Method And Apparatus For Analysis Of Impurities In Air And Other Gases

Abstract

Mass spectrometric analysis of trace impurities including particulate matter in air or other gas utilizes a heated metallic filament on which a sample of the gas is impinged to generate short bursts of ions of the impurity (in the case of particulates) by surface ionization. The gas sample is introduced to the heated filament by passage through a thin tube and impingement on a very small hole prior to entering the chamber in which the filament is located. A pump removes most of the gas between the tube and hole and the particles passing through the hole are directed at the heated filament. An ion lens generates an electric field which withdraws the ions from the heated filament surface and a conventional mass analyzer is used for mass analysis of the ions.

Description

My invention relates to mass spectrometric analysis of impurities in a gas, and in particular, to a method and apparatus utilizing surface ionization of the impurities on a heated filament for producing the impurity ion which is analyzed by a conventional mass analyzer.

Mass spectrometers have long been used for the analysis of trace gaseous impurities in air and other gases, and the simple mass spectrometer usually utilizes a conventional electron bombardment ion source for generating the ions of contaminant molecules to be analyzed. A mass spectrometer using only a single stage of mass separation is limited in its application by reduced sensitivity due to the generally broad background of scattered ions, principally N.sub.2 and O.sub.2 that are ionized as normal gases in an air sample and arrive at the detector of the mass spectrometer either by gas scattering or bouncing off the walls of the analyzer tube. The magnitude of this background depends on mass position and gas pressure which typically makes analysis below one part per million (p.p.m.) difficult. More complicated mass spectrometers using additional stages of mass analysis and, or energy analysis, to more effectively filter out these unwanted gas ions can reduce the background to the p.p.b. range or lower, but at this level, discrete ion peaks due to a multitude of various kinds of ion-molecule reactions and other side effects becomes a major problem and the instrument is no longer small and simple. The ion-molecule reactions occur, for example, by reactions between the various ions of N.sub.2, O.sub.2, Ar, H.sub.2 O and other common air constituents. Electrons hitting the surfaces of the ion source can also produce unwanted ions. To eliminate these problems, it would be desirable to ionize only the impurities in the air or other gas and not the more common major components in such air or other gas sample.

Continuous analysis of particulate matter in the gas sample presents additional problems for the conventional mass spectrometer since the introduction of the sample cannot be accomplished with a simple gas leak. The only previously known method has been to collect the sample on a filter and then by mechanical means or by chemical treatment, transfer the sample from the filter to the ion source of the spectrometer. Electron bombardment ion sources can only be used if the sample is vaporized separately and this would probably decrease the net yield of ions. Finally, the particles which can be analyzed by means of electron bombardment ion sources are generally limited to molecules and do not include particulate matter.

Therefore, one of the principal objects of my invention is to provide direct mass spectrometric analysis of trace impurities in a gas by utilizing surface ionization to produce the ions to be analyzed.

Another object of my invention is to provide mass spectrometric analysis of particulate matter in a gas sample by utilizing the surface ionization process.

Another object of my invention is to obtain the mass spectrometric analysis on a continuous basis.

A further object of my invention is to obtain the mass spectrometric analysis by ionizing only the contaminant particles and not the major components of the gas sample.

A still further object of my invention is to provide a method and apparatus for the mass spectrometric analysis of trace impurities in a gas sample which has a higher sensitivity than conventional simple mass spectrometers and is of simple construction.

Briefly stated, my invention is a method and apparatus for the analysis of impurities in air and other gases which utilizes surface ionization to produce the ions of the contaminant particles without ionizing the major components in the gas sample. Particles are defined herein as including both molecules and particulate matter. The gas sample is introduced to a heated filament by passage through a thin tube and impingement on a very small hole in the wall of the chamber in which the filament is located. A rough pump removes most of the gas between the tube and hole and the particles passing through the hole are directed at the heated filament. An ion lens positioned in the chamber adjacent the heated filament generates an electric field which withdraws and focuses the ions from the heated filament surface, and a slit aperture defines the particle ion beam which is directed to a conventional mass analyzer. Each contaminant particulate generates one short burst of the ions by surface ionization, and the number of ions per burst is a measure of the amount of the particular contaminant per particulate as measured by a suitable electron multiplier at the output of the mass analyzer. Continuous mass spectrometric analysis is obtained with my method and apparatus. If continuous analysis is not necessary, the gas sample can be impinged on the cold filament for a given length of time, the gas then pumped out and the filament heated. The integration over the time of the gas stream impingement on the cold filament produces increased sensitivity and does not require the rapid pumping of large amounts of the gas as under continuous analysis conditions. The heated filament may be in the form of a ribbon of a metal such as tungsten or rhenium. An alternative form of the heated filament is a long heated platinum capillary through which the gas sample is introduced into the chamber and the impurity particle ions are produced at the heated surface of the capillary by surface ionization and withdrawn by the ion lens.

The features of my invention which I desire to protect herein are pointed out with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation, together with further objects and advantages thereof may best be understood by reference to the following description taken in connection with the accompanying drawings wherein like parts in each of the several figures are identified by the same reference character and wherein:

FIG. 1 is a diagrammatic view of a first embodiment of the ion source chamber of my mass spectrometer in accordance with my invention but rotated 90.degree. counterclockwise from its normal orientation;

FIG. 2 is a top view of my mass spectrometer;

FIG. 3 is a side view, partly in section, of the mass spectrometer illustrated in FIG. 2;

FIG. 4 is a detailed side view, partly in section, of the ion source chamber illustrated diagrammatically in FIG. 1 but oriented in its normal position as shown in FIG. 3;

FIG. 5 is an exploded view of the ion source and ion beam forming electrodes depicted in FIG. 4; and

FIG. 6 is a diagrammatic view of a mass spectrometer illustrating a second embodiment of the ion source.

Referring now to FIG. 1, there is illustrated in diagrammatic form the essential elements of the ion source of my mass spectrometer and which constitutes the novel aspects of my invention. The ion source is enclosed in a chamber 10 which is partitioned to include a gas sample inlet chamber 10a therein. Although my spectrometer is adapted for use with various types of gases, its operation will be described with reference to an air sample containing various particle contaminants wherein, as defined hereinabove, the particle may be merely molecular in size, or be the larger size particulate matter.

The interior of chamber 10 is maintained at a relatively high vacuum in the order of 10.sup.-5 torr and gas samle inlet chamber 10a is maintained at a rough vacuum of approximately 1 torr. Due to the vacuum within chamber 10a, an air sample is drawn into chamber 10a through a length of small diameter metal tube 11 which passes through an outer wall of chamber 10a. A valve 17 in tube 11 controls the air inflow. Aligned with the end of tube 11 is a very small hole 12 in the partitioning wall of chamber 10a, and the rough vacuum in chamber 10a removes most of the air which passes through tube 11 and is impinged on hole 12. Some of the particles (molecules or particulates) in the air sample pass through hole 12 and impinge on a heated filament in the form of a U-shaped metal ribbon 13 which is heated directly by d.c. electric current to a temperature in the range of 700.degree. to 1,700.degree. C. by means of electrical conductors 13'. Filament 13 is fabricated of a metal which yields a high work function, especially when heavily oxygen covered, such as tungsten, rhenium, iridium or platinum. The metal filament 13 is oriented such that particles passing from tube 11 through hole 12 impinge upon a major surface of the metal filament 13 at an angle which is not critical, but is such as to have the particles impinge at or somewhere near the center of the filament major surface. The particles impinging on the heated filament are evaporated as ions of the particles, i.e., the ions are generated by surface ionization.

The ions produced as a particle impinges on the surface of filament 13 are generated as a short burst of ions for each particle, and the number of ions per burst is directly proportional to the amount of the particular contaminant element or compound per particle. Thus, one molecule can generate no more than one ion and several ions per burst are generated for each particulate. The ions are withdrawn from the filament 13 surface by an electric field produced by two pairs of ion beam focussing electrodes forming an ion lens 14 which is closely spaced to filament 13 and aligned therewith. Elements 14a and 14b of the first pair of the focussing electrodes located closest to filament 13, are connected to a common d.c. voltage by means of conductor 14a'. Elements 14c and 14d of the second pair of focussing electrodes are connected to two slightly different d.c. voltages, for centering the ion beam due to any system misalignment, by means of conductors 14c' and 14d'. A pair of ion beam defining slits 15 for defining the width of the ion beam directed toward a mass analyzer are positioned on the opposite side of the ion lens 14 from filament 13 and are aligned with both the filament and ion lens. The two slits 15a, 15b, are at ground potential, and the slit 15a closer to the ion lens 14 is wider than the slit 15b which is closer to the mass analyzer. The width of the ion beam, as determined by defining slits 15 is one of the primary characteristics in determining the resolution of a magnetic sector mass analyzer. The short burst of ions after passing through defining slits 15 enters a mass analyzer 30 which, as one example, is a conventional small, portable, 90.degree. sector, magnetic deflection-type analyzer, and which functions to separate the ions according to their mass, the analyzer being controlled for selecting the ions of the particular element or compound in the gas (air) sample sought to be analyzed. The output of the mass analyzer is coupled to an ion detector in the form of a high gain electron multiplier for sensing the particular ions being analyzed.

The ions produced by the surface ionization have only thermal energy and thus only a simple mass analyzer is required for mass analysis, that is, an energy analyzer is not required in my spectrometer. The success of my particular method for analysis of impurities in the gas depends to a great extent on the presence of oxygen in the sample. The oxygen in the air sample is adsorbed on the heated filament 13 and increases its work function. The higher the oxygen pressure and the lower the temperature, the greater the work function and hence, the greater the ionization efficiency. However, the rate of vaporization and emission of ions from the filament surface decreases with decreasing temperature so that a compromise must be made between the efficiency of ionization and rate of emission. The background of undesired ions also changes with temperature and is a consideration when choosing the optimum filament temperature. Various metallic elements including alkali metals, alkaline earths and rare earths such as Mg, Al, Pb, Bi, Cu, Nb, Zr, Ti, Cr, Mn, Fe and Ni among others and metallic compounds of these elements such as LiCO.sub.3, KNO.sub.3, CrO.sub.3, BiCO.sub.3 and CsNO.sub.3 as well as organic compounds such as ethanol, toluene, aniline and acetone having ionization potentials below 9 electron volts can be ionized and detected with my spectrometer. The common components of air have high ionization potentials (N.sub.2 = 15.5 eV, O.sub.2 = 12.2 eV, Ar = 15.7 eV) and therefore have very little probability of being ionized. The background due to undesired organic compounds existing in chamber 10 due to any number of reasons may be significant and in order to reduce such background, a liquid nitrogen surface adjacent the ion source is a desirable feature. For this purpose, a liquid nitrogen cooled plate 16 in the shape of a toroid is located at one end of chamber 10 adjacent filament 13 for reducing the background by a factor of 3 to 10. The nitrogen is supplied to plate 16 by means of inlet tube 22. Complete cooling of the entire inner surface of housing 10 could be possible for further reduction of the background, but would be expensive and present difficulties in fabrication. A viewing port 18 may be utilized at the end of chamber 10 adjacent filament 13 fo purposes of viewing the filament and the surrounding elements.

Referring now to FIGS. 2 and 3, there are respectively shown top and side views of my mass spectrometer and illustrate all of the components except for the electrical power supply, rough pump which develops the rough vacuum in chamber 10a and diffusion pump which develops the high vacuum in chamber 10. The chamber 10 containing the ion source is a thin walled device and utilizes heavy flanges for obtaining sufficient pressure on copper gaskets for vacuum sealing purposes. A pair of first flanges 20 are located along the entire side of the chamber in which the viewing port 18 is located. The inlet tube 22 for supplying the liquid nitrogen to the toroidal plate member 16 within chamber 10 passes through outer flange 20 and is welded thereto. The nitrogen may be poured directly into tube 22, or via a suitable means such as a funnel 23 as illustrated. Chamber 10, and all of the flanges and tubes described herein are fabricated of a metal which typically is a stainless steel. A short length of tube 19 passes through an opening in the top wall 10c of chamber 10 and the interior of such tube forms the air sample inlet chamber 10a. Tube 19 is welded to the top wall 10c of chamber 10 and the outer end of the tube is provided with a first flange which is sealed to a second flange forming a second pair of flanges 25. A tube 24 is welded to the second flange 25 and the other end of tube 24 is connected to the rough pump (not shown) which may be of the mechanical vacuum type and which establishes the rough vacuum of approximately 1 torr in the gas sample inlet chamber 10a. Gas sample inlet tube 11 is also connected to second flange 25 for passage into the interior of inlet chamber 10a. Tube 26 passes through an opening in a side wall of chamber 10 and is welded thereto. The outer end of tube 26 is provided with a first flange which is sealed to a second flange forming a third pair of flanges 27. A tube 36 is welded to the second flange 27 and the other end of tube 36 is connected to a diffusion pump (not shown) or other suitable high vacuum generating pump for developing the vacuum in chamber 10 around the ion source of approximately 10.sup..sup.-5 torr. A third tube 28 passes through an opening in, and is welded to, the side wall 10b of chamber 10 opposite the side wall defined by flanges 20. The outer end of tube 28 is provided with a first flange sealed to a second flange forming a fourth pair of flanges 29. Tube 28 provides the exit passage for the ions from chamber 10.

The input end of the mass analyzer designated as a whole by numeral 30 is connected to the second flange 29. Mass analyzer 30 is of the conventional magnetic type and consists of a 90.degree. sector fabricated as a 90.degree. stainless steel elbow 31 and includes an entrance slit for the ion beam which enters through passage 28 and an aligned collector slit at the analyzer output. To achieve the high sensitivity desired in my spectrometer, an electron multiplier 32 is used as the ion detector and consists of a conventional 10 stage electron multiplier having a gain of approximately 10.sup.7. The secondary emission electron multiplier permits the signal level to be raised to a point at which a low output load resistance may be used to obtain short response times to thereby make it possible to display the spectrometer output on an oscilloscope. The short response time permits detection of the ion bursts representing the contaminant particles that are spaced apart by intervals as short as microseconds. The duration of an ion burst is primarily a function of the filament temperature and may be in a range from microseconds to minutes. The output current of the electron multiplier can be read by a suitable device such as an electrometer which is capable of reading in the order of 10.sup..sup.-13 amperes.

Insulated electrical conductors 13' and 14a', 14c', 14d' which respectively supply the d.c. power to filament 13 and the elements of the ion lens 14, pass through a bottom wall 10d of chamber 10 within fourth tube 33 which passes through an opening in such bottom wall and is welded thereto. The outer end of tube 33 is provided with a first flange sealed to a second flange forming a fifth pair of flanges 34. A suitable means 37 for sealing the conductors feedthrough in the second flange 34 is provided, such as a glass seal or potting compound formed on the outer surface of second flange 34. The remote ends of the conductors 13', 14a', 14c', 14d' are connected to the d.c. power supply (not shown).

Referring now to FIG. 4, there is shown a detailed view of the ion source chamber 10, and in particular, illustrates the manner in which the ion source 13, ion beam lens 14 and ion beam defining slits 15 are retained within chamber 10 in alignment with each other and with tube 28 through which the ion beam passes enroute to the mass analyzer 30. The side 10b of chamber 10 through which tube 28 passes, serves as a base for supporting an integral filament 13, ion lens 14 and slits 15 structure. Four ceramic electrically nonconductive posts 41 have first ends closely fitted into holes in an annular member 40 rigidly connected to chamber side wall 10b. Two or more screws pass through slit plate 15a and into member 40 to rigidly fix posts 41 within member 40 and thereby provide the support for each of the ion source and beam focussing and defining elements 13, 14 and 15. Only two of the posts 41 are illustrated since this portion of the drawing is in section through the center of chamber 10. The four ceramic posts 41 are each provided with four cylindrical insulating spacers for separating the adjacent elements of the ion source and ion beam focussing and defining means from each other. Ion beam defining slits 15 comprise two rectangular metal plates 15a and 15b generally maintained at ground potential and with a slit in each plate of length approximaely equal to the length of filament 13. Plates 15a and 15b are parallel to each other and to filament 13 and the slits are aligned with filament 13. In a typical application, the slit 15b disposed furthest from filament 13 has a width of 0.010 inch, and the other slit 15a is of 0.040 inch width. The narrower width beam defining slit 15b is preferably rigidly fastened to a major surface of the annular member 40 in any convenient manner such as by means of two or more screws passing through plate 15b into member 40, or by being spot welded thereto. The beam defining slit plates 15a and 15b are each provided with four holes through which the four ceramic posts 41 pass for aligning such elements with the ion beam focussing electrodes 14a, 14b, 14c and 14d and with filament 13. Each of the ion focussing electrodes 14a-d is provided with two holes through which two of the ceramic posts pass. The four focussing electrodes form two separate electrostatic ion beam focussing members for focussing the ions to a fine line beam, the first pair 14a and 14b being connected to a common voltage as described hereinabove. Each of the electrodes includes a semicircular portion oriented parallel to the slit plates 15 and filament 13 and through which the two holes are formed, and a rectangular portion normal to the semicircular portion and which primarily determines the ion beam focussing function. The spacings between the rectangular portions of each pair of electrodes (14a, 14b) and (14c, 14d) are aligned with the slits in plates 15a, 15b and with filament 13. A circular electrically nonconductive support member 42 is spaced from the ion beam focussing electrodes 14a and 14b by means of the cylindrical spacers on the ceramic posts 41 and forms the support and aligning member for filament 13. The four holes are also formed through circular support member 42 through which the ceramic posts 41 are installed. In addition, a large hole which preferably is of rectangular shape is formed through the center of member 42 for permitting viewing of filament 13 through viewing port 18. Finally, two pairs of other holes are formed through support member 42 and a first pair of screws passes through a first pair of ceramic spacers 43 for rigidly securing a first electrically conductive plate member 44 in a predetermined orientation parallel to support member 42 and filament 13. In like manner, a second pair of screws passes through a second pair of ceramic spacers for securing a second plate member in a predetermined orientation coplanar with the first plate member 44 (not shown since they are directly behind the first pairs of screws, spacers and plate member as viewed in FIG. 4). These two plate members 44 are each of rectangular shape and equally spaced apart along a long edge thereof, the orientation of plate members 44 being such that the spacing between them is aligned with and approximately equal to the width of the large rectangular centrally located hole in member 42. The conductors 13' which supply the D.C. power to filament 13 are attached to the major surfaces of plate member 44 adjacent support member 42. A pair of short L-shaped electrical conductors 46 each has a first leg thereof rigidly attached to the opposite major surface of plate members 44 along lines adjacent the near edges of the two plate members 44 and each second leg of the L-shaped conductors 46 projects toward the other plate member 44. The ribbon filament 13 is of U-shape and has its outer two legs rigidly attached along the second legs of the L-shaped members 46. Obviously, other means can be utilized for supporting the ion source in chamber 10. Thus, ribbon filament 13 is retained in fixed alignment with the ion lens 14 and beam defining slits 15 by means of the hereinabove described structure, and the filament can be viewed during operation of my spectrometer by peering through viewing port 18. The viewing port 18 is a small glass plate which is sealed to an iron-nickel-cobalt alloy tube 47 that is conventionally employed for providing viewing ports in vacuum systems. The alloy tube 47 is welded to the outer flange 20.

FIG. 4 also indicates the form of gas sample inlet chamber 10a. In one specific embodiment, gas sample inlet tube 11 is of 1/16-inch inner diameter but reduced at the entrance for obtaining a gas sample flow rate of one cubic centimeter per second, and hole 12 is of 0.002-inch diameter. Tube 19 which is provided with flange 25 at the outer end thereof, is provided with an inner end wall 19a through which hole 12 is formed, this end wall preferably being normal to the path traversed by the particles issuing from tube 11.

Referring now in particular to FIG. 5, there is shown an exploded view of the orientation of the ribbon filament 13, the ion beam focussing electrodes 14a-d, and the ion beam defining slits 15a and 15b with the holes therethrough for the ceramic posts 41. The direction of the particles which pass through the small hole 12 in the gas sample inlet chamber 10a and impinge upon filament 13, and the subsequent ion path are illustrated by the lines with the arrowheads.

A second embodiment of the ion source is illustrated in FIG. 6 wherein a capillary tube 60, fabricated of a metal such as platinum which can be heated in air without oxidizing, has a first end disposed outside the top wall of the apparatus and the remainder is located within a first chamber 61. Tube 60 is heated by means of electrical conductors connected to a suitable D.C. power source (not shown) and the gas sample is supplied to the first end of the capillary tube 60 to cause surface ionization of the particles in such gas sample which are emitted from the inner surface of the tube 60 and withdrawn therefrom by a first slit 66 maintained at a fixed D.C. voltage. Slit 66 also provides a means for separating the ions from the gaseous medium within chamber 61 and thereby allows only a small flow of the gaseous medium into the adjoining chamber 62 in which are retained the ion lens 14 and the beam defining slits 15. The mass analyzer 30 and ion detector 32 may be connected by means of a suitable tubing to the output of chamber 62 as in the case of the FIG. 1 embodiment, or alternatively, as illustrated in FIG. 6, the mass analyzer and detector may be contained in a third chamber 63 which is interconnected with the output of chamber 62 by means of the narrower of the beam defining slits 15. In the case of the FIG. 6 embodiment wherein the three chambers 61, 62 and 63 are within one housing, each chamber is provided with a separate pump for maintaining the inside of such particular chamber at a predetermined low gas pressure level. Thus, a first stage pump is connected to chamber 61 for maintaining such chamber at a pressure level of approximately 1 torr, a second stage pump is connected to chamber 62 for maintaining such chamber at a pressure level of approximately 10.sup..sup.-3 torr, and a third stage pump is connected to chamber 63 for maintaining such chamber at a pressure of approximatey 10.sup..sup.-5 torr.

The process by which the specific particles in the gaseous medium being sampled are ionized is the same in both the FIGS. 1 and 6 ion source embodiments and results from surface ionization by impaction of the particles (molecules or particulate matter) on a heated electrode which may be in the form of a U-shaped ribbon or capillary tube. The advantages of my method and apparatus over conventional mass spectrometric analysis are (1) an increase in sensitivity because it does not ionize normal gases in air or the major components of the gaseous medium, (2) the method of sample introduction does not filter out the particles which are to be analyzed under a continuous sampling, (3) analyzes particles including particulate matter as to the number of particles per volume of gas sample and the amounts of the impurity (i.e., number of molecules) per particulate, and (4) provides for continuous analysis of particles in a gaseous medium wherein the particles have moderate-to-low ionization potential less than approximately 9 electron volts and includes various elements and compounds.

Another advantage of my method and apparatus for the analysis of impurities in gaseous media is that in the case wherein continuous analysis is not required, an even greater sensitivity of mass spectrometric analysis may often be obtained by an integration technique which avoids the problem of rapid pumping of large amounts of the gas that is required for the analysis on the continuous basis. In the noncontinuous analysis technique, the gas stream is impinged on the cold filament 13 for a given length of time over which the integration is to be made, then the gas sample input flow is reduced, or completely stopped, and the pressure in chamber 10 decreases to about 10.sup..sup.-5 torr, and finally, filament 13 is heated to cause surface ionization of the particles which had impinged thereon. This integration technique can lead to increased mass spectrometric analysis sensitivity since a greater number of the particles have collected on the filament over the integration period and also permits the use of smaller pumps since the gas sample need not be rapidly pumped out of chamber 10 but is only pumped down to 10.sup..sup.-5 torr after the integration time over which the gas stream has impinged on the cold filament.

The power supplied to the ion source (filament 13 or capillary tube 60) is adjusted to obtain the desired operating temperature in the range of 700.degree. to 1,700.degree. C. which is determined by the particular contaminant particles to be ionized. The voltages applied to the ion lens 14 are adjusted for obtaining both the desired fine line beam and a maximum ion current. The rate of particle impingement on the filament for a given particle density in the gas sample can be improved by increasing the pumping speeds so that a greater flow of the gas sample can be maintained through tube 11 in the FIG. 1 embodiment or through the capillary 60 in the FIG. 6 embodiment. The background which is of a low level is due primarily to contamination of metal parts adjacent the filament and can be even further reduced by a periodic heating of such parts so as to clean them. The background due to organic compounds may be reduced to a low level by baking the system to approximately 400.degree. C., the longer the bakeout period, the greater the reduction in partial pressure of organic compounds. The organic compounds are also trapped directly at their source by utilizing the liquid nitrogen surface 16 described hereinabove with reference to FIG. 1.

From the foregoing description, it can be appreciated that my invention makes available a new method and apparatus for spectrometric analysis of impurities including particulates in the gaseous medium which is especially well adapted for providing a high sensitivity since it does not ionize normal gas components and it can be utilized for continuous analysis or for increased sensitivity by integrating over a large volume of the gaseous medium. Having described two particular embodiments of my invention, it should be obvious by those skilled in the art that various changes in form and detail such as in the structure of the filament upon which the contaminant particles are impinged and the use of other well known ion beam focussing and beam defining techniques and the use of other conventional mass analyzers and ion detectors may be made without departing from the scope of my invention as defined by the following claims.

Claims

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A mass spectrometer for the analysis of impurity particles including particulates in a gaseous medium utilizing surface ionization to produce the ions to be analyzed and comprising

a first chamber,

first pump means for developing and maintaining a vacuum in the order of 10.sup..sup.-5 torr within said first chamber,

a metal filament of a metal having a high work function especially when oxygen covered and positioned within said first chamber,

a second chamber located in said first chamber and partitioned therefrom, said second chamber maintained at a vacuum having a pressure of magnitude several orders higher than the vacuum in said first chamber,

a tube having a first end external of said first and second chamber and open to a gas sample being analyzed for particular impurity particles including particulates, and a second end located in said second chamber, the second end of said tube aligned with a small hole in a wall partitioning the second chamber from the first chamber to thereby substantially reduce passage of the gaseous medium into the first chamber while permitting passage of the impurity particles thereto, the second end of said tube also aligned with a major surface of said filament for directing the impurity particles toward said filament for impingement thereon, the gas sample passing continuously through said tube for obtaining continuous analysis of the particular impurity particles including particulates.

means for heating said filament to a temperature in a range between 700.degree. and 1,700.degree. C., the particular operating temperature of said filament being determined by the particular impurity particle to be analyzed,

means positioned within said first chamber adjacent the heated filament for establishing an electric field for withdrawing from a surface of said filament ions of the impurity particles in the gas sample generated as short bursts of ions by surface ionization, and for focussing the ions into a thin line beam thereof,

means positioned within said first chamber adjacent the electric field means and remote from said filament for defining the width of the thin line ion beam,

mass analyzer means in communication with said ion beam width defining means for separating the ions according to their mass and selecting the ions of the particular impurity desired to be analyzed, and

ion detector means in communication with said mass analyzer means for sensing the selected ions desired to be analyzed, the number of ions per burst sensed by said ion detector means being a measure of the amount of the particular impurity per particulate, a single ion representing a single molecule of the impurity.

2. The mass spectrometer set forth in claim 1 wherein

said second chamber is maintained at a vacuum in the order of 1 torr.

3. The mass spectrometer set forth in claim 1 and further comprising

second pump means connected to said second chamber for developing and maintaining a vacuum in the order of 1 torr therein.

4. The mass spectrometer set forth in claim 1 and further comprising

a valve in said tube for controlling the flow of the gas sample into said first chamber whereby the aseous medium is sampled over selected time intervals.

5. The mass spectrometer set forth in claim 1 and further comprising

means for reducing background of ions occuring in said first chamber from sources other than the gas sample,

6. The mass spectrometer set forth in claim 1 wherein

the small hole is of diameter approximately 0.002 inch.

7. The mass spectrometer set forth in claim 1 wherein

said metal filament is a u-shaped ribbon.

8. The mass spectrometer set forth in claim 7 wherein

said metal ribbon filament is fabricated from a metal selected from the group consisting of rhenium, tungsten, iridium and platinum.

9. The mass spectrometer set forth in claim 7 and further comprising

means for rigidly supporting said metal ribbon filament, said electric field means and said ion beam width defining means in fixed alignment.

10. The mass spectrometer set forth in claim 9 wherein

said supporting means comprise

a plurality of electrically nonconductive posts having first ends rigidly connected to a first side of said first chamber,

an electrically nonconductive member rigidly connected to second ends of said posts,

said electric field means and said ion beam width defining means rigidly connected along the length of said posts, and

a pair of electrically conductive plate members rigidly connected to said electrically nonconductive member in spaced apart coplanar relationship, ends of said u-shaped metal ribbon filament rigidly connected to said electrically conductive plate members,

said filament heating means consisting of a pair of electrical conductors connected to said electrically conductive plate members and to a source of electric power.

11. The mass spectrometer set forth in claim 10 wherein

said electrically nonconductive member is provided with an aperture therethrough oriented in alignment with the spacing between said pair of electrically conductive plate members, and

a viewing port located in a second side of said first chamber opposite the first side for permitting viewing of said metal ribbon filament through the aperture in said electrically nonconductive member during operation of the spectrometer.

12. A method for the analysis of impurity particles including particulates in a gaseous medium comprising the steps of

introducing a gas sample being analyzed for particular impurity particles into a irst end of a tube located external of both a first chamber and second chamber located in the first chamber but partitioned therefrom,

exiting the gas sample from the second end of the tub located in the second chamber,

developing and maintaining the second chamber at a rough vacuum having a pressure of magnitude several orders higher than the vacuum in the first chamber,

directing the gas sample exiting from the second end of the tube at a small hole in a wall partitioning the second chamber from the first chamber to thereby substantially reduce passage of the gaseous medium into the first chamber while permitting pasage of the impurity particles thereto,

directing the impurity particles upon passage through the small hole toward a filament positioned in the irst chamber for impingement thereon,

developing and maintaining a vacuum in the order of 10.sup..sup.-5 torr within said first chamber,

heating the filament to a temperature in a range between 700.degree. and 1,700.degree. C., the particular operating temperature of the filament being determined by the particular impurity particle to be analyzed and causing surface ionization of the impurity particles impinging on the filament,

establishing an electric field in the vicinity of the filament for withdrawing ions of the impurity particles in the gas sample from the surface of the filament and for focussing the ions into a thin line beam thereof, the ions being generated as short bursts of ions, the number of ions per burst being determined by the amount of the particular impurity per particulate and a single ion representing a single molecule of the impurity,

passing the thin line ion beam through at least one slitted plate member for defining the width of the thin line ion beam,

passing the defined ion beam into a mass analyzer for separating out the particular ions desired to be analyzed from other ions which may also occur in the first chamber, and

detecting the particular ions desired to be analyzed to thereby determine the amount of the impurity particles in the gaseous medium.

13. The method set forth in claim 12 wherein

the step of exiting the gas sample from the second end of the tube consists of continuously passing the gas sample through the tube for obtaining continuous analysis of the gas medium.

14. The method set forth in claim 13 wherein the steps of impinging the gas sample on the filament, heating the filament and developing the vacuum may all be done simultaneously.

15. The method set forth in claim 12 and further comprising the step of

controlling the flow of the gas sample into the first chamber for obtaining analysis of the gaseous medium over selected time intervals.

16. The method set forth in claim 15 wherein the steps of impinging the gas samle on the filament, heating the filament, and developing the vacuum consist of

impinging the gas sample on the filament for a predetermined interval in the absence of the 10.sup..sup.-5 torr vacuum in the first chamber, then developing the 10.sup..sup.-5 torr vacuum at the termination of the air sample interval, and finally heating the filament upon developing the 10.sup..sup.-5 torr vacuum.

17. The method set forth in claim 12 and further comprising the step of

developing and maintaining the rough vacuum in the order of 1 torr in the second chamber.