Direct Coupling of Capillary Columns to a Mass Spectrometer-Technique Rapid Column Interchange
Allowing
Kurt Grob Gas Chromatography Laboratory of the University, Raemistr. 54,8001 Zurich, Switzerland
Hansjurg Jaeggi Gas Chromatography Laboratory, 9043 Trogen, Switzerland
Introducing the total effluents that leave a capillary column into the ion source of a mass spectrometer has become a widespread practice. It is almost exclusively employed with high resolution gas chromatography where the single substances are present in amounts around 1 nanogram. Narrow bore columns with a carrier flow rate below 1 ml/min can be directly coupled to most types of mass spectrometers without modification. The related literature has been reviewed by Kienitz (I, 2 ) . With wider bore columns, effluent splitters have been used-e.g., by Nota et al. ( 3 ) .Directly introducing the total effluents at a rate of 1-10 ml/min involves a more efficient pumping system as described by Schultze and Kaiser ( 4 ) and working in our laboratory (5). While the basic technique of direct coupling no longer presents any serious problems, the practical connection between the capillary column and the mass spectrometer still remains the critical detail in many laboratories. Introducing the exit end of the capillary itself into the ion source is laborious and difficult, especially with glass capillaries. Column change becomes a relatively important action. Thus, an intermediary capillary between the GC oven and the ion source is normally preferred. Although we had good results with a piece of glass tubing, we now use a platinum capillary because of better mechanical stability. Other inert metals, such as gold, do the same service. Column interchange, then, becomes a matter of loosening and reestablishing the connection between column and intermediary tubing. For such connections, several elaborate techniques have been described-e.g., by Guiochon et al. (6). While working perfectly they are, again, time consuming. With glass capillaries, column change normally involves breaking the connected capillary parts. The only material allowing easy mounting and interchanging without breakage is PTFE shrink tubing.
PREPARATION OF PTFE CONNECTIONS We use PTFE shrink tubing from Du Pont. The proper size for capillary tubing with outer diameter of 0.8-1.0 mm is WG 24. Previously, we have produced the shrunk connections in situ, Le., in the GC oven, with the aid of a blowpipe flame or of a hot air stream (the shrinking temperature is ca. 320 "C). For more than a year we have used, with best success, the following simplified procedure. We prepare, on the bench, the junction between the exit end of the column and a short auxiliary piece of tubing having the same diameter as the platinum capillary. While the surface of the former must be clean, the latter is lightly lubricated
(1) (2) (3) (4) (5) (6)
Kienitz. Fresenius' Z.Anal. Chem., 252, 350 (1970). Kienitz, Fresenius' Z. Anal. Chem., 252, 118 (1970). G. Nota, G. Marino and A. Malorni, Chem. Ind. (London) 1970, 1294. P. Schultze and K. H. Kaiser, Chromatographia, 4, 381 (1971). K. Grob and G. Grob, J. Chromatogr.,62, 1 (1971). Merle d'Aubigne, J . Landault and G. Guiochon, Chromatographia, 4, H. H.
309 (1971).
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by dipping in a 1% solution of a silicone oil, such as SF 96, in hexane. After the shrinking procedure (Le., simple heating over a small flame), the auxiliary tubing is easily pulled out of the Teflon collar, which is then cleared from adhering silicone material by introducing an end of yarn moistened with carbon tetrachloride. The mechanical strength of the wanted connection can be controlled by the proper choice of the moment when the auxiliary tubing is pulled out of the warm fitting. The earlier this is done, the more the Teflon will tend to shrink, thus yielding a slightly reduced internal diameter. The Teflon fitting is now ready to be simply pushed over the end of the platinum capillary, in the GC oven. Provided the surface of the metal is perfectly smooth, the junction is ideally high vacuum tight. Column interchange means nothing more than pulling downwards the end of the capillary column, on which the Teflon collar adheres strongly, and pushing over the platinum capillary the new Teflon fitting, that has been prepared, on the bench, a t the end of the column to be coupled. The complete procedure is feasible within a few minutes and does not involve any deformation of both connected parts.
PREVENTION OF AIR DIFFUSION THROUGH THE PTFE FITTING In the first part of our two-year long practice with the above mentioned method of coupling, we encountered one serious problem. During programmed temperature runs, the total ion current suddenly started rising when the oven temperature approached 170 "C. Sometimes it settled again on an elevated level, sometimes it continued rising, thus yielding an erratic base line for the gas chromatogram. The mass spectra revealed an excessive concentration of air. Lowering the oven temperature caused the leakage to disappear completely just as suddenly as it had shown up. A two- or threefold PTFE layer (shrink tube size WG 24, WG 20, and WG 18 successively) retarded the appearance of the leakage to a 5-10 "C higher temperature without effectively reducing it. Obviously the PTFE material becomes permeable for the surrounding air at temperatures above 170 "C. We tried to overcome the trouble by covering the PTFE collar with a layer of an epoxide resin, or with a cut of silicone gum tubing. We also immersed it in silicone oil. Each of these attempts showed serious drawbacks. Finally, we found the simple solution which has been working perfectly for more than a year. Shown schematically in Figure 1 and as a photograph in Figure 2, it consists of a T-piece allowed to produce a helium atmosphere around the PTFE fitting. At elevated temperatures, helium instead of air diffuses through the Teflon. Theoretically, this means an increase of the carrier gas flow rate. We were, however, not able to detect it. We tested the device in the following way. To eliminate the influence from column bleed we coupled an empty glass capillary using the described method. The helium flow rate a t 100 "C oven temperature was 4 ml/min, causing the vacuum to fall down to 5 X 10-6 Torr. Programming the oven temperature 5"/min up to 280 "C produced
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Figure 1. T piece giving H e atmosphere around the gliding con-
nection between glass capillary column and metal capillary leading to ion source. Flow sheet of He supply
Figure 3. Right: Gas chromatograph (Carlo Erba. Milan. Mod. GI) on racks, on mobile support. Center: direct probe leading to lock (arrow) with connection to prevacuum (below lock). Left: Ion source and 600-1. pump of MS (Varian MAT. Mod. CH5) to reduce gas diffusion. Under these conditions, a continuous helium flow through the T of 3-5 ml/min proved to be sufficient. The major part of the T is a glass tube, since it is essential to ohserve whether the connected ends are in axial position and touching each other. Our f i s t version of the T, which we considered mechanically too delicate, was entirely made of glass. The T is linked to platinum tubing by a gliding PTFE fitting. Thus, it can easily he removed to allow periodical cleaning of the platinum tubing hy rinsing with solvents.
INSTRUMENT DESIGN PROVIDING FULL USE OF TRE COUPLING SYSTEM
Figure 2. GC oven opened. Upper right: lower part 01 injection block with inlet to capillary column. 20-m glass capillary on support. Upper left: T piece with gliding connection in center of glass tube and with He supply tubing (PTFE. white). Top left: Platinum capillary leaving GC oven a continuously falling total ion current, due to the continuously decreasing helium flow rate. After keeping the oven temperature a t 280 "C for 10 minutes, we increased the carrier gas inlet pressure to reestablish the original flow of 4 ml/min. This produced a total ion current less than 10% greater as compared with the current observed at 100 "C. The mass spectra showed the slight increase to be due to a small increase of the background, caused by trace impurities released by the column andfor the fittings. In order to maintain a pure helium atmosphere, the T is mounted vertically. Its lower end is restricted in diameter
The primary merit of our coupling system is very rapid column interchange. An instrumental precondition for this is the opportunity to cut the gas flow into the ion source during the interchange procedure. Figure 3 shows our setup. The intermediary platinum capillary, starting in the gas chromatograph on the right side, leads through a polished and internally heated steel rod (direct probe) into a lock (at point of white arrow) connected to the prevacuum. This is the position for column interchange and for column conditioning. T o start operation, the valve a t the left of the lock is opened and the gas chromatograph is shifted, on tracks, 20 cm t o the left. No leakage is observed while the steel rod glides through the PTFE fitting a t the entrance of the lock. A second precondition is the suitable construction of the gas chromatograph. Quick and safe manipulation of the glass capillary column is possible only in a large oven with .a large front door, as shown in Figure 2. The column is suspended on a support while 6-8 turns of the coil remain free-hanging on both sides, providing the necessary
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elasticity to establish the connections. [For details of handling glass capillary columns, see (7).] It is desirable, furthermore, that the same type of gas chromatograph be used for GC/MS as for pure GC. This allows a given sample to be run first on GC with FID. When the optimal analysis conditions have been found,
(7) K. Grob and H. J. Jaeggi, Chrornatographia, 5, 382 (1972).
the column is moved, without any modifications of the glass parts, to the MS. After identification of interesting substances, it is often moved back again to GC for quantitative estimation. Received for review February 26, 1973. Accepted March 23, 1973. All above-mentioned work has been made possible through the generosity of F. J. Burrus & Cie, Boncourt, Switzerland.
Coupling of Permeation and Exponential Dilution Methods for Use in Gas Chromatographic Trace Analysis
Fabrizio Bruner, Claudio Canulli, and Massimiliano Possanzini Laboratorio lnquinamento Atmosferico del C. N.R., c / o lstituto di Chimica Analitica, Universita 00785 Roma, ltaly
Trace analysis by gas chromatography is becoming more and more important since air pollution and similar topics became the objects of extensive research and of routine determination as well. In this connection a precise knowledge of the reliability of chromatographic columns is very important in quantitative determinations and calibration curves must be carried out rather often. Thus, a standard device to make calibration curves is desirable as a necessary accessory for any gas chromatograph used for trace analysis. In a recent paper, the use of the exponential dilution method in the analysis of sulfur compounds in air has been described ( I ) ; the results showed that by making very careful measurements, a relative standard deviation of about 5% for the SO2 curve slope could be achieved. However, syringe injection of the sample into the exponential dilution flask can often be a definite source of error, in some cases affecting severely the accuracy of measurements, and the curve obtained is completely out of the 5% indicated above. Advantages in the determination of the initial concentration Co are obtained when permeation tubes are used. On the other hand, the necessity of mixing the effluent of the permeation tube with additional gas a t different flow rates in order to vary the sample concentration according to the need of making a calibration curve, can be the source of considerable errors. The combination of a permeation tube (P.T.),which ensures a high degree of accuracy in obtaining the value of initial concentration, and of the exponential dilution flask (EDF), to obtain precise concentrations between C, and the detection limit is a technique which grants the advantages of both methods. In this paper, an apparatus serving for this aim is described. The SO2 permeation tube was prepared in this laboratory according to the usual procedure (2); the exponential dilution flask, constructed entirely in Teflon (Du Pont) was the same as described in Ref. (1). The total system is described (1) F. Bruner, A. Liberti, M. Possanzini, and I. Ailegrini, Anal. Chem., 44, 2070 (1972). (2) F. P . Scaringelli, A. E. O'Keeffe, E. Rosenberg, and J. P . Bell, Ana/. Chern.. 42, 871 (1970).
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in Figure 1. The gas line, including EDF and sampler, is saturated with the mixture coming from the permeation tube. The diluting gas is kept a t a constant flow with the system described in Ref. (1). In our experimental conditions, the SO2 concentration in the gas entering the EDF was 29.30 ppm. Once the EDF and the entire line are saturated with the SOz-containing mixture, flowing at a rate of 60 ml/min, the 4-way and 8-way valves are rotated at the same time and exponential dilution takes place. Area us. concentration points are taken in the same way as in Ref. (1).
The flow rate of the nitrogen diluter is kept at the same value (60ml/min). The calibration curve is made according to the procedure of Ref. ( I ) . Particular care must be taken to be sure that the initial concentration of SO2 is reached in the EDF. This was tested by connecting the EDF directly to the Flame Photometric Detector (Tracor, Inc., Austin, Texas) and by recording continuously the signal obtained. In this way, the first part of the graph of Figure 2 is obtained. When the SO2 level reaches a constant value, the 4-way valve is rotated and the dilution starts. The second part of the graph of Figure 2 is obtained in such a way. One hour or less is sufficient to obtain a constant signal (EDF saturation). By running the calibration curve for SO2 according to the procedure described, we could note the following improvements: The relative standard deviation on all the points obtained by plotting the calibration curve using all 20 dilutions was 1%.The slope of the calibration curve (log peak area us. log concentration) was 1.85 [the column used was the same described in Ref. ( I ) ] . The accuracy of measurements does not depend on the operator, and this is important if many apparatus of this type should be used for monitoring purposes. The error connected with the fact that by using the conventional operation of EDF, the initial concentration of the sample is not preserved in the sampler (3) is practically eliminated. In our system the saturation involves the (3) G. Greco, Jr., F. Gioia, and F. Alfani, Chim. Ind. (Milanj, 53, 1133 (1971).
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