Anal. Chem. 1982, 5 4 , 1893-1895
time Flgure 3.
" Flgure 1. Circuit for constant current operation of ECD (A) where positive pulses are applied to the anode and (B) where negatlve pulses are applied to the cathode. Individual circuit elements are described in the Experimental Section. 30
pulse period
Flgure 2.
background clean air sample is shown in Figure 3. The chromatogram shown was obtained with a home-built GC and ECD where whole air samples are introduced via a 2-mL stainless steel sampling loop operated by a 8030 Carle valve. The column (10 f t X l/s in. stainless steel packed with 10% SF 96 on Chromosorb W) was operated at ambient temperature and the detector at 200 "C. The carrier gas was 10% CH4 in argon with a flow of 60 mL/min. In this 2-mL clean air sample, the concentration of CFC13can be assumed to be about 200 pptr (v/v) and that of CC14 about 100 pptr (5). We have analyzed numerous clean air samples using the same chromatographic components but with various commercial ECDs and find the quality of the signal-to-noise response shown in Figure 3 compares favorably with those obtained previously. ACKNOWLEDGMENT The authors thank Richard Geer for assistance with circuit designs. LITERATURE CITED
Circuit for fixed1 frequency operation of ECD.
and could be increased by use of another with higher frequency capability. Little improvement, however, would thereby be gained in our case because at 100 kHz the period between pulses is then only 10 times greater than the width of each pulse and linearity of response becomes progressively lost due to more fundamental causes. In order to demonstra.te that the sensitivity inherent in ECD detection is maintained with this circuit, an analysis of a
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Analysis of 2-mL whole air sample using CC-ECD.
(1) Maggs, R. J.; Jaynes, P.L.; Davies, A. J.; Lovelock, J. E. Anal. Chem. 1971, 43, 1966. (2) Sullivan, J. J.; Burgett, C. A. Chromafographia 1975, 8 ,176. (3) Lovelock, J. E.; Watson, A. J. J . Chromafogr. 1978, 158, 123. (4) Grlmsrud, E. P.;Knighton, W. B., Anal. Chem. 1982, 5 4 , 565. (5) Goidan, Paul National Oceanic and Atmospheric Administration, Bouider, CO. personal communication, March 19, 1982.
RECEIVED for review March 20,1982. Accepted May 21,1982. This work is supported by the National Science Foundation under Grant No. CHE-7824515.
On-Column Injector for Capillary Gas Chromatography T. L. Peters," T. J. Nastrlck, and L. L. Lamparskl Analytical Laboratories, Do w Chemical Company, Midland, Michigan 48640
An on-column injection device for capillary columns was recently described ( I ) and made commercially available on Carlo Erba gas chromatographs. With this injection system, a long (10 cm) 32-gauge needle is inserted through a cooled 0003-2700/82/0354-1893$01.25/0
narrow bore injection valve and into the capillary column. A narrow bore guide and a system of stops for the syringe so the valve can be opened and closed allow injection while maintaining column head pressure. 0 1982 Amerlcan Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 11, SEPTEMBER 1982
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Figure 1. Diagram of fixed needle injector: (1) fused silica needle, (2) septum, (3) ’ I 4in. to ‘ I l e in. reducing union, (4) oven wall, (5) ’ I 4 in. 0.d. glass tublng, (6) ’I4 in. to ’ I r ein. reducing union, (7) carrier gas inlet, (8) capillary column.
The advantages of this system, which have been described in the literature (2-4), make it the technique of choice when doing trace analysis and the ultimate in sensitivity and efficiency is required. Other manufacturers (SGE,HewlettPackard, Siemens) also offer variations of the basic on-column injector to fit virtually any instrument. Unfortunately, the cost of this type of injector has limited its use considerably. We have designed a variation of the capillary on-column injector that can be used with most gas chromatographs with a minimum of modification to the instrument. We shall refer to this injector as a “fixed needle” injector.
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EXPERIMENTAL SECTION The basic construction of the injector is very simple (Figure 1). Parts needed are common Swagelok fittings and glass tubing. One of the 1/4 to ‘/I6 in. reducing unions has ‘/le in. stainless steel tubing silver soldered in through one of the “hex”sides. This line serves as the carrier gas inlet and is connected to the head pressure regulator. The 1/4 in. Pyrex tubing can be of any inside diameter since its function is only as a support for the fused silica “needle”. The lengths of the fused silica tubing and the Pyrex tube will be dictated by the instrument in which they will be used. The fused silica needle should extend -2 cm into the capillary column and 5 mm above the septum. The diameter of the fused silica needle is somewhat dependent on the capillary columns that will be used. A good general purpose size is 0.10 mm i.d. fused silica which is available from J & W Scientific, Inc., Orangevale, CA. This works well with columns 0.25 mm i.d. and larger. For injection in this system, a luer tip syringe is used (Hamilton No. 701-LT). The volume of liquid to be loaded into the syringe will depend on the volume of the fused silica tubing. Only the material that exits the fused silica needle will actually be injected. Before an injection is made, the temperature of the entire system must be slightly below the boiling point of the solvent. If this is not done, the solvent will flash inside the fused silica needle depositing the organics there rather than in the capillary column. For injection of an appropriate volume, solvent is loaded into the syringe and pulled back until an air space is observed. The syringe tip is placed over the fused silica needle and gently seated against the septum to form a seal and the injection made. After the plunger is slowly depressed, 2 s is allowed to pass before slowly withdrawing the syringe plunger to collect the solvent in the needle. The syringe is then removed and the capillary column temperature programmed to elute the components. During N
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Flgure 2. Comparison of injectors using a chlorinated biphenyls mixture: (A) fixed needle injector, (B) commercial on-column injector.
analysis, the fused silica needle is left open, venting carrier gas.
RESULTS AND DISCUSSION For one comparison, a standard mixture of polychlorinated biphenyls was analyzed using both the fixed needle injector and a commerical on-column injector. The same column and as close as possible to the same conditions were used for the comparison. Figure 2 illustrates that the fixed needle injector (A) and the on-column injector (B) are for all practical pur-
Anal. Chem. 1982, 5 4 , 1895
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with blank solvent injections. Multiple injections have indicated that the reproducibility is limited mainly by operator skill. With external standardization and a Clo hydrocarbon, a relative standard deviation ( l a ) of 3% was typically achieved. For best accuracy and precision in most instances, internal standard techniques should be used with this injector.
poses equivalent for this separation. The slight apparent superiority of run A over run B is believed to be due to the detector configurations. In “A” the column terminated at the flame tip but in “B” the column terminated 3 cm from the flame tip. Due to the nature of this injection technique, one would expect serious cross-contamination problems. However, since that portion of sample not introduced into the column is pushed back out of the fused silica needle as a liquid, very little cross-contamination can occur. Also, since the majority of the needle is subjected to the oven temperature during programming and is constantly flushed with carrier gas, material not back-flushed with the solvent is thermally volatilized from the needle. Experiments have shown less than 5% cross-contamination when standards of chlorinated benzenes and phenolics at the 10 ng/yL level were compared
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LITERATURE CITED (1) Grob, K.; Grob, K., Jr. J. Chromatogr. 1978, 151, 311-324. (2) Galll, S.;Trestlanu, S.; Grob, K., Jr. HRC CC,J. High Resolut. Chromatogr. Chromatogr. Commun. 1979, 6 366-370. (3) Grob, K.; Neukom, H. P. J. Chromatogr. 1980, 189, 109-117. (4) Grob, K. HRC CC J . High Resolut. Chromatogr. Chromatogr. Commun. 1978, 1 1 , 263-267. I
RECEIVED for review March 14,1982. Accepted May 3,1982.
Recovery of Clogged Glass dets in Molecular Separators for Combined Gas Chromatography/Mass Spectrometry Wayne E. Wentworth, Ya-Chi Chen, and Albert Zlatkis Department of Chemlstry, Unlversity of Houston, Houston, Texas 77004
Brlan S. Middleditch“ Department of Biochemlcisl and Blophysical Sciences, University of Houston, Houston, Texas 77004
Of the various molecular separators used to couple a gas chromatograph to a maw spectrometer, the most satisfactory is the jet-type separator (1-3). The original version of this separator (as manufactured by LKB Produkter, Sweden) incorporated removable ntainless steel jetn. During the use of this device i t was not uncommon for one of the jets (usually the inlet jet) to become clogged with materials such as column packing and particles of the O-ring or ferrule used to connect the column to the separator. It was a relatively simple procedure to remove a jet from the separator and dislodge the obstruction. On the rare occasion when the blockage could not be removed, it was necessary to replace only the individual jet. The jet separators of most modern imitruments, however, are constructed of glass or glass-lined stainless steel. It is much more difficult to remove blockages from the jets and, if the procedure fails, it is usually necessary to replace the entire separator. The inlet jet typically is as much as 10 cm long and is tapered to an internal diameter of 0.1 mm. It is not an easy task to remove stubborn obstructions from the tip of such a jet. We were recently faced with this problem. The inlet jet of the separator in our Finnigan 1020/OWA mass spectrometer became clogged with a material which appeared to be either Carbopack C (graphitized carbon black, a gas chromatograph column packing) or graphite (from a ferrule). I t resisted conventional methods of removing it.
EXPERIMENTAL SECTION We followed the manuf‘acturer’srecommendationsfor removing the blockage. We filled the separator three-quarters full with a 1:l mixture of acetone arid methanol. Application of vacuum to the gas chromatograph end of the inlet jet failed to draw solvent through the jet. Application of air pressure (20-30 kPa) to the mass spectrometer end of the separator with the neck of the separator blocked was insufficient to push solvent through the inlet jet. Alternate appliultion of vacuum and pressure was equally unsuccessful. We could not dislodge the blockage using a fine tungsten wire: any wire long enough to reach to the tip of the jet and with a small enatugh diameter to fit inside the jet had 0003-2700/82/0354-1895$0 1.25/0
insufficient rigidity to move the blockage (this operation was performed under a binocular microscope at X 15 magnification). Among the less conventional methods that failed was an attempt to pyrolyze the material blocking the jet. We placed the separator in a glassblower’s annealing oven with air flowing in through a tube inserted in the neck of the separator and left it overnight at 565 “C. By this time it seemed almost inevitable that the separator would have to be discarded so we resorted to a more drastic method, fully aware of the possibility that the device might be irrepairably damaged. We clamped the separator in a vertical position, with the inlet jet uppermost, and transferred 10% hydrofluoric acid (CAUTION corrosive, causing painful sores on the skin usually noticed on the next day only; avoid inhaling the vapors) into the jet using a Pasteur pipet. The quantity of acid used was just sufficient to cover the blockage to a depth of 2 mm. After 15 min, water was added to dilute the acid and application of a vacuum to the gas chromatograph end of the inlet jet was sufficient to dislodge the obstruction.
DISCUSSION If the blockage was, as we suspected, carbonaceous in nature it would not have dissolved in the hydrofluoric acid. It seems likely that sufficient glass in the region of the blockage was dissolved to release the blockage. Microscopic examination of the jet failed to reveal any evidence of damage and, when replaced in the instrument, its function seemed to be unimpaired. We have not had the opportunity to repeat this procedure, so we are unable to describe the optimum acid concentration. The use of excessive concentrations of hydrofluoric acid may alter the operating characteristics of the separator, so we suggest that sequential treatments with increasing concentrations of acid be applied until the blockage is removed. LITERATURE CITED (1) Ryhage, R. Anal. Chem. 1964, 36, 759. (2) Ryhage, R.; Wikstrorn. S.; Waller, G. R. Anal. Chem. 1965, 3 7 , 435. (3) Ryhage, R. Ark. Keml 1987, 26, 305.
RECEIVED for review March 4, 1982. Accepted May 3, 1982. 0 1982 American Chemical Society
