An Integrated Gas Chromatograph-Mass Spectrometer System with Carrier Gas Separator Max Blumer Woods Hole Oceanographic Institution, Woods Hole, Mass. 02543
MOLECULAR SEPARATORS, differing in their permeability for carrier gas and heavier molecules, are widely used for interfacing gas chromatographs (GC) with mass spectrometers (MS) (I, 2 ) . The Watson-Biemann enricher (3, 4) makes use of the preferential diffusion of helium through a glass frit. We purchased a modified commercial version of this device. Its performance in the recovery of microgram amounts of polar high boiling materials was disappointing. Thus, methyl palmitate was almost completely retained by the separator. The performance was improved by treatment of the frit with trimethylchlorosilane; however, esters above CIS were still retained as shown by increasing peak distortion, peak delay, and a drop in yield. This specific separator was constructed with a fixed glass leak at the entrance to the analyzer; therefore the selection of an optimum flow rate required venting and glassblowing. Also, the enricher could not be isolated from the spectrometer. A heated porous Teflon separator (5) was constructed here. Its operation was satisfactory; however, as previously noted ( I ) , appreciable delays and distortion of peak shapes occurred in the elution of the peaks. Leaks occurred frequently, both in the capillary and at its junction to the hypodermic steel tubing. An enrichment device using porous silver membranes, brazed to a circular chamber has been described (6). In this design, the active surface area, approximately 31 cm2, is nearly as large as in the original Biemann-Watson device (25 cm2) and the commercial version tested by us (50 cm2). Aside from increasing the danger of adsorptive losses, such a large surface area requires a separator with large dead volume and leads to a loss of resolution by remixing of the chromatographic peaks. I wish to describe an integrated GC-MS system constructed from standard commercial parts, with a silver membrane of 0.1 cm*surface area as the active component. EXPERIMENTAL. An Aerograph Model 550 B GC-oven with flame detector and separate amplifier, recorder, and temperature programmer is used. A balanced flow splitter is inserted between column and flame detector base. It consists (Figure l ) of two pieces (TI, Tz)of narrow bore stainless steel tubing, packed with glass microbeads (“siliconized,” or treated with trimethylchlorosilane, Applied Science Laboratories, State College, Pa.), held in place by short lengths of kinked 20gauge Chrome1 A wire. This has less influence on the flow (1) M. A. Grayson and C. J. Wolf, ANAL.CHEM., 39, 1438 (1967). (2) F. A. J. M. Leemans and J. A. McCloskey, J . Am. Oil Chemists, SOC.,44, 11 (1967). (3) J. T. Watson and K. Biemann, ANAL.CHEM., 36, 1135 (1964). (4) ibid., 37, 844 (1965). ( 5 ) S . R. Lipsky, C. G. Horvath, and W. J. McMurray, ibid., 38,
1585 (1966).
(6) R. F. Cree, Pittsburgh Conf. on Anal. Chem. and Appl. Spectr., March 1967, Abstr. of Papers, p 96, No. 188.
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rate than the more common asbestos or quartz wool plug. The dimensions given (Parts list) are for an approximate 1 :2 split between GC and MS. The split ratio is easily altered by inserting different lengths of packed tubing. Use of a balanced splitter, in which both arms have approximately the same flow resistance, minimizes the temperature dependence of the split ratio, since it partly cancels variations in flow resistance due to differences in expansion coefficient between glass and steel. The flow line T3 to the enricher and valve VI are heated with heating tape and insulated with fiber glass. The enricher, exit valve VZand connection to the forepump line are mounted in a forced circulation air oven, located close to the source area of the spectrometer. The silver membrane (6-mm. o.d., 2 mil thick, 3-micron max. pore dia.) is cut with a No. 3 cork borer from a silver membrane filter (No. FM-13-3, 83483-05, Selas Flotronics, Spring House, Pa.), It is held between Teflon washers (6-mm o.d., 3.5 mm i.d.) cut with No. 1 and No. 3 cork borers from Teflon sheet (0.2 mm thick, Bel Art Products, Pequannock, N. J.). Careful assembly of the enricher prevents the formation of leaks and of crimps and tears in the membrane. For assembly, the lJ4-inchSwagelok union tee is held in a vise, and the membrane and washers are seated against the shoulder in the side arm. The Swagelok reducer, held in a Vise Grip, is inserted and firmly pressed down while the nut and ferrules are tightened. Rotation of the reducer should be avoided. The exit valve is preset at the factory for dead stop at positive minimum flow. It is supplied with instructions for resetting to complete shutoff. This will permit isolation of the enricher from the spectrometer. The enricher is exhausted into a mechanical pump. The dimensions of the flow line are critical, restrictions should be avoided as they limit the gas flow and affect the efficiency of the separator. Connection of the enricher to the MS (Consolidated Electrodynamics, Model 21-104, Monrovia, Calif.) is made through a glass line; a metal fitting into the source region would be preferred, but no difficulties have been experienced with a Swagelok or Kovar metal to glass connection. The split ratio of the GC splitter is determined from the carrier gas flow rates measured at the flame detector base with the shutoff valve to the enricher either open or closed. The enricher split ratio is determined from the carrier gas flow rates measured with a bubble flow meter at the exit of the enricher and analyzer forepumps; a correction is made for any flow due to leaks which remains with closed enricher shutoff and exit valves. To measure separator yield and enrichment, the sensitivity of the mass spectrometer for each test substance has to be determined as follows: A known weight of the sample is introduced into the heated glass inlet system and the total ion current is recorded during a known time interval while helium is simultaneously introduced into the analyzer via G C and separator. The initial and final ion current readings are proportional to the initial and final sample concentrations in the reservoir. The sensitivity is calculated from these values and the area enclosed by the decay curve. Another known sample is then introduced via the gas chromatograph at the same helium flow rate and the total ion current is re-
ENRICHER
SPLITTER
\
0
ag Membrane-
-0 0
b-7 Flame
I
M. s
to
Mach. Pump
Figure 1. Construction detail of the GC-MS interface
corded. The weight of sample entering the separator is calculated from the weight injected and the G C split ratio. The yield of the separator is obtained from the weight of sample entering the separator and its ion current response ( I ) . The enrichment (1) is calculated from the yield and the enricher split ratio.
RESULTS AND DISCUSSION Optimum He flow for our columns ( 1/8-incho.d., 0.035-inch wall) is near 13.5 ml/min. Typically, of this 10 cc/min enter the enricher and 0.1-0.3 mllmin pass into the MS. The enricher exit valve is opened to 20-50 divisions; this results in an analyzer pressure of 0.6-2 x 10-5 Torr and a pressure at the analyzer forepump of 5-15 Torr. Higher or lower flow rates can be accommodated, if necessary, with changes in the dimensions of the Teflon washers, the size of the Tee and the porosity of the membrane which is available in a pore size range from 0.2-5 microns. Thus, by substituting a 318inch or a 1I2-inchTee, the cross section of the silver membrane and, correspondingly of the throughput, can be increased by factors of two and four. It should be noted that changes on splitter and enricher can be made without the need for venting the analyzer and without having to resort to glass blowing. The separator has been operated between 100 and 250 "C: operation outside these limits should be possible. Peaks appear simultaneously at the flame detector and at the source of the spectrometer ; identical plate efficiencies are measured from the chromatograms of the G C and of the MS detector. At lower valve settings, the peak delay increases and some peak broadening is observed. Separation yield and enrichment factors (Table I) are similar to those measured by Grayson and Wolf (I) for a fritted glass separator at identical flow rates. It should be noted that this performance is retained in spite of a reduction of the surface area of the separator by several orders of magnitude. The enrichment is above, the yield slightly below that of the Teflon enricher; however, the problems of lag time, peak
distortion, and proneness to leaks are absent. The silver separator has been in continuous operation for several months; operation has been reproducible and uninterrupted. The membrane has been exposed repeatedly to differential pressures of 1 atmosphere without rupturing. Operation has been found satisfactory with saturated and olefinic hydrocarbons to Czo,with saturated and unsaturated aldehydes, ketones and dicarbonyl compounds, and with fatty acid esters to Czz. While it has not yet been tested with higher boiling compounds, there appears no reason that its use cannot be considerably extended to higher temperatures and to compounds above Czo. No preferential retention of olefinic compounds by the porous silver membrane has been noted. The separator has been tested for possible reactions with halogen- and sulfur-compounds. Spectra obtained for nbutylbromide, see-butylchloride, and diethyl-sulfide at an enricher temperature of 240 "C were identical with those obtained on the same samples introduced through a glass inlet system. The intensity ratio of the molecular and other halogen- or sulfur-containing ions to hydrocarbon ions was identical for both types of sample introduction. This ratio is a sensitive measure of any removal of halogen or sulfur which might occur in the separator. Thus, in spite of the unnecessarily high test temperature, no reaction took place between the silver membrane and the halogen- and sulfurTable I. Separation Yield (Y) and Enrichment (N) Obtained with Porous Silver Separator at 210 "C Flow, Wmin 4 7.5
10 12 18
N-Pentane N 33 4
Y ,7Z
N-Decane N 52 6
Y ,Z,
53
18
59
20
32
15
34
16
N-Hexadecane Y ,Z, N 47
12
56
24
VOL. 40, NO. 10, AUGUST 1968
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compounds. This is attributed to the low sample pressure, the high flow rate, and the very small surface area of the membrane. CONCLUSIONS
The system permits G C operation independently or simultaneously with MS scanning at maximum chromatographic efficiency. The elution of a G C peak can be observed both on the hydrogen flame recorder and the total ion current monitor of the MS; the latter is disabled during an MS scan but the flame signal can still be monitored. The silver membrane carrier gas separator is easily constructed from standard commercial parts. The rigid metal construction requires no glass blowing or soldered or brazed joints; all parts are easily exchanged within minutes and contaminated flow lines may be disassembled and cleaned ultrasonically in a solvent bath. Compared to previous designs, the active area of this exchanger has been reduced by more than two orders of magnitude. The thickness of the silver membrane and therefore the surface area of its internal pores is another two orders of magnitude less than that of the glass frits previously used. As a consequence, this design has led to a striking reduction in adsorptive losses, in dead volume, in peak delay and peak broadening. The design is flexible and should permit operation outside the conditions described here. Parts List 202-1-316 Nuts, 16 reqd. 402-1-316 Nuts, 10 reqd. 812-1-316 Nuts, 1 reqd.
200-6-316 Unions, 2 reqd. 400-6-316 Unions, 2 reqd. 200-R-4-316Reducers, 8 reqd. 810-6-4-316Reducing union, 1 reqd. 200-3-316 Union Tee, 1 reqd. 400-3-316 Union Tee, 1 reqd. All fittings Swagelok, Crawford Fitting Co., Solon, Ohio Silicone Rubber 0-Ring, ‘/*-inch i.d. and brass backup ring (Varian Aerograph, Walnut Creek, Calif.) Teflon washers, 2 reqd., see text Silver membrane, 1 reqd. see text TI = 11-cm Tubing, l/s-inch o.d., 0.049-inch wall, packed with 100-140 mesh glass beads, siliconized, see text Tz = 3-cm Tubing, l/e-inch o.d., 0.049-inch wall, packed with 1OC-140 mesh glass beads, siliconized, see text T3 = Tubing, l/s-inch o.d., 0.049-inch wall Tubing, ’/*-incho.d., 0.015-inch wall Tubing, l/a-incho.d., 0.065-inch wall All tubing 304 stainless, Tube Sales, Englewood, N. J. VI = Shutoff valve, No. 413 HT, Hoke, Inc., Cresskill, N. J. V z = Bellows metering valve, SS-4BMW, Nupro Co., Cleveland, Ohio Gas chromatograph oven, e.g., Aerograph Model 550-B (Varian Aerograph, Walnut Creek, Calif.) with flame detector, separate amplifier, and recorder. Temperature programmer optional Enricher Oven, heatable to 300 “C, forced circulation. Vacuum pump, e.g. Welch Model 1400, Duo Seal, Welch Scientific Co., Skokie, Ill. RECEIVED for review February 26, 1968. Accepted May 8, 1968. Supported by ONR (N0014-66-contract CO-241), by NSF (GA-539) and by API (85 A). Contribution No. 2080 of the Woods Hole Oceanographic Institution.
Induction Heating in Zone Melting of Organic Compounds Henry Plancher, J. C. Morris, and W. E. Haines Laramie Petroleum Research Center, Bureau of Mines, U.S . Department of Interior, Laramie, Wyo. INDUCTIONGENERATOR.The induction generator, deAN INVESTIGATION was undertaken to study the feasibility of signed for continuous use, has an output of 2.5 kW, and the using induction heating in the zone melting of organic compower output frequency is a nominal 450 kHz. The capacity pounds. In using induction heating in the processing of of the generator is sufficient to produce simultaneously molten organic materials, a conductor is employed as an intermediate zones in 10 samples with melting points as high as 440 “C. heating device or susceptor. Pfann (I) suggested that the LOADCOILS. The pancake, or plate-concentrator, load susceptor might be imbedded in the sample when using this coil with two loops is shown in Figure 2. Plate coils were heating technique for zone melting. used because they concentrate the radio frequency field in a This investigation was part of a search for a means of proshallow horizontal plane, conducive to flat, low-volume molten zones. Coils were made from 3/16-inchcopper tubing ducing and maintaining low-volume molten zones necessary in which the inner turn of the planar spiral was silver-soldered for using zone melting as a separation tool for complex mixto the l/s-inch copper sheet that serves as the secondary tures such as petroleum fractions and residues. A theoretical inductor or base. A radial slot was cut through the secondary discussion of such a process has been published (2), and the inductor to prevent electrical continuity about the periphery. most difficult of the prescribed conditions is maintaining lowThe coil assembly was insulated with Glyptal, and the genvolume molten zones. Attempts to reduce zone lengths in erator leads were further protected with asbestos or rubber conventional, externally heated systems have been reported tubing. ( 3 , 4 ) ;however, molten zones with lengths less than the radius For the work reported here, various sizes of coils were used of the container have been difficult to maintain. to accommodate tubes of different diameters, and as many as A technique and an apparatus for using induction heating 10 coils were connected horizontally in series, permitting up to 10 samples to be processed simultaneously. with an imbedded susceptor to produce and maintain lowTUB=. Glass tubes ranging from 1 to 10 cm in diameter volume molten zones in the zone melting of organic materials and of various lengths were used. Precision-bore glass tubes have been devised. were used to permit closer tolerances between the susceptor EXPERIMENTAL and the tube. For zone melting under vacuum, a side-arm Apparatus. Figure 1 is a schematic drawing of the ascontaining susceptors for the desired number of passes of a sembled apparatus. molten zone was attached near the top of the tube. Susceptors were disks of 1%mesh, galvanized SUSCEPTORS. (1) W. G. Pfann, “Zone Melting,” Wiley, New York, 1958, p 75. wire screen whose cross linkages were soldered to ensure (2) W. G. Pfann, ANAL.CHEM., 36,2231 (1964). rigidity. The disks were cut with a punch and die 0.39 mm (3) F. Ordway, ibid., 37,1178 (1965). smaller than the i.d. of the tube to be used. (4) W. G. Pfann, “Zone Melting,” 2nd ed., Wiley, New York, Each susceptor was balance-tested and matched to the tube. 1966, p 97. 1592
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