of a second column of different polarity to separate compounds with similar retention characteristics may be used for final confirmation of an analysis. ACKNOWLEDGMENT
The authors gratefully acknowledge the technical assistance of Margaret Saville, E. A. RIeigs, and Gerald Hou li han
LITERATURE CITED
(1) Ambrose, D. A., Keulemans, A. I. M., Purnell, J. H., ANAL. CHEM.30, 1582 (1958). (2) Anders, M. W., Mannering, G. J., Ibid., 34, 730 (1962). (3) Cieplinski, E. W., Ibid., 35,256 (1963). (4) Fales, H. M., Pisano, J. J., Anal. Biochem. 3,337 (1962). (5) Horning E. C., hloscatelli, E. A., Sweeley, 6. C., Chem. & Ind. (London) 751 (1959).
(6) Lloyd, H. A., Fales, H. M., Highet, P. F., VandenHeuvel, W. J. A., Wildman. W. C.. J. Am. Chem. SOC.82. 379f(1960). ’ (7) Parker, K. D., Fontan, C. R., Kirk, P. L., ANAL.CHEM.34, 757 (1962). (8) Ibid., 35,356 (1963). (9) Parker, K. D., Kirk, P. L., Ibid., 33, 1378 (1961).
RECEIVED for review December 19, 1962. Accepted May 29, 1963.
Gas Chromatographic Determination of Traces of Ethanol in Methanol KARL J. BOMBAUGH and WILLIAM E. THOMASON Research and Developmenf Division, Spencer Chemical Co., Merriam, Kan.
V A
method to determine ethanol in methanol at the 10-p.p.m. level is described. By using a column containing d-sorbitol on acetylated White Chromosorb at 100’ C., ethanol is eluted before methanol into a clearly resolved peak. Solid support treatment and proper column temperature are critical factors in obtaining resolution at the parts-per-millionlevel.
M
than 20 p.p.m. ethanol is desired in certain chemical syntheses, such as in the synthesis of dimethyl isophthalate where ethanol produces undesirable side reactions which result in an off-colored product. A method was needed, therefore, to determine ethanol in methanol a t the 10 p.p.m. level. The determination of ethanol in methanol a t trace levels is much more d s c u l t than would be inferred from the relative ease with which ethanol may be determined a t the 1% level. The limitation in this determination is not only the sensitivity of the detector but also the separation ability of the column. What appeared to be complete separation a t detector sensitivities suitable for analysis a t the 1%level was only a partial separation when monitored by a high sensitivity ionization detector. A wide variety of polar substrates such as polyglycoh, polyesters, and polyfunctional surfactants suitable for alcohol separations at higher ethanol concentrations were not suitable at the 10 p.p.m. level because ethanol eluted on the tail of the methanol peak. This work showed that by using dsorbitol as the stationary phase, a t the proper column temperature, ethanol was eluted before methanol. When the solid support was pretreated with glacial acetic acid (1, %’),base-line resolution was attained even a t the 10 p.p.m. level. 1452
ETHANOL CONTAINING less
ANALYTICAL CHEMISTRY
Table 1.
Retention of Ethanol Relative to Methanol
Source Fisher Scientific Co. Fisher Scientific Co. Fisher Scientific Co. Fisher Scientific Co. Fisher Scientific Co. Fisher Scientific Co. Fisher Scientific Co. Fisher Scientific Co. Fisher Scientific Co. Fisher Scientific Co. Fisher Scientific Co. Dow Chemical Co. Union Carbide Chemical Co. Armour Industrial Chemicals Co. Armour Industrial Chemicals Co. Nonylphenoxypolyoxyethylene Rohm & Haas Co. ethanol Polyethylene glycol 6000 Union Carbide Chemical Co. Substrate d-Sorbitol d-Sorbitol &Sorbitol d-Sorbitol &Sorbitol d-Sorbitol d-Sorbitol d-Sorbitol 2% HaPo, Glycerol Glycerol Glycerol Polypropylene glycol UCON HB 660 Armeen SD Ethofat 60/25
+
EXPERIMENTAL
The work was done on a BarberColman Model 20 gas chromatograph equipped with an ionization detector using a tritium foil source. The signal was read out on a 5-mv. Wheelco recorder. The Barber-Colman split type sample injector was replaced with a direct sample injector prepared from a I/8-inch Swagelok Bulkhead fitting. The body of the fitting was drilled with a no. 52 drill and a piece of l/s-inch 0.d. tubing was silver-soldered into it. The sharp leading edge of the Swagelok fitting was ground flat to provide a seat for the septum. A resistance heater was mounted on the column side of the assembly and thermally insulated. Liquid phases investigated are shown in Table I. All liquids were loaded on the solid support to 20 weight yo. Solid supports investigated were White Chromosorb (Johns-Manville Products Corp.), acid-washed White Chromosorb, sold under the trade name Gas-Chrom P. (Applied Science Laboratory, Inc.),
Temperature, O C. 42 50 59 74 90 100 107 100 43 66 78 85 70 85 80 67 100
Relative retention 1.36 1.23 1.00 0.80 0.78 0.76 0.78
Dehydrated 1.02 0.92 0.91 1.55 1.50 1.43 1.47 1.40
1.24
and White Chromosorb acetylated in this laboratory. The acetylated solid support was prepared by digesting 60- to 80-mesh White Chromosorb for 1 hour in glacial acetic acid. After digestion, the excess acid was removed by decantation and by repeated washings with distilled water. The support was dried at 100’ C., coated and screened to select the 60to 80-mesh material. The analytical column was prepared from a 35-foot length of ‘/*-inch copper tubing, coiled after packing. It was packed with acetylated White Chromosorb which had been loaded to 20 weight yowith d-sorbitol. Samples were injected with a Hamilton microliter syringe equipped with a Chaney adapter. Ethanol-free methanol was prepared by distillation on a 30-plate Oldershaw column operated at total reflux for several hours and followed by a brief collection period a t a 30 to 1 reflux ratio. Standard calibration solutions containing known amounts of ethanol were prepared from the ethanol-free base stock.
Figure 1 . Compari!ion of ethanol-free methanol and methanol containing 10 p.p.m. ethanol Peak identities 1. UnidentiRed impurities 2. Ethanol 3. Methanol Operating conditions Top chromatogram: column, 3 5 feet X '/4 inch 20% sorbitol an acetylated White Chromosorb; temperature, 1 OOOC.; flow, 39 ml./min. argon; voltage, 1 2 5 0 ; sample size, 6 pl. Bottom chromatogram: same as top except 100' C. column temperature and 58 rnl./minute flow rate
Chromatograms of these solutions were prepared a t the operating conditions shown in Figure 1 which also shows a comparison of (chromatograms of ethanol-free methanol and of methanol containing 10 p.p.m. ethanol. Ethanol peak areas were measured with a polar planimeter. Peak €,reas obtained for the standard solutions are shown in Table 11. A plot of the data yields a linear calibration line with a zero intercept over the concentration range tested. The sensitivity as determined from the slope of the calibration line was 27.5 X 107 sq. cm./gram. DISCUSSION
Influence of Solid Support Treatment. The solid support had a significant effect on the scparation of traces of ethanol from methanol. Columns using untreated V.hite Chromosorb or hydrochloric acid-washed White Chromosorb (Gas-Chrom P) as the solid support and sorbitol as the stationary phase failed t o resolve ethanol from methariol a t levels below 400 p.p.m. However, a similar column in which the White Chromosorb was treated with glacial iacetic acid prior to coating with the liquid phase effected
2
I TIME +
Figure 2. Comparison of acetylated support with HCItreated support Peak identities as in Figure 1 Sample size Top chromatogram, 6 pl. with 28 p.p.m. ethanol Bottom chromatogram, 0.35 pl. with 400 p.p.m. ethanol
base-line resolution at levels below 10 p.p.m. ethanol. The influence of support treatment is shown in Figure 2. The column used to prepare the bottom chromatogram contained Gas-Chrom P while the column used for the top contained acetylated White Chromosorb. Base-line resolution is shown in the top chromatogram with a sample load of 1.41 X lo-? gram of ethanol at a concentration of 28 p.p.m. in methanol. Without acetylation of the support only a shoulder peak was obtained for an ethanol load of 1.11 X 10-7 gram a t a 400 p.p.m. concentration. Sorbitol loads of 10, 15, 20, and 35 weight % were tried on nonacetylated, but acid-washed support. No improvement in separation was shown over that obtained with 20y0 (Figure 2, bottom). Sample injections from 0.1 pl. to 6 pl. were tried mit,h the same nonacetylated support and no improvement in separation was obtained. A 2001, sorbitol 2% phosphoric acid column was unsuccessful because dehydration of the sorbitol occurred a t 100' C. After a few hours of operation a t 100° C., water was observed bleeding
from the column accompanied by a n odor of burnt sugar. Influence of Temperature. The separation was studied a t six different temperatures using sorbitol as a substrate and a t three different temperatures using glycerol as the substrate. The results are shown in Table I. With the sorbitol column a t 5 9 O C., a relative retention of 1 is obtained. At this temperature ethanol and methanol elute in a single peak. Below this temperature methanol elutes before ethanol while above this temperature elution order is reversed. The results show that resolution was improved with increased temperature. This is a departure from the widely used concept that a separation is Table 11.
Calibration Data for Ethanol in Methanol Ethanol Sensitivity, sq. cm./ injected; Peak area, gram X Ethanol, grams X 10' sq. cm. lom7 p.p.m. 28 49 92 173
1.41 2.35 4.36 8.24
38.2 64.3 122.7 224.8 Average
VOL. 35, NO. 10, SEPTEMBER 1 9 6 3
27.02 27.40 28.14 27.27 27.5
0
1453
improved by IOU (>ringcolumn teniperature. Although the separation factors increase both above and below 59" C. resolution was not improved below 59" C. because of excewive peak broadening a t lower temperatures. Other Substrates. Several other substrates which have been widel)used for alcohol separations were screened for their suitability for this separation. Table I shows the retention of ethanol relative t o methanol. S o n e of the other substrates was as They either suitable as sorbitol. offered smaller separation factors or eluted ethanol after methanol. Although the separation temperature was not optimized with these substrates, the separation factors should apply over a reasonable temperature range. This
may be assumed since the heats of solution of ethanol and methanol are more nearly alike in these less polar substrates than in sorbitol and glycerol. As a result relative retention in these substrates is less temperature sensitive than in sorbitol and glycerol. Although glycerol eluted ethanol brfore methanol it was not as desirable as sorbitol because it bled severely at its optimum separation temperature and because it provided a lower separation factor. The severe base-line interference caused by the glycerol bleed was minimized by adding a 1.b-foot section of 55Z0 polyethylene glycol 400 onto the end of the glycerol column to act as a getter. This system was used to obtain the retention-temperature relationship for glycerol shown in Table I.
Several of the substrates listed in Table I should be quite suitable for determining traces of methanol in ethanol since they elute methanol first. Since polypropylene glycol offers the largest separation factor, this substrate loaded on acetylated White Chromosorb should be a n excellent column for determining traces of methanol in ethanol. LITERATURE CITED
(1) Bombaugh, K. J., Bull, CHEM.
34, 1237 (1962).
IT.C., AS.41..
( 2 ) . hIcIteynolds, IT.O., Pittsburgh Conference on And) tical Chemistry and
Applied Spectroscopy, Pittsburgh, Pa., 1961.
RECEIVED for review December 26, 1962. Accepted Rlay 20, 1963.
Analysis of Polyether and Polyolefin Polymers by Gas Chroma tog ra phic Dete rmina t ion of the Vo Ia ti le Prod uc ts Resu Iting fro m Co nt roI Ie d Pyro Iys is EDWARD W. NEUMANN and HERBERT G. NADEAU
O h Research Center, O h Mathieson Chemical Corp., New Haven 4, Conn.
b Pyrolysis and gas chromatography are used to determine the composition of various ethylene oxide-propylene oxide copolymers and various ethylene-butene copolymers. The pyrolyses are performed in an evacuated glass vial at 360' and 410" C. Analysis of the gases produced is accomplished by gas chromatography, using a flame ionization detector. Interpretation of the gas chromatographic data in most cases enables identification of the polymer. Quantitaiive analysis i s possible for the polyether mixed polymers. Possibilities of determining branched chains in the polyolefins are discussed.
T
HE FIRST pyrolysis
studies on polymers were made by Killiams in 1862 (11). This work was concerned with natural rubber, from which the basic isoprene unit was isolated. The earliest work on the pyrolysis of polymers in which gas chromatography is used appeared in 1954 in a paper by Davison, Slaney, and Wragg ( 2 ) . They pyrolyzed at 650" C. and used the chromatograms to identify qualitatively the polymers of butadiene-styrene, butadiene-acrylonitrile, isobutene, ethyl and methyl acrylates, and vinyl acetate. The component peaks were not identified, and the chromatograms were used in only the strictest qualitative n'ay. 1454
ANALYTICAL CHEMISTRY
Since this early work, there have been many reports on the qualitative identification of polymers. Haslam, Hamilton, and Jeffs (6), Haslam and Jeffs ( 6 ) , Radell and Strutz (8), and Strassburger et al. (9) have identified acrylate and methacrylate polymers singly and in mixtures by means of pyrolysis chromatograms. The latter paper also describes a method for determining quantitatively the composition of various acrylatemethacrylate copolymers. There have been several papers describing pyrolysis techniques, which range from simple vacuum distillations at high temperatures to flash pyrolysis on a hot wire which is an integral part of a gas chromatographic instrument. Each technique has its own drawbacks and advantages, and the choice is largely a matter of convenience. While the pyrolysis chromatograms will vary ~~~~
Table 1.
PolyethyIene0187" 5.2 Polyethylene018TD i i . 1 Polyethylene0187b 5 . 7 a Pyrolyzed the same day. b Pyrolyzed 3 weeks later.
~~
with technique and temperature of pyrolysis, they are all quite reproducible under the same conditions. The aim of this study was to determine the feasibility of a pyrolysis-gas chromatographic method for analyzing the composition of ethylene oxidepropylene oxide homopolymers and copolymers and to determine the composition of ethylene-butene copolymers. EXPERIMENTAL
Apparatus. A Perkin-Elmer gas chromatograph, Model 154 D, with flame ionization detector, was used along with a 2-meter Perkin-Elmer C column (Silicone DC-200). Column temperature was 30' C., and helium flow rate was 35 ml. per minute. Pyrolysis was done in a cylindrical Glas-Col heating mantle, approximately 4 x 3 inches in size.
~
Range of Polyethylene Pyrolysis
6.1
6.5 5 2
13 4 12.2 14.0
47.4
12.3
47.0 43.0
13.2 13.2
12.3 13.7 15.0
3.1 2.4 4.0
-
