Gas Chromatography for Trace Analysis JAMES D. BOGGUS and N. G. ADAMS Ethyl Corp., Baton Rouge, l a .
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Although gas-liquid chromatography
is now widely used for quantitative analysis, little has been reported on the use of this very sensitive method in dealing with trace components. A technique is presented which shows the adaptability of gas-liquid partition chromatography to the analysis of a plant stream of ethyl chloride (chloroethane) of 99% purity. The method utilizes a concentration step, accomplished b y venting as much as possible of the major component, followed b y an analysis of the concentrated traces. Materials present in concentrations as low as 1 p.p.rn. can b e determined. Both operating conditions and necessary apparatus a r e discussed.
T
H o u x a coiiiparatively recent development, gas-liquid chromatography has already proved to be a highly successful method for both analytical separations and quantitative analysis. Approximately 400 publications have outlined various techniques and equipment for effecting specific separations and analyses, and recently several books ( 2 , 4,5 ) corer the field very ivell. Few, hon-ever. hal-e dealt with the problem of trace components (1. 3 ) . This paper presents techniques and apparatus used on a specific plant stream for the determination of trace components in concentrations as low as 1 p.p.m. The application of the method to other high purity strcams also is discussed. The ability of gas-liquid chroniatography to effect virtually complete separations between components of a sample suggested its use for a concentration step. It has now become common practice to collect selected fractions for further study by infrared, mass spectrometric, or repeated gas chromatographic techniques The technique described utilizes a large diameter column for a concentration step in nhich a large portion of the major component of the sample is discarded. The concentrate of contaminants thus obtained is subsequently analyzed u-ith a small diameter column. APPARATUS
Two Perkin-Elmer Model 154 Vapor Fractometers provide the basic instrumentation. The first instrument is used for the concentration and collec-
tion of the trace constituents of the sample. Modifications necessary on this instrument include the installation of a Perkin-Elmer gas-sampling valve with a 1-ml. sample volume. and a tn-o-way vent (Figure 1,A) for the collection or venting of the effluent vapors. The gas sampling valve is employed without modification to deliver approximately 1 ml. of liquid sample. The column is a 10-foot copper tube inch in outside diameter. This column is packed with 100 to 20 weight ratio of crushed C-22 firebrick (40 to 70 mesh) and Sujol mineral oil. and bent into a TI- to fit into the Fractometer oven. The instrument is operated n i t h a n oven temperature of 40" C. and a carrier gas flow rate of 150 ml. per minute. Both commercial nitrogen and helium have been used as the carrier gas. Honever, because of the increase in sensitivity, helium gives more satisfactory results. K i t h the latter, the major trace components can be observed during the concentration step. These trace components are collected in a U-tube (Figure l , A ) attached t o the two-way vent and immersed in liquid nitrogen. The second instrument is used for the analysis of the collected concentrate obtained from the first instrument. It has been modified by the addition of a bypass sample introduction system (Figure 1B), constructed so that the U-tube containing the condensed traces can be fitted directly onto the system. Thus, the collected sample can be analyzed without further transferring. T o provide added sensitivity, the Leeds & Northrup recorder was converted from 10 to 2.5 mv. full scale. Although this modification increased the warm-up equilibration time somewhat, the noise level remained low enough to be of no consequence. The column in the second instrument is a 10foot stainless steel tube '/4 inch in outside diameter, with the same packing as above. The oven temperature is 30" C. and the helium flow rate is 50 nil. per minute. EXPERIMENTAL
I n the past, the determination of trace components in the ethyl chloride product stream (99+% purity) presented a sizable task. The procedure involved a long careful distillation step, followed by mass and infrared spectrometric analyses of the fractions. Because of its inherent tedium, this analysis was conducted only during plant operational difficulties. The present method requires approximately
GLASS W U O L - l ' q FILLING
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Figure 1. Apparatus for sample collection and rerun A.
Cold trapping tube on vent of first instrument
E. Trapping tube position on bypass of second instrument
one tenth as much time with no sacrifice of accuracy. The stream is sampled and stored in iionshatterable stainless steel bombs of 104-cubic inch capacity. The bombs are filled with sample close t o capacity to provide a favorable liquid t o vapor ratio. n-Hexane is added to each bomb as an internal standard in a concentration of 0.02% by weight. The n-hexane &-as deemed a satisfactory internal standard after experimentation had shown that it was not a coniponent present in the sample. Also, because the greatest portion of the total impurities present consisted of saturated hydrocarbons, the selection of n-hexane was desirable from the standpoint of similar detector response. Figure 2 shows an adapter used to add this internal standard. The adapter is made by brazing together two standard l/r-inch brass hose ends nhich have had the fitting ends machined down to accommodate a '/4-inch 0 ring. This feature allon4 the adapter to be connected to the sample bomb with only finger-tight pressure. An evacuated bomb is connected t o the lower end of the adapter and the desired volume of n-hexane is deposited into the adapter from a n dgla syringe equipped with a Shardlow micrometer (Burrell Corp., Pittsburgh, Pa.). The bomb containing the sample is then connected to the upper end of the adapter and the transfer is accomplished VOL. 30, NO. 9, SEPTEMBER 1958
1471
by opening the two needle valves. To minimize a possible loss of internal standard into the packing of the needle valve, the flow direction of the valve is toward the bomb. The filled sample bomb is attached above the perkin-Elmer gas sample valve by means of an adapter similar to the transfer adapter. To prevent rust particles from fouling the valve, two screen filters and a glass wool plug are placed in the adapter. The procedure for filling the sample tube is to turn the selector knob counterclockwise and open the upper needle valve. The lower needle valve is then cracked and a small quantity of the liquid vented. After a short interval of time, to allow the pressure to equilibratein the sampler, the selector knob is turned clockwise and the sample is introduced into the column. The top valve is then closed and the excess sample is vented. A typical chromatogram of the concentration step is presented in Figure 3. The ethyl chloride emerges first, along with any Cd hydrocarbons or lower boiling substances, and is vented. The collection of the trace components is begun on the tail of the ethyl chloride peak at the point where the height of the curve is 1/82 of the peak height. This cut point was chosen after experimentation showed that beginning the collection earlier yields no more of the traces. The collection is continued until the heaviest component emerges. The collection tube with the condensed material is then attached to the bypass type sample introduction system of the instrument used for the analysis. The liquid nitrogen used as coolant is removed and the sample is allowed to vaporize. Heaters designed t o fit the U-tube are used to speed up this process. After the sample is vaporized, the stream of carrier helium is directed through the collection tube and the sample is introduced to the second column for analysis. Figure 4 gives a typical chromatogram of the analysis step. Ethyl chloride is still by far the largest component, but those compounds originally present as undetectable traces now show clearly. The sensitivity of the technique can be realized by considering the peak area of n-hexane which represents only 0.02 weight yo of the original sample. The very small peaks, such as the ethyl bromide (bromoethane) peak, indicate a concentration in the order of 1 p.p.m. There was some concern at first that, in the concentration and collection step, the cold trap used might not completely recover the trace components. It was speculated that the good precision obtained from the repeatability runs (Table I) merely indicated that the trapping losses were consistent for each constituent. I n order to check the trapping efficiency, two cold traps, in series, m-ere employed in several runs. The material in each of the 1472
ANALYTICAL CHEMISTRY
cold traps was then run through the second instrument. I n no case did the ethyl chloride peak of the second trap exceed 0.1% of that in the first trap. No peaks were detected for the other components present in the mixture. It was thus concluded that the trapping of the trace components in this manner was quantitative and that the contribution of any trapping losses to the over-all accuracy of the method was negligible.
CALCULATION
Each of the component peaks was characterized by standard methods (known retention times, infrared analysis, and mass spectrometer analysis). Integration of the peaks was accomplished by both square count and a compensating polar planimeter. For the purpose of adding n-hexane volumetrically, the density of the sample was assumed to be equal to that of ethyl chl’oride. The calculation of the concentration of all of the hexane isomers, 2,3-dimethylbutane and 2 - methylpentane (calculated as one component), 3methylpentane, and 2,2dimethylbutane, were made on the assumption that the instrument response to these was the same as for n-hexane. Thus, the simple proportionality relating the known weight and area of the peak of n-hexane (the internal standard) yielded the weight per cent of the hexane isomers. Pentane isomer peak areas were related to the internal standard by the use of an experimentally determined factor. All other components of the sample were present in such extremely low concentrations that they were calculated as having the same instrument sensitivity as the internal standard. To check the precision of this analysis, a series of analyses was made on one sample. The data obtained from these runs are presented in Table I.
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FILLED BOMB/SAMPLE
SAMPLE
BOM8
Figure 2. Adapter used for addition of internal standard
0
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Figure 3.
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Typical chromatogram of concentration step R-values refer to sensitivity ranges
C TIME
Figure 4.
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Typical chromatogram of collected fraction of concentration step
I n general, these data show that the method has approximately the same precision over the range of 5 to 3000 p.p.m. One exception is noted in the case of 3-methylpentane with a coefficient of variation of 17.8%. The cause of this apparent anomaly is not known. The 1,ldichloroethane is listed as less than its detection limit of 0.5 p.p.m. on this particular sample; however, its concentration has run as high as 50 p.p.m. in others. Samples which contain as low as 1.5 p.p.m. have been analyzed, although a t this concentration it becomes more difficult to integrate the curve with good precision. SUMMARY AND DISCUSSION
Although this method was developed for one specific analysis, it can be applied to many other systems. Unfortunately, with the operating conditions and column used in this analysis, any butanes or lighter components are lost. It is known that butanes are present in these samples on the order of 0.1% by weight. Determination of these materials is possible if a column which will delay the emergence of the ethyl chloride is chosen. One column which mill do this is a 10-foot copper tube, I/2-inch in outside diameter, packed with 100 to 20 weight ratio C-22 firebrick and tricresyl phosphate. On this column the ethyl chloride emerges with the hexanes and the CC traces can be collected before the major component. It should be possible to find a column liquid substrate which would give a retention time for ethyl chloride enough
Table 1.
Concentration of Traces and Repeatability of Analyses.
Coefficient Std.
Component Isopentane n-Pentane 2,2-Dimethylbutane 2,3-Dimethylbutane 2-Methylpentane 3-Methylpentane Ethyl bromide 1,l-Dichloroethane
Dev., P.P.M. 118.5 46.9,42.2,44.9,45.5,43.4,43.4,47.2 1.89 11.3, 11.4, 11.3, 11.9, 10.9, 11.7, 11.5 0.32 177.4, 169.8, 169.6, 179.1, 179.4, 178.5, 183.7 5.22 7.9, 8 . 4 , 11.6, 11.9, 12.4, 11.0, 12.8 1.94 5.3,5.4,6.6, 5.9, 6.2, 5.8, 6.4 0.49
