Efficiency of Molecular Separators for Interfacing a Gas Chromatograph with a Mass Spectrometer Michael A. Grayson and Clarence J. Wolf Research Division, McDonnell Douglas Corp., St. Louis, Mo. 63166
Two systems, a fritted glass tube and a porous Teflon capillary tube, were investigated as interfaces for combining a mass spectrometer with a gas chromatograph. In both systems the concentration of the effluent in the carrier gas stream is increased by preferential removal of the carrier through the walls of the separator. The separation yield and enrichment factor for the separators were studied as a function of temperature, carrier gas flow rate, and molecular weight of sample.
THECOMBINATION of a mass Spectrometer with a gas chromatograph forms an extremely powerful analytical tool. The gas chromatograph can separate complex mixtures quantitatively into individual unidentified components. The mass spectrometer is excellent for identification of complex compounds provided that they are relatively pure. A gas chromatograph normally uses a carrier gas at atmospheric pressure whose flow rate varies between 5 and 200 ml/min. The mass spectrometer operates best at a pressure of less than torr. Thus, it is desirable to remove the carrier gas from the chromatographed sample prior to admittance into the mass spectrometer. Several systems have been described for the batch-wise trapping of each individual component as it emerges from the chromatograph (1, 2). The carrier gas is removed while the sample is held at low temperature. The trap is warmed and the unknown admitted to the spectrometer. McFadden discusses some of the problems associated with combining gas chromatographs with mass spectrometers (3). Under special conditions where large samples are available and low flow rates are used in the chromatograph, it is possible to go directly into the spectrometer ( 4 ) . In this latter technique 99% of the sample is vented to the atmosphere and 1 % goes into the spectrometer. Many samples are not conveniently trapped because of their chemical reactivity or because they are present in minute concentration. In addition, in a complex chromatogram several hundred peaks may appear, thereby imposing a n undue burden on the operator. The ideal technique for obtaining a mass spectrum of a chromatographed effluent consists in joining the two instruments directly with a molecular separator in which the carrier gas is preferentially removed and the sample passes undisturbed into the ion source of the spectrometer. Five different techniques for performing this separation have been described: the jetmolecular diffusion system (9,the fritted glass tube (6), the heated porous Teflon tube (7), a
Rotameter
4 ft heated stainless ,010 in. i.d.
All lines and v from GC to MS are heated
Figure 1. Block diagram of gas chromatograph and mass spectrometer
heated thin organic film (8), and porous metal membranes (9).
We restricted our studies to the separators described by Watson and Biemann (6) and Lipsky, Horvath, and McMurray (7) because they can be easily fabricated in the laboratory and are, therefore, readily accessible. The separator yield and enrichment of both separators were studied as a function of temperature, carrier gas flow rate, and molecular weight of sample. The separator yield, Y,is defined as the ratio of the amount of sample entering the spectrometer (QM8)t o that entering the separator ( Q G c ) ; that is, amount out divided by amount in: y = -Q- r s QGC
Y is usually expressed as a per cent rather than as a fraction. It represents the ability of the particular device to allow organic material to pass into the ion source of the mass spectrometer. Therefore, some authors have referred to the separator yield term ( Y ) as efficiency. Note that in the absence of a separator, the yield would be 100 because all the sample exiting the chromatograph would enter the mass spectrometer. Hence, the objection to the use of the term efficiency for Y . The enrichment is defined as
where Qxs/HeMs is the ratio of sample to carrier gas on the MS side of the separator and Qcc/HeCc is the ratio of sample to carrier gas on the GC side of the separator. Rearranging Equation 2 , N =
( I ) A. A. Ebert, Jr., ANAL.CHEM., 33, 1865 (1961). (2) J. W. Amy, E. M. Chait, W. E. Baitinger, and F. W. McLafferty, Zbid., 31, 1265 (1965). (3) W . H. McFadden, Separutiorz Science, 1, 723 (1966). (4) R. S. Gohike, ANAL.CHEM., 31,535 (1959). (5) R . Ryhage, Ibid., 36, 759 (1964). (6) J. T. Watson and K. Biemann, Zbid.,p. 1135; 37, 8446 (1965). (7) S. R . Lipsky, C . G. Horvath, and W. J. McMurray, Zbid.,38, 1585 (1966).
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ANALYTICAL CHEMISTRY
Separator
Detector gate set at characteristic mass of effluent
QXS Hecc QCCH e r s
(3)
When the definition of yield, Equation 1, is noted, we obtain (8) P. M. Llewellyn and D. P. Littlejohn, Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, February 1966. (9) R. F. Cree, Zbid.,March 1967.
(4) When Equation 4 is expressed in terms of flow rates, we have
N = Y
He flow into separator H e flow into spectrometer
(5)
The enrichment may be large but it is a meaningless parameter, if the amount of sample reaching the ion source is too small t o produce a good spectrum. Thus, a complex relation exists between the separator yield and enrichment with respect to maximum use in any particular system. EXPERIMENTAL
An F M Model 500 gas chromatograph modified with a Carle microthermal conductivity detector was used to determine the purity and concentration of each sample entering the separator. The column in the chromatograph was usually 5 ft X lis inch packed with SE-30 on 60- t o 80mesh Chromasorb-W. This system could be used with helium flow rates greater than 2 and less than 125 ml/min. The mass of material exiting from the separator was measured in a Bendix Model 12-101 time-of-flight mass spectrometer. The spectrometer was adjusted t o record only the ion current from one particular peak (usually the molecular ion) of the fragmentation pattern as the sample emerged from the separator. The ion current peaks are similar t o those obtained with a chromatograph, and the area under the ion peak is proportional t o the sample concentration. The sensitivity t o each compound was directly determined by passing a known amount of material into the spectrometer without using the separator. A 6-foot heated stainless steel capillary tube (0.010-inch i.d.) was inserted in place of the separator to obtain the proper pressure drop between the chromatograph and the mass spectrometer. A micrometer needle valve was placed between the capillary and the spectrometer t o limit the gas flow into the ion source t o 1 t o 2 mlimin. The ion current peaks were recorded on magnetic tape and their areas were determined with a n Infotronics CRS-100 digital integrator. The ion current areas were a linear function of mass for liquid and gas samples of less than 0.2 pl and 200 p l , respectively. However, for a given amount of material the areas varied with the helium pressure in the ion source of the spectrometer. The helium pressure was measured in all experiments by noting the ion current a t mass 4 (with a separate gating circuit) while simultaneously measuring the molecular ion. The compensating magnets, horizontal and vertical deflection plates, ion focus control, and ionizing voltage
5z
(70 eV in all experiments) of the spectrometer were carefully adjusted prior t o each measurement t o ensure reproducibility. Because the ion current areas were pressure sensitive, partorr, the areas ticularly for pressures greater than 8 X were corrected for the pressure. effect in the ion source. A block diagram of the experimental arrangement is shown in Figure 1 . A 4-foot heated stainless steel capillary (0.010inch i.d.) serves as a pressure reducer t o the separator entrance. A mechanical pump (speed 100 liters/min) provides a vacuum for the separator envelope. The micrometer needle valve between the separator exit and the spectrometer provides a variable leak; all other fixed leaks and constrictions in the mass spectrometer were removed. All valves and lines between the chromatograph and the spectrometer were heated t o prevent sample condensation. The fritted glass separator (6) consisted of an 8-inch length of porous glass tube (nominal pore size 1 p) enclosed in a glass vacuum envelope. The entire separator was enclosed in a n oven and its temperature was controlled to =t3" C. The capillary tubing a t the entrance and the needle valve a t the exit of the separator replaced the glass constrictions described by Watson and Biemann (6). The porous Teflon separator (7) consisted of 7 feet of thin-walled Teflon capillary (0.012-inch i.d. with 0.006-inch wall thickness) enclosed in a glass vacuum envelope. The tubing was wrapped around the outside of a glass finger containing a cartridge heater immersed in silicone. The temperature of the' bath was controlled t o =t2" C. The needle valve a t the exit of the separator controlled the amount of sample reaching the ion source. The separation yield, enrichment, and pressure in the ion source are a direct function of the opening in the variable micrometer needle valve. The three are interrelated and cannot be easily separated. With the fritted glass separator the needle valve was approximately open. If the chromatographic peak was a light organic-for instance, ethanethe needle valve was opened less than 2 0 x t o prevent a severe pressure rise in the mass spectrometer and subsequent depression of the mass spectrum. Helium pressures of 1-2 X 10-6 torr are readily obtained with helium flow rates as high as 100 ml/min by proper adjustment of the needle valve. With the Teflon separator the opening in the needle valve is extremely critical. If the opening is t o o large, the helium pressure in the ion source exceeds torr; if the opening is too small, the gas flow through the chromatograph is reduced. Thus, the minimum opening consistent with unhampered carrier gas flow is desired. The lowest obtainable helium pressure in the ion source under these constraints was 1-2 X 10-5 torr for flow rates up t o 15 ml/min. This pressure range is approximately the same as that reported by Lipsky et al. (7) with their system.
T able I. Separation Yield (Y)and Enrichments (N) Obtained with the Fritted Glass Separator as a Function of Carrier Gas Flow Rate, Temperature, and Compound Flow, Ethane N-hexane Benzene Isononane Temp, "C ml/min y, N Y, N y, N y, 72 N 5 19 4 54 11 36 7 64 12 75 10 5 53 38 10 18 15 60 17 15 6 56 18 18 39 13 65 21 5 15 3 47 9 7 34 52 10 150 10 4 13 14 46 10 34 52 14 15 15 5 48 16 12 36 53 18 5 15 3 41 8 33 6 45 9 225 10 16 4 13 46 10 35 14 49 15 14 5 42 14 35 12 16 48 5 14 3 43 8 31 6 46 9 300 10 4 14 41 11 32 9 42 12 15 5 13 38 10 13 29 42 14
z
z
z
VOL. 39, NO. 12, OCTOBER 1967
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Table 11. Separation Yield ( Y )and Enrichments (N)Obtained with the Porous Teflon Separator as a Function of Carrier Cas Flow Rate, Temperature, and Compound Ethane N-hexane Benzene Isononane Flow, y, 72 N y, 72 N y, N ml/min y, % N Temp, "C
z
280
5 10 15
69 51 46
I .4 2.0 3.5
89 68 61
1.8 2.7 4.3
78 63 51
1.6 2.4 3.5
90 70 62
1.8 2.8 4.3
300
5 10 15
72 61 53
1.4 2.4 3.7
92 82 75
1.8 3.3 5.2
84 64 51
1.7 2.5 3.5
94 77 65
1.9 3.1 4.6
320
5 10 15
75 64 54
1.5 2.5 3.8
94 88 82
1.9 3.5 5.7
84 66 54
1.7 2.6 3.8
95 86 70
1.9 3.4 4.9
Table 111. Operating Characteristics of the Fritted Glass Tube and Porous Teflon Tube Molecular Separators Fritted Glass Teflon 1 to >lo0 ml/min 1 to 15 ml/min Usable flow rates
