Gravimetric Gas Chromatography Elmar V. Piel, Department of Chemistry, Norwich University, Northfield, Vt.
ravimetric analysis, the most direct G and obvious method of quantitative analysis, appears t o have been neglected in gas chromatographic analyses. Yet, it may be applied with great accuracy and is more universally applicable than other integral measurement methods in this field. The achievement of high quantitative accuracy with present differential detection methods is not simple. Probably the most accurate method involves preparation of a calibration curve relating peak area to weight of the component. Carefully purified samples of the component are a necessity here, and in addition, precise control over detector and column variables must be maintained ( 2 ) . The use of an internal standard requires even more preparation for accurate analyses, and as indicated by Rosie and Grob (6) may nevertheless allow relative errors of 3 to 6%. The gravimetric method, however, yields absolute results of high accuracy as good as or better than those of the above methods without previous information on the component analyzed for other than retention time, and without regard to chromatograph conditions other than these leading to separation. Mixtures of pure benzene, toluene, and m-xylene of known composition were easily and accurately analyzed by the gravimetric method. These compounds were chosen to check the gravimetric method because of their stability and ready availability in the pure state. The analyses were carried out by using an absorption tube containing ordinary gas liquid chromatographic (GLC) packing to collect quantitatively the eluted components whose retention times in the absorption tube were relatively long. The absorption tube was used at room temperature. Quantitative injection of the weighed liquid samples was accomplished with a slightly modified syringe and injection technique. The syringe modification involves the placing of one or more constrictions in the syringe and/or needle, positioned back from the needle tip. This causes air-liquid seals to be formed at the constrictions. The seals prevent the motion of the column of liquid in the syringe during handling, and keep the liquid from lying in the tip section of the needle. A suitable constriction in the 696
ANALYTICAL CHEMISTRY
needle is readily obtained by leaving a section of needle cleaning wire in the base of the injection needle. A section of slightly out-of-round wire or glass rod jammed into the exit end of the glass syringe may serve for a second constriction. The technique of accomplishing quantitative transfer of the weighed sample is as follows: the sample is drawn into the syringe with an air column between it and the plunger. It is then drawn up a little further into the syringe to emtpy the needle tip, and to cause air-liquid seals to be formed a t the constrictions. Upon injection, both liquid sample and the air columns are forced into the chromatograph column. When the needle is withdrawn through the septum, the needle tip is held within the septum material while the plunger is pulled back. This is to assure that the pressure within the syringe is below atmospheric before completing withdrawal of the needle through the septum. Thus expulsion of sample from the needle by gas pressure within the syringe is avoided after needle withdrawal. Present methods of sample introduction with syringe and volumetric measurement demand a knowledge of sample density, hence also syringe temperature (to within about 1' C.) for accuracy. The introduction of a weighed sample sealed in a glass ampoule and crushed within the chromatograph, though accurate, is cumbersome (3).
The difficulties in the way of quantitative collection of pure eluates are attested to by recent literature describing the use of cold traps, rotated coolers, electrostatic precipitation, etc., for the purpose (1, 4, 8) However, quantitative collection of the eluate in weighable form with a minimum of equipment is easily accomplished by using a simple absorption tube containing ordinary GLC packing. During collection of a component, it is attached to the exit of the chromatograph column and held there during the time that the component is emerging from the column. The dimensions of the absorption tube are not critical. However, the tube must be large enough so that the component t o be collected will not start to be eluted from it before all of the component has emerged from the chromatograph column. The quantitative nature of the injection and collection systems was tested by injecting weighed amounts of pure toluene into a chromatograph and comparing these weights with weights of the samples after passage through the chromatograph and collection. Samples 60 to 80 mg. in weight were used. All
weighings were taken on an ordinary chainomatic analytical balance, and only chain settings were changed in estimating the weights. The chromatograph column used was a 1.1meter by 5-mm. i.d. column packed with 60-to 80-mesh firebrick containing 30% by weight of silicone oil. The column was held at 110' C. with a carrier gas flow rate of 120 ml. per minute. The absorption tube, measuring 25-em. by 5-mm. i d . , was packed with 4 grams of 60- to 80-mesh firebrick containing 3501, by weight of tricresyl phosphate. The connection between chromatograph column and absorption tube was a short section of Teflon tubing, This connection was heated during collection of the eluate to prevent condensation in this section and to avoid possible fog formation. A preliminary run with thermal conductivity detection established the time of elution of the toluene. The absorption tube was connected to the column outlet 30 seconds before start of emergence of the toluene and disconnected 30 seconds after emergence of the toluene from the column had ceased. Cnder these conditions, for a series of 10 injections, the average deviation of the weight collected from the weight injected was 0.1 mg., and the error range was 0.3 mg. Thus it appears that the injection and collection system is quantitative within the limits of error of the ordinary analytical balance. Xitrogen was either used as the carrier gas or used to flush out the collection tube before weighing to avoid buoyancy errors caused by allowing gases with a density much different from air to remain in the absorption tube. The absorption tube was reused in each successive determination after warming with steam and flushing with a rapid stream of nitrogen for a few minutes to remove collected toluene. RESULTS
The quantitative nature of the collection in the above manner has also been checked for a large variety of volatile hydrocarbons in an independent method ( 5 ) . This involves heating the collection tube after component collection, attaching it to the column entrance, and passing the carrier gas through both absorption tube and column in series. Thus components collected may be repeatedly sent through the column. As many as 10 recirculations caused no diminution in peak area for compounds of normal stability.
,
Analysis of prepared mixtures of known composition of pure benzene, toluene, and m-xylene was then carried out gravimetrically, using the same sample size, column, conditions, and method of eluate collection BS above. A series of 10 analyses for each of the three components was carried out on a mixture in which no component was present in less than 20% nor more than 50% concentration. The average relative standard deviation of the analyses from the prepared mixture was 0.5%, and the relative error range was 1.4% It may he noted that Scott (7) has recently shown that relatively complex mixtures can he separated on the standard 4-mm. analytical column with sample charges as high as ahout 150 mg. Since gravimetric accuracy d e pends on sample size, use of his techniques is advantageous with analytical
sized columns. The use of the larger diameter preparative columns offers the possibility of still further increasing the accuracy of the gravimetric method. The chief limitations of the gravimetric method are that analyses must be limited t o well separated components present in accurately weighable amounts-i.e., at least about 10 mg. of component per sample charge. The primary usefulness of the method would appear t o he as an adjunct t o the usual differential detection methods wherever high quantitative accuracy is required. An especially favorable application of the method would he in accurately determining reaction yields of volatile products in exploratory work where isolation of samples of pure component and meparation of a calihra-
ACKNOWLEDGMENT
I thank my senior students in “In strumental Analysis,’’ particularly York Doerr and Carl R. Suhr, for their help in this project. LITERATURE CITED
I% P., Tuey, G. A. P., “Gas Chromatography 1958,” D. H. Desty, ed., p. 284, Butterworths, London, 1958. (2) Dal Nogare, S., Juvet, R. S., “GasLiquid Chromatography,” pp. 256-9, Interscience, 1962. 2 a,, pp. 177-178. (1) Atkinson,
(3) IWd., pp. 177-8. (4) Xirkland, J. J., “Gas Chromittography,” J. J. Coates et al., eds., p. 203, Academic Press, New York, 1958. ( 5 ) Piel, E. V., Norwich University, Northfield, Vt., unpublished work, 1963. (6) Rosie, D. M., Grob, R. L., ANAL. CHEM.29, 1263 (1957). (7) Scott, R. P. W., Nature 198, 782 114R.1> \_l_l,_
(8) Wehrli, A., Xovats E., J . Chmmatog. 3, 313 (1960).
Preparation and Application of a Perfluorocarbon Vacuum Sealant Carl R. Wellman, Ballistic Research Laboratories, Aberdeen Proving Ground, Md. NEED EXISTS in
the laboratory for a
A vacuum sealing and/or gasketing material possessing versatility and simplicity in preparation and application. The present state of the art of preparing vacuum seals is time consuming, expensive, and sometimes unreliable. The nature of our work has involved mixtures that are corrosive and explosive in contact with moist air. Therefore, our analytical devices had t o give reliable vacuum performance. This report describes a vacuum sealing material with indefinite shelf life based on chemically inert perfluorocarhon type polymers that has been successfully employed in our laboratory for the past two years.
weight 600,000 t o 800,000, and average basic particle size of 0.2 to 0.3 micron in 300-micron agglomerates. Trifluoromonochloroethylene polymer is commercially available from the Chemical Division of Minnesota Mining and Manufacturing Co. Various grades are produced and known as Kel-F oils and waxes. Those found satisfactory for this application were Grade 10 polymer oil, 40 and 210 polymer waxes. Suitable solvents for Kel-F polymer oils and waxes are aromatic, aliphatic, and chlorinated hydrocarbons, alcohols, ketones, esters, and fluorocarbons. We have found methylene chloride or acetone satisfactory for the laboratory preparation.
AATERIALS
trafiuoroethylene polymer, available irom Dupont as Teflon, Grade 6, has characteristics as follows; specific gravity 2.2 to 2.23, niolecular
Figure 1 ant
Application of vacuum seal-
PREPARATION AND PROPERTIES OF VACUUM SEALING MATERIAL
Fifty grams of Kel-F, Grade 40 wax, were completely dissolved in 200 cc. of methylenechloride a t room temperature in a 500-cc. Erlenmeyer flask. Fifty grams of powdered Teflon were added t o the above solution but did not dissolve. The slurry was thoroughly mixed with a rotary evaporator for 15 minutes and the solvent removed with a warm water bath at reduced pressure. The material as initially prepared, is a white substance of dough-like consistency. It possesses a rather unique property of being subject t o a progressive increase in tensile strength by stretching and folding in a “taffypulling” manner. With experience, a multitude of consistencies can he ohtained depending on the desired application. The composition of the vacuum sealant produced is homogenous
and has tensile properties markedly different from either of the starting components. It is obvious that the resulting product is something more than Teflon coated with Kel-F wax. Attempts to prepare a material with similar physical properties using Grade 5 Teflon (higher molecular wt., density, and basic particle size) with Kel-F oils and waxes, were unsuccessful, Also, the suitable Grade 6 Teflon with Dow Corning 703 Silcone Oil proved unsatisfactory. APPLICATION
When employed as a gasketing or sealing compound for vacuum application, it has been possible with the simplest technique to prepare within minutes vacuum-tight, chemically inert, and moderately heat resisting (150’ C.) gas cells suitable for infrared, visible, and ultraviolet studies. Cell bodies have been constructed of horosilicate glass metal, Teflon, and polyethylene and the window materials
Figure 2.
Dual purpose cell
VOL. 36, NO. 3, MARCH I964
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