Compounds containing oxygen or halogen also are adsorbed more strongly than their su1face tensions would suggest. ddsorbabilities on the nitrile of fourteen such compounds also are given in Table 111. These polar solutes also must be adsorbed strongly because of electrostatic attraction between solute and liquid phase. All of them except carbon tetrachloride, which has no dipole moment, have highrr adsorbabilities than the hydrocarbons. The order of adsorbabilities follows no single property of the solutes. Some combination of dipole moment and empirically determined polarity (9) appears to govern the adsorbability qualitatively. CONCLUSION
Whether solutes will adsorb preferentially on the surface of the liquid or the surface of the support presumably will depend largely on the polarity of the liquid phase. With polar liquid phases, adsorption on the liquid should predominate, the adsorption being greatest on liquids of high surface energy. With nonpolar liquid phases, adsorption on the support should be more important; adsorption on nonpolar liquids was not expected and was not observed, although a small amount would have been difficult to measure. Adsorption on the liquid also should not be prominent with capillary columns, because the surface area per unit volume of liquid normally mill be much less than with packed columns.
Besides affecting elution orders, adsorption on the liquid also affects the efficiency of columns. Those nitrile columns in which adsorption on the liquid predominates have several times the theoretical plates of those in which solution is the more important. Attempts t o prepare highly efficient columns working entirely on the principle of adsorption on the liquid should be made. Adsorption on the liquid in gas chromatography is troublesome in some cases and beneficial in others. Partition and activity coefficients determined by gas chromatography for polar liquid phases will be in error unless corrected for the contribution from adsorption. Also, adsorption mili complicate the job of cataloging retention volumefi; it often will be necessary to specify the percentage and surface area of the liquid phases, and values of IC, as well as k, will have to be evaluated. On the other hand, adsorption is helpful when making columns to separate multicomponent mixturrs, because its contribution can be varied to avoid overlapping of peaks. -41~0.this newly discovered phenomenon should permit the techniques of gas chromatography to be used for fundamental study of adsorption of gases on liquids. LITERATURE CITED
(1) Ambrose, D., Keulemans,
A.I.N., Purnell, J. H., AXAL.CHEU. 30, 1582 (1959). (2) Baker, W.J., Lee, E. H., Wall, R. F., Second Biannual Gas Chromatography
Symposium, Michigan State University, June 1959. (3) Eggertsen, F. T., Knight, H. S., ANAL.CHEM.30,15 (1958). (4) Fukuda, T., Japan Analyst 8 , 627 (1959). \--
- I
(5) Glasstone, S., “Textbook of Physical Chemistry,” p. 1194-202, Van Nostrand, New York, 1946. (6) Hoare, M. R., Purnell, J. H., Trans. Faraday SOC.52,222 (1956). (7) James, A. T., Martin, A. J. P., Biochem. J. 50,679 (1952). (8) Johns, T., “Gas Chromatography,” V. J. Coates, H. J. Noebels, and I. S. Fagerson, eds., p. 31, Academic Press, New York, 1958. (9) Koso--er, E. M., J. Am. Chem. SOC. 80,3253 (1958). (10) Kvantes, A., Rijnders, G. W. A., “Gas Chromatography,” D. H. Desty, ed., p. 131, Butterworths, London, 1958. (11) Littlewood, A B , Phillips, C. P. G , Price, D. T., J . Chem. SOC. 1955, 1480 (12) Martin, R. L., Winters, J. C., ANAL.CHEM.31, 1954 (1959). (13) Nelsen, F. AI., Eggertsen, F. T., Ibid., 30, 1387 (1958). (14) Ottenstein, D. bl., Detroit Anachem Conference, Wayne State University, October 27. 1959. (15) Porter, ’P. E., Deal, C. H., Stross, F. J., J . Am. Chenz. SOC.78, 2999 (1 !?iii’i ,-_--,.
(16) Winters, J. C., Jones, F. S., Martin, R. L., Section 17, 5th Forld Petroleum Congress. New York. June 1959. (17) Fqolf, F., Ternon-, A., Kolloid Z. 166, 38 (1959). (18) Zlatkis, A., Ling, S., Kaufman, H.R., A 4 ~ . CHEX 4 ~ . 31, 945 (1959). RECEIVED for review September 8, 1960. Accepted November 28, 1960. Division of Analytical Chemistry, 137th Meeting, ACS, Cleveland, Ohio, April 1960; Gas Chromatography Discussion Group, Liverpool, England, October 21, 1960.
Gas-Liquid Chromatographic Analysis of Trace Impurities in Styrene Using Capillary Columns 0.L.
HOLLIS
Organic Busic Research laboratory, Texas Division, The Dow Chemicul Co., Freeport, Tex.
b Trace analyses using capillary columns have not been reported except those in which a t least one concentration step was included. The difficulty of separation of the xylene isomers and other impurities in 99.65% styrene makes the analyses of finished styrene excessively long on packed columns. The technique described made use of capillary columns in conjunction with the triode argon detector of Lovelock to separate and detect the xylenes and other impurities in finished styrene in concentrations below 25 p.p.m. without a concentration step. The method is rapid and gives good separations of the impurities from the major components. 352
ANALYTICAL CHEMISTRY
I
of aromatic hydrocarbons, certain difficult separations, such as m- and p-xylene, have been studied (S, 4, 6, 0, l o ) , and within the last two years the capillary column has made these separations less of a problem. However, the challenge of the m-, p-xylene split has been accepted as the acme of column efficiency by most makers of capillary columns Due to limitations in the detection of exceedingly minute amounts of material, the analysis of trace impurities in finished products (99.0% punty) by capillary column gas-liquid chromatography (GLC) has not been considered feasible previously. Such analyses have been done primarily on packed columns with large sample loads ( 1 , 8) N THE ANALYSIS
or with some prior concentration step, such as distillation or preparative scale chromatography with trapping of trace components (2). For qualitative identification the trapped components were submitted to infrared or mass spectrometric analysis. The excessive column lengths required for these difficult separations and the associated long times for the analysis of finished styrene on packed columns led us t o consider capillary columns. The invention by Lovelock (Y) of the triode cell provided a detector of sufficient sensitivity that the problem of determining impurities in styrene could be attacked with capillary columns. The xylenes, suspected as minor impurities, are difficult to separate from styrene and the
t
1000
- 2000 V. '\
Teflon
Bfass of Slalniess Steel
Source-
Gas Diffuser Three layers 100 mesh metal g a m
Source Size alpha or beta 1.5 x 10'. ampere
Sufficient to give 1.0
lo dl
Figure 2. Figure 1.
Triode argon detector of Lovelock
Electronic changes for diode and triode opera tion
Diode-triode reversible polarity power supply
major impurities, ethylbenzene and cumene. Therefore, they were of primary interest in this study. EXPERIMENTAL
Apparatus. T h e instrument used was a Barber-Colman Model 10 gas chroniatograph and was modified for this use n i t h capillary columns. T h e original detector was replaced with a triode argon detector of Lovelock's design ( 7 ) , details of which are shown in Figure 1. This detector was fabricated in our omn shops. The power supply was modified to reverse the polarity and the electrometer zero circuit was changed to give a center zero on the Helipot, as shown in Figure 2. A sample splitter m s used before the column. Tno sample splitters of different designs n ere tried, but neither was satisfactory for linear splitting n i t h samples of different sizes; homever, fair reproducibility was obtained with a standard sample size. Preparation of Columns. Three columns were prepared from 304 stainless steel DHP annealed tubing, one of 25-gage needlestock, and the other two of 0.062-inch outer diameter and 0.010-inch inner diameter tubing manufactured by J. Bishop Co. The capillary tubing was washed n ith solvents just prior to coating in the folloning order: pentane, methylene chloride, acetone, diethyl ether, and the solvent for the stationary phase. The solvents were forced through the tube under pressure with argon, and the tubing was dried in the stream of argon after the final wash. The stationary liquid in lOy0 solution by volume was then forced through the tubing with argon a t 20 p.s.i.g. Purging with argon was continued until all the solvent was evaporated from the column. The stationary liquid phases were bis(phenoxypheny1) ether (The
Dow Chemical Co.), Ucon 50-HB-2000 (Union Carbide Chemicals Co.), and Apiezon L (Metropolitan-Vickers Electrical Co., Ltd.) dissolved in diethyl ether, absolute methanol, and methylene chloride, respectively. The bis(phenoxyphenyl) ether and Apiezon L columns were 200 feet long and the Ucon column, the needlestock, was 125 feet long. PROCEDURE
Operating Conditions.
TableIl.
Compound Benzene Toluene Ethylbenzene p-Xylene m-Xvlene o-XS;lene Styrene Cumene n-Propylbenzene m-Ethyltoluene p-Ethyltoluene 1,3,5-Trimethylbenzene o-Ethgltoluene a-Methylstyrene tert-But ylbenzene Isobuty lbenzene sec-Butylbenzene @-Methylstyrene Indane m-Cymene p-Cymene o-Cymene m-Vinyltoluene p-Vinyltoluene o-Vinyltoluene
Tempera-
tures and flow rates were selected t o give a satisfactory separation of ethylbenzene from m- and p-xylene and of cumene from o-xylene, and to give elution times of less than 30 minutes when the impurities eluted before styrene were of interest. For the other impurities the conditions were adjusted to give as rapid elution as possible consistent with the desired separation. All columns were usable up to operating temperatures of 200" C., thus allowing convenient examination of components boiling up to 300" C.
Retention Data for Aromatic Compounds
B. P.," c. 80.1 110.6 136.2 138.3 139.1 144.4 145.2 152.5 159.45 161.3 162.0 164.7 165.1 165.4 168.8 171.0 173.5 176.0 177 0 175 2
Relative Retention Times (Benzene = Apiezonb UconC Bis(phenoxy)d 1.00 1.00 1.oo 1.13 1.37 1.41 1.64 2.26 2.18 2.39 2.22 1.75 2.45 2.30 1.93 2.94 2.72 1.93 3.78 3.08 2.04 3.02 2.94 2.35 3.67 2.48 3 80 4 00 2.53 3.72 2.77 2.77 2.75 2.99 3.61 3.87 3 15
6 08 4.43 4.69 4 86
5.16 4.34 4.46 4 75
a Retention time in minutes is obtained by multiplying relative retention times by 9.2 for Apiezon, 4.9 for Ucon, and 9.1 for bis(phenoxy) column. 19 p.s.i.g., 100" C. 20 p.s.i.g., 65" C. 40 p.s.i.g., 95" C.
VOL. 33, NO. 3, MARCH 1961
353
f 3 0
5
Figure 3.
10
15 10 ElUllON TIME lminulei)
25
4
30
Chromatogram of styrene on Apiezon L Conditionr:
5
0
15
10
20
30
ELUTION TIM€ jminul.0
100' C., 19 p.r.i.g.
Figure 4.
Chromatogram of styrene on Ucon column Conditions: 70' C., 20 p.s.i.g.
A satisfactory set of operating conditions for each of the columns is given in Table I. Calibration of Known Mixtures. For qualitative identification, retention times of authentic samples of known and suspected impurities were determined singly and in mixtures. For quantitative calibration, standard mixtures containing from 100 to 1000 p.p.m. each of benzene, toluene, m-xylene, isobutylbenzene, and secbutylbenzene were analyzed. Amounts of other impurities were estimated on the basis of these data. Peak height was used as the measure of quantity. In the analyses, the unknown samples were run intermittently with standardized mixtures and peak heights were compared. Uniformity of sample size was assessed by the use of toluene as an internal standard. In no sample of finished styrene was toluene present in amounts greater than 5 p.p.m., so 50 p.p.m. was added as the standard. Samples of 0.5 and 1.0 pl. measured with a Hamilton 1.0-p1. syringe were introduced into the sample splitter with a split flow of 150 to 200 cc. per minute which gave a split ratio of 100 to 200, or a total sample to the column of not more than 0.005 pl.
The chromatogram of a different sample of finished styrene run on the Ucon column is shown in Figure 4. The concentrations of individual components are different from those of the styrene of Figure 1 and this illustrates the variability observed among different samples. This column separates ethylbenzene and m- and p-xylene well. Compounds with unsaturated side chains are retained longer than those with saturated side chains as shown by the wide separation of styrene and cumene which have elution times and boiling points in opposite order, 18.5 and 14.8 minutes, and 145.2" and 162.5' C., respectively. I n finished styrene the m- and p-ethyltoluene peak was obscured by the styrene peak. However, in synthetic mixtures they could be detected on the trailing edge of the styrene peak at concentrations of 0.5% or greater. Isobutylbenzene, sec-, and tert butylbenzene were well separated from one another and from other components. Figure 5 shows a chromatogram of finished styrene run on the bis(phenoxy-
phenyl) ether column. Essentially all components in finished styrene were well separated. This column is unusual because it separates p- from mxylene much better than it separates ethylbenzene from p-xylene and because the structurally very similar mand p-ethyltoluenes were eluted in opposite order of their boiling points and of their elution from the Apiezon L column. This column also gave the best separation of o-xylene from cumene in finished styrene. Table I summarizes the elution data obtained on known and suspected styrene impurities. Those compounds which could be obtained were added to styrene as standards and their elution times were determined. The table shows that the thiee chromatograms of finished styrene on these columns will have essentially every impurity represented by a t least one single-component peak. Six samples of finished styrene from different sources were analyred in this study and had essentially the same quali-
RESULTS AND DISCUSSION
A chromatogram of finished styrene (99.65% pure) run on the Apiezon L column is shown in Figure 3. This column gave essentially a boiling point separation. It did not separate p from m-xylene, or e-xylene, styrene, and cumene from one another a t the existing concentrations, or isobutylbenzene from sec-butylbenzene. It did, however, separate m- and p ethyltoluene which were not separated on the Ucon 50-HB-2000 column. The concentration figures beside each peak show the total amounts of impurities in parts per million. 354
ANALYTICAL CHEMISTRY
t E -
Figure 5.
Chromatogram of styrene on bis(phenoxyphenyl) ether column Conditions:
105' C., 40 p.s.1.g.
tative composition. Table I1 indicates the impurities actually found and their concentration ranges. I n the right hand column are shown concentrations a t which components were actually dctected under the conditions of these esperiments. In many of these cases the response from less material could certainly have been differentiated from base line noise but no eff 01t was made to determine actual minimum detcctablc limits. The values represent rather the order of magnitude of the limits under the conditions used, and together with data of sample size nnd split ratio, they indicate the o d e r of magnitude of the mass of material which the appaiatus will detect. For many of the impurities which were well sepnrated a larger sample size could have been used, thus enabling the determination of even smaller quantities. CONCLUSIONS
In this type of essential analytical work both highly efficient columns and highly sensitive detectors are necessities. The invention of capillary chromatographic columns by Golay (6) provided the separating tool, and the triode cell
Table II. Concentration of Trace Components in Six Samples of Finished Styrene
Compound Benzene Toluene ' Ethylbenzene p-Xylene m-Xylene o-Xylene Styrene Cumene Unknown n-Propylbenzene m-Ethyltoluene p-Ethyltoluene a-Xethylstyrene Unknown tert-Butylbenzene Isobutylbenzene sec-Butylbenzene m-Viny ltoluene p-Vinyltoluene &Methylstyrene
Range, P.P.M.
OMDL,@ P.P.M.
