Automatic High Temperature Vent System for a Gas Chromatograph/ Mass Spectrometer Interface Douglas W. Kuehl, Gary E. Glass, and Frank A. Puglisi U S . Environmental Protection Agency, National Water QualityLaboratory, 6201 Congdon Boulevard, Duluth, Minn. 55804 The gas chromatograph interfaced to a mass spectrometer (GC/MS) has become a widely used analytical tool for identification of volatile compounds (1-4). In addition, the ability to quickly vent unwanted GC effluent from entering the MS has increased the versatility of the GC/MS system. Previous venting systems have employed metal valves (5) which may result in peak tailing and decomposition of sensitive components. Magnetically operated glass valves have also been used (6). However, in these systems, the valves have been mounted in the detector oven of the gas chromatograph and the magnets tend to lose some strength. Commercially available multipoint valves equipped with a filled polytetrafluoroethylene slide have been used, but are limited to temperatures below 240 "C. This communication reports the design of an all-glass, remotely controlled, high temperature system for venting. This system uses an electrically-timed high vacuum solenoid that offers millisecond response. The all-glass system reduces the possibility of sample decomposition as well as allowing temperatures up to 400 "C. Venting allows injections of up to 25 p1 of solvent to be easily handled.
EXPERIMENTAL INSTRUMENTATION The GC/MS system consists of a Varian Aerograph gas chromatograph (Model 1740), a dual stage Watson-Biemann separator with a 30-cm glass transfer line connecting the GC to the separator, and a Varian MAT CH-5 single focusing mass spectrometer equipped with a pressure measuring source for total ion monitoring (TIM). Each stage of the separator has been equipped with a valve (Varian MAT, Model No. NW8 451-001) between the separator and fore-pump and a vacuum thermocouple which has been connected to a vacuum gauge (Kinney Vacuum Gauge, Model No. I11 KTPG-3). A Varian SpectroSystem 100 MS computer system is used on line to process the MS data. The glass transfer line consists of a 15-cm x 0.02-mm i.d. capillary line at the separator fused to a 15-cm x 2-mm i.d. length of glass fitted with a glass socket to accept a GC column. At the junction of the two 15-cm pieces, a 10-cm x 2-mm i.d. length of glass tubing has been attached at an angle of 90' to the transfer line, and extends through the top of the detector oven, see Figure 1. The connection to the high vacuum straight-through solenoid valve (Vacuum Accessories Corp. of America, Model No. SOV050-01-PB fitted with Y+-inch Swagelok connection) is made using flexible stainless steel to glass tubing (Cajon G321-4-2 and G3214-2-X4). This prevents strain and glass breakage and moves the solenoid valve away from heated areas. The vacuum side of the solenoid is connected to a fore-vacuum pump (Edwards ED-75) which also serves as the pump for the separator. The solenoid is wired into an electronic timing unit (Sol-vent unit, Analytical Biochemical Labs.). The electronic timer allows the solenoid to be opened manually or automatically from 0-300 seconds with millisecond response. The solenoid is mounted behind the GC on an arm extending from the GC stand, and the "Sol-vent" unit is mounted on the side of the GC.
(1) R. Ryhage,AnaL Chem., 36,759 (1964). (2) J. T. Watson and K. Biemann,Ana/. Chem., 36, 1135 (1964). (3) P. M. Llewellyn and D. P. Littlejohn, Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Pittsburgh, Pa., February 1966. (4) R. Venkalaraghaven and F. W. McLafferty, Chem. Tech., June, 1972. p 364. (5) J. P. Mieure, J . G. Converse, M. W. Dietrich. and Lewis Fowler, Anal. Chem., 44, 1332 (1972). (6) S.P. Markey, Anal. Chem., 42, 307 (1970).
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ANALYTICAL CHEMISTRY, VOL, 46, NO. 6 , MAY 1974
DISCUSSION During the normal GC/MS operation, the solenoid is closed and all GC effluent passes to the separator. The pressure at each stage of the separator is reduced by the fore-pump to 1-5 Torr for a column flow of ca. 25 ml/min. When venting is required, the vent may be opened and closed manually by the operator or it may be closed by the electronic timing unit. The opened solenoid increases the vacuum in the "T" of the transfer line and essentially effluent diffuses through the "T" toward the vacuum pump. The vacuum then drops to 0.3-0.5 Torr, as only a trace amount of effluent passes toward the separator. The vent may be opened for a precise time period that can be accurately reproduced (*0.5 sec). The length of time required for venting can be determined by examining an FID trace of the sample. Decreasing the solvent venting time allows the operator to obtain the solvent shoulder on the TIM trace without any effect upon ion source vacuum. MS data can then be obtained on early eluting peaks. Experiments with a dual solenoid system, that provides the addition of a make-up gas to the separator while the vent valve is open, have been conducted. Using this method, a constant base line may be maintained for the TIM (reference 4 ) . However, it was felt that this was not necessary because pen response time for the TIM to reach its zero point was less than 10 seconds after the vent was closed. The ability to quickly vent GC effluent has several advantages in GC/MS operation. Venting of the solvent allows continuous MS operation without the possibility of damage to the filament or electron multiplier. Spectral interpretation may be simplified by the elimination of background peaks due to the solvent or a large peak which appears later in the chromatogram. Moreover, opening the vent allows the GC to be maintained at working temperatures and carrier gas flows when not in use without allowing column bleed to enter the ion source. This procedure improves the ion source vacuum which increases filament life.
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Figure 1. Diagram of solvent venting system
The venting system has been in use for over 18 months, under a wide variety of conditions. Data have been gathered with column temperatures ranging from 50 to 325 "C, column flows from 15 to 60 ml/min, and column lengths that varied from 2 to 12 feet. No adverse effects of the
performance of the GC or the efficiency of packed columns have been observed. Received for review December 21, 1972. Accepted November 23, 1973. Manufacturers' names are used for product identificationonly.
Internal Standard Addition Method for Preparation of Homogeneous Powder Mixes J. F. Jaworski, R. A. Burdo,l and G. H. Morrison D e p a r t m e n t of Chemistry, Cornell University. Ithaca, N. Y . 14850
These solution doping procedures fall into two categories. An internal standard solution is added to the powone of the major difficulties of calibration in spark source mass spectrometry (SSMS)( 1 ) . Because of the lack of homogeneity resulting from the addition of internal standard in solid form followed by successive powder dilutions, a number of solution doping procedures have been proposed (2-8). These solution doping procedures fall into two categories. An internal standard solution is added to the powdered sample, the solvent is evaporated by heating or freeze drying. and the resulting doped sample is blended with graphite or other conducting medium to form the mix from which electrodes are pressed (2-4). The internal standard and sample, both in the same solution, are added to graphite, the solvent is evaporated, and the mix is blended with the powdered sample (3, 5-8). Neither of these two approaches was found by this laboratory t o be universally applicable to the preparation of conducting electrodes from complex samples such that the electrodes contained a homogeneously dispersed internal standard. The first of the above approaches is subject to inhomogeneities resulting from specific interactions occurring between the sample and the ions of the internal standard-ie., ion exchange. The second approach results in poorer detection limits because of the limited solubility of many samples in solution. If not completely in solution, sample precipitates will cause selective adsorption with resultant inhomogeneity. It also fails to utilize the main advantage of SSMS,the direct analysis of solid samples. A relatively rapid method is described here which allows the preparation of homogeneous internal standardsample mixes in SSMS without the limitations of previous methods. The method involves solution doping of graphite with internal standard(s) followed by freeze drying and blending. In this way, a large amount of graphite may be processed into a stable doped conducting medium which may be tested for homogeneity as well as contamination. This stock graphite may then be used to provide exactly the same amount of internal standard in the electrodes of both sample and calibration standard. Present address, Department of Chemistry, University of Rhode Island, Kingston, R.I. 02882. (1) (2) (3) (4)
G . D.Nicholsetai.. Anal Chem.. 39, 584 (1967). D. A Griffith eta/.. Talanta. 18,665 (1971). J. P. Yurachecketai..Anal. Chem , 41, 1666 (1969). P. F . S. Jackson and J. Whitehead, Analyst (London), 91, 418
(5) (6) (7) (8)
( 1968), A. W . Fitchettand R. P. Buck, Anai. Chem.. 45, 1027 (1973). G . H . Morrison and B. N . Colby. Anal. Chem . 44. 1206 (1972). S. S. C. Tongetai.. Anal Chem.. 41, 1872 (1969). I . H . Crocker and W . F. Meritt. Water Res.. 6, 285 (1971).
The precision of the internal standard lines measured within a single photoplate is approximately *5% RSD while the precision of analysis using a standard for sensitivity calibration is approximately *lo% RSD or better for 80% of 42 elements analyzed in geological samples. In addition to the preparation of internal standard mixes, the procedure can be used to prepare homogeneous solid synthetic samples for SSMS and emission spectrometry. EXPERIMElNTAL Ten grams of high purity graphit.e, suitable for making pellets (Spex Industries, Metuchen, N.J.), were weighed into a clean 60-ml polyethylene bottle. High purity In metal and high purity Re03 were each dissolved in a minimum of reagent grade nitric acid and diluted with double distilled water to yield solutions containing 365 ppm In and 19.6 ppm Re. Four ml of the In solution, 3 ml of the Re solution, as well as 3 ml double distilled water and 5 ml spectral grade methanol were mixed in a polyethylene beaker and then added to the graphite in the polyethylene bottle. Several polyethylene balls were added and the mixture was shaken in a Spex mixer mill for 10 t o 15 minutes. Tlie resulting slurry was inspected for even wetting as well as for the absence of a liquid/graphite partition ( i . e . , phase separation) and was then quick-frozen in liquid nitrogen and freeze-dried in a conventional acetone-Dry Ice freeze-drying apparatus for 18 to 24 hours. The dried powder containing 146 ppm In, 5.88 ppm Re was placed in a vacuum oven for 3 hours a t 60 "C to drive off any residual moisture, then tumbled for a n hour to break up lumps. Three such batches of graphite 'were prepared simultaneously from the same internal standard stock solutions. One gram of graphite from each batch was added to 1 gram of LSGS standard rock W-1 and blended as reported earlier (9) to yield thrte mixes from which electrodes were pressed. Triplicate analyses were made on each batch, the results being recorded on Ilford Q2 photoplates and read using a Jarrell-Ash Model 23-100 scanning microdensitometer interfaced with a PDP-11/20 computer ( 1 0 ) .
RESULTS AND IIISCUSSION Indium and Re were chosen as internal standards because they are normally present in very low concentration in most complex samples. Other criteria for the selection of suitable internal standards have been discussed by Taylor (11). Their concentrations were chosen to permit analysis of higher level elements by comparison with Il5In, while low level elements were compared with 11%, IS5Re, and IS7Re. Use of two or more widely spaced internal standards makes it possible to determine whether variations in results are due to changes in plate sensitivity or sample homogeneity. (9) G . H. Morrison and A . M . Rothenberg, A n a l . Chem , 44, 515 (1972). (10) R. A. Burdo, J. R. Roth and G . H. Morrison, Anal. Chem , 46, 701 (1974). (11) S. R. Taylor, Geochim. Cosmochim. Acta. 29, 1243 (1965) A N A L Y T I C A L C H E M I S T R Y , VOL. 46, N O . 6 , M A Y 1974
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