Anal. Chem. 1983, 55,2009-2011
The results are presented in Table I. The precision and accuracy data were comparable to those of the methods previously reported ( I , 2). The standard deviations ranged. from 0.0002 to 0.0060 w t OJO and the precentage accuracy values were inferior to 12.00%. Considering the small concentrations involved, this error is quite low. Finally, this improved GLC procedure for determining the trace solvents in lube oils and waxes proved to be simple, fast, and accurate, providing an adequate means of controlling dewaxing refinery plants.
2009
Registry No. Methyl ethyl ketone, 78-93-3;toluene, 108-88-3. LITERATURE CITED (1) Durrett, L. R. Anal. Chem. 1959, 3 1 , 1824-1825. (2) Rawat, 6. S.; Prasad, G. Anal. Chem. 1981, 5 3 , 1144-1145.
RECEIVED for review April 14,1983. Accepted June 20, 1983. The author thanks Petr6leo Brasileiro S/A for permission to publish this work.
Low-Splitting-Ratio Injector for Capillary Gas Chromatography T. J. Nestrick,* L. L. Lamparski, and T. L. Peters The Dow Chemical Company, Michigan Division Analytical Laboratories, Building 574, Midland, Michigan 48640 Although the technique of split injection is perhaps the most convenient method of sample introduction for capillary gas chromatography, a variety of problems limit its applicability to many quantitative situations. Recent papers by Grob and Neukom ( I , 2) and Schomburg et al. ( 3 ) , review several technical problems encountered when using variable splitting ratio injectors of conventional design. Summarizing, the problem of poor quantitative reproducibility can be attributed to physical design parameters of conventional injectors which influence the mechanism of sample vaporization and the subsequent proportional introduction of the sample into the column. Although many designs for injector vaporization tubes have been described (3-5) for optimizing homogenization of the sample vapor-carrier gas mixture, careful selection of injector temperature, solvent, splitting ratio, and internal surface activity are normally required for each sample to be examined. Even when optimum vaporization procedures are used with conventional splitters, additional variations of the preset splitting ratio can result from the pressure wave initiated within the injector by sample vaporization ( I ) and also by significant recondensation of sample components in the column inlet (2). Because of these limitations, we have developed a low splitting ratio injector which exhibits characteristics more suitable to general-purpose quantitative application than can easily be obtained with conventional splitters. If the problems of injection induced pressure waves and/or component recondensation in the analytical column seriously affect the carrier flow rate in both the high-flow vent line and the capillary column in conventional splitters, then we surmised that if these two flow paths were similar in physical characteristics and could be subjected to identical temperature conditions, then such flow rate disturbances might in theory be more effectively balanced.
EXPERIMENTAL SECTION The structural components of an operationally fixed, low splitting ratio injector are shown in Figure 1. As indicated, a 0.25 in. (6.35 mm) 0.d. glass or quartz vaporization tube of appropriate length (inside diameter discussed later) is sealed into a heated injection port equipped with a septum inlet and a regulated constant pressure, preheated, carrier gas feed. The following component descriptions relate to applications of analytical capillaries having internal diameters from -0.2 mm to -0.35 mm. The analytical column and a segment of 0.10 mm i.d. X -40 cm fused silica tubing are coupled to the vaporization tube with a 'I4 in. to '/16 in. reducing union (low dead volume type not
necessary). Although two-hole ferrules are available, we have found that one-hole graphite ferrules provide an adequate seal when both capillaries are inserted through the single hole. Prior to sealing, the narrow-bore restrictor tube (split-tube) is adjusted t o protrude through the ferrule -5-10 mm with the analytical column inlet -25 mm beyond the split-tube inlet. Depending upon the length of the injection port, proper alignment of the capillary tubes should provide a distance of -3-4 cm from the tip of the syringe needle to the analytical column inlet for optimum performance. Next, the splitting ratio of the injector is adjusted to provide optimum chromatographic performance from the analytical column. Typically, this is accomplished by visually examining the peak shape obtained for a selected test compound under convenient isothermal operating conditions. In order to obtain the broadest operational range, the test compound should be dissolved in a solvent whose boiling point is -50 "C (or more) below the analytical column temperature. Employing this procedure, the length of the split-tube is reduced until peak tailing of the test compound is acceptable. It should be noted that reducing the split-tube length has the affect of increasing the splitting ratio. We have found that fixed splitting ratios between -3:l and -7:l provide the best compromise of sensitivity and column performance for 1-2 mm i.d. vaporization tubes with -0.2-0.35 mm i.d. X -20-60 m analytical columns. Caution. The split-tube is permitted to vent directly into the column oven during optimization runs for all carrier gases except hydrogen. When optimized, it is attached to a minimum 2 mm i.d. metal tube that exits the column oven and leads to an appropriate fume vent. Adsorbent materials should not be placed in this vent line; even the slight back pressure resulting from their presence can significantly affect injector performance.
-
RESULTS AND DISCUSSION In our environmental analysis laboratory we are routinely asked to quantitatively analyze for a variety of compounds which may be present in complex matrices at trace concentrations. While capillary columns provide superior separation potential for such analyses, reproducible sample introduction has been a significant problem. We have concluded from our use and evaluation of several commercially available injectors (Le., variable ratio split, Grob-splitless, and on-column) that none of these devices are without fault from a general-purpose point of view. When it is advantageous to introduce as much sample into the column as is conveniently possible for sensitivity reasons, each of these injection systems impose constraints upon the analysis conditions. Variable ratio splitters offer poor reproducibility at splitting ratios
