zene ratio in the fuels used is about 5 to 1,whereas that in the exhaust is about 2 to 1 (Table 11). This same change in ratios has been observed with the same fuels in another recent investigation (19). I n a third study, in which various blending stocks associated with particular refining processes were used, similar shifts in toluene to benzene ratios were evident (14). The available data strongly suggest that benzene may be synthesized in appreciable amounts in combustion chambers of automobiles. The alternate possibility is that toluene is selectively destroyed. However, i t is well known that benzene is formed readily from acetylene and other aliphatic hydrocarbons by pyrolytic processes. Higher aromatic hydrocarbons also are formed but probably in much smaller amounts. It is also possible that in the combustion process, benzene and some toluene are formed under some operating conditions by dealkylation of heavier aromatics ( I C ) . The decrease in the concentration of aromatic hydrocarbons during irradiation could have wide significance. The rates of reaction found for the aromatic hydrocarbons fall between that for ethylene and those for the higher olefins in the same systems (21). Consequently, the rate of loss associated n ~ i t hthese materials certainly is sig-
nificant in terms of the over-all atmospheric reactions in "smoggyJJ atmospheres. ACKNOWLEDGMENT
The authors thank A. G. Altenau for his assistance in obtaining calibration data. LITERATURE CITED
(1) Sltshuller, A . P., Cohen, I. R., ANAL.
CHEM.32,802 (1960). (2) Altshuller, A. P., Wartburg, A. F., Cohen, I. R., Sleva, S. F., Intern. J . Air Pollution 5, KO.2, in press. (3) Andreatch, A. J., Feinland, R., ANAL. CHEM. 32,1021 (1960). (4) Dwty, D. H., Geach, C. !,., Goldup, il. in Gas Chromatography, R. P. Scott, ed., p. 46, Butterworths, London, 1960. (5) Eggertsen, F. T., Nelsen, F. M., ANAL. CHEAT. 30, 1040 (1958). (6) Ettre, L. S., Claudy, H. X., Chem. in Can. 12,34 (1960). ( 7 ) Farrington, P. S., Pecsok, R. L., Meeker, R. L., Olson, T. J., 4 x - 4 ~ . CHEM. 31, 1512 (1959). (8) Feinland, R., Andreatch, A. J., Cotrupe, D. P., Ibid., 33,991 (1961). (9) Ferrin, C. R., Chase, J. O., Hum, R. W., in IS-4 Proceedings, 1961 International Gas Chromatography Symnmiiim. n __.~.., =. 267 - _. . (10) Halasz, I., Schneider, W., A4NAL. CHEM.33,978 (1961). (11) Halasz, I., Schneider, W., 1961 Gas Chromatography Symposium, Michigan State Univ., June 1961; I S A Proceedings 3, 195 (1961).
(12) Heaton, IT'. B., Wentworth, J. T., AXAL.CHEM.31,349 (1959). (13) Hughes, K. J., Hurn, R. W., J . -4ir Pollution Control Assoc. 10. 367 11960). (14) Hurn, R. W., Davis, T. C.,' Pro;. Am. Petrol. Inst. 38 (31, 353 (1958): (15) Hurn, R. W., Davis, T. C., Tribble, P. E., Twenty-fifth Midyear Pvleeting of hmerican Petroleum Institute's Division of Refining, Detroit, Mch., May 11, 1960. (16) Meigh, D. F., Sature 186,902 (1960). (17) Neligan, R. E., Joint Research Conference on Motor Vehicle Exhaust Emissions and Their Effects, Los Angeles, Calif., Dee. 5, 1961. (18) Neligan, R. E., Brunelle, 11. F., Saeli, D., Fourth Conference on Methods in Air Pollution Studies, Los Angeles, Calif., Dec. 5-6, 1960. (19) Keligan, R. E., hlader, P. P., Chambers, L. A , , J . Azr Pollution Control dssoc. 11, 178 (1961). (20) Rose, A. H., Brandt, C. S., Zbid., 10, 331 (1960). (21) Schuck, E. R., Doyle, G. J., Report N o . 29, .4ir Pollution Foundation, San Marino, Calif., Oct. 1959. (22) Schuck, E. R., Ford, H. W., Stephens, E. R., Report No. 26, Air Pollution Foundation. San Rlarino, Calif., Oct. 1958. (23) Tuttle, W. X., Feldstein, AI., J . ilir Pollution Control Assoc. 10,if27 (1960). (24) Van der F,raats, F., in Gas Chromatography, R. P. Scott, ed., p. 46, Butterworths, London, 1960. RECEIVED for review September 13, 1961. Accepted February 5, 1962. Division of Water and Waste Chemistry, 140th Meeting, .4CS, Chicago, Ill., September 1961.
Programmed Temperature Gas Chromatography with Glass Microbeads Quantitative Analysis of a Homologous Series of Alcohols and Hydrocarbons J. G. NIKELLY Esso Research and Engineering Co., linden, N. J.
,A programmed temperature gas chromatography column of lightly coated glass microbeads has been used to separate a reaction product containing up to 30 components consisting of even-numbered normal alcohols, alkanes, and alkenes, ranging from c6 through C26. The mixture, boiling up to 400" C., is resolved in 30 minutes. Comparisons of Carbowax, Apiezon L, and silicone gum rubber as substrates for separating alcohols indicate that Carbowax is most efficient whether it is used on Chromosorb or on glass microbead supports. For quantitative analysis, correction factors were determined for the conversion of peak areas to concentrations.
472
ANALYTICAL CHEMISTRY
G
CHROhL4TOGRAPHIC SUppOrtS, such as glass microbeads, which as a result of their lower surface area and weaker adsorption effects can be coated with low amounts of substrate Fvithout causing peak tailing, have been used successfully in place of conventional supports (3, 4, 9). They are particularly useful when polar compounds are eluted on nonpolar substrates which are less effective in neutralizing the adsorption sites of the support. The resulting reduced retention times permit the separation of high boiling substances a t lower operating temperatures; this allows the use of more polar, lower boiling substrates, and widens the range of 1017-cost inAS
strunients which do not provide for high temperature operation. U p to the present, relatively little work has been published in this area, especially in the field of applications. I n this work, a 2-meter microbeads column coated with 0.570 by weight Carboxax 20,000 was suitable for a single-run quantitative analysis of a 30-component mixture of normal alcohols, alkanes, and alkenes ranging from 6 to 26 carbon atoms and having normal boiling points ranging from 50" to over 400" C. When the same mixture was analyzed on a 2-meter conventional column of 6 n t . yo Carbonas on Chroniosorb (an amount of substrate sufficiently low to reduce retention times
without causing peak tailing), the highest component eluted satisfactorily was C14alcohol. Laramy and Lively ( 7 ) have resolved a similar mixture using a temperature programmed, silicone gum rubber column, 30 weight yo on Celite, but t h e alkenes were not separated from the corresponding alkanes and the molecular weight range of the eluted components was l o w r . Separations of alcohols up to CI8, without the corresponding alkanes and alkenes, were also shown by Faley ( 2 ) who used a column of 1% silicone gum rubber on Chromosorb and by Link and llorissette (8) who used a column of 20% Apiezon L on Chromosorb. which was specially treated with potassium hydroxide. I n this study, the efficiency of microbead and Chroniosorb columns with various substrates m-a+compared. Peak area correction factors rwre also deterniinrd; this was particularly necessary lmaiise of the range of molecular typos and w i g h t ? separated in a .ingle run. EQUIPMENT A N D MATERIALS
Equipment. A linear programmed temperature gas chromatograph, F & AI Scientific Corp., Model 300-B, wzs equipped n i t h a four-filament hot nire detector. T h e sample injection port and detector block temperature were maintained at 200" t o 250" C. The detector signal was supplied to a 1-mv. Daystrom-Reston recorder, Model 6701, with a 1-second full scale response and a chart speed of 30 inches per hour. Materials. Helium dried by passing through a bed of 5A Linde LIolecular Sieves, n a s used as the carrier gas. There 11ere no significant changes in flow rate or base line drift during the linear programmed temperature runs when t h e helium inlet pressure was maintained at 50 t o 60 p.s.i.g. at t h e flow regulator of the instrument. G1:tss microbeads, 60- to 80-mesh size (Alicrobends Inc., Jackson, Miss.) were used untreated ( 3 ) . SLtmples of the higher n-alcohols, Cla through C20, were purchased from Lachat Biochemical Co., 2202 JT'est 107th Place, Chicago 43, Ill. Samples of the higher hydrocarbons were furnished by the Petroleum Refining Laboratory, The Pennsylvania State University, University Park, Pa. Preparation of Columns. Columns m-ere prepared in t h e usual procedure b y packing the coated support in 1t o 2-meter lengths of soft copper tubing, 0.25 inch O.D. Although t h e iron filings n hich are invariably prese n t in t h e beads (approsimately 0.2 n-t. yo)are harmless in most applications, they are removed easily by stirring t h e beads n i t h a rotating magnetic stirring bar. Carbowax Columns. Three hundred milligrams of Carbovax 20,000 \VAS dissolved in about 50 ml. of warm
RETEhTlON TIME. Y#CU-ES
Figure 1. Typical gas chromatographic recording obtained with Carbowax on microbeads 1.5-meter column of 0.5 wt. % Carbowax 20,000 on 200-micron glass microbeads; temperature programming from 55' to 250' C. a t 9' per minute; He flow rate 50 cc. per minute; 0.7-pl. somple of alcohol mixture containing added hydrocarbons
sufficiently fluid, they were introduced into the copper tubing in small portions, and packed by pressing with a 0.125inch steel rod. The columns were conditioned at 250" C. for a few hours and the base line shift due to substrate bleeding was less than 0.1 mv. at the maximum operating temperature of each column (255" C. for the Carbowax and Apiezon columns; 340'. C. for the silicone gum rubber column). Alcohol - Hydrocarbon Mixtures. T h e alcohol - hydrocarbon mixtures which n-ere t o be analyzed quantitatively consisted of the normal even carbon-numbercd alcohol., ranging from C6 through C26, including small amounts of the corresponding alkanes and alkenes. This mixture is a product of the alkyl metal process (11) for preparing higher alcohols used in the de-
methanol a n d t h e solution thoroughly mixed with 60 grams of beads. T h e mixture was heated with frequent stirring until t h e solvent was evaporated. T h e resulting packing of 0.5 wt. yo Carbowax contains the minim u m amount of substrate necessary t o prevent tailing of the alcohol peaks. Fifty-five grams of packing is sufficient for a 2-meter column. Proportionately larger amounts of Carbowax were used for columns containing 1, 5, and 10% of Carbowax on beads. Silicone G u m Rubber a n d Apiezon L Columns. T h e packings for these columns were prepared as above, in the same concentration. H o t benzene was used for dissolving t h e silicone gum rubber (General Electric 30) a n d ether Silicone gum grease, S.E. for dissolving the Apiezon grease. As t h e resulting packings nere not
c C
I
70
3c
0 I MILLIVOLTS
c
II
I 28
20
24
6
12
8
I
1 4
0
RETENTION T I M E - MINUTES
Figure 2. Typical gas chromatographic recording obtained with silicone gum rubber on microbeads
yo
2-meter column of 0.5 wt. silicone gum rubber on 200-micron glass microbeads; programmed temperature from 55' to 31 5" C. at 9' per minute; H e flow rate 5 0 cc. per minute; 1-pl. sample of alcohol mixture contoining added hydrocarbons
VOL. 34, NO. 4, APRIL 1 9 6 2
473
1 ~
-1. 1'0
-~ 6"
RETE\
2-0
Oh T E l l LR.
LC
LR'
DrG
-
03
~'I
C
Figure 3. Boiling points vs. retention temperatures; Carbowax on microbeads column
tergent and plasticizer fields. Larger amounts of alkanes and alkenes were added to the reaction product t o study their separation for the development of the analytical method. Experimental Conditions. The starting temperature for each r u n was 55" C. I n practice, because of instrumental limitations, this was the lowest temperature a t which t h e column would reach thermal equilibrium and yet be low enough so t h a t the separations were acceptable. I n choosing the temperature programming and helium flow rates, some compromise was necessary. After about 12 different experimental runs, the optimum heating rate found was 9' per minute and the He flow rate was 50 cc. per minute. The He tank pressure (inlet of the gas chromatographic instrument) was 50 p.s.i.g. At this pressure, the constant-differential flow controller of the instrument (Moore Products Go.) maintained the He flow rate constant, within 3%, during each programmed temperature run. Such constancy was necessary for quantitative results. RESULTS A N D DISCUSSION
Typical gas chromatographic recordings obtained with the Carbowax and silicone gum rubber columns appear in Figures 1 and 2. A recording obtained with the Apiezon L column is not shown since it wa. similar t o the recording obtained with the silicone gum rubber column (Figure 2). Although all three columns have the same elution range, namely up through the C24 alcohol, the Carbowas column is superior. This column resolves the close boiling alkanes and alkenes (except for the C6 and Cs compounds) and separates the alcohols up to C,, with little peak tailing. T n o other columns, of 1 and of 7 wt. yo Carbowax 20,000 on Chromosorb R,60- to 80-mesh, 2 meters long, were 474
ANALYTICAL CHEMISTRY
also tested for separating the alcoholhydrocarbon mixture. While their efficiency and selectivity were adequate and in fact better than for the Carbowax on microbead column (Figure l), their application range for the elution of high boiling components was limited due to the larger amount of substrate. The highest alcohol eluted was Ci6. K h e n the amount of substrate was lowered, the 1 n-t. % Carbowas column, the alcohol peaks tailed severely. Selectivity. T h e relative ability of the Carbowax and silicone gum rubber glass microbead columns t o separate close boiling components of different compound type can be compared in Figures 3 and 4 which show the relation of boiling point t o retention temperature. T h e retention temperatures were determined from t h e retention times in Figures 1 a n d 2. T h e boiling points of t h e higher alcohols were estimated from retention d a t a or extrapolated from boiling points taken a t reduced pressure. On the Carbowax eoliimn (Figure 3)) the close boiling components of different coinpound types are separated according to their increasing polarity: alkanes, alkenes, and alcohols, while on the less polar silicone gum rubber column (Figure 4), the separation according to polarity is less completei.r., the close boiling alkenes and alkanps are not resolved. ThP plots of boiling point tis. retention temperature are straight lines except in the lower temperature range in the case of the silicone guin rubber column. Here the lines are curved downward because the starting temperature in the programmed temperature runs was not sufficiently low for this particular column. Also, the elution order is reverscd for the alcohols and hydrocarbons of similar boiling points. This may be due to a difference of temperature effects on the relative contributions of adsorption and solution of the coinponcnts on the column (9). Such an effect is not surprising since silicone gum rubber is sufficiently less polar than Carbowax to produce tailing in the alcohol peaks as seen in Figure 2. The Apiezon column (0.5 wt. yo $piezon L grease on 200-micron glass microbeads) was even less selective than the silicone gum rubber column. Fnrtliermore, the alcohol peaks showed more serious tailing, as Apiezon L is a less polar substrate than silicone gum rubbcr ( I O ) . Efficiency. There are significant differences in t h e efficiency of the Carbo!{-au and silicone gum rubber glass microbead columns when the two columns are compared with each other and with conventional columns of the same substrates. For simplicity, the comparisons were made with constant temperature runs, using
Table I. Comparison of Column Efficiencies (Theoretical Plates) for Four Different Columns (Sample charge 0.3 ~ 1 . ) NO. of Theoretical
Plates
1-
1-
1-Meter Column Hexanol Dodecanol 207, Carbowax on Chromosorb 2110 1400 3%Carbomax on glass microbeads 605 864 207, Silicone gum rubber on Chromosorb 180 106 0.5% Siliconegum rubber on glass microbeads 263 348
0.3 MI. of 1-hexanol and I-dodecanol a t temperatures and helium flow rates under xhich the elution was completed in a reasonably short time with satisfactory peak shapes. I n each case, the efficiency was measured as the number of theoretical plates calculated from peak sharpness and retention time ( 6 ) . The results for 1-meter columns are summarized in Table I from which the following observations can be made. Silicone gum rubber, which is often used as a substrate for the separation of higher alcohols on conventional columns ( 2 , 7 , 8),is less efficient than Carbon-ax on glass microbead columns as well as on conventional coluinns (20 n.t. % on Chromosorb). This may be expected since alcohols as polar components are snmeu-hat adsorbed a t the interface of the nonpolar substrate and support ( 9 ) , thus decreasing the efficiency. This concept may be supported by the unexpected higher efficiency shorn with the column of
320-
lbl; 1"120-
1oc-
I
Figure 4. Boiling points vs. retention temperatures; silicone gum rubber on microbeads column
Table II. Precision and Accuracy Analysis of a Synthetic Alcohol-Hydrocarbon Mixture Average Composition Deviation bY froni Synthesis, Composition by Analysis, JVt. % Mean, Component Wt. % Run 1 Run 2 Run 3 Mean % 16.8 Butanol 17.2 17.0 15.8 16.6 h2.0 Decanol 16.3 15.8 16.8 16.2 zt2.7 15.9 Hexadecanol 16.5 16.8 16.4 16.8 f2.0 17.2 16.7 16.7 16.6 dz0.6 16.8 16.5 Decane Hexadecane 17.3 17.5 17.4 17.5 17.5 f0.2 Tetradecene 15.8 16.2 16.8 16.4 16.1 11.6
hil.’etR
Cc C:REC?S
PER FMOLECLLE
Figure 5. Correction factors [ = (weight per cent)/(area per cent)] for normal alcohols, alkanes, and alkenes as function o f number of carbon atoms per molecule
silicone gum rubber on the less absorbing glass beads as compared to Chromosorb. Another difference between the two column supports, as shown in Table I, is that in the case of conventional columns the efficiency is lon-er for 1dodecanol than i t is for 1-hexanol; on the other hand, in the case of glass microbead columns, the reverse is true-Le., t h e efficiency is higher for 1-dodecanol than it is for 1-hexanol. This difference indicates that lightly coated glass microbe:ad columns may be preferable for the separation of higher alcohols. The increased efficiency for 1-dodecanol results from the lover column temperature required for it. rlution from the glass microbeads column a t conditions which were optimized to provide a reasonable elution time. The glass niicrobead support mag also compare favorably with Teflon. A coating of about 10 n-t. yo of Carboxax on tlir glass microbead support (60- to 80-mesh1 was sufficient to prevent tailing of the m t e r peak. il 1-meter column of such a packing, operated a t 80” C. was equivalent to approvimately 150 theoretical plates when tested with a 0.5-J. sample of methyl isobutyl ketone. Under the same conditions, an equal length of a commercially made column of Carbowav on Teflon was equivalent to approvimately 100 theoretical plates. Correction Factors. Because of t h e wide range of components separated
in each single run, i t was particularly necessary t o determine correction factors for the conversion of peak areas t o weight composition. Several synthetic samples of alcohols, alkanes, a n d alkenes were prepared from pure compounds ranging from butanol to eicosanol and from hexane-hexene to octadecane-octadecene. I n most cases the purity of each compound was better than 99%; however, some compounds, mostly the higher alkanes were available at less than 99% purity and were either purified gas chromakographically (by trapping the pure component in a capillary tube) or their concentration in the synthetic samples was corrected on the basis of peak areas using estimated correction factors. The calculated weight per cent of each component in the synthetic samples was then divided by the corresponding peak area per cent to give the correction factors which are shown graphically in Figure 5 , T h e regular increase of the correction factors with increasing molecular weight in the homologous series is in agreement with data on higher alcohols and some hydrocarbons of Link and Morissette (8),Laramy and Lively ( 7 ) , and Alm et al. ( I ) , but it appears to conflict with data on lower alcohols calculated from response per mole values obtained by Jamieson ( 5 ) . The correction factorr of the higher alcohols were determined with constant as well as XTith programmed temperature runs. The correction factors in Figure 5 can be used for the quantitative analysis of mixtures containing primary n-alcohols, alkanes, and 1-alkenes. T h e peak area of each component is simply multiplied by the correspondin,0‘ correction factor. The corrected peak areas are then normalized in the usual procedure. The factors are relatively independent of any particular sample composition because they were determined from several synthetic samples of different composition. An example
Average Error,
70 -4.1 -0.9 +2.4
-1.3 f1.0 +3.7
of their application is shown in Table I1 which summarizes the analysis of a six-component mixture. The estimated error in the determination of each component is 2 to 4y0relative. ACKNOWLEDGMENT
The author is grateful to the I ~ P O Research and Engineering Co. for permission to publish this nork. Thanks are due to A. A. Austin for furnishing the test samples and to AI, Dooley and G. Kanter for eyierimental work. LITERATURE CITED
(1) Alm, J., Driscoll, J. F., Smith, IT7. R., Gudzinowicz, B. J., Division of Petroleum Chemistry Symposium on Gas Chromatography, 139th Meeting, ACS, St. Louis, Mo., March 1961. (2) Faley, R., Gas Chromatography Symposium, Southwest Regional Meeting, ACS, Oklahoma City, Okla., Dcceniber 1-3, 1960. (3) Frederick, D. H., Cooke, TI‘. D., Eastern Analytical Symposium, Xew York, Xovember 2-4, 1960; Proceedings, Instrument Society of America, International Gas Chromatography Symposium Michigan State University, June 13-16, 1961. (4) Hishta, C., riesserly, J. P., lieschke, R. F., h A L . CHEJZ. 32, 880 (1960). ( 5 ) Jamieson, G. R., J . Chromatog. 3 , 464-70 (1960). (6) Neulemans, A. I. M., “Gas Chromatography,” p. 113, Reinhold, Ken. York, 1947. (7) Laramy, R. E., Lively, L. D., Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, February 27-March 3, 1961. (8) Link, IT. E., Morissette, R. A,, J . Am. Chem. SOC.37, 668-71 (1960). (9) Martin, L., ANAL.CHEM.33, 347-52 (1961). (10) McNair, H. M., Devries, T., Division of Petroleum Chemistry, Symposium on Gas Chromatography, 139th Meeting, ACS, St. Louis, Mo., March 1961. (11) Ziegler, Karl, U. S. Patent 2,892,858 (June 30, 1959). RECEIVED for review September 18, 1961. Accepted December 26, 1961.
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