discharge plasma. Driven by the flow driving force 3, the wastewater 1 flows along the hydrophilic solid substrate surface 3 with a layer thickness of 0.1-3 mm. The substrate surface can be any plane or curved surface or any combination of these, the only constraint on shape being that the flowing wastewater should not separate from the surface in the proximity of the plasma. An example of a combination of planes is the V-shaped channel, while curved surfaces can be for instance cylindrical, conical, or spherical surfaces. The wastewater is connected through the cathode contact 6 to the negative pole of the power supply applied for sustaining the plasma, the metallic anode 5 connected to the positive pole being disposed 0.5-5 mm above the surface of the solution. The plasma 4 gets stabilized after being ignited by spark or any other means at the location of the narrowest inter-electrode gap between the anode 5 and cathode 1.

The cathode contact connecting to the flowing wastewater film can be made of metal or graphite, but the preferred implementation is an ionic conductor having a conductivity sufficient for carrying the discharge current of 50-100 mA. An advantage of establishing the contact through an ionic conductor is that the formation of hydrogen (which would disturb flow stability and deteriorate plasma behaviour) can be avoided.

Details of the ionically conducting cathode contact arrangement can be seen in Fig 5. As it can be seen in Fig. 5/a, the wastewater film 1 flowing on the substrate 3 is in contact with the ionically conducting cathode contact 7, with a high-conductivity electrolyte solution 5 (preferably a 2-50% solution of a strong acid or its salt) being disposed in a reservoir element 4 behind the cathode contact 7. The electrolyte solution 5 is connected to the negative pole of the power supply through a non-corrodible metal electrode 6 (preferably Pt), or a graphite electrode. When discharge current passes through the electrode 6 a small amount hydrogen is formed which is vented into the air through port 2 in a safe way. The ionically conducting cathode contact 7 can be implemented as a gelatinous closure plug (made e.g. from agar-agar), a solid porous plug (e.g. grass frit, ceramic, etc.), or a hydrophilic membrane saturated with the electrolyte 5. The hydrophilic membrane can be a semipermeable membrane (e.g. a cellulose-based membrane, PVC membrane, dialysis membrane), or an ionically conducting membrane containing chemically bonded charged groups. Fig. 5/b shows an electrolyte-free connection arrangement, where a porous electron-conducting film or mesh 5 (metal or graphite) is mounted on the back of the ion-conducting membrane 7, with the metal electrode 6 being connected to this film or mesh. At the contact of the ion-conducting membrane 7 and electrode mesh 5 hydrogen is formed, which has to be vented out through port 2. The advantage of this solution is the electrolyte-free configuration, but it has the drawback of smaller current transfer capability, which makes it necessary to increase conductor cross section.

In the simplest of cases the driving force of flow is gravity, in which case no external energy is needed. In the arrangement shown in Fig. 2/a the flow of the flowing film 1 is sustained on the hydrophilic substrate surface 3 (set at an angle α with respect to the horizontal plane) with the help of continuous infeed 2 by the gravitational force. The cathodic contact is established through an ionic conductor 7 and a metallic contact 6 connected thereto.

As it can be seen in Fig. 2/b, in the practically feasible angular range α=10-90 the flowing film has a widening profile as the solution flows from the infeed point 2 downwards. At the ionically conducting contact 7 (which determines the position of the plasma) established opposite to the metallic anode the flow profile already has a stationary size and shape. We have found in our experiments that the plasma drives deposited material to the periphery of the wastewater flow, where it has a significantly smaller flow disturbing effect, and thus operating time between two cell cleaning pauses may be as long as 24-48 hours.

The application of the solution utilizing a hydrophilic membrane cover can further lengthen maintenance-free running time by a factor of 10-30. The requirement for a hydrophilic substrate surface and the establishment of cathode contact by means of a hydrophilic contact-membrane give a further possibility for providing a quick and automatizable solution for cleansing the discharge area. This can be achieved utilizing the arrangement shown in Fig. 6, where the hydrophilic membrane 7 covers the entire flow holding surface of body 3. The membrane 7 sticks to the surface of the body 3 due to a slight tensioning, but for the sake of increased safety clamping elements 9 (e.g. spring rollers or rods) can also be applied. This way the wastewater flows along the surface formed by the hydrophilic membrane, which has one of the cathode contacts detailed in Fig. 5 mounted at a suitable location. The solution is illustrated in Fig. 6/a by the cathode contact comprising an electrolyte. In case the hydrophilic membrane is applied in the above manner, the body 3 need not be made of a hydrophilic material, which means that any chemically resistant and not bulk conducting structural material is suitable. According to the arrangement shown in Fig. 6/b the hydrophilic membrane can be slid further into any direction, but preferably into the flow direction of the wastewater sample either in a pre-programmed manner or in case an increase of contamination degree is detected. For wastewaters without greasy contaminants the utilization of a wedge-like cleaning plate or any other suitable cleaning element provides that the surface portion of the hydrophilic membrane that has been removed from the plasma region is cleaned automatically and thus it is possible to apply an endless membrane comprising a looped return side 10. With this solution membrane demand can be reduced by as much as an order of magnitude, depending on the type of contaminants.

Alternatively, the wastewater sample can be forced to flow in a film-like layer by means of a centrifugal driving force. In this case the flow substrate surface is generally a rotationally symmetric surface, with a shape ranging from conical to spherical and even to a rotating disc. Rotating the surface about its axis of symmetry will effectively cause the wastewater introduced at the axis of rotation to spread evenly on the surface, with floating contaminants getting drifted to the periphery of the flow surface due to the significant force acting on them.

Fig. 3 shows a possible embodiment of the ELCAD cathode with a rotating-body flow guide surface, where wastewater sample is introduced at the top of the rotated conical base body. In case of a continuous in-feed 2 the wastewater is spread over the entire surface in the form of a liquid film 1, intensively flowing along the surface towards the circumference. To make cathodic contact for the liquid film 1 two solutions are provided, both of which are illustrated by the figure. A stationary slip ring 6 made of metal is arranged outside the rim of the surface with a narrow gap in such a way that it is in contact with the entire circumference of the liquid film 1, with the radial-direction flow preventing deposits from getting near the plasma 4 ignited between the anode 5 (disposed leaving a discharge air gap) and the water film. Contaminants building up on the collar electrode 6 can be removed easily.

A more advantageous (lower-resistance) cathodic contact to the liquid film can be provided utilizing an ionic conductor. In this case the ionically conducting liquid-free contact shown in Fig. 5/b is mounted inside a circular channel 7 recessed into the surface of rotating body 3 conforming to the position of the plasma. Contact to the electrode is provided through sliding contact 8, with the hydrogen that is formed on the contact being released through bores 9.

Another configuration of the ELCAD cathode with rotating-body flow guide surface is shown in Fig. 4. The wastewater sample 2, being fed through the rotating tubular axle, is introduced to the top portion of the cone from the interior of the rotating body, with the centrifugal force spreading the sample 2 evenly on the surface in the form of a liquid film. The plasma 4 is maintained between the anode 5 disposed axially and the water introduced to the tip of the cone. Cathodic contact to the water film can be provided by either one of the two solutions illustrated in the drawing. According to the first solution contact is provided to a slip ring 9 by means of a sliding contact 6, and the slip ring 9 is connected to the water passing through the axle tube through a solution-free ionically conducting contact 7 (details of the latter are shown in Fig. 5/b). The other solution involves an electrode 8 mounted in the outer surface of the rotating body, with the electrode 8 being connected through a metallic connection to the slip ring 9. The main advantage of the arrangement shown in Fig. 4 is that contaminants are removed from the base region of the plasma, and there is no other point on the surface where deposits could be accumulated.

The preferred optical arrangement applied for image transformation is illustrated in Fig. 7. Fig 7/a shows preferred position of the slit of the polychromator 3 in the general case when photons emitted by the plasma source 2 are collected by means of an ellipsoid mirror 1. Setting the slit at a non 90 degrees angle with respect to the midline of the light beam requires that the detection slit 5 should be arranged in the polychromator at an oblique position conforming to the image transformation to detect the image provided by the concave grating 6. Fig. 7/b shows a preferred arrangement of the prism body 4, indicating light paths. In this case the focusing element and the prism are implemented as a single combined element.

Claims

1. Method for measuring the metal content of solutions, characterized by forcing the flowing solution of uniform flow velocity and suitable electrical conductivity to assume a free-surface stationary flow shape by means of a suitable hydrophilic boundary surface, igniting a glow discharge between a metallic anode and the solution acting as a cathode, the metallic anode being disposed at a predetermined distance from the surface of the solution, measuring the intensity of light emitted by the glow discharge at characteristics wavelengths of metals and background emitting components, and determining the concentration of metals utilizing previously recorded calibration diagrams.

2. Implementation of an appliance working according to the method of Claim 1, characterized by that the stationary flow shape of the solution is produced on a surface disposed at an angle of 10-90 degrees with respect to the horizontal plane utilizing the balance of gravity and surface tension.

3. Implementation of an appliance working according to the method of Claim 1, characterized by that the stationary flow shape of the solution is produced on a rotating surface, utilizing the balance of centrifugal force and surface tension, where the centrifugal force is dominant over gravity, and where the liquid is introduced into the appliance at an arbitrary point, preferably at the axis of rotation, of the surface, the liquid spreading over the rotating surface and leaving the surface at the rim thereof.

4. Implementation of the appliance according Claim 2, characterized by that said surface is a slanting surface with a size of 0.5-10 cm , whereon the liquid is introduced from an upper and/or lateral direction.

5. Implementation of the appliance according to Claims 2 and 3, characterized by that the cathodic contact connecting to the solution is disposed in an arbitrary configuration in any region outside the vicinity of the base point of the plasma.

6. Implementation of the appliance according to Claim 4, characterized by that the cathodic contact connecting to the solution is disposed in the slanting surface under the base point of the plasma.

7. Implementation of the appliance according to Claims 5 and 6, characterized by that the cathodic contact connecting to the solution is made of an electron conducting material, preferably of precious metal or precious metal alloy.

8. Implementation of the appliance according to Claims 5 and 6, characterized by that the cathodic contact connecting to the solution is made of an ion-conducting material and comprises a closure consisting preferably of a hydrophilic membrane.

9. Implementation of the appliance according to Claim 3, characterized by that the cathodic contact connecting to the solution is constituted by a circular electrode of any conduction type disposed at the circumference of the rotating surface.

10. Implementation of the appliance according to Claim 3, characterized by that the cathodic contact connecting to the solution is constituted by a circular electrode of any conduction type disposed in the rotating surface.

11. Implementation of the appliance according to Claim 3, characterized by that the cathodic contact connecting to the solution is constituted by an electrode of any conduction type disposed in the solution inlet portion.

12. Implementation of the appliance according to Claim 3, characterized by that the rotating surface is a conical surface with a vertex angle of 10-270 degrees, where the axis of rotation can be set at any angle with respect to the perpendicular direction.

13. Implementation of the appliance according to Claim 3, characterized by that the rotating surface is the inner surface of a cylinder with a diameter of 10-200 mm.

14. Method for measuring the metal content of a solution with electrolyte cathode iy

glow discharge spectrometry, characterized by producing the cathodic contact necessary for the ignition of glow discharge on the surface of the solution of suitable electric conductivity at the base point of the plasma through a mechanically slideable separator layer and thus providing that contaminants can be removed from the liquid-contacting portion of the discharge cell, located under the plasma region, as frequently as is necessary.

15. Implementation of an appliance based on the method of Claim 14, characterized by that the movable ionically conducting layer is a porous ceramic element with a thickness of 0.5-2 mm, adapted for being cleansed after removing it by sliding from the discharge location.

16. Implementation of an appliance based on the method of Claim 14, characterized by that the movable ionically conducting layer is a hydrophilic film applied in a tensioned state between two storage rolls.

17. Method for measuring the metal content of a solution with electrolyte cathode glow discharge spectrometry, characterized by that a dispersing effect is applied in the step producing the solution having suitable electric conductivity, the dispersing effect being brought about by mechanical energy, preferably by the introduction of ultrasound.

18. Implementation of an appliance based on the method of Claim 17, characterized

9 by that the dispersing effect has a specific value of 1-100 W/cm .

19. Method for measuring the metal content of a solution with electrolyte cathode glow discharge spectrometry, characterized by that sample preparedness detection is carried out by measuring the specific conductivity of the acidified sample.

20. Implementation of the appliance based on the method of Claim 19, characterized by that the measurement of specific conductivity is accomplished through measuring the inductivity of a prepared sample that is passed through a flow pipe having suitable geometry.

21. Apparatus for detecting ELCAD emission, characterized by that the optical axis of the optical element applied for collecting the beam to be detected, or, in case a complex imaging system is applied, the optical axis of at least one optical element of the imaging system, is arranged at an angle other than 90 degrees with respect to the normal line of the sample surface, or, in case of a curved sample surface, with respect to the surface normals of said sample surface.

22. Implementation of the apparatus according to Claim 21, characterized by that an optical arrangement comprising a prism or prisms and/or a lens or lenses is disposed in the optical train after the beam collecting element, the prism(s) and lens(es) transforming the images of object point groups located at different distances from the optical axis set at an angle with respect to the normal lines of the sample such that the transformed virtual or real image points emit light with a given numerical aperture in a direction near perpendicular to the image plane.

23. Implementation of the apparatus of Claim 21, characterized by that a focusing optical element/arrangement is applied inside the polychromator in the vicinity of the detection site.