US3563083A - Automatic interface for gas chromatograph-mass spectrometer system - Google Patents

Abstract

AN ANALYTICAL SYSTEM IS DESCRIBED INCLUDING A COMBINATION OF A GAS CHROMATOGRAPH AND A MASS SPECTROMETER ALONG WITH AN AUTOMATIC INTERFACE APPARATUS FOR RENDERING THE TWO DISSIMILAR COMPONENTS HIGHLY COMPATIBLE IN A CONTINUOUS FLOW QUANTITATIVE-QUALITATIVE ANALYZING SYSTEM. THE INTERFACE INCLUDES AN AUTOMATIC CONTROL CIRCUIT FOR DETECTING THE OUTPUT OF THE GAS CHROMATOGRAPH AND SELECTING FROM EACH GC PEAK A PREDETERMINED QUANTITY OF SAMPLE FOR PRESENTATION TO THE ION SOURCE OF THE MASS SPECTOMETER.

Description

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United States Patent O1 3,563,083 Patented Feb. 16, 1971 hcc 3,563,083 AUTOMATIC INTERFACE FOR GAS CHROMATO- GRAIH-MASS SPECTROMETER SYSTEM Hanspeter Benz, Palo Alto, Calif., assignor to Varian Associates, Palo Alto, Calif., a corporation of California Filed Jan. 15, 1968, Ser. No. 698,019 Int. Cl. G01n 31 08 U.S. Cl. 7323.1 6 Claims ABSTRACT OF THE DISCLOSURE This invention relates in general to analytical instruments for deriving qualitative as well as quantitative information from a gaseous material and more speciiically to an apparatus for enabling the marriage of two different types of investigative instruments into a precision analytical system so as to facilitate the highly sensitive investigation of certain materials.

PRIOR ART Among the most useful and sensitive instruments available to the chemist are the lgas chromatograph (GC) and the mass spectrograph. The basic characteristic of the gas chromatograph is its ability to separate and make quantitative measurements. Its qualitative measurement capability however, is limited and is dependent on calibration and further experimentation. The principal advantage of the mass spectrometer resides in its great sensitivity and the amount of qualitative information which it is capable of providing.

These differing capabilities of the respective devices are the factors which lead to the desirability of a combination of the methods of gas chromatography and mass spectrometry in a single analytical system. Fortunately, the two devices have an important facet in common; the amount of sample material which each can handle is in roughly the same range, i.e., from nanograms to fractional milligrams.

However, the two devices diifer in other important aspects. The gas chromatograph for example, operates at near atmospheric pressure while the mass spectrometer operates at a high vacuum. Furthermore, the carrier gas and sample mixture in the chromatograph ows at a rate far in excess of the permissible ilow of material in the ion source of the mass spectrometer. These dissimilarities would tend to make the two devices incompatible were it not for the use of a suitable interface apparatus allowing the automatic selection of only a portion of the output of the chromatograph to be supplied to the ion source of the mass spectrometer. Various interface devices have been developed which allow a portion of the ecluent of a gas chromatograph to be directly introduced into a 'gas analyzer. These devices are disclosed in the articles: Quantitative and Qualitative ionization Detector for Gas Chromatography, pp, 195-205, Proc. Instr. Soc. Am. 1961; Use of a Mass Spectrometer as a Detector and Analyzer for Eluents Emerging from High Temperature Gas Liquid Chromatograph Columns, pp. 759- 764, Anal. Chem., 36(4) April 11964; and High-Resolution Mass Spectra of Compounds Emerging from a Gas Chromatograph, pp. 1135-7, Anal. Chem., 36(6) May 1964.

Another apparatus having substantial advantages over the above devices is disclosed in the copending Llewellyn application Ser. No. 511,756 now Pat. No. 3,455,092 'filed Dec. `6, 1965 now Patent 3,455,092 issued July 15, 1969 and assigned to the assignee of the present invention. This apparatus includes a permeable membrane type separator which serves the dual function of separating the material to be analyzed from the carrier gas (thus reducing the ilow rate) while at the same time enabling the introduction of the sample material into the mass spectrometer at a greatly reduced pressure.

In subsequently iled applications Ser. Nos. 626,193, and now abandoned, 626,194 now Pat. No. 3,398,505 and 626,196 now Pat. lNo. 3,471,692 issued Oct. 7, 1969 (all iiled on Mar. 27, 1967 and assigned to the assignee of the present invention) various improvements to the above mentioned Llewelyn invention are disclosed. This invention relates to still a further improvement to the previously disclosed gas analysis system, however, the present invention is more particularly directed toward an automatic control system for enabling the extraction of a predetermined quantity of sample material from each sample peak in the eiliuent of a chromatographic column or any other suitable source of gaseous material.

OBJECTS The principal object of this invention is the provision of an automatic interface for increasing the utility and compatibility of a gas chromatograph-mass spectrometer system.

Another object of the invention is the provision of a means for automatically selecting from the output of a gas chromatographic column predetermined quantities of sample material for presentation to the input of a mass spectrometer.

A further object of the invention is to provide a sensitive detection and control circuit for operating a valve means which diverts from the output iiow path of a gaseous material separating apparatus :into the input flow path of a gas analyzer means or fraction collecting means a predetermined quantity of sample material.

A still further object of the invention is to provide a sample selection control circuit for automatically opening a valve means in the fllow path between a gas chromatograph and a mass spectrometer upon sensing a quantitative peak in the output of the gas separating means, integrating the output until a predetermined quantity of gas has passed through said valve means and then automatically closing said valve means.

Other objects and advantages of the present invention will become apparent after having read the following detailed disclosure of a preferred embodiment shown in the drawings.

DRAWING FIG. 1 is a schematic diagram of a gas analysis system incorporating the present invention,

FIG. 2 is a chromatograph exemplary of the output of the gas chromatography of FIG. l.

FIG. 3 is a sequencing chart indicating the operational sequence of the control system components with relation to the exemplary chromatogram of FIG. 2.

DESCRIPTION In FIG. 1 there is shown a gas analysis system including a sample selection and control circuit in accordance with the present invention. The system includes a source which supplies a continuous ow of a suitable carrier gas to the gas chromatograph 12. Typical carrier gases include the permanent gases such as He, H2, N2, Ar. At the input 11 to the column of the gas chromatograph the sample gas is mixed with the carrier gas. As the mixture passes through the column the various gaseous constituents are time separated within the ow stream and may be detected at the column output as a series of peaked curves (GC peaks) of varying amplitudes and durations such as is depicted in FIG. 2. The respective areas under these GC peaks are representative of the relative proportions of the various constituent gases in the sample. Depending on the gaseous materials in the sample and the type of column packing, etc., the peaks may be separated by as little as a few seconds or by as much as several minutes. In some instances where the separation of two or more constituents is less distinct, an unresolved, plural maxima peak `will be detected.

The output of the gas chromatograph 12 is connected through a splitter 14 to a valve means 16 including a spool 17 which selectively directs the flow stream through either a vent port 18 or through a conduit 20 which communicates with the input of a fluid separating means 22. A suitable structure for the valve means 16 is disclosed in the aforementioned applications Ser. Nos. 626,- 193 and 626,196.

In the normal operating position (shown in dashed lines in the drawing) spool 17 of the valve means 16 directs the flow stream from chromatograph 12 out through vent 18 where it may be collected, exhausted or introduced into other analytical equipment. When the valve is actuated into its other position, las shown in solid lines in the drawing, the iiow stream is directed through a conduit 20 to a separator 22, which may be of the 2- stage permeable membrane type `disclosed in the aforementioned application Ser. No. 511,756. As the gaseous mixture passes through the separator 22 the excess material is vented while a portion of the sample material is separated out to be introduced into the ion source of the mass analyzer 24. A suitable vacuum pump 26 is provided for evacuating the analyzing chamber of the analyzer 24 down to a pressure of approximately 10-6 torr.

In order that a continuous low of gas be provided to the input of the separator 22 a bypass conduit 13 is connected from the gas source 10 to the valve means 16.

Referring again to FIG. 2 it will be noted that the form of each sample GC peak differs in some respect from the other GC peaks. Since the calibration of the output of the mass spectrometer 24 depends on the quantity of sample input thereto it is desirable that the quantity of sample material selected from each peak for introduction into the mass analyzer be the same. This necessi tates opening the valve 16 for a different time At for each GC peak due to the disimiliarity in the characteristics of the respective peaks.

One method of selecting a sample is to manually control the valve 16 in accordance with the projected or observed output from the column. Another method is to provide a timing means which causes the spool 17 of valve 16 to be placed in its sample selecting position for a predetermined period of time. By referring to the exemplary curves shown in FIG. 2 it will be apparent however, that such methods cannot reasonably be expected to select, with any degree of accuracy, substantially equal quantities of sample material from the non-similar peaks.

Returning now to FIG. 1 there is shown a valve control system, in accordance with the present invention, which provides a means for automatically selecting from each GC peak an equal quantity of material. The Splitter 14, which may be of the type disclosed in copending application Ser. No. 666,618 filed Sept. 1l, 1967 now Pat. 3,498,027 issued Mar. 3, 1970 and assigned to the assignee of the present invention, diverts a small percentage (approximately 1%) of the ow stream to a suitable detector apparatus 27 which may, for example, ,be a flame detector 27. The flame detector 27 produces an electrical output signal which varies in accordance with the quantitative distribution of the sample material in the efliuent iiow stream of the chromatograph 12 (see FIG. 2).

By means of a lead 28 this electrical signal is supplied to a valve actuating and control circuit in accordance with a preferred embodiment of the invention which is comprised of a minimum detector 31), a maximum detector 36, a valve actuator 42 and switching means S4, an integrator 48, a signal comparator 50 and a control amplier 52. The signal from detector 27 is fed directly to the input of the minimum detector 30, the maximum detector 36 and through the switch means S1 to the integrator 48.

For purposes of illustration the minimum detector 30 is shown in its most basic schematic form including an operational amplier 31, a diode 32, a resistor 33 and a capacitor 34. The operational amplifier 31 is of the type that produces an output signal only when the voltage on its input leads are not substantially equal. When a signal appears on lead 28 and the switch S1 is closed, as shown, connecting the capacitor thereto through resistor 33, the voltage on the capacitor 34 will follow the signal voltage so that the potential on both of the input terminals of amplifier 31 are approximately equal and no output signal is generated thereby. When the switch S1 is opened the capacitor will follow only excursions of signal voltage in the negative direction due to the polarity of the diode 32 and upon the first excursion in the positive direction, as at points c, f, etc. in FIG. 2, the voltages applied to the input of the amplifier 31 will differ and an output will be generated thereby. This output is connected through a lead 35 so as to open switch S2 and close switch S3 as illustrated in the drawing.

The maximum detector 36 is schematically illustrated in the manner of minimum detector 30 previously described except that the polarity of the diode 37 is reversed so that the operational amplifier is sensitive to signal excursions in the negative direction when the switch S2 is in its open position as shown. The output of amplifier 38 is connected through a lead 39 to switch S1 where it acts to close switch S1 when the signal in line 28 begins to decrease in magnitude, as at points a, d, g, etc. in FIG. 2 of the drawing.

The output of amplifier 38 is also connected to a triggerable power supply means 40 of valve actuator means 42 which is turned ON by a signal generated by amplifier 38. The output of power supply 40 is connected through switch means S3 to a solenoid 44 which, when energized, causes the spool 17 of valve 16 to be displaced into its upper position allowing sample gas to ow into separator 22. A spring means 46 is provided for returning the spool 17 to its lower position when switch S3 is opened.

As the spool 17 is displaced upwardly in response to the detection of points a, d, g, etc. in FIG. 2, switch means S4 is closed connecting the input of integrator 48 to the output of llame detector 27. During the time that switch S1 is closed integrator `48 effectively provides a summation of the area under each peak between the points a-b, etc., which corresponds to the shaded areas shown in FIG. 2. The output of integrator 48 is connected to a comparator 50 which is presettable so as to generate an output signal when the integration has reached a predetermined value corresponding to the amount of sample which is to be supplied from each GC peak to separator 22 for transmission to mass spectrometer 24.

The output signal from comparator 50 causes control amplifier 52 to generate a control signal which, Via lead 54, is fed back to open switch S1, to close switch S2 and to open switch S3 (while also de-energizing power supply 40) thus de-energizing value actuator 42 so as to allow spool 17 of valve 18 to return to its lower position. At the same time switch S4 is opened and integrator 48 is automatically reset and made ready to start another cycle upon the next closing of switch S4.

OPERATION The operation of the detection and control circuit will be explained with particular reference to FIG. 3 which graphically indicates the operational sequence of the circuit.

Initially Sl is closed, S2 is opened, S3 is closed and the spool 17 is in its lowermost position. (Shown in dashed lines.) Fluid flowing into the valve 16 is exhausted out vent 18 and only carrier gas from line 13 is supplied through the valve 16 to separator 22. As peak P1 emerges from chromatograph 12 and is detected by flame detector 27, the maximum detector is poised and ready to detect the rst maximum point cz. By design the ow impedance between splitter 14 and valve 16, and between splitter 14 and flame detector 27 is such that the maximum point a (see FIG. 2) of GC peak P1 reaches the detector 27 slightly before the same point of the peak P1 reaches the valve 16. (The difference in eifective ow distance allows for the time lag resulting from the sequence of detection and valve actuation.) When point a is detected a signal is generated by detector 36 which opens switch S1 (enabling detector 30 to detect the next minimum) and activates the power supply 40 which in turn energizes solenoid 44 thus raising the spool 77 to its upper position. The sample flow is now being diverted through conduit 20 to separator 22 where a portion thereof is separated out and introduced into mass spectrometer 24.

At the same time switch S4 is closed and the integrator 48 is activated. These conditions are indicated in FIG. 3 wherein the solid lines indicate respectively, closed switches and activated components.

When a predetermined quantity of sample material (indicated by the shaded area between a and b in FIG. 2) has been allowed to pass into separator 22, as determined by integrator 48 and comparator 50i, amplifier 52 is activated to close switch S2, open switch S3, deactivate power supply 40 and return valve spool 17 to its lower position opening switch S4 and shutting down integrator 48.

When the signal from llame detector 27 reaches the point c at the end of the first peak "P1, the minimum detector 30 generates a signal which opens S2 and closes S3 thus activating maximum detector 36 and completing a current path from the yet inactivated power supply 40 to solenoid 44.

When the maximum detector 36 detects the maximum point d power supply 40 is activated and the previously described sequence is automatically repeated introducing into separator 22 the predetermined quantity of sample material represented by the shaded area of peak P2 between points d and e in FIG. 2. This sequence is automatically and repeatedly executed by the control apparatus so that the same quantity of sample material is extracted from each peak for introduction to separator 22 and subsequent analysis.

As an added feature to the embodiment of the invention disclosed, it is contemplated that a threshold detector or any other suitable detector means could be substituted for one or more of the detectors shown in the drawing. Moreover, it is further contemplated that an even more selective computing type of apparatus could be incorporated in the circuit along with the detectors so as to further increase the selectivity of the valve control systern. It is also contemplated that the input to the peak detector could be taken directly from the detector circuitry of the gas chromatograph 12.

This sample selection and control apparatus will undoubtedly iind application in other analytical systems besides that depicted in the drawing. One such application contemplated is in fraction collection apparatus where it is desirous to be able to collect certain quantities of material from a gaseous mixture. Another similar applica tion is between a rst chromatographic column and one or more other chromatographic columns.

After having read the above disclosure it will become apparent that many alterations and modications may be made to the system without departing from the invention and it is to be understood that this description is for purposes of illustration only and is in no manner intended to be limiting in any way and that I intend that the appended claims be interpreted as covering all modifications which fall within the true spirit and scope of my invention.

What is claimed:

l1. In a gas handling system including, means for separating a gaseous material into a series of time separated constituent peaks of differing heights carried along in a carrier gas stream, a plurality of ilow path means selectively connected to said separating means by a flow directing valve means, -means connected to at least one of said ow path means for receiving at least a portion of said gaseous material, gas dletector means coupled to said separating means for detecting said constituent peaks, valve control means responsive to said detector means to open said valve .means in response to the detection of predetermined characteristics of said peaks, the improvement wherein said valve control means further includes an integrating means responsive to said detector means for determining how much. separated gaseous constituent material has been allowed to flow to said receiving means, said control means responsive to a predetermined output of said integrator to close said valve means so that only a predetermined quantity of said separated gaseous constituent material is allowed to flow to said receiving means.

2. Apparatus as recited in claim 1 ywherein said integrator means and said valve actuator means are operatively coupled together by a comparator means which causes a control signal to be generated `and supplied to said valve actuator in response to a predetermined output of said integrator.

3. In a gas handling system as set forth in claim 1 wherein lsaid valve control means .further includes a comparator circuit responsive to said. integrating means for determining when a predetermined quantity of gaseous constituent material has been allowed to flow to said receiving means.

4. yIn a gas handling system as set forth in claim 3 wherein said valve control means further includes a signal detector means responsive to said gas detector means for rendering said integrator operative only after the detection of a predetermined -peak characteristic.

S. Apparatus in accordance with claim 4 ywherein said signal 1detector means includes a rst means responsive to a maximum signal value and a second means responsive to a minimum signal value.

6. In a gas handling system in accordance with claim 1 wherein said means for separating the gaseous materials is a gas chromatograph and said gas receiving means is a mass spectrometer.

References Cited UNITED STATES PATENTS 3,301,040 1/1967 Levy et al. 73-23.l '3,376,731 4/1968 Cross et al 7.3-23.1 '3,396,870 8/1968 Diamond et al. Z22-14 3,402,972 9/ 1968 Cooper et al 137-487.5X 3,405,549 10/1968 Finley 73--23.1 3,049,908 8/ 1962 Kindred et al 73-23 3,421,292 1/ 1969 Llewellyn 73-23.1 3,245,269 4/ 1966 Ivie 73-23.1

RICHARD C. QUEISSER, Primary Examiner E. J. KOCH, Assistant Examiner

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