US3087112A

US3087112A - Trace gas analyzer

Info

Publication number: US3087112A
Application number: US862813A
Authority: US
Inventors: William C Pfefferele
Original assignee: Engelhard Industries Inc
Current assignee: Engelhard Industries Inc
Priority date: 1959-12-30
Filing date: 1959-12-30
Publication date: 1963-04-23
Anticipated expiration: 1980-04-23
Also published as: GB916590A; DE1158290B

Description

April 23,

Samp/e aan 44 W. C. PFEFFERLE TRACE GAS ANALYZER Filed Dec. 50. 1959 0f Gas 7b Be Haifa/pt 60k/mf? United States Patent O 3,087,112 TRACE GAS ANALYZER William C. Pfeferele, Middletown, NJ., assigner, by

mesne assignments, to Engelhard Industries, Inc., Newark, NJ., a corporation of Delaware Filed Dec. 30, 1959, Ser. No. 862,813 16 Claims. (Cl. 324-33) This invention relates to gas analyzers, and more par- -ticularly to analyzers for detecting traces of gases in proportions signiiicantly less than one part per million.

In the field of gas chromatography, gases are customarily analyzed by the application of a sample to an elution or chromatographic column; the component gases in the sample are separated by the different speeds of migration through the column; and the amount of gas of each type can be detected during successive time intervals at the output of the column. Up until recently, however, the sensitivity of analyzers employing the principles of gas chromatography has been relatively low and generally has been considerably poorer than one part per million. Furthermore, some of the higher sensitivity devices are restricted in application.

Accordingly, a principal object of the present invention is to improve the sensitivity of gas analyzers, without sacrificing versatility.

iFrom a comprehensive aspect, one illustrative embodiment of the invention includes two successive chromatographic columns, a source of carrier gas such as hydrogen or helium having less than one part per million of extraneous gases of significantly lower ionization potential than that of the carrier gas, and a secondary ionization detector having a sensitivity considerably greater than one part per million. 'Ihe carrier gas may be hydrogen or helium purified by diffusion through palladium or quartz, respectively.

The secondary ionization detector in the illustrative embodiment of the invention may be an electronic tube which includes at least four electrodes. These electrodes include a cathode, rst and second grids, and a plate. The potential between the cathode and the iirst of the two grids, the accelerating grid, is posit-ive and has a value significantly greater than the ionization potential of the gas which is being detected, and a value which is insuicient to ionize the carrier gas. Thus, for example, when oxygen having an ionization potential of 12.5 volts is being detected, and when hydrogen having an ionization potential of 15.6 volts is used for the carrier gas, the potential between the cathode and the accelerating grid may Irbe about 14 volts, slightly less than the ionization potential of hydrogen. rIhe second or shielding grid which is provided has a negative potential with respect to the first grid. This potential may, for example, be approximately 30 volts negative with respect to the first grid. A negative voltage sufficient to ionize the carrier gas may be applied to the plate of the tube.

-In operation, the electrons from the cathode are accelerated toward the firs-t grid and ionize the oxygen in the region between the cathode and the first grid. Positive ions are then drawn toward the shielding grid and the plate which are negatively charged. The high negative potential of up to about 300 volts between the screen grid and the plate permits secondary ionization of the carrier gas, thus providing considerable amplification within the tube. The use of the shielding grid is desirable to avoid penetration of the eld provided by the high negative voltage at the plate into the region between the cathode and the first grid.

In accordance with a feature of the invention, a gas to be detected is carried to a tetrode secondary ionization electron tube detector by a carrier gas having a higher ionizai-ton potential than the gas to be detected.

ice

In accordance With additional features of the invention, the carrier gas described in the preceding paragraph may be hydrogen or helium purified by diffusion through palladium or quartz, respectively, and the carrier gas and the sample to be analyzed are passed through one or more chromatographic columns to separate the gas to be quantitatively analyzed from other gases.

Other objects, features and advantages of the invention will become apparent from a consideration of the following detailed description and from the drawing, in which,

FIG. 1 is a block diagram of a gas analyzing system in accordance with the present invention;

FIG. 2 is a schematic circuit diagram of the secondary ionization gas detector in accordance with the invention which may be utilized in the system of FIG. 1; and

FIG. 3 is a detailed Iblock diagram of a portion of the system of FIG. 1.

With reference to the block diagram of FIG. l, it will be assumed that it is desired to measure the concentration -of a gas such as oxygen in a gas stream indicated -by block 12. The principal components of the system include the secondary ionization detector 14 and its associated recorder 16, the cycle timer 18, and a source of .pure carrier gas which includes the cylinder of hydrogen 2) and the palladium diffusion purifier 22. 'Ihe hydrogen purifier may, for example, be of the form shown in U.S. Patent No. 2,911,057, granted November 3, 1959 to R. B. Green et al. In addition to conventional shutoit valves (not shown), the system includes lthe six port sampling valve 24 and two four port valves 26 and 28. The columns 30 and 32, which are provided for separating 4different types of gases, also form an important part of the system.

Suitable pressure regulators

34 and 36 a-re provided in the input lines for the purified carrier gas.

Before considering the mode of operation of the system of the present invention in detai-l, reference is made to the following reference materials dealing with the subject of gas chromatography. These include Gas Chromatography edited by Vincent I. Coates, Henry I. Noebels, and Irving S. Fagerson, Academic Press, Inc., New York, 19'58; Vapour Phase Chromatography edited by D. H. Desty, London, Butterworths Scientific Publications, 1957; An Ionization Gauge Detector for Gas Chromatography by S. A. Ryce and W. A. Bryce, pp. 1293 to 1297, Canadian Journal of Chemistry, vol. 35, 1957.

The sample Valve assembly 24 ris connected to the gas stream 12 to be monitored by the tubes 3S. The input tube 40 supplies carrier gas, and the output tube 42 from valve 24 .is connected by valve 26 to the first column 30. In addition, :the valve assembly 24 has a local sampling loop 44 associated with it. In the position sholwn in FIG. 1, the gas from the source 12 circulates through the sample loop 44. Upon rotation of the core of the six port valve 24 by 1/6 of a turn in either direction, the sample loop 44 is connected in series with tubes 40l and 42, but disconnected from the source 12. This operation introduces 'a sample of accurately determined volume into the first column 30 which volume is that defined within the sample loop.

The use of elution columns for the separation of gases is well known in the field of gas chromatography, and a number of different forms of columns are described in the texts cited above. In the present case, it is desirable to make a preliminary separa-tion of gases in the first column 30 and a final separation of gases in the second column 32. The first column 30 could therefore be a partition column, whereas the second column 32 could be an adsorption type column. Both types of columns are disclosed in the texts cited above. The partition column might characteristically include liquid such as 3 dibutylphthalate or silicone oil `on -a solid such las granular fire brick. The adsorption column 32 could suitably be of the type disclosed on pages 247 et seq. of the Desty text cited above. y

The system as disclosed in FIG. l is particularly applicalble to the analysis of xed gases such -as oxygen, yfor specic example. As mentioned above, separation is obtained in chromatographic columns by the different times required for gases lof different types to migrate through the column. Two columns are employed in order to prevent poisoning of the adsorption column 32 by gases which are not reversibly adsorbed. The partition column 3% permits the rapid migration of gases such as oxygen, argon, nitrogen, etc., while other heavier gases having higher boiling points, such as propane, traverse the partition column k30 at a slower rate. The heavier gases migrate reversibly through the partition column and' ymay therefore be vented from column 30 in the course of a reverse ilushing operation, while separation of nitrogen, oxygen and similar gases is occurring in the adsorption column 32. It` may be noted that Vthese last mentioned gases are not separated 'in time to any considerable extent as they diffuse through the partition column. The use of the two columns therefore minimizes aging of the adsorption column by avoiding contamination or poisoning by the heavier gases. In certain cases in which the quantity of oxygen or ano-ther gas is to lbe determined in gas streams having a known composition analyzed. Characteristically, both columns ycould ibe approximately ve feet in length, and -the time of diffusion for oxygen about two minutes. Passage of the oxygen through the column 30 takesplace in the direction indicated by the dashed

line arrows

46 and 48, valves 26 and 28 being in a position one quarter turn from the position 'shown in FIG. 1. Thus the oxygen passes through valve 26, the partition column 30, and valve 28 into the -adsorption column 32. Following the passage of the oxygen from column 30, the valves 26 and 2.8 are turned to the position shown in FIG. 1 and the partition column is reverse flushed, with the gas proceeding through the partition column in the direction indicated by the solid arrows 47 and 49 and nally out t-he vent connected to valve 28. Pressure is maintained on the column 32 by the source of carrier gas provided through the pressure regulator 36 and line 50.

The chromatographic columns 30` and 32 are preferably operated at atmospheric pressure or slightly above. In this manner errors which could result from slight leaks in the system are avoided. To accommodate the atmospheric pressure in the columns and the low pressure required in the detector 14, a pressure reducer 51 is pro-- vided. This reducer could be in the form of a suitable constriction in the flow line, a Ableed-off vent, or could include both of these structures.

The secondary ionization detector 414. includes an electronic tube and its associated circuitry as described in detail l'below in connection with FIG. 2. Electrical output signals from the detector 14 iare supplied on electrical lead 52 to the recorder 16. The output gas line '54 from Vthe detect-or 14 leads to a pump (not shown).

The mode of operation of the system of FIG. 1 is controlled by a conventional cycle timer 18 which may be of Iany standard form. A synchronous clock motor with adjustable electrical contacts may suitably be employed. The cycle timer 118 controls the position of the six vport valve '24 as indicated by the dashed line 56, the position of valves 26 and 28 by a common mechanical connection as indicated by dashed lines 58, and the operative periods of the recorder .16 as indicated schematically by the dashed line 60. Conventional electromechanical operating devices maybe employed. The sequence of operation of the cycle timer 18 is indicated by the following steps:

Step l The six port sample valve 24 is 4tur-ned to the sample position, one-sixth turn from the position shown in FIG. 1.

Step 2 Step No. 2 occurs following a time interval sufficient for the gas being analyzed to pass through partition column llt, and into adsorption column 32. At -this time valves 26 and 28 are rotated to reverse gas flow in partition' column 30 and to vent gases from the partition column (the position shown in FIG. 1). Pressure is maintained on the adsorption column 32 from line 5d.

Step 3 Immediately following Step 2, the six port sample -valve 24 is'returned to its original position as shown in FIG. l of the drawings.

Step 4 For a time interval bracketing the time at which the ygas in question leaves the adsorption column 32, the

'recorder 16 is enabled and provides a quantitative indication ofthe amount of the gas in question in the sample.

Step 5 partition column 30 by reverse flushing.

Steps 6 et seq.

The cycle set forth above is now repeated. In addition tothe use of hydrogen puried by diffusion through palladium as discussed above, diffusion `purified helium may also be used as a carrier gas. In

the case of helium, however, diffusion through quartz is preferred to diffusion through palladium. Argon,

'nitrogen or any other suitable gas or combination of gases may be employed whenV purified to the required extent (signilicantly lless of more readily ionized impurities than one part per million). The ionization potentials for these various gases are 24.6 volts for helium, 15.7 volts for argon, and 15.6 volts for hydrogen. It is, of course, necessary to select a carrier gas having an ionization potential which is greater than that of the gas or gases which are being analyzed.

Considering the matter of reversing the ow of carrier gas through the partition column 30, this is done to eliminate portions of the original sample which may still be within the partition column. 'The flow of gas through ya partition column is normally a reversible process. Accordingly, the time for 'black flushing need only be approximately equal to the time of ow in the forward direction. A slight additional time period is secondary ionization detector 14 of FIG. 1 is set forth in some detail. In FIG. 2, the electron tube may be of the general type known commercially as a 6CB6A, provided with input and output connecting tubes. However, unlike Ithe commercial tube, the heater and the heated cathode must be resistant to oxidation. Accordingly, the cathode is preferably of gold or silver and may also be of iridium, and platinum Ialloyed with 10 to 40 percent of rhodium may be used for the heater filament. Any other suitable materialswhich are sutlciently resistant to oxidation may be used.

The tube 60 in FIG. 2 preferably includes an indirectly heated cathode 62, an laccelerating gridv 64, a

shielding grid 66, and a plate 68. A carrier gas, such. as hydrogen, and the gas, such as oxygen, which is being detected, are supplied through the inlet and

outlet tubes

70 and 72.

The detector, in accordance with an important aspect of the present invention, is based on the principle of selective ionization of trace components in a gaseous carrier, wherein the carrie-r has an ionization potential which is higher than the gas which is to be detected. The potential between the cathode 62 land the grid 64 is positive so that electrons from the cathode 62 are accelerated toward grid 64. The potential is adjusted to a level such that the carrier gas will not be ionized to any appreciable extent but that a portion of the gas which is being detected is ionized. Thus, for example, when oxygen having an ionization potential of 12.5 volts is carried through tube 60 by a carrier gas such as hydrogen having an ionization potential of `about 15.6 volts, the potential from cathode to grid may be about 14a' volts. As this value is just below the ionization potential for hydrogen, the electrons from cathode 62 will not attain the energy required to ionize hydrogen, and no appreciable amount of hydrogen will be ionized. As the oxygen reaches the ionization detection tube 60, however, appreciable ionization of the oxygen takes place. The positively charged oxygen ions are drawn toward the shielding grid 66 which is at a negative potential with respect to both the accelerating grid 64 and the cathode 62. A high negative potential of up to about 300 volts is applied between the shielding grid 66 and the plate 68. As the positive oxygen ions are accelerated through shielding grid 66 toward the plate 68, the carrier gas is also ionized and considerable amplification of the signal is obtained. This secondary ionization is linear or a monotonie function of the primary ionization and produces a device having much greater sensitivity than is possible with the ionization detectors such as those discussed in the references cited above.

As noted above, the negative voltage applied to the plate is of the order of magnitude of up to 300 volts. Higher voltages may also be employed up to the breakdown point of the tube. This breakdown point will depend among other things on the geometry of the tube,

the types of gases which are present, 'and the pressure in the tube. When the plate voltage is adjusted to a value somewhat less than the breakdown point of the tube, maximum amplification and sensitivity of the apparatus is secured.

The use of the shielding grid 66 is considered to be particularly important; without this shielding grid, it is impossible to obtain significant amplification 'by secondary ionization with high voltages applied to the plate. When even moderate voltages are applied to the plate in triode arrangements, the iield penetrates the region between the cathode and grid and produces a net reduction in output. In one known case, this reduction in output started at a plate Voltage level of about 15 volts.

With `regard to the potentials applied to the Various electrodes, it should be noted that grid 64 must be positive with respect to the cathode 612 and have such a magnitude as to produce ionization lin the gas being detected but not in the carrier gas to any significant extent. The plate 68 must be negative with respect to the cathode 62 and should have a potential with respect to the grid 64 which is significantly greater than the ionization potential of the carrier gas. In this manner, secondary ionization is provided. The shielding grid 66 should have a potential which is between that of grid 64 and plate 68, preferably closer in potential to the accelerating grid 64 than to the plate 68, for example volts` with respect to the cathode.

The pressure within the tube 60 must -be suiciently low to permit electron flow from cathode 62 toI grid 64. The gas pressure should generally be less than about 1 centimeter of mercury, and it is desirable that the pressure be approximately in the range of 0.1 to l millimeter of mercury, for most tube geometries. In particular, the pressure should be such that the mean free path of electrons emitted from the cathode is of the same order of magnitude or greater than the spacing between the cathode and grid.

An indirectly heated cathode is to be preferred over a lament cathode. An arrangement employing an indirectly heated cathode permits closer control of the potential between the cathode and grid for selective ionization purposes. In contrast, a self-heated cathode has a signicant voltage drop from end to end and therefore has a variable cathode-to-grid potential from end to end. For selective ionization, where diiierences of a few volts become critical, accurate fixing of the cathode-to-grid voltage is essential. The

adjustable voltage sources

74, 76 and 78 are shown schematically as variable batteries. It will be understood that suitable power supplies of the type well known in the art may be employed. The output block 80 in the plate circuit of tube 60 represents any suitable amplifier or microammeter, for example. In the system of FIG. 1, the output device 80` would be the recorder 16 and its associated apparatus.

The detector 60` is capable of detecting traces of oxygen in the range of 1/1000 or 1;/10000 of one part per million of oxygen in a carrier gas of hydrogen. This corresponds to the detection of one part in 109 or 1010 parts. With regard to the sensitivity in the detection of oxygen present in the source 12 of FIG. 1, the entire apparatus is capable of detecting approximately 1/100 or 1/1000 of one part per million of oxygen present in the input stream. This corresponds to the detection of one part in 108 or 109 parts. In this regard, it is considered that the limiting factors on sensitivity are the noise, arising from impurities in the carrier gas, Voltage fluctuations or the like.

In the foregoing description, the apparatus of lFIG. 2 has been disclosed as performing the function indicated by block 14 in FIG. 1. In some cases, however, the ionization detection circuit of FIG. 2 -may be employed without the complete apparatus as shown in FIG. l. Thus, for example, it is applicable directly to the detection of traces of oxygen in pure nitrogen. Purified nitrogen normally contains as impurities traces of oxygen and argon. As both nitrogen and argon have ionization potentials which are significantly higher than that of oxygen, the presence of oxygen can be detected directly with the apparatus of FIG. 2. With argon having an ionization potential of 15.7 volts, that of nitrogen being 15.5 volts, and oxygen being of only 12.5 volts, the voltage applied to the accelerating grid 64 in FIG. 2 would be approximately 14 volts in such a system.

As disclosed above, the detection unit 14 of FIG. 1 may be implemented by a single ionization detection tube and circuit as shown in FIG. 2. FIG. 3 represents an embodiment of the circuit of FIG. 1 in which the detector unit 14 is implemented by the use of two

detectors

14 and 14" -which lead to a dual recorder 16. In accordance with the embodiment of FIG. 3, oxygen and argon may be detected simultaneously. These two gases are particularly good examples as they may pass through chromatographic columns in approximately the same time interval. In the case of the simultaneous analysis for both argon and oxygen, a helium carrier may be employed. The accelerating grid potential of the detection tubes in the

detectors

14 and 14" are then set at respectively different levels. Thus, for example, in the low detector 14 the control grid potential is adjusted to a level between the 12.5 volt ionization potential of oxygen and the 15.7 Volt ionization potential of argon. The tube in detector 14" has a control grid potential somewhat higher than 15.7 volts but less than the 24.5 volt ionization potential of helium. The high detector 14" having the higher accelerating grid potential provides an output -signal indicating the presence of both oxygen and argon, while the low detector `14' is only responsive to the

Vdetectors

14 and 14.

presence of oxygen. The difference in the readings between the two detection devices indicates the quantity of argon present in the mixture. The dual recorder la simultaneously registers the signals from each of the two In accordance' with conventional practice, the readings may be recorded on a moving sheet of graph paper so that the precise reading at each mornent'of time is indicated.

Various other possible combinations of gases and the vnecessary changes in the system to accommodate them will now be considered. To analyze for the presence of nitro- Agen, it is desirable to employ a carrier gas such as helium having an vionization potential of 24.5 volts, rather than a hydrogen carrier having an ionization potential of 15.6

volts, just 1/10 of a volt higher than the ionization potential of nitrogen. In cases where it is desired to analyze for traces of benzene in a gaseous stream, a hydrogen carrier may be employed, as lbenzene has an ionization potential of only 9.6 volts. As benzene is a heavier gas,

one or more partition columns may be employed for separation, and the absorption column shown in FIG. 1 is not required.

In connection with the system of FIG. 1 of the drawings, a standard source o-f oxygen containing gas may ybe provided. Under these circumstances, the instrument maybe calibrated periodically by inserting a sample from 'the standard source.

' It is to be understood that the above described arrangements are illustrative of the application of the principles of the invention. Numerous other arrangements may be devised by those skilled in the art without departing from the spirit and `scope of the invention.

What is claimed is:

l. In 4a gas analyzing system, means for diiusion purifying -a carrier gas having a high ionization potential so that it has less Ithan one part per million impurities havving a lower ionization potential than the carrier gas, means `for combining said carrier gas with a composite gas sample to be analyzed which has a lower ionization potential than that of said carrier gas, a plurality of chromatographic separation columns, means lfor applying the caraccelerating grid of said tube which `is greater than the ionization potential of the `gas -being analyzed and less than the ionization potential of the carrier gas, means for applying a potential to the plate of Said tube which is signicantly greater'than the ionization potential of 4the carrier gas, and means for applying a potential to the shielding grid adjacent said plate which is intermediate between the potential of said accelerating grid and that lof said plate.

2. In a gas analyzing system, an electron tube having a gas inlet and a gas outlet, means for diffusion purifying a carrier gas having a high ionization potential so that it has less than one part per million of impurities having a lower ionization potential than the carrier gas,

`means for combining said carrier gas with a gas sample to be analyzed which has a lower ionization potential than that'of said carrier gas, means for supplying to said tube the carrier `gas, and the gas to be analyzed, a cathode, an accelerating grid, a shielding grid and a plate mounted within said tube, means for establishing an efective potential (between the cathode and the accelerating grid of said tube which is greater than the ionization potential of the gas being detected but less than the ionization potential of the carrier gas, means lfor applying a potential to the plate `of said tube which is significantly :greater than the ionization potential of the carrier gas,`and means for applying a potential to the vshielding grid adjacent said plate which is intermediate between the potential of said accelerating grid and that of said plate.

3. In a gas analyzing system, an electron tube having a gas inlet and a gas outlet, means for supplying to said tube a carrier gas having a high ionization potential and a gas to be analyzed having an ionization potential which is less than that of the carrier gas, an indirectly heated cathode, an accelerating grid, a shielding grid and a plate mounted within said tube, means for establishing a potential between the cathode and the accelerating grid of said tube which is greater than the ionization potential of the gas being detected but less than the ionization potential ofthe carrier gas, means for applying a potential to the plate of said tube whi-ch is significantly greater in magnitude than the ionization potential of the carrier gas, and means for applying a potential to the shielding grid adjacent said plate which has a value between the Vpotential of said -accelerating grid and that of -said plate.

`4. In a gas analyzing system, an electron tube having a gas inlet and a gas outlet, means for purifying a carlrier gas having a 'high ionization potential so that it has less than one part per million of impurities having a lower ionization potential than the carrier gas, means for combining said carrier gas with a gas ysample to be analyzed which has a lower ionization potential than that of said carrier gas, means for supplying to said tube the carrier gas and the gas to be analyzed, a cathode, an accelerating grid, lay shielding grid and a plate mounted within said tube, means for establishing an eifective p otential between the cathode and the accelerating grid of said tube which is greater than the ionization potential olf-the gas being detected but less thany the ionization potential of the carrier gas, means for applying a potential to the plateA of -said tetrode which is signicantly greater than the ionization potential of the carrier gas, and means Ifor applying a potential to the shielding grid adjacent said plate which is intermediate between the potential of said accelerating grid and that of said plate.

5. In a gas analyzing system, means for diffusion puriple to be analyzed which has a lower ionization potential Ithan that of said carrier gas, at least one chromatographic separation column, means Afor applying the carrier gas and the gas sample to be analyzed to said column, an electron tube having a gas inlet and a gas outlet, means for coupling the gas from said column to said electron tube, a cathode, an accelerating grid, a shielding grid and a plate mounted within said tube, means for establishing a potential between the cathode and the accelerating grid of said tube which is greater than the ionization potential of the gas being analyzed and less than the ionization potential of the carrier gas, means for applying a potential to the plate of said tube which -is significantly greater than the ionization potential of the carrier gas, and means for applying a potential to the yshielding grid adjacent said plate which is intermediate between Ithe potential of said accelerating grid and that of said plate.

6 In a gas analyzing system, means for diffusion purifying a carrier gas having a high ionization potential so that it has less than one rpart per million impurities having a lower ionization potential than the carrier gas, means for combining said carrier gas with a composite gas sample to be analyzed which has a lower ionization potential than that of said carrier gas, va plurality of chromatographic separation columns, means for applying the carrier gas and the gas sample Ito be analyzed to said columns in series, an electron tube having a gas inlet and a gas outlet, means for coupling the4 gas from said columns to said electron tube, a cathode, an accelerating grid, a shielding grid and a plate mounted within said tube, means for establishing a potential between the cathode and the accelerating grid of said tube which is greater than the ionization potential of the gas being detected but not significantly greater than the ionization potential of the carrier gas, means for applying a potential to the plate of said tube which is significantly greater than the ionization potential of the carrier gas, and means for applying a potential to the shielding grid adjacent said plate which is between the potential of said accelerating grid and that of said plate, a recorder coupled to receive output signals from said electron tube, and timing means for operating said combining means and for enabling said recorder at a predetermined time following the operation of said combining means.

7. In a gas analyzing system, an electron tube having a gas inlet and a gas outlet, means for supplying to said tube a carrier gas having a high ionization potential and a gas to be analyzed having an ionization potential which is less than that of the carrier gas, a cathode, an accelerating grid, a shielding grid and a plate mounted within said tube, means for establishing -a positive potential between the cathode and the accelerating grid of said tube which is greater than the ionization potential of the gas being detected but less than the ionization potential of the carrier gas, means for applying a negative potential to the plate of said -tetrode which is signicantly greater with respect to said accelerating grid than the ionization potential of the carrier gas, and means for applying a potential to the shielding .grid located between the plate and the accelerating grid which has a value between the potential of said accelerating grid and that of said plate.

8. In a gas analyzing system, an electron tube having a ,gas inlet and a gas outlet, means for supplying to said tube a background gas including one or more gases having high ionization potential and a gas to be analyzed having an ionization potential which is less than that of the background gas, a cathode, a plate and at least two grids mounted within said tube, means for establishing a positive potential between the cathode and the accelerating grid of said tube which is greater than the ionization potential of the :gas being -analyzed and less than the ionization potential of the background gas, means for applying a negative potential to the plate of said tetrode which is signiiicantly greater than the ionization potenti-al of at least one of the gases of the background gas, and means for applying a potential to the shielding -grid adjacent said plate which is intermediate between the potential of said accelerating grid and that of said plate.

9. In a gas analyzing system, means for diffusion purifying a carrier `gas having a high ionization potential so that it has less than one part per million impurities having :a lower ionization potential than the carrier gas, means for combining said carrier gas with a composite ,gas sample to be analyzed which has a lower ionization potential than that of said carrier gas, 1a plurality of chromatographic separation columns, means for applying the carrier gas and the gas lsample to be analyzed to said columns in series, an electron tube having a gas inlet and a gas outlet, means or coupling the gas from said columns to said electron tube, a cathode, an laccelerating grid, a shielding grid and a plate mounted Within said tube, means for establishing a potential between the cathode and the accelerating grid of said tube which is greater than the ionization potential of the gas being detected but less than the ionization potential of the carrier gas, means for applying a potential to the plate of said tube which is signicantly greater than the ionization potential of the carrier gas, means for applying a potential to the shielding grid adjacent said plate which is intermediate between the potential of lsaid 4accelerating grid and that of said plate, a recorder coupled to receive output signals from said electron tube, and .timing means for enabling said recorder flor a time interval encompassing the ltime period 10 during which the gas sample to be analyzed is leaving said columns yand entering said electron tube.

l0. In a gas analyzing system, means for -diiusion purifying a carrier gas having a high ionization potential so that it has less than one part per million of impurities having a lower ionization potential than the carrier gas,

vmeans for combining said carrier gas with a composite gas sample to be analyzed which has a lower ionization potential than that of sai-d `carrier gas, a plurality of chromatographic separation columns, one of said columns being a partition column and another of said columns -being an adsorption column, means for applying the carrier gas and the gas sample to be analyzed to said columns in series, an electron tube having a gas inlet and a gas outlet, means for ycoupling the gas from said columns to -said electron tube, a cathode, an accelerating grid, a shielding grid and a plate mounted with said tube, means for establishing a potential between the cathode and the accelerating grid of said tube which is greater than the ionization potential of the gas being analyzed and less than the ionization potential of the carrier gas, means for applying a potential to the plate of said tube which is significantly greater than the ionization potential of the carrier gas, and means for applying a potential to the shielding grid adjacent said plate which is intermediate between the potential of said accelerating grid and that of said plate.

1l. In a gas analysis system, at least one chroma- `tographic column, means for applying a gas sample to said column including two gases which have approximately the same rate of migration through said column, at least two ionization detectors coupled to the output of said column, means for establishing an ionization potential in one yof said detectors which is between the ionization potentials of said two gases, means for establishing an ionization potential in the other of said detectors which is above the ionization potential of both of said gases, each ldetect-or including a plate for collecting ions to produce output signals, and means for recording the output signals from both of the detectors.

l2. -In a gas analyzing system, an electron tube having .a gas inlet and a gas outlet, means for supplying to said tube a carrier gas having a high ionization potential and a gas to be analyzed having an ionization potential which is less than that of the carrier gas, an indirectly heated cathode, an accelerating grid and a plate mounted within said tube, means for establishing a positive potential between the indirectly heated cathode and the accelerating grid of said tube which is greater than the ionization potential of the gas being detected but less than the ionization potential of the carrier gas, and means for applying a potential to the plate of said tube which is negative with respect to the potential of said grid.

13. In a gas analysis system, at least two ionization detectors, means for applying a gas sample including two gases which have different ionization potentials to both of said detectors, means for establishing an ionization potential in one of said detectors which is between the ionization potential of said two gases, means for establishing an ionization potential in the other of said detectors which is above the ionization potential of both of :said gases, each detector including a plate for collecting ions to produce 4output signals, and means for recording the output signals from both of the detectors.

14. In a gas analysis system, at least on chromatographic column, a carrier gas having a relatively high ionization potential, a gas sample including two gases which have approximately the same rate of migration through said column and which have lower ionization p-otentials than said carrier gas, means for combining said sample with said carrier gas and for applying the combined gases to said column, at least two ionization detectors lcoupled to the output of said column, means for establishing an ionization potential in one of said detectors which is between the ionization potentials of said two sampled gases, means for establishing an ionization potential in the other of said detectors which is above the ionizationpotential of both of said sampled gases, each detector including a plate for collecting ions Vto produce output signals, and means for recording the outputl signals from both ofthe detectors.

15. In a gas analysis system, at least one chromaltographic column, a carrier gas having a relatively high ionization potential, a gas sample including two gases which have approximately the same rate of migration through said column and which have lower ionization potentials than said ycarrier gas, means `for combining said sample with said carrier gas and for applying the combined gases to said column, at least two ionization detectors coupled to the output of said column, means for establishingl an ionization potential in one of said detectors whichvis between the ionization potentials of said Atwo sampled gases, means for establishing an ionization potential in the other of said detectors which is above the ionization potential of both of said sampled gases, each detector including a plate for collecting ions to produce output signals, Iand means for recording the output signals from both of said detectors.

16. In a gas analysis system, at least one chroma- Ytographieh column, means for applying a gas sample yto said column including two gases which have different ionization potentials, at least two ionization `detectors coupled to the output of said column, means for establishing an ionization potential in one 4of said detect-ors `which is between the ionization potentials of said two gases, means for establishing an ionization potential in -means for recording the output signals from both of the detectors.

References Cited in the tile of this patent UNITED STATES PATENTS 2,579,352 white Dec. 18, 1951 2,761,976 Obermaier Sept. 4, 1956 20 2,770,772 Foulkes Nov. 13, 1956 FOREIGN PATENTS 805,034` Great Britain Nov.16, 1958

Claims

1. IN A GAS ANALYZING SYSTEM, MEANS FOR DIFFUSING PURIFYING A CARRIER GAS HAVING A HIGH IONIZATION POTENTIAL SO THAT IT HAS LESS THAN ONE PART PER MILLION CARRIER GAS, MEANS ING A LOWER IONIZATION POTENTIAL THAN THE CARRIER GAS, MEANS FOR COMBINING SAID CARRIER GAS WITH A COMPOSITE GAS SAMPLE TO BE ANALYZED WHICH HAS A LOWER IONIZATION POTENTIAL THAN THAT OF SAID CARRIER GAS, A PLURALITY OF CHROMATOGRAPHIC SEPARATION COLUMS, MEANS FOR APPLYING THE CARRIER GAS AND THE GAS SMAPLE TO BE ANALYZED TO SAID COLUMNS IN SERIES, AN ELECTRON TUBE HAVING A GAS INLET AND A GAS OUTLET, MEANS FOR COUPLING THE GAS FROM SAID COLUMNS TO SAID ELECTRON TUBE, A CATHODE, AN ACCELERATING GRID, A