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Anal. Chem. 1984, 56,2997-2999
in the distribution. The inset in Figure 5 is an oscilloscope picture of an iterated map (11) of the position of spark N (horizontal axis) vs. the position of spark N + 1(vertical axis) in the train, where N runs from 1to 2047. The diffuse group of points in the upper right of the inset corresponds to those instances in which two successive sparks were centered in the right lobe of the distribution. Eighteen such points can be counted. For purely stochastic behavior, in which each spark attacks the electrode independently, the probability of having two successive sparks in the right-hand lobe should be the square of the probability of any spark being so located. One would thus predict that 17 pairs of sparks would sample the right lobe sequentially. Thus, it is demonstrated in this instance that wander is random within the f l limit inherent in asynchronous counting measurements. The data are consistent with independence of each spark in attacking the electrode. While similar measuremenb have been made for different gas flow rates, they have not yet been attempted for different spark repetition rates. Were two centroid-monitor assemblies available, they could be arranged to view the discharge at a 90° separation, allowing for the position of the cathode spot of each spark to be located in a Cartesian plane. Such an arrangment would be useful for surface mapping of electrode inhomogeneity (8) in situations where the desired discharge positional stability could not be achieved.
positional stability of a spark or arc during routine analyses. By condensing raw video data before digitizing for computer storage, positional data for entire trains of sparks can be realistically stored.
CONCLUSION
RECEIVED for review June 11,1984. Accepted August 27,1984. This work was supported by the National Science Foundation (Grant CHE-81-21809). Portions of this work were presented at the 10th Annual Federation for Analytical Chemistry and Spectroscopy Societies Meeting, Philadelphia, PA, September, 1983.
A simple device has been demonstrated which can compute the centroid position of a non-point-light source or shadow. An example application has been given where the positional stability of a train of spark discharges has been monitored. The device could be employed for on-line monitoring of the
Note Added in Proof. Similar hardware has recently been reported (12).
ACKNOWLEDGMENT Engineering advice by C. Hawley and software assistance by M. A. Lovik are appreciated. LITERATURE CITED Sacks, R. D.; Walters, J. P. Anal. Chem. 1970, 42, 61-84. Waiters, J. P.; Goldstein, S. A. ASTM Spec. Tech. Publ. 1973, 540, 45-7 1. Walters, J. P.; Eaton, W. S. Anal. Chem. 1983, 55, 57-64. Barnhart, S. G. Ph.D. Thesis, University of Wisconsin-Madison, 1983. Washburn, D. N.; Walters, J. P. Appl. Spectrosc. 1982, 36, 510-519. Ekimoff, D.; Walters, J. P. Anal. Chem. 1981, 53, 1644-1655. Walters, J. P.; Goldstein, S. A. Spechochlm. Acta, Part 8 1984, 398, 693-728. Oiesik, J. W.; Walters, J. P. Appl. Spectrosc. 1983, 37, 105-119. Tran, T. V.; Scheeline, A. Appl. Spectrosc. 1981, 35, 536-540. Coleman, D. M.; Walters, J. P.; Watters, R. W. Spectrochim. Acta, Part B 1977, 328, 287-304. Hardas, B. R.; Scheeiine, A. Anal. Chem. 1984, 56, 169-175. Bertani, D.; Cetlca, M.; Ciliberto, S.; Franclni, F. Rev. Scl. Instrum. 1884, 55, 1270-1272.
Interface for the Direct Coupling of a Second Gas Chromatograph to a Gas Chromatograph/Mass Spectrometer for Use with a Fused Silica Capillary Column James F. Pankow* and Lome M. Isabelle Department of Chemical, Biological, and Environmental Sciences, Oregon Graduate Center, 19600 N .W. Walker Road, Beauerton, Oregon 97006 For a laboratory which employs a gas chromatograph/mass spectrometer/data system (GC/MS/DS), many days of expensive instrument time can be lost each month while reconfiguring the GC for the differenttypes of analyses performed. Such reconfigurations can take the form of changing columns, mounting other equipment on the GC (e.g., a purge and trap apparatus), etc. The time lost during these equipment changeovers can be costly to research and service analytical laboratories alike. If two GCs were interfaced to the same MS in a given GC/MS/DS system, then such reconfigurations could take place on one GC while analyses were proceeding and being completed on the other. Thereafter, the next type of analysis could begin with minimal delay. Such thoughts have led Kalman (1) to interface a second GC to a Finnigan 4000 GC/MS/DS. The interface was heated resistively by using an electrical current. This interfacing was facilitated by the presence of a normally unused flange on the right-hand side of the Finnigan 4000 MS source manifold. In its standard configuration, the flange’s only purposes are to (1) allow the introduction of MS calibration gas and (2) provide a measurement port for the pirani gauge which senses the MS source pressure. This paper describes a GC/MS interface that is similar in function but fundamentally different in design. It was constructed primarily of copper. This metal was selected due to its high thermal conductivity. External to the MS, heating 0003-2700/84/0356-2997$01.50/0
of the interface is provided with cartridge heaters. Heat is transmitted to the portion of the interface which is internal to the MS by conduction down the axis of the interface. Conductive heating has been used rather than direct electrical resistive heating (1) or circulating hot oil heating (2) due to its greater simplicity. In this manner, the difficult-to-access portion of the interface inside of the MS is heated from an external source. The interface has been designed for use primarily with fused silica capillary columns in a directly coupled mode. It has been built into the right-hand side manifold flange of a Finnigan 4000 GC/MS/DS, and it allows the continued introduction of MS calibration gas from that side of the MS source. In this paper, the term “GC-1” will refer to the original GC fitted to the GC/MS/DS; the term “GC-2”will refer to the retrofit GC connected to the auxiliary GC/MS interface to be described.
EXPERIMENTAL SECTION The interface is presented in Figures 1 and 2. Its positional relationship to the MS source pirani gauge and the calibration gas inlet is shown in block diagram form in Figure 2. It was built around a 19.1 cm long copper tube with an 0.d. of 0.295 cm and an i.d. of 0.0572 cm. The tube was constructed from a 19.1 cm long piece of 0.159 cm (0.0625 in.) o.d., 0.0572 cm (0.0225 in.) i.d. copper tubing and a 19.1 cm long piece of standard 0.318 cm (0.125 in.) o.d., 0.173 cm (0.068 in.) i.d. copper tubing. The smaller 0.d. tube was inserted into the larger 0.d. tube, and the ends were 0 1984 American Chemical Society
2998
ANALYTICAL CHEMISTRY, VOL. 56, NO. 14, DECEMBER 1984 I To Variac 1
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carefully silver soldered together to yield a single 0.0572 cm i.d. tube. The 0.d. was then reduced to 0.295 cm by sanding. The 0.295-cm value was selected so that the tube would clear the i.d. (0.312 cm) of the variable conductance assembly which is mounted to the inside surface of the flange. The 0.0572 cm i.d. allowed a 0.041 cm 0.d. fused silica column to pass easily through the interface and into the MS source region. A 5.1 cm long section of 0.635 cm 0.d. (0.250 in.), 0.295 cm i.d. brass tubing was then placed over one end of the copper tube and silver soldered in place. Since the brass tube provides a sealing surface for ferrules in the interface, its outer surface was polished to a smooth finish. Brass was selected over copper for this part due to its greater ability to withstand high temperatures without oxidizing in air. After a stainless steel Swagelok fitting (Crawford Fitting Co., Solon, OH) for a 0.635 cm 0.d. tube was welded onto the preexisting stainless steel elbow on the flange, the brass/copper tube was inserted into the fitting and passed through the variable conductance assembly. It was held in place with a Swagelok nut and a 0.635 cm i.d. vespel/graphite ferrule (Alltech Associates, The length of the brass/copper tube was such that Deerfield, E). it extended almost to the end of the variable conductance line: it ended within 0.50 cm of the right-hand side of the source. A Swagelok stainless steel 0.635-0.159-cm (0.250-0.0625 in.) reducing union was mounted on the end of the brass section with a vespel/graphite ferrule, thereby allowing the connection of the interface to the oven of GC-2 by means of a 0.159 cm 0.d. stainless steel tube transfer line. The union was drilled out to 0.159 cm i.d. so that the stainless steel transfer line could butt up against the copper tube and thereby allow a fused silica column to pass
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unhindered into the copper tube of the interface. The stainless steel transfer l i e was connected to the union with brass ferrules. A Hewlett-Packard (Palo Alto, CA) 5790A GC was used as GC-2. A 30-m, fused silica, SE-54, 0.25-rm-film thickness capillary column was obtained from J & W Scientific (Rancho Cordova, CAI. The transfer h e was passed into the GC oven. In the oven, the outlet end of the column was connected to the transfer line with a Swagelok union for 0.159-cm tubing and a vespel/graphite ferrule and passed directly through to the end of the copper tube of the interface. The inlet end of the column was connected to an on-column injector (Hewlett-Packard). The transfer line was wrapped with a heat tape and maintained at 280 "C. An aluminum heater block, also maintained at 280 "C, was used to surround the fitting welded to the preexisting elbow on the flange, the exposed portion of the brass tube, and the major portion of the reducing union. The high thermal conductivity of aluminum ensured that all of the interface surrounded by the heater block was also at or near 280 OC. The heat which conducted down the copper line provided a hot interface to the MS source. The heater block caused the outside surface temperature of the stainless steel flange to rise to 70 "C, Le., not an excessively or inconveniently high temperature. The relatively low value of 70 "C is a partial result of the low thermal conductivity of stainless steel. While a major disadvantage if one desired to provide heat for the interface by heating the entire right-hand-side flange, this low thermal conductivity is an advantage here since it prevents excessive heating of the flange. Figure 2 shows how the calibration gas vial, calibration gas metering valve, and pirani gauge have been reconfigured to accommodate the interface. In the normal (GC-1) operation of the Finnigan 4000 GC/ MS/DS with capillary columns and electron impact (EI) ionization, the GC-1 transfer line is mated to the MS source and the variable conductance line is pulled away from the source. Since the interface presented here brings the capillary column flow in through the variable conductance line rather than the GC-1 transfer line, the positions of the two lines relative to the source had to be made reversible, depending upon whether (1)GC-1 and the existing interface were to be used or (2) GC-2 and the new interface were to be used. To accomplish this reversal, in an approach similar to that taken by Kalman (I), a double-throw toggle switch was installed in the CI controller module in power lines J1-6 and 51-3 running to solenoids V11 and V12. When the "ionizer mode" switch was in the "EI/Direct" (EI/Dir) position, in position 1of the toggle switch, the GC-1 transfer line was mated to the source, the variable conductance line was open, and GC-1
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Figure 3. Chromatogram for coronene (MW = 300) eluting from capillary column at 300 O C : (a)mass chromatogram (mlz 300);(b) reconstructed ion chromatogram (RIC, or total ion chromatogram). Peak width = 4.3 s. high boiling compounds that elute from a chromatographic could be used. In position 2 of the toggle switch, the GC-1 transfer line was open, the variable conductance line was mated to the column at high temperatures will be transmitted efficiently, source, and GC-2 could be used. in turn, through the interface. Figure 3 shows the sharp (4.3-s width at half-peak height) m/z 300 mass chromatogram and RESULTS AND DISCUSSION reconstructed (or total) ion chromatogram (RIC) peaks for The fact that the interface provides a direct coupling to the 15 ng of coronene injected on-column in 1.5 FL of 60:40 aceMS with the fused silica column leading all the way to the tone/hexane. This compound (MW = 300) eluted from the source ensures excellent chromatography with a complete GC-2 column at 300 "C. The peak sharpness and absence of avoidance of unnecessary glass or metal interface surfaces. tailing provide direct evidence that the copper tube in the Since the copper tube of the interface would not itself provide interface was conducting adequate heat (from the heater block much restriction to flow, however, the ionizer/analyzer portion region) toward the MS end of the interface. In the absence of the MS must be brought up to atmospheric pressure before of the heater block, even fluorene (MW = 166) exhibited changing the column. If the changing of the column in GC-2 severe tailing. was desired while data acquisition from GC-1 w a still desired, The reliance of this interface on heating by cartridge heaters that data acquisition could be briefly interrupted while the and thermal conduction make it both simple and efficient. MS was vented with helium and the column change proceeded The process of thermal conduction down the axis of the copper in GC-2. Following the rapid evacuation of the MS (1-2 h interface provides the facile heating of a difficult-to-access for diffusion pumps, faster for turbomolecular pumps), data region in the MS. The installation of an additional interface acquisition with GC-1 could continue. Alternatively, with a of this type on a GC/MS/DS system extends the flexibility few minor modifications, a chromatographically inert flow of that system by allowing the use of a second GC. restrictor could be incorporated into the interface. That is, a small-diameter fused silica tube leading to the MS source ACKNOWLEDGMENT could be mounted in the copper tube. Outside of the MS, the We express our appreciation to David Kalman for several inlet of the restrictor could slip -2 mm into the outlet end very helpful comments. of the column. When a column was disconnected, the restrictor would allow the MS vacuum pumps to accommodate Registry No. Copper, 7440-50-8. the air flow into the MS during a column change. As a second LITERATURE CITED alternative, the interface could be modified to incorporate the (1) Kalman, D.,persoAal comrnunlcation, 1963. full features of an open-split-type interface as described by (2) Friedll, F. HRC CC, J . Hlgh Resolut. Chromatogr. Chromatogr. ComHennenberg et al. (3) and others (4-73. Either approach would mun. 1981. 4 . 495. remove the need for MS venting and subsequent pump-down (3) Hennenberg, D.; Henrlchs, U.; Husmann, H . ; Schomburg, G. J . Chromatoor. 1978. 167. 139. for column changes. (4) Kollec W. D.;Tressl, G. HRC CC. J . H!ah Resolut. Chromatow. - ChroThe functions of the calibration gas lines and the MS source matogr. Commun. 1980,3 , 359. pressure pirani gauge have been maintained. The source (5) Schmid, P. P; Muller, M. D.; Slmon, W. HRC CC, J . High Resolut. Chromatogr. Chromatogr. Commun. 1979,2 , 359. pressure may read somewhat high, however, when carrier gas (6) Jamieson, 0. C. HRC CC , J . H/gh Resolut . Chromatogr . Chromatogr . is flowing in GC-2. This is due to the release of carrier gas Commun. 1982, 5,632. (7) Hurley, R. B. HRC CC, J . High Resolut. Chromatogr. Chromatogr. into the variable conductance line which then leads back to Commun. 1980, 3, 147. the pirani gauge. An important test as to whether or not a GC/MS interface is operating properly may be carried out by examining whether RECEIVED for review June 1, 1984. Accepted July 30, 1984.
