US3530290A - Field emission ion source for a mass spectrometer having relatively movable anode and cathode electrodes - Google Patents

Description

3,530,290 FIELD EMISSION ION SOURCE FOR A MASS SPECTROMETER HAVING Sept. 22, 1970 G. G. WANLESS ET AL RELATIVELY MOVABLE ANODE AND CATHODE ELECTRODES Filed Sept. 15, 1 967 3 Sheets-Sheet l ROS/fl VE VOZMGE C ONI'ROZ CA THODE VOZ 746E CON TROZ A MP1 //-'/ER5 AND REC ORD/NG APP/1 RA 71/5 DETECTOR FIG. I

INVENTORS 6. A. Glad, Jr:

PATENT ATTORNEY :5 Sheets-Sh'": 2

Sept. 22, 1970 G. G. WANLESS ET AL FIELD EMISSION ION SOURCE FOR A MASS SPECTROMETER HAVING RELATIVELY MOVABLE ANODE AND CATHODE ELECTRODES Filed Sept. 15, 1967 PATENT ATTORNEY w GI I m m I W I I I I. s z E f n C Mm 64 I I 6 6 I a I R I Q I I N f R & I Q R mm R I I I II II v \II a 6 @EERQO II I ,|l sll II 11 1 IF I $t 41 v I I QM, h N x r I ww M Q 7 r6 7/4 7! W I 6528 II SE28 $36; 6953 k 68:9. with? h I %AAV/ Q Sept-22, 1970 G. G. WANLESs ET Al. 3,530,290

FIELD-EMISSION ION SOURCE FOR A MASS SPECTROMETER HAVING RELATIVELY MOVABLE ANODE AND CATHODE ELECTRODES Filed Sept. 15, 1967 3 Sheets-Sheet 5 FIG. 6

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40 90 v ANGIE, Degrees G. G. Wan/ass G. A. Glocl', Jr] INVENTOPS FIG. 4

QMKQMW PATENT ATTORNEY United States Patent 3,530,290 FIELD EMISSION ION SOURCE FOR A MASS SPEC- TROMETER HAVING RELATIVELY MOVABLE ANODE AND CATHODE ELECTRODES Graham G. Wanless, Westfield, N..I., and George A. Glock, Baltimore, Md., assignors to Esso Research and Engineering Company, a corporation of Delaware Filed Sept. 13, 1967, Ser. No. 674,040 (Filed under Rule 47(a) and 35 U.S.C. 116) Int. Cl. H013 39/36 US. Cl. 250-413 4 Claims ABSTRACT OF THE DISCLOSURE An improved ionization source for a mass spectrometer which sources employ a tip array anode consisting of a multiplicity of individual anode tips arrayed on a support means. The anodes contemplated may consist of a row of evenly spaced tips or alternatively there may be two or more such rows.

FIELD OF THE INVENTION This invention relates to mass spectrometry and to means and methods for increasing the sensitivity of a mass spectrometer. More particularly, the invention relates to the use of field ionization in mass spectrometers and is specifically concerned with improved field-ionization anodes.

Mass spectrometry is one method of determining the chemical or elemental composition of a material. In the conventional mass spectrometer, the material to be examined is introduced into an ionization chamber and subjected to electron bombardment which causes the material to ionize. The ions thus generated are accelerated in a mass tube by an electrostatic field and are resolved into successive groups of individual mass by the dispersing power of a magnetic field. The relative abundance of each mass group is then measured and recorded in peaks on an oscillographic chart. Every ionized mass group has a distinct mass-to-charge (m/e) ratio and, therefore, should appear as a separate peak on the oscillographic record.

The present invention is directed to providing an improved apparatus and method of mass spectrometry. In contrast to the more conventional spectrometers which use electron bombardment for providing an ion source, the instant invention relates in particular to improvements in those spectrometers which make use of a field ionization source. Field ionization is a preferred kind of mass spectrometry which lead to simpler spectra. In this process, the sample of material to be analyzed is vaporized and the vaporized molecules are allowed to pass into a region of very high electrostatic field strength between an anode and a cathode which are about 2 mm. apart. In the region between the anode and the cathode an electrostatic field gradient on the order of about volts per centimeter is generated. This field gradient results in a stress which is sufficient to abstract an electron from a molecule. As a result a positive ion is formed and this ion is then quickly ejected from the region of high stress by the repulsion of the field-ion anode. From this point the mixture of ions is analyzed by any of the conventional mass spectrometric methods known to those skilled in the art. The field ionization technique yields a greater abundance of parent ions and lesser amounts of fragment ions as compared to the electron bombardment techniques and, hence, is of greater advantage in the analysis of mixtures of molecules.

The large field gradient of about 10 volts per centimeter referred to above may be established by using an "ice anode possessing a very small radius of curvature. F example, it may be calculated that a field-ion anode i the form of a thin wire having a diameter of 0.25 micro can create a field stress of 76 10 volts per centimete near the surface of the wire, while using a potential diffe; ence between the thin wire anode and cathode of 10,00 volts.

In the past three basic types of field-ionization anode have been used in mass spectrometers. The first of thee is in the form of an individual small radius tip made c some refractory material such as tungsten, platinum c gold, the radius of curvature of the tip being about 50 to 1000 angstroms and being obtained by electrolyticall etching the tip. Most of the published literature in thi field is based on such a tip anode. The second type make use of a razor blade as an anode; however, the use of thi type anode has been restricted because of limitations 0 the field stress which may be applied to the blade. Th third type uses the small diameter wire referred to above It should be appreciated that these fine anode wire: known in the art as Beckey-Wollaston wires, are ver fragile and difficult to make and to use. Off-setting thes disadvantages in the past has been the fact that sensitivf ties using such wire anodes could be to 1000 time greater than those obtained with single anode tips. A fur ther limitation is imposed on the use of each of th prior art anodes in that they can not take advantage 0 the angular dependence of ion abundances which corn from the tip of the source in order to increase signa strength, as may the anode of the instant invention, a will be discussed hereinafter. Thus, a specific purpose 0 the instant application is to provide a more durable an practical device which produces a stronger signal to serv as the field-ionization anode. This device will be referre to as a tip array anode.

SUMMARY OF THE INVENTION The improved anode of the instant invention comprise a multiplicity of individual anode tips side by side an. mounted on a horizontal support rod. The individual wire before they are sharpened by electrolytic etching may b 0.010 to 0.025 mm. in diameter and their spacings ma be in the range of from about 2 mm. to 10 mm. betweei individual wires. The tip-array anodes of the instant in vention may consist of one such row of evenly spacer tips or alternatively may be of two such rows in paralle or several such rows. In the latter case the device wil be referred to as a pin cushion array. While certai1 arrays have been used in the past in X-ray tubes and elec tron microscopes, their function therein has been that of field-emitter. At this point it is important to distinguisl between field-emission and field-ionization. A field-emitte is a cathode capable of providing a very high density nega tive electrical discharge. Field ionization, as hereinbefon discussed, is a newer development and uses reverse elec trical polarity so that the small radius device is an anodl in this case. Thus, the instant invention is directed to th application of a tip array device as a practical anode ii the field ionization mass spectrometer source.

Thus, an object of the invention is to provide a tip array anode for use in a field ionization mass spectrom eter source.

Another object of the invention is to provide a men durable anode with capabilities for greater sensitivitie: than can be obtained with the anodes of the prior art.

Yet another object is to provide an anode which, because of the way it forms ions, yields a greater abundanct of desired primary metastable ions relative to the abundance of parent ions than is obtainable with the presentl used anodes of the art.

Still another object of the instant invention is to provide an anode which can take advantage of the angular 3 Iendenee of the ion abundances which come from the rce in order to increase signal strength. these and further objects as well as a fuller underiding of the invention may be had by reference to the ompanying detailed descriptions and by referring to drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS IG. 1 is a simplified schematic illustration of a con- .tional mass spectrometer incorporating the improved de of the instant invention.

IG. 2 is an enlarged View of the area enclosed by dotted line in FIG. 1.

*IGS. 3A, 3B and 3C depict preferred configurations the improved anode of the instant invention.

IG. 4 is a graph depicting the angular dependency of rg-y coming from a single field-ionization anode.

IG. 5 depicts schematically the energy cones emergfrom the tips of a plurality of anode tips.

16. 6 indicates schematically the preferred orienta- 1 of the tip-array anode with respect to the slit in the lode.

BSCRIPTION OF PREFERRED EMBODIMENTS eferring now to the figures in detail and particularly FIG. 1, a schematic diagram of a simplified mass specneter employing the improvement of the instant inven- 1 is shown. That portion of FIG. 1 not enclosed within dotted lines operates in a conventional manner. It is lprised of a mass spectrometer tube 10 provided with ionization end 11, a magnet 12, and an ion collector detector end 16. An ionization head generally indi- 2d at 13 is afiixed to the ionization end 11 of mass :trometer tube 10. The operations of head 13 will be :ussed in greater detail in conjunction with the dission of FIG. 2.

:1 operation, the material to be analyzed is admitted the ionization head 13 by means of a suitable inlet Fhere the sample is ionized and the ions produced in head are expelled from the ionization chamber into tube 10. As the ionized particles travel through the as tube 10, they are resolved into separate homoous beams such as 15, 17 and 19' by means of a magc field produced between the poles of the electrognet 12. The strength of the magnetic field in magnet may be controlled and is a function of the current ring through the winding 14. At any given strength of imposed magnetic field, only that homogenous ion m, e.g. beam 17, whose mass-to-charge ratio (m/e) responds to the strength of the magnetic field can otiate the curved section of the mass tube and pass )ugh the collector slit 21. The ions passing through the ector slit impinge on a collecting electrode (not Wn) within the detector head 16- and produce a small rent. This current is amplified and recorded by means 1 suitable apparatus 18. An oscillographic chart (not wn) produced by the recording apparatus 18 indi- :s the extent of resolution.

referring now to FIG. 2, an enlarged view of the ionion head 13 of FIG. 1 is shown. The head 13 may be veniently referred to as a dual field-ionization and :tron impact source. In this regard head 13 consists of upper portion 6 and a lower portion 8, portion 6 con- .ing What shall be designated as a tip array field ionion source while portion 8 contains an electron-impact rce of ions of the conventional type which is known he art as a Nier-type source. Referring further to the array field ionization source contained in portion 6 head 13, an anode support rod 26 is maintained in ition in the upper part of portion 6 by the insulated nector 24. At its distal end, support rod 26 carries anode holding member 28, which is secured to rod 26 means of the set-screw 30. In a preferred embodiment de holding member 28 may have its lower portion 31 )tably mounted on its upper portion 29, so that the uded angle between these portions may be adjusted.

The significance of this arrangement will be discussed hereinafter. The lower portion of anode holder 28 supports a tip-array anode generally indicated at 35, which is comprised of a mutiplicity of individual anode tips 34, which are mounted side-by-side on a tip support rod 32. The individual wires comprising the said multiplicity of anode tips may be from 0.0100 to 0.0250 mm. in diameter (before electrolytic etching) and their spacings may be in the range of 2 to 10 mm. between the individual wires. As previously indicated, a tip-array anode may consist of one such row of evenly spaced tips or alternatively there may be two such rows in parallel or several such rows. Preferred configurations of such tip-array anodes will be discussed in further detail pursuant to the discussion of FIG. 3.

Positioned below tip-array anode 35 is a cup-shaped cathode 36, defining in its bottom portion a slit 41. Through the use of a suitable control means 5, cathode 35 may be maintained at a negative potential in the range of --500 to -10,000 volts by the lead 9. It is to be appreciated that the cathode is carefully insulated from the walls of head 13. Similarly, the potential of the tip-array anode may be suitably controlled within the range of +1000 to +l2,000 volts by use of positive voltage control 7 connected to holder 28 by lead 43. With the tiparray anode having a potential of +3000 volts and with the cathode potential set at -7000 volts, it can be calculated that each one of the individual ion anode wires comprising the tip-array can create a field stress of approximately 73 10 volts per centimeter near the surface of the wire. This stress is sufficient to abstract an electron from a molecule of the sample entering the head through the inlet line 4. As a result, a positive ion is formed and this is quickly ejected from the region of high stress by the repulsion of the field-ion anode. A multiplicity of ions so formed will thus emerge from the slit 41 in the cathode 36. This beam then passes through the slot 58 of the box 42 of a conventional electron-impact source. With the tip-array anode in use, box 42 provided with tap terminal 56 may be maintained in the voltage range of 0 to +2000 volts and preferably in the range of to +200 volts by the variable resistor 47 which is connected via the lead 45 to control 7. The ion beam then continues through the box 42 and passes through the slit 66 located in the bottom thereof. It then enters a slit 69 defined by a pair of drawout plates 68 and 70. For purposes of discussion these plates will be designated with respect to their relative position as indicated in FIG. 1; that is, plate 70 will be referred to as the upper drawout plate and plate 68 shall be designated as the lower drawout plate. Each of these plates is provided with a separate terminal 72 and 74 respectively, so that each plate may be maintained at a different potential by the use of resistor 47 and control 7. The function of this pair of plates is to straighten out the beam as it passes between them. Thus, for example, if the beam was following the curvilinear path indicated by the dotted arrow a in FIG. 2 as it approached the plates 68 and 70, its path could be made to follow the arrow 12 by suitably adjusting the potentials on plates 70 and 68. In the preferred embodiment the upper plate may be maintained at a voltage in the range of 1400 to 1500 volts. The lower plate may be maintained at a potential of about 400-500 volts less. However, it is to be appreciated that the difference in voltage between these two plates will be subject to continuing adjustments so as to translate any curvilinear motion of the beam into rectilinear motion as the beam passes through slit 66 between the two plates. The ion beam then passes through the slit defined by the plates 78 and 80, which may also be maintained at different potentials by resistor 47. This pair of plates functions in a similar manner to plates 68 and 70 and may be represented as providing fine tuning for the focusing of the beam, whereas plates 68 and 70 provide the initial adjustment. These plates may be maintained in a lower voltage range, for example, 100 to 200 volts and the voltage difference between them may be in the range of 50 to 150 volts, which difference is also subject to adjustment to provide the required focusing. The straightened beam then passes through the slit defined by the plates 88 and 90. These plates are usually maintained at 0 voltage. It will be appreciated that the pairs of plates previously described serve to adjust the path of the beam in the plane of FIG. 2. Adjustment of the direction of the beam in a plane perpendicular to that of the figure is accomplished by means of the plates 92 and 94, which may be maintained with a suitable voltage difference so as to center the beam in the middle of tube While the ionization head 13 may be equipped solely with the tip-array anode device discussed in the above paragraphs, it may also be conveniently equipped with an electron-impact source of the conventional Nier type. This gives a somewhat extended flexibility of operation in the analysis of molecules which vary in their ability to be ionized. The workings of the Nier-type electronimpact source may be conveniently understood by once again referring to FIG. 2.

A current is passed from the filament controller 3 through the filament 50 via the terminals 52 and 54. This flow of current causes the filament to emit a steady stream of electrons. The material to be analyzed is introduced through the inlet 4 and works its way down into box 42. There the sample is subjected to the electron beam C emitted from the filament 50. As the electrons of beam C impinge upon the molecules of the sample, ions are formed. These ions which are, of course, positively charged particles are ejected from the box 42 and through the slots defined by the various pairs of plates 68 and 70, 78 and 80, and 90 and 88. To insure that the electron beam follows substantially the path indicated by the dotted line C, a collector plate 62 provided with a terminal tap 64 is maintained at a voltage supplied by control 7 through the lead 51, which is more positive than that of the filament or the walls of the chamber 42. Thus, in a typical application the filament would be maintained at a voltage of approximately +2930 volts. The walls of the box 42 would be maintained at a voltage of approximately +3000 volts and the electron collector 62 would have a voltage in the range of +3080 volts. When the electron-impact source is in use, the pairs of plates located under box 42 would function in much the same manner as previously described; however, they would be operated in somewhat different voltage ranges. Thus, plates 68 and 70 would be operated in the range of 2000 to 3000 volts, whereas plates 78 and 80 would be operated in the range of about 150 to 300 volts.

Reference will now be had to FIG. 3 to illustrate three of the preferred embodiments for the improved tip-array anode of the instant invention. FIG. 3(A) represents a single row tip-array anode. As may be seen, it consists of a multiplicity of individual small diameter wires 34 which are arranged side-by-side on a support rod 32 and in good electrical contact therewith. The individual wires preferably have diameters in the range of 0.0100 to 0.025 mm. and each wire has a tip which has a radius of curvature preferably in the range of about 500 to 1000 angstroms, this radius being obtained by electrolytically etching the wires. The spacings between the individual wires are preferably in the range of 0.0005 mm. to 0.0025 mm. Another form of the tip-array anode is indicated in FIG. 3(B). The anode of FIG. 3(B) is similar in construction to that of FIG. 3(A) except that a double row of individual wires is provided. These rows are designated e and f in FIG. 3(B). Stille another preferred form of the tip-array anode is depicted in FIG. 3(C). This has previously been referred to as the pin cushion configuration and it consists of a multiplicity of rows of individual wires 34 which are maintained in position by a conductive block 37 in which the wires are embedded.

Experiments to be herein subsequently discussed wer conducted using tip arrays which were fabricated fror tungsten. However, in this regard it is to be appreciate that inert materials such as gold, platinum, irridiun rhodium, and rhenium are particularly suitable for fabr cating these anodes.

A particular advantage of the anode of the instar invention is its durability as compared to the devices c the prior art. The Beckey-Wollaston Wires hereinbefor discussed of the usual 2.5 micron diameter are extremel fragile and are susceptible to breakage even due to sligl'. jolts during handling. Their use over any period of tim depends on following the most exact operating procedur and allows very little room for deviation. By comparisor a tip array of the type herein described and shown i FIG 3B was used under a wide variety of condition over a period of 48 days with no problems. In additior it has experimentally been established that the tip-arra anodes of the instant invention can deliver a larger signa than one can obtain using a Beckey-Wollaston wire 0 an individual anode tip in the same mass spectromete source. In this regard reference should be had to th first two entries in Table I below, which presents th sensitivity of acetone in chart divisions of an oscillc graphic readout chart per micromole of acetone using Beckey-Wollaston wire and using a tip-array anode o the type depicted in FIG. 3B. It may be seen that th sensitivity has been increased by approximately 39%.

TABLE I Type of field-ion anode:

(l) Beckey-Wollaston wire 39,000 chart division per micromole.

Sensitivity of acetone A comparison of entries 2 and 3 leads to the discussio1 of yet another improvement which may be achievet utilizing the teachings of the instant invention. It has bee] shown in the prior art that the energy coming from single field ionization anode; that is, the pattern of posi tive ion density has an angular dependency. As the elec trostatic field stress is increased (as previously indicatee this is a function of the potential difference between thr anode and the cathode) there is an increase in the tota number of positive ions. In addition, with increasing fielr stress the locus of the energy distribution of these ion; emerging from the tip becomes the surface of a solie cone. As the field stress becomes greater, the interna angle of this cone becomes larger until it maximizes a a given angle for a given field stress. This angular depend ency is depicted in the graph of FIG. 4. Thus, it may be seen, for example, that with a potential between the anode and the cathode of 16,000 volts, the greatest pos sible ion density will be defined by a cone having at apex angle of that is, when viewed in cross-sectior the sides of the cone will form an angle of 40 with the line which defines the direction of the individual wire anode. If one now arranges many such tipe in a double row configuration as shown in FIG. 5 (this represents the specific embodiment of the anode of FIG. 3B), one should expect a certain circle of confusion in the fielc' between each of the rows of individual anode tipe where the cones intersect. Also, the net result will be a max imization of energy in the planes designated HLJK anc' LMP of FIG. 5. Thus, the geometry indicates that the locus of maximum energy will be in the range of about 40-50 from the vertical under the conditions above :d. The configurations of the anodes depicted in 38. 3A and 3B allow this angular energy distribution be efiectively utilized. Thus, if the anode of the instant ention is located with respect to the slit in the cathas diagrammatically indicated in FIG. 6, an improvent in the signal strength may be achieved. The result y be demonstrated by referring to the second and third ries in Table I above. Thus, as indicated in entry 2 ere the tip array was mounted vertically and parallel the slit in cathode 41, the sensitivity of acetone was asured at 54,000 chart divisions/micromole. However, an the tip array was mounted at 40 to the vertical, indicated in FIG. 6, and still being parallel to the in cathode 41, an increase of approximately 52% the sensitivity was achieved. This finding demonstrates referred way in which these tip arrays may be oriented h respect to the cathode slit to yield increased signal :ngth. It also teaches that the mounting angle of the array should be correlated with the operating con- .ons of the source; that is, the potential difierence existbetween the cathode and the anode. By having the I61 portion of holder 28 pivotably mounted on the )er portion 29 as hereinbefore recited, the mounting gle of the tip array may be easily adjusted to produce Jal strength. itill another advantage of the instant invention is that :an make better use of metastable ions for purposes of lecular structure determinations. This may be exined by the fact that field ionization of the type herein cribed may be considered to be a more gentle way forming ions. That is to say that the individual molees of the sample will be less subject to increases in gmentation where an electron is withdrawn by the use field ion anodes, than Will be the case where the molee is bombarded by a stream of electrons. It has been rblished that the tip-array anodes of the instant inven- 1 Will deliver a greater abundance of metastable ions ltive to the abundance of the parent ions than will a e anode of the Beckey-Wollaston type. This may be u by comparison of entries 1 and 2 in Table II. There is indicated that starting with a sample of n-octane, ich produces a parent ion having a mass-to-charge ratio 114, more of the metastable ion having a mass-to- .rge ratio of 63.38 will be produced using the tip array 11 will be produced using the Beckey-Wollaston wire.

TABLE II Abundanees of ions in n-octane Parent ion Metastable ion e of field-ion anode (m/e) 114 (in/e) 63.38

eokey-Wollaston wire 100. 11. 2 ip-array anode (type 3B) 100. 0 15. 4

portion being fixedly mounted and said second portion being operably mounted on said first portion, one of said first member and said second portion being pivotably mounted for movement relative to the other of said first member and said second portion to allow adjustment of the angle defined between facing surfaces of said first member and said second portion; and

(c) an anode comprising a support rod having a central portion and end portions substantially perpendicular to said central portion, and a multiplicity of in dividual anode tips arrayed from said central portion to form at least a single row of said tips, each row defining a plane which is substantially parallel to the plane defined by said end portions, said end portions being affixed to said second portion of said anode holder, and means for maintaining a potential between said anode and said cathode, said angle be ing adjusted as a function of said potential.

2. The source of claim 1 further characterized in that said cathode is fixedly mounted and said second portion is pivotably mounted on said first portion thereby to define an adjustable mounting angle between said portions, and said potential is in the range of from about 7000* to about 17,000 volts and said mounting angle is in the range of from about 30 to about 50.

3. In a mass spectrometer an improved field ionization source which comprises in combination:

(a) an anode comprising a support rod and a multiplicity of individual anode tips affixed to said support rod and in electrical contact therewith, said tips defining at least one row;

(b) a cathode located below said anode and having a slit which runs substantially parallel to said support rod;

(c) said row being disposed at an angle in the range of from about 30 to about 50 from a plane drawn along the length of said slits and perpendicular to the plane of said slit; and

(d) means for maintaining a potential field gradient between said cathode and said anode.

4. In an improved field ionization source for use in a mass spectrometer comprising in combination:

(a) an anode holder having first and second portions, said second portion being pivotably mounted on said first portion to allow for adjustment of the mounting angle defined between said portions; and

(b) an anode including a support means afiixed to said second portion and having an outer surface, and a multiplicity of individual anode tips arrayed from said outer surface to form at least a single row of said tips, each row defining a plane which is substantially perpendicular to said outer surface.

References Cited UNITED STATES PATENTS 2,809,314 10/1957 Herb 250-413 3,405,263 10/1968 Wanless et a1 250'-41.9

FOREIGN PATENTS 1,171,641 6/1964 Germany.

RALPH G. NILSON, Primary Examiner E. CHURCH, Assistant Examiner

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