For more volatile samples cooling is necessary while evacuating the vacuum lock to prevent sample loss. However, for such samples volatilization from the packing can usually be carried out. in an auxiliary high vacuum system connected directly to the ion source, simplifying introduction.
As a representative system a mixture of steroid derivatives was separated on a n Aerograph 600-C Hy-Fi gas chromatograph with a flame ionization detector, hydrogen generator, nitrogen in. stainless steel carrier gas, 6 ft. X column packed with 3% SE-30 on Gas Chrom Q at 250’ C., and a 1 O : l splitter allowing simultaneous collection and detection of the effluent’. Trapped fractions gave spectra of good quality from three different mass spectrometers: Bendix time-of-flight, using a modified Gohlke inlet ( 4 ) ; Hitachi RMU-6A with a Microtek solid introduction system entering between the molecular leak and the ion source; and Consolidated Electrodynamics Corp. 21-110 double-focusing with the 21-086 direct introduction probe. A preliminary check of absolute sensitivity with the latter mass spectrometer was made with collectors containing measured amounts of pregnanediol diacetate. Heating samples of 0.1 pg. gave fairly steady ion currents for 10-20 minutes, so that good qualita-
tive high resolution spectra could be obtained by mass scanning with the electron multiplier detector (Figure 2) , with time for exact mass measurements by peak-matching (9) for several ions. Using photographic plate recording, samples of 0.01 to 10 pg. a t 150’ C. gave total ion currents of 3 X l o + to 1 X coulombs. Thus a G C moles can give fraction of about a high (>10,000) resolution photoplate spectrum which is useful for qualitative analysis. Direct introduction into the ion source of the separated fraction on its chromatographic substrate is also a convenient technique for other separation methods such as liquid-liquid, pa,per, and thin layer chromatography. We are currently investigating such applications, the extension of sensitivity limits, and the automation of this technique. Note added in proof: Since our original report of this work, trapping of G C effluent on column packing for additional G C analysis has been described
(4a). LITERATURE CITED
(1) Beroza., R L ,. J . Gas Chrom. 2, 330 \
,
(1964). (2) Colson, E. R., ANAL.CHEM.35, 1111 (1963). (3) Gohlke, R. S., Ibid., 31, 535 (1959).
S.,Chem. and Ind. 1963, 946. (4a) Kemp, W., Itogne, O., Ibid., 1965,418. (5) Rluller, D. O., ANAL.CHEM.35, 2033 (1963). (6) Novak, J. et al., Ibid., 37, 660 (1965). (7) Ross, W. D., Moon, J. F., Evers, R. L., J . Gas Chrom. 2, 340 (1964). (8) Ryhage, R., ANAL.CHEM.36, 759 (1964). (9) Saunders, R. A., Williams, A. E., “Mass Spectrometry of Organic Ions,” F. W. McLafferty, ed., pp. 354, Academic Press, New York, 1963. (10) Scott, R. P. W., personal communication, April 1965, Unilever Research Laboratory, Sharnbrook, England. (11) Watson, J. T., Biemann, Klaus, ANAL.CHEM.36, 1135 (1964); 37, 844 (1965). (12) widm mark, K., Widmark, G., Acta. Chem. Scand. 16,575 (1962). J. W. AMY E. AI. CHAIT W. E. BAITINGER F. W. RICLAFFERTY
(4) Gohlke, R.
Department of Chemistry Purdue University Lafayette, Ind. RECEIVED for review June 16, 1965. Accepted July 2, 1965. Presented at the Anniversary Meeting, The Chemical Society, Glasgow, April 7, 1965, and at the 13th Annual Meeting on hIass Spectrometry, ASTM E-14, St. Louis, May 1965. Work supported by research grants from the National Institutes of Health (GM 12755) and the National Science Foundation (GP 4335).
Determination of Vinyl Acetate in Ethylene Vinyl Acetate Copolymers Based on High Energy Radiolysis SIR: Recent work in our laboratories on the effect of high energy radiation on several ethylene vinyl acetate copolymers has shown that radiolysis can be an effective tool in the quantitative analysis of their composition, even when the vinyl acetate is present in amounts as little as 1 to 2%. Radiolysis has been used before for studying ethylene polymers but these studies (1-3) have been primarily directed toward analyzing ethylene a-olefin copolymers and identifying the short chain branches in the so-called conventional high pressure polyethylene. Work described here is concerned with a successful extension of this technique in determining vinyl acetate in ethylene vinyl acetate copolymers quantitatively. EXPERIMENTAL
Samples of copolymer in finely divided form and in amounts equal to roughly 1 gram per sample are accurately neighed into glass break-seal tubes of approximately 20-nil. capacity and are sealed off under very high vacuum (at pressures less than 10-5 mm. Hg) after pumping the tubes for a t least 1 hour under this vacuum to rid the 1266
ANALYTICAL CHEMISTRY
samples of any adsorbed oxygen. The samples are then subjected to a measured dose (in the neighborhood of 100 megarads) of gamma radiation from a cobalt-60 or some other suitable source. Following this, the volatile portion of radiolysis product which consists principally of low molecular weight hydrocarbons, carbon monoxide, carbon dioxide, and hydrogen is analyzed by gas chromatography for its carbon monoxide content and its G value determined in the usual manner. G value is the number of molecules produced per gram of sample per 100-e.v. incident radiation dose. RESULTS AND DISCUSSION
When a series of samples of pure polyethylene and ethylene vinyl acetate copolymers of differing vinyl acetate content was subjected to radiolysis under conditions described above with rigorous exclusion of oxygen and their volatile products were analyzed by gas chromatography the copolymers alone gave carbon monoxide as a major component. Further, the amount of carbon monoxide so produced increased s i t h increase in the vinyl acetate content indicating a good possibility of radiolysis affording
a convenient tool for analyzing vinyl acetate content in these copolymers. A calibration curve was constructed by using a number of copolymers of known composition as standards, subjecting the same to known radiation dosage, and calculating their carbon monoxide G values by analyzing the volatile products from each sample for CO content. The CO analysis was carried out using a gas chromatograph. The vinyl acetate content of standard copolymers was determined and crosschecked by three independent methods of analysis, namely, infrared, nuclear magnetic resonance, and saponification. The observed data are given in the upper half of Table I. The calibration curve so constructed from this datai.e., plotting wt. yo vinyl acetate against carbon monoxide G value-is observed to be a straight line that fits the following equation: Wt.
yo vinyl acetate
=
842 X G (carbon monoxide) The method of analysis has been tested using several blends of copolymers and pure polyethylene of known composition, and the ensuing results are
Table 1. Radiolysis of Ethylene Vinyl Acetate Copolymers G (Carbon Monoxide) vs. Vinyl Acetate Content
Wt. % Vinyl acetate In co-
Dolvmer ~" 3.6 9.5 19.9 28.0 3.6 9.5 19.9
Carbon black ... ... ... ... 2.5 2.5 2.5
Carbon Vinyl monoxide, acetate G value insamde X lo* 3.6 9.5 19.9 28.0 3.2 8.6 17.9
0.53 1.21 2.40 3.54 0.52 1.06 1.90
within & l % of those calculated from knowledge of the vinyl acetate content of the copolymers and the composition of the blends. An observation of immense practical value made during the course of this work is that filler; like carbon black which are often incorporated in such resins do not interfere with the analysis. This is evident, for instance, from the excellent manner in which carbon monoxide G data of samples in which carbon black was deliberately incorporated in known amounts prior to radiolysis (see lower half of Table I) fits the calibration line obtained from the data of samples containing no
carbon black. I n contrast, such fillers not only interfere in other methods of analysis, but quite often actually prevent the analysis; the latter is particularly true with infrared method. LITERATURE CITED
(1) Boyle, D.A., Sirnpson, W., Waldron, J. D., Polymer 2, 323 (1961). (2)Kamath, P. M., Barlow, A. Unpub-
lished data.
(3) Wilbourn, A. H.,J . Polymer Sci. 34, 569 (1959). P. hI. KAYATH ANTHONYBARLOW
Research Division U. S. Industrial Chemicals Co. Cincinnati, Ohio 45237 RECEIVEDfor review June 3, 1965. Accepted July 2, 1965.
Effect of Solvent Composition on the Separation of Thiourea, Thiocyanate, and Sulfide by Paper Chromatography SIR:Thiourea and its derivatives are important analytical reagents, used both for the determination of numerous metal ions (7) and for masking cations such as copper. Thiourea is converted to thiocyanate on heating ( 7 ) and to hydrogen sulfide on hydrolysis. Thiourea has been separated from thiocyanate by dissolution of the mixture in a selective solvent, such as water, a t a suitable temperature ( 5 ) , by addition of mercuric chloride in the presence of ammonium acetate (S), and by adduct formation with carbon tetrachloride [ I ) . Ion exclusion has also been used to separate thiocyanate from sulfide (9), and sulfide has been determinrd in the presence of thiourea and thiocyanate by titration with sodium o-hydrosymercuribenzoate ( 1 0). \Ye are aware of no published methods for the separation of thiourea, thiocyanate, and sulfide by paper chromatography. The relationship between R/ value and solvent composition has received increasing attention in recent years ( 2 , 4, 6 , 6 ) . I n this paper we summarize the solvent systems studied for the separation of thiourea, thiocyanate, and sulfide. The effect of dielectric constant on the R , values of the three substances is also described and discussed.
done in a Kawerau apparatus. Schleicher and Schuell number 2043a paper was used throughout the study, except for circular chromatography where Whatman number 1 circles, 12.5 cm. in diameter, were used. Reagents. Chemicals and solvents were either E. Merck (Darmstadt) or British Drug House analytical reagent grade. DETECTORS.An approximately 0.1M solution of mercurous nitrate was used to detect thiourea. Thiocyanate was detected with 0.12M ferric nitrate. If formic acid were used to develop the thiocyanate chromatogram it was necessary to wash the paper successively with anisole and diethyl ether prior to applying the detector. For sulfide, the chromatograms were first dipped in 0.1M AgNO, and then washed with 4111 HKO1 followed by distilled water to remove excess silver nitrate. TEST SOLUTIONSOne per cent aqueous solutions of thiourea, ammonium thiocyanate, and sodium sulfide were prepared by dissolving the required quantity of each in distilled water and diluting to the required volume. Procedure. T h e strips were conditioned for 5 minutes and then developed, using ascending techniques, until the solvent front had reached a suitable distance from t h e starting line.
EXPERIMENTAL
RESULTS
Apparatus. Glass jars 20 b y 5 cm. were used to develop paper strips 15 b y 4 cm. Strips 26 b y 4 cm. in size were developed in jars 30 by 5 . 5 cm. Circular chromatography was
T o proceed systematically, the R, values of the three substances were obtained in numerous pure organic solvents and in solvents containing
complexing agents. Some of the more interesting results are summarized in Table I. R/ values of all the three were found to be zero in dioxane, butyl acetate, diphenyl oxide, isopropyl ether, chloroform, cyclohexane, carbon disulfide, ethylene trichloride, and anisole. Thiocyanate and sulfide did not move in acetophenone, methyl acetate, ethyl acetate, ethyl acetoacetate, diethyl malonate, and diphenyl oxalate. However, thiourea showed some movement accompanied with tailing in these solvents. Separation of Binary and Ternary Mixtures. Binary mixtures were prepared from 0 . 1 X solutions of t h e three substances in ratios varying from (1:lO) to (1O:l). Mixtures were also prepared from 0.01JI solutions to study any dilution effect. In all cases satisfactory separations were achieved. Representative results are shown in
Table
I.
R, Values Rt Thio-
Solvent Thiourea cyanate AIethanol 0.34 0.48 Ethanol 0.23 0.27 Formic acid 0.70 0.68 1%. Aq. lead nitrate 0 70 0.81 Distilled water 0.71 0.93 Pyridine 0.69 0.3ga Tails.
VOL. 37, NO. 10, SEPTEMBER 1965
Sulfide 0.Oga 0.00 0.61 0.36 0.99 0.00
1267
