p. 30, R. P. W.Scott, ed., Butterworths, 1960. (8) Giddings, J . C., J . Chromatog. 3, 520 (1960). (9) Glueckauf, E., Analyst 77, 931 (1952). (10) Golay, M. J. E., “Gas Chromatography,” p. 36, D. H. Desty, ed., ileademic Press, Sew Tork, 1958. (11) Golay, M. J. E.. “Gas Chromatography,” p. 315, V. J. Coates et al., eds., Academic Press, Sew York, 1958.
(12) James, .4. T., Martin, a. J. P., Analyst 77, 915 (1952). (13) Jones, W. L., .;ira~.CHEM. 33, 829 (1961). (14) Kieaelbach, R.,Zbid.. 33, 23 (1961). (15) Ibid., p. 806. (16) Kieselbach, R., private communication. (17) Litt,lmood, A. B., “Gas Chromatography, p. 22, D. H. Desty, ed., Academic Press, Sew York, 1958.
(18) Purnell, J. H., J . Chem. SOC.1960, 1268. (19) Safranski, L. W., Dal Nogare. S.. Chem. Eng. Xews 39, 102 (1961). 120) Scott, R. P. W., Hazeldean, G. S. F., “Gas Chromatography,” p. 144, R. P. W. Scott, ed.. Butteraorthe. London, 1960. RECEIVED for review February 19, 1962. Accepted -4pril 24, 1962.
Determination of Hydrocarbon Types in Gasoline by Gas Chromatography RONALD L. MARTIN Research and Development Department, American Oil Co., Whiting, lnd.
b Gas chromatography provides a new approach to hydrocarbon-type analysis. Two separation steps, combined in a single 30-minute gas chromatographic run, resolve gasolines into aromatics, olefins, and saturates. In the first step, aromatics are separated from saturates plus olefins with a P,fl’-thiodipropionitrile column, and saturates plus olefins are measured as they emerge; the column is then backflushed to remove and measure the aromatics. In the second step, olefins are separated from saturates in the column effluent by reaction with mercuric perchlorate. The saturates pass unaltered through the mercuric perchlorate, are collected in a liquid nitrogen trap, sent back through the column, and measured again. Olefins are determined b y difference between the chart areas for saturates plus olefins and for saturates alone. Analyses of gasolines and naphthas by the new method check well with those by liquid chromatography and mass spectrometry. High accuracy was attained on known synthetics. The new method does not require removal of components with five or less carbon atoms, as do the conventional methods. Elimination of the depentanization step permits direct analysis of full range gasolines.
A
of gasolines for three major hydrocarbon types-aromatics, olefins, and saturates-is often required in petroleum laboratories. Such analyses are used to determine product quality and to guide refining processes. Three methods are commonly used: the fluorescent indicator adsorption (FIA) method (2, 6 ) , mass spectrometry (3, 4, IO), and chemical methods, typified by ASThI method D-875 ( 1 ) . I n the FIA method, the sample is chromatographed into the three hydroKALYSIS
896
ANALYTICAL CHEMISTRY
FLOW P d T T E R N I
Figure 1.
1
FLOW P I T T E R N 2
1
Analysis scheme
carbon types on silica gel; two fluorescent dyes make the three hydrocarbon zones distinguishable under ultraviolet light. This method is simple to perform and has been applied directly to samples boiling as high as 600’ F. Accuracy usually is satisfactory, but rests on the assumption that the unavoidable overlap a t the color boundaries is equal for each type. Mass spectrometric methods are of two kinds. K i t h the older method (a, 4,two runs and a treatment n i t h acid for removal of olefins are required. I n a recent method ( I O ) , parent-ion and fragment-ion spectra are combined to give a faster analysis. Mass spectrornetry can give more information than the other methods; saturates and olefins can be divided into subgroups, and aromatics and olefins determined by carbon number. Disadvantages are espensive equipment and tedious calibrations. Bccuracy usually is good but depends heavily on calibrations with the type of sample to be analyzed. In chemical method D-875 (I), aromatics and olefins are determined together by reaction with sulfuric acid, and olefins are determined separately by reaction with bromine. The method is less popular than the others because it is long and the olefin determination is often in error.
A11 three methods suffer from the same disadvantage in the analysis of gasolines; for reliable results, components through four or five carbon atoms must be removed before analysis. With the FIA method, the sample should be depentanized and the lower boiling fraction analyzed separately for saturates and olefins. K i t h the mass spectrometric methods and the chemical method, debutanization is needed. =1 gas chromatographic method that would not be subject to the disadvantages of the other methods has been developed. The low boiling components need not he removed, and the analysis is fast and easily performed. PRINCIPLE OF METHOD
Gasolines are separated into aromatics, olefins, and saturates using two separate steps combined in a single run. Aromatics are separated from saturates and olefins with a &?’-thiodipropjonitrjle column; olefins are then separated from saturates by reaction 17ith mercuric perchlorate (5, 8, 1 4 ) . iichieving both separations in a single run requires a liquid nitrogen trap for storage and a system for changing the direction of carrier gas flow. The scheme is illustrated in Figure 1. This figure shows the apparatus components and the order in which they are arranged in each of two flow patterns. The sample is added with the apparatus in flow pattern 1. A11 saturates and olefins are eluted from the column together in one large unsymmetrical peak before the Ion-est boiling aromatic, benzene. The saturates and olefins are detected as they emerge from the column. After passing through the detector, the olefins are caught by the mercuric-perchlorate absorber; the saturateq pass through the absorber and are collected in the liquid nitrogen trap.
After the saturates and olefins have left the column but before benzene emerges, the gas flow is changed to flow pattern 2. This backflushes the aromatics from the column in one peak. Finally, the liquid nitrogen trap is warmed, and the saturates return through the column and are now detected alone. Figure 2 shows a typical chromatogram. The saturates and aromatics are determined directly from their chart areas; olefins are determined by difference between the areas for saturates plus olefins and for saturates alone. SPECIAL FEATURES
OF METHOD
The column, the absorber, the liquid nitrogen trap, and the detector all have unusual demands placed upon them. The column must selectively retain aromatics and yet not bleed; the absorber must trap all olefins but none of the saturates; the liquid nitrogen trap must collect the saturates quantitatively; and the detector must give a response for each hydrocarbon type t h a t is convertible to per cent by liquid volume. Column. Many liquid phases were tested for separation of aromatics from saturates and olefins, b u t only P,P'-thiodipropionitrile satisfied both the selectivity a n d stability requirements. T h e nitrile bleeds slightly b u t n o t seriously a t t h e 115" C. operating temperature. 1,2,3-tris(2cyanoeth0xy)propane ( I S ) showed the vime selectivity as the nitrile, but bled more. N,S-bis(2-cyanoethyl)formamide, reported recently ( 9 ) , may also have the selectivity and stability required. The selectivity requirement is stringent. Gasoline has an end point as high as 400" F. ; therefore, saturates and olefins through as many as 11 carbon atoms must be eluted from the A li-foot column before benzene. column containing 38y0 thiodipropionitrile on Chromosorb, operated a t 115" C., accomplishes these separations ; this is shown in Table I -:1 the retention times relative to benzene of several saturates and olefins. %-Paraffins boiling below 400" F. are eluted well before benzene; olefins beloiv 400' F. also are eluted before benzene, but the margin is smaller. Substituted cyclic paraffins and olefins were not available for study; the highest boiling ones might not fully elute before benzene, but fortunately they are most often absent in gasoline. Saturates and olefins in gasolines with end points well above 400" F. would not be fully eluted before benzene. I n this uncommon case, the change in flow pattern can be detained until just before toluene emerges. Retention of toluene is 1.5 times t h a t of benzene, which allows paraffins boiling to about 510' F. and olefins to 460' F. to he eluted before
I 0
,
- - __ - 5
- - -Y-
J 10
I5
~
20
25
x1
MINUTES
Figure 2. Chromatogram separation by types
showing
backflushing is begun. The analysis is then continued in normal fashion, except that the area of the benzene peak is measured and added to that of the aromatics peak. The benzene peak is easily distinguishable on the tail of the saturates and olefins. 9 column containing 38 + 395 nitrile m s chosen after observing that columns with less liquid phase did not retain benzene past all of the saturates and olefins in gasoline. Retention of benzene relative to saturates and olefins increases greatly with the amount of nitrile. This change in relative retention likely results from a changing contribution of solute adsorption on the surface of the liquid phase (12). Amounts much above 38% are not practical because the packing becomes \vet and column efficiency drops. The column temperature of 115' C. keeps the hydrocarbons from condensing, gives short elution times, and still allows satisfactory separations. Absorber. Solid mercuric perchlorate on Chromosorb has been used pre\-iously in mass spectrometry ( 5 ) and gas chromatography (8,14) to remove olefins and acetylenes. It does not react with saturates b u t holds unsaturates as complexes of mercuric ions and pi electrons ( 5 ); oxidation by perchlorate probably follows. To remove all olefins with mercuric perchlorate, the operating temperature and the reagent composition had to be changed from those ordinarily used ( 6 ) . The absorber temperature is maintained a t 60" C., instead of 100" C., and mercuric perchlorate is fortified with perchloric acid. All olefins in gasoline, including ethylene, are retained indefinitely when the absorber is maintained a t 60" C., whereas a t 100" C., many low boiling olefins are eluted after only a few minutes. The 60" C. temperature is adequate to move the saturates through the absorber to the liquid nitrogen trap. Perchloric acid minimizes the formation of interfering volatile reaction products. These reaction products, saturated hydrocarbons of one to seven carbon atoms, are formed b y the reaction of certain olefins with material absorbed from previous runs. Many olefins do not enter into this type of
reaction; lion ever. others yieltl 0.1 to 27, of these products when perchloric acid is not present. Propylene is the worst; up to ten saturate peaks n i t h areas equivalent to 20Y0of the propylene have been observed. The formation of these products is reduced about %old by including a 50% excess of perchloric acid with the mercuric perchlorate. The life of the absorber is increased by keeping it a t room temperature, except during run'. To ensure complete olefin removal. it is advisable t o install freqh absorber packing e>ery second day, regardlws of the number of samples added. Trap. T h e liquid nitrogen t r a p must collect and store all saturates passing the absorber. An empty t r a p or even one packed with glass no01 allows some saturates t o pass; however, all saturates except methane are collected quantitatively T\ hen t h e t r a p is packed with 0.2 gram of Chromosorb coated with 2%) of General Electric SE-30 silicone rubber. Detector. H y d r o c a r b o n - t y p e analyses conventionally are reported in terms of per cent by liquid volume for each hydrocarbon type. Kone of the gas chromatographic detectors, honever, gives a response t h a t is exactly proportional t o liquid volume for all hydrocarbons. Fortunately, thermal conductility detectors are acceptable; they give a reiponse nearly
Table I.
carbon Tuniber 10 11 12 13
Relative Retention Times
n-Paraffins n-Olefins RetenReten!.p., tion B.p., tion F. times" F. times" 339 0 55 345 0 38 O
385 421 456
0 51 0 69 0 94
379 416
0 77 1 02
* Retention times relative to benzene =
1.00. Table II. Thermal Conductivity Response for Synthetic Blends
(5-pI. sample of each blend) Average ('arbon Car- RelaSumbon tive Hydrocarbon her Num- ReType Range ber sponse n-Paraffins 5-11 6.1 100
Branched paraffins Cycloparaffins n-Olefins .4romatics
5-11 5-11 5--11
7.2 8.4 3.3
99 97 95
5-9 5-8 5-10 5-10 5-10 6-10 GlO 6-10
6.6 6.6 6 1 7 3 8 4 6.5 8.0 9.3
95 100 101 97 93 115 111 1Oi
VOL. 34, NO. 8, JULY 1962
897
proportional to liquid volume for the members within each of the three hydrocarbon types. Measurements of response for 12 synthetic blends are given in Table 11. These data were obtained by measuring peak areas for a 5.0-p1. sample of each of the blends. The blends contained members of a single hydrocarbon type with average carbon numbers from 6.1 to 9.3. The response from all blends containing the same hydrocarbon type varies only a few per cent between the evtremes of average carbon number. For the saturates, the response varies only a fen- per cent among n-paraffins, branched paraffins, and cyrloparaffins. Becau3e these differences are relatively small, the thermal conductivity response is taken as a direct measure of per cent by volume for all members within each hydrocarbon type. The error thus introduced usually is small-especially with gasolines-because composition stays relatively constant within each hydrocarbon type. The aggregate chart area for each hydrocarbon type, hoii-ever, requires a thermal conductivity factor for conversion to liquid volume present. With the knon-ledge that the average carbon numbers of the saturate, olefin, and aromatic fractions of gasolines average, respectively, about 7, 6.5, and 8, factors of 1.00, 1.00. and 0.87 were calculated from the data in Table 11. These factors can be used with all full range gasolines, because they are relatively insensitive to the differences in composition normally encountered. For samples of unusual composition or where
Table 111.
IeaOllTI
1I.P
3.
Schematic
high accuracy is desired, special factors could be determined. DETAILS
OF METHOD
All apparatus components, except the absorber and the trap, are housed in a n oven operated a t 115" C. The oven is supported on legs so the absorber and trap, which extend below, are easily accessible. The sample introduction zone consists of a silicone disk in a '/r-inch Swagelok compression nut that extends through the oven wall. The column is 17 feet of '/(-inch copper tubing packed with 38% by weight of P,P'-thiodipropionitrile on 28to 35-mesh acid-washed Chromosorb. Support of large particle size is used to minimize pressure drop. The nitrile was deposited on the Chromosorb from benzene solution. To prepare the absorber packing, 100 grams of 28- to 35-mesh Chromosorb is mixed thoroughly with a solution composed of 60 ml. of water, 19 ml. of 70% perchloric acid, and 18 grams
Analyses of Seven Naphthas b y Gas Chromatography, FIA, and Mass Spectrometry
Gas Chromatography 49.0 f 0.7b 30.0 f O.gb 21.0 f 0 . 3 b 52.7 26.9 20.4 61.2 f 0.6b 10.0 f 0 . 7 b 28.8 =k 0 . 4 b 39.5 f 0.4b 1.1f 0.6b 59.4 f 0.5b 8.5 91 .o 0.5 5.8 0.0 94.2 91.7
Mass Spectrometry 49.8 32.0 18.2 54.5 29.9 15.6 59.0 9.7 31.3 37.1 1.3 61.6
FIA" Types 51.4 Saturates 27.0 Olefins 21.6 hromatics 55.5 Commercial Saturates 24.5 gasoline B Olefins 20.0 Aromatics 62.5 Saturates Commercial 9.5 Olefins gasoline C 28.0 iZromatics Experimental Saturates 39.7 0.0 Olefins gasoline 60.3 rlromatics Saturates 6.0 9.8 Heptene 94.0 90.2 polymer Olefins 0.0 0.0 Aromatics Saturates 6.0 7.8 Catalytic 0.0 0.0 Olefins reformate 94.0 92.2 Aromatics Saturates 94.0 86.6 Virgin 0.0 0.0 5.1 Olefins naphtha rZromatics 8.3 6.0 8.3 a FI.4 result on material boiling above 120" F. combined with gas chromatographic component analysis on material boiling below 120" F. Standard deviation compiled from eight replicate determinations.
Sample Commercial gasoline A
898
ANALYTICAL CHEMISTRY
LeaOIBC"
FLOW PATTERN 2
FLOW PATTERN I
Figure
7l.P
valving
diagram
of mercuric oxide. The wet brick is spread thinly on n atch glasses and dried for l'/z hours at about 92" C. The packing is usable indefinitely if stored in a closed container. The absorber tube, 8 inches of 9-mm. glass tubing, is packed with 3 inches of mercuric perchlorate on Chromosorb followed b y 2 inches of anhydrous magnesium perchlorate, 2 inches of Ascarite, and 1 inch more of magnesium perchlorate. The magnesium perchlorate and ilscarite are added to catch the mater and carbon dioxide that are slowly released by the absorber. An 8-inch piece of '/4-inch 0.d. Teflon tubing is used for the liquid nitrogen trap, The Teflon tubing minimizes heat conduction between the oven and the liquid nitrogen, and seals we11 with compression fittings. A thermal conductivity cell of the filament type was used for detection. It was operated with a conventional bridge system and 1-mv. recorder. The change in flow patterns is accomplished with two Circle Seal 8-port valves, the handles of which project through one side of the oven. The apparatus is assembled as shonn in Figure 3 ; valve positions are shown for both flow patterns. The gas flows are indicated by the darkened areas. The arrows indicate the direction of flow. Each 8-port valve consists of two 4-way valves operated n-ith a single stem and is so depicted. The apparatus must be built free of leaks. T o perform the analysis, the oven is brought to 115" C. and maintained to 0.5" C. The t n o valves are positioned to give flow pattern 1. The helium flow rate is adjusted to about 100 cc. per minute. A beaker containing oil at 60" C. is positioned around the absorber tube, and a Dewar flask containing liquid nitrogen is positioned around the trap. About 6 pl. of sample are added by syringe. Elution is continued in flow pattern 1 until just before benzene is to be eluted, then the two valves are positioned to give flow pattern 2. The Aow rates before and after the valve switch should not differ by more than 1 cc. per minute. After the aromatic peak is eluted, the liquid nitrogen is removed from the trap and a beaker containing oil at about 140" C. is positioned around it. Flow is continued until the saturates are eluted. Chart areas for saturates plus olefins, aromatics, and saturates alone are
measured. The area for saturates is subtracted from saturates plus olefins to determine olefins area. The areas are multiplied b y thermal conductivity factors and normalized to 100%. ANALYTICAL RESULTS
Accuracy was determined by analyzing two synthetic blends, each containing about 50 pure grade hydrocarbons. ‘The blends were prepared to simulate gasoline as closely as possible. The analyses, as averages of three runs, and the prepared compositions n-ere : Blend 1, Prepared Found Saturates 49 7 49 9 Olefins 22 7 22 4 Aromatics 27 6 27 7
Blend Prepared 41 4 34 3 24 3
2.
5
Found 41 0 34 6 24 4
The difference betn een the prepared and found values average about 1% of the amount found. The excellent accuracy indicated by these analyses is higher than nould be evpected for routine analyses w t h this method. As a further indic:ition of accuracy, seven naphthas oi widely differing conipositions were analyzed by gas chromatography, FIA, and mass spectrometry-each in duplicate. The analyses by the thrc>e methods, given in Table 111, show relatively good agreement. Actually, the gas chromatographic method agrecs more closely with
either of the other two methods than the other tn-o agree between themselves. These analyses of naphthas of widely differing compositions demonstrate the broad applicability of the gas chromatographic method. The standard deviations average about 0.670 absolute; they are highest for olefins and lowest for aromatics. The deviations are about those expected from measurement of peak areas. CONCLUSIONS
Advantages of the new method are short analysis times and applicability to full range gasolines and other hydrocarbon mixtures containing any amount of low boiling hydrocarbons. Disadvantages are the determination of olefins by difference. the limitation to naphthas with end points not far above 400’ F., and the failure to separate saturates into paraffins and naphthenes. Several evtensions of the method are apparent. One is to divide the saturate fraction further. %-Paraffins could be separated from branched paraffins and naphthenes with 31olecular Sieves (’7). Separation of naphthenes from paraffins by selective dehydrogenation of naphthenes ( 2 1 , l4j might also be possible. The present gas chromatographic method could be simplified and improved if an absorber were available that would remove aromatics from sat-
urates plus olefins. The nitrile column then would not be needed. The method also could be improved if a n olefin absorber were available that could be made to release olefins quantitatively for direct determination. LITERATURE CITED
(1) Am. SOC.Testing Materials, “ASTM Standards, 1961,” Method D-875-60T,
Part 7,p. 361. (2) Ibid., Method D-1319-61T, Part 7, p. 702. (3) Ibid., “ASTM Standards on Petroleum Products and Lubricants,” p. 1128, 1961. (4) Brown, R. A , A s ~ L .CHEM.23, 430 (1951). (5) Coulson, D. >I., Ihzd., 31,906 (1959). (6) Criddle, D. IT., LeTourneau, R. A., Ibid., 23, 1620 (1951). (7) Eggertsen, F. T., Groennings, S., Ibid., 33, 1147 (1961). (8) Ferrin, C. R., Chase, J. O., Hurn, R. W.,Intern. Gas Chroniatography Symp., Michigan State Cniversity, June 1961. (9) Frisone, G. J., ,Vature 193,370 (1962). (10) Frisque, A. J., Grubb, H. M., Ehrhardt, C. H., T’ander Haar, R. W., ANAL.CHEM.33, 389 (1961). (11) Keulemans, A. I. M., Voge, H. H., J . Phys. Chem. 63,476 (1959 ). (12) Martin, R. L., AXAL. CHEW 33, 347 (1961). (13) McNair, H. hl., DeVries, T., Ibid., 33, 806 (1961). (14) Rowan, R., Ibid., 33, 658 (1961). RECEIVEDfor review January 31, 1962. Accepted Bpril 20, 1062. Division of Analytical Chemistry, l i l s t Meeting, A4CS,JT7ashington, 11. C.. March 1062.
Identification of Substituted Barbituric Acids by Gas Chromato g ra phy of The ir Pyrolysis Produc ts DONALD F. NELSON‘ and PAUL L. KIRK School o f Criminology, University o f California, Berkeley, Calif
b The pyrolysis products of 27 substituted barbituric acids, which may be used therapeutically, have been studied with the Pye argon gas chromatograph. Free acids, their sodium salts, and mixtures of free acid with anhydrous potassium carbonate were compared. Pyrolysis offers a practical method of identifying barbiturates.
T
HE retention time of a barbiturate, measured under specified gas-chromatographic conditions, has been shown (4) to be a useful addition to the list of properties which are available for its identification. By combining pyrolysis with gas chromatography, each one of a
On leave from the Dominion Laboratory, X e n Zealand.
group of substituted barbituric acids has been made to yield, under given conditions, a reproducible succession of peaks whose retention times and relative sizes form a recognizable pattern and which, when considered together, approach the requirements for individualization. Study of the pyrolysis products of organic substances by gas chromatography has been reported by a number of workers, including Davison, Slaney, and Wragg ( I ) , who suggested it as a method for the identification of polymers. Janak ( 2 , s) described the apparatus and procedure by which he pyrolyzed substances in the carrier gas stream so that the products immediately entered the column. H e recommended its use for the identification of organic substances, and described the
results of pyrolyzing certain natural products such as fats, proteins, and alkaloids and synthetic compounds such as amino acids and barbiturates. He found quantities of sample below 0.1 mg. sufficient for analysis. From the study of 14 barbiturates he reported t h a t fragments of the molecules present are always highly specific, both qualitatively and quantitatively. I n the work here presented the pyrolysis products of 27 substituted barbituric acids were separated by gas chromatography. EXPERIMENTAL
Procedure a n d Apparatus. T h e barbiturates which a r e t h e subject of this study a r e listed in Table I, together with common synonyms, t h e VOL. 34, NO. 8, JULY 1962
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