mechanism for precipitation from homogeneous solution mentioned by Gordon (6). 8-Quinolinol Methods. The quinolinol methods exhibit t h e least interference by coprecipitation of sodium. Potassium behaved approximately the same as sodium. The importance of efficient washing is demonstrated by a few of the runs marked ( b ) (Table I). I n these precipitations the solid phase was difficult to wash, because it had a tendency to float. The supernatant solution could thus not be washed out efficiently, resulting in much higher sodium levels. The amount of sodium coprecipitated by the homogeneous method was only one third of that coprecipitated by the conventional method. Table I11 shows that the floating precipitate is more highly contaminated than the settled precipitate. Again, as in the oxalate method, the first precipitate which appears during precipitation from homogeneous solution contains more sodium than that precipitating later. ils expected, raising the p H by the direct addition of ammonia causes the resulting precipitate to be more highly contaminated. Over the range of time studied, which approximated or exceeded the customary times for the respective analyses, no evidence n a s found for
post precipitation in any of the methods studied. Table IV shows some quantitative data on the accuracy and precision of the method of precipitation of magiiesium-8-quinolinolate from homogeneous solution by the preferred Procedure A as compared to the procedure of Hahn. The precipitates were dried a t 105” C. to the dihydrate, generally the less desirable procedure, because any coprecipitated reagent would not be volatilized a t 105” but would sublime at 150” C. The average error is -0.01 mg. of magnesium as compared to -0.10 mg. for the Hahn procedure. The standard deviations are 0.11 and 0.13 mg., respectively, for the homogeneous and Hahn procedure.
(4) Gordon, L., Caley, E. R., A h 4 ~ . CHEX.20,560 (1918). (5) Gordon, L., Salutsky, 11.E., Willard, H. H., “Precipitation from Homogeneous Solution,” Kilcy, New York, 1959. 16) . . Hahn, F. L., Z. anal. Chem. 86. 153 (1931). ‘ ( 7 ) Hahn, F. L., Vieweg, K., Ibid., 71, 12-30 i1927). (8) Hahn, F. L., Vieweg, K., hIeyer, H., Ber. 60, 971 (1927). (9) Hahn, O., “Applied Radiochemistry,” Cornel1 University Press, Ithaca, N. I-., 14% ^YVI.
W. F., Lundell, G. E. F., “Applied Inorganic Analysis,” 2nd ed.. ed., milev. miles, Kenr York. York, 1953. (11) Kahn,“ Kahn, B., Lyon, W. S., Phys. Rev. (2) 91, 1212 (1953). (12) Kolthoff, Kolthoff, I. M., Sandell, E. B., “Textbook of Quantitative Inorganic Analysis,” 3rd ed., p. 360, hlacmillan, New York. 1952. (13) hIoser, (10) Hillebrand,
ACKNOWLEDGMENT
A. H. ,4.Heyn acknowledges the support of the Atomic Energy Commission during his stay at the Brookhaven National Laboratory as a summer visitor and research collaborator during the summers of 1955 and 1956. LITERATURE CITED
(I) Bernstein, W., Chasr, R. L., Schardt,
A. W., Rev. Sci. Instr. 24, 437 (1953). W. -4..
( 2 ) Chase, R. L., Hirinbotham, ’ Ibid., 23,34 (1952).
(3) Elving, P. J., Caley, E. R., IND.ENG. CHEX, ANAL.ED. 9, 558-62 (1937).
Short-Lived Radioisotopes,” Proceedings of International Conference of Radioisotopes in Scientific Research, 1957. (17) Willard, H. H., ANAL. CHmi. 22, 1x72 (1950). RECEIVEDfor review April 19, 1957. A4ccepted h’ovember 2, 1959. Division of Analrtical Chemistrv, 131st hleeting, ACS, Miami, Fla., -4prii 1957.
Rapid Method for Semiquantitative Determination of Volatile Aldehydes, Ketones, and Acids Flash Exchange Gas Chromatography JACK
W. RALLS
Western Regional Research laboratory, Albany 7 0, Calif.
b A technique of flask exchange gas chromatography was developed for the determination of volatile components of vegetables. Derivatives of volatile compounds are heated with reagents in glass capillary tubes and the products volatilized directly into a gas chromatography unit. 2,4Dinitrophenylhydrazones (2,4-DNPH) are exchanged with excess a-ketoglutaric acid by heating at 250” C. for 10 seconds. Mono-2,4-DNPH prepared from Cz to Cg compounds give rapid regeneration. Bis(2,4-DNPH) are not regenerated. Potassium salts of C1 to Cs monobasic, aliphatic carboxylic acids react with potassium ethyl sulfate a t 300” C. to produce ethyl 332
ANALYTICAL CHEMISTRY
esters. The technique has particular application to the analysis of trace amounts of compounds in large volumes of water. It has a precision of 10 to 20% and requires 10 mg. of mixed derivatives for multiple analyses.
A
problem in the chemistry of vegetable flavor is the isolation and identification of trace constituents present usually in parts per million concentrations in aqueous systems. Considerable progress has been made by using larger quantities of material to obtain enough sample to fractionate and identify. The methods commonly used for fracMAJOR
tionation and identifieation of the trace components-adsorption, partition, paper, and gas chromatography-have limitations especially in the isolation of the material prior to analysis. A more rapid and efficient method was sought by employing gas chromatography Kith the readily available solid derivatives of the volatile compounds (6). After abortive attempts a t pyrolysis of 2,4-dinitrophenylhydrazones (2,4DNPH) and partially successful efforts a t rapid hydrolysis with p-toluenesulfonic acid monohydrate, attention was turned t o the ketoacid exchange reaction (4). The ketoacids previously used for exchange (or hydrolysis) are relatively volatile and/or are unstable at high tem-
peratures. a-Ketoglutaric acid was stable and nonvolatile a t 250" C. for short periods. Furthermore, mixtures of 2,4-DNPH with excess solid a-ketoglutaric acid regenerated the parent carbonyl compounds when heated at 240" to 260" C. for 10 to 15 seconds. The general concepts of this technique should apply to other classes of compounds. The requirements for a useful method are: ready preparation of solid derivatives, a general regeneration reaction n hich is rapid a t 200' to 300" C., and chromatographic columns for efficient separation. The technique is a n alternative t o paper chromatography for identification of small amounts of material but the former is more rapid and has a higher degree of resolution; semiquantitative results are obtained directly. A very old reaction of organic chemistry involves the production of ethyl esters by dry distillation of potassium salts of acids n-ith potassium ethyl sulRCOOK
+ C2HaSOaK
-+
+
RCOOCtHb KzSOd
fate (7, 13). This reaction satisfied the second requirement above and was the basis for the method of determination of volatile C1 to Ce carboxylic acids. APPARATUS
The gas chromatographic unit was a commercial model with a four-filament, thermal conductivity cell and a 1-mv. recorder. The columns were inch X 5 or 10 foot stainless steel packed with silicone (Dow-Corning 550) and stearic acid (15% of the silicone by weight) (25 parts) on firebrick (30 to 60 mesh) (75 parts), or LAC 446 (glycol-adipate polymer) (30 parts) on firebrick (TO parts), or Carbowax 1540 (30 parts) on firebrick (TO parts). Temperature was controlled to 1 1 " C. Gas (helium) flow rate was 32, 46, or 50 ml. per minute with a n inlet pressure of 8 to I 1 p.s.i.g. and outlet pressure atmospheric. The filament current was 250 & 2 ma. MATERIALS
2,4-DSPH were prepared using 2,4dinitrophenylhydrazine (2 grams per liter) in 2.V hydrochloric acid. They n-ere crystallized t o a constant melting point and in all cases agreed with the literature values. The potassium salts were prepared from purchased acids except for dimethylacrylic acid which was synthesized (9). The potassium ethj-1 sulfate was made following the directions of Evans and Albertson ( 3 ) . PROCEDURE
The potassium formate, acetate, and isovalerate were very hygroscopic and were weighed in a low humidity room, The potassium salts were stored over phosphorus pentoxide in desiccators a t room temperature and pressure. The
Table I.
Retention Times and Volumes for Ethyl Esters of Aliphatic Carboxyiic Acids
Helium flow rate, 50 ml. per minute LAC 446, 80" C.a Retention Silicone-Stearic, 80' C. * Retention volume time, uncorrected, Retention Retention Parent Acid min. ml. time volume 1.7 85 2.1 105 Formic 2.1 105 3.7 185 Acetic 3.1 155 6.9 345 Propionic 4.6 230 12.9 645 Butyric 8.2 410 26.0 1300 Valeric 14.0 700 53.6 2680 Caproic Isobutyric 2.9 145 8.5 425 Isovaleric 5.2 255 16.3 815 10.4 520 36.8 1840 Isocaproic Trimethylacetic(pivalic) 2.9 145 10.8 540 -4crylic 3.6 180 6.5 325 Methacrylic 4.8 240 11.8 590 9.9 495 18.2 910 Crotonic Dimethylacrylic 5.7 285 27.3 1365 a Theoretical plates = 500 (ethyl propionate). b Theoretical plates = 1384 (ethyl propionate).
unknown mixtures of potassium salts were equilibrated under identical conditions, so that the same hydrated forms were used as for the knowns. Recommended Procedure. A mixture of potassium salts (or 2,4-DKPH) is weighed ( h 0 . 1 mg.) and mixed with a n equal weight of potassium ethyl sulfate (or 3 parts by m-eight of CYketoglutaric acid). The compounds are ground and mixed on a watch glass with a Nichrome spatula until a uniform mixture results. One end of B borosilicate glass capillary tube, 1.5 mm. in outside diameter X 115 mm. X 0.25 mm. in mall thickness, is sealed and the tube is Reighed. For any set of analysis, the weights of the capillary tubes are selected t o correspond within 1 6 % . The sample (4.1 t o 4.9 mg.) is introduced by filling the open end of the tube, moving the sample t o the closed end by gentle scraping with a triangular file, and packing by light tapping on a wood surface. After weighing the sample, the capillary tube is forced through two '/*-inch thick circular, silicone rubber gaskets until the top gasket is l l j s inchcs from the open end of the tube. The tube is placed in a l/a-inch elongated chamber packing nut with the lower gasket seated against the back face of the nut. The tube is then bent 90' a t the point where it exists from the back of the nut using a microburnw flame for a minimum time. The tube is mounted in the sample injection chamber of the gas chromatographic unit. The nut is tightened until the capillary tube rotates with some difficulty. After the instrument returns to equilibrium (10 t o 15 minutes), the lower part of the capillary tube up to the bend is heated for 10 seconds a t 300" C. (240' t o 260" C. for 10 to 15 seconds for 2,4-DNPH) by a silicone oil (DowCorning 710) bath contained in a 14mm. in outside diameter X 100 mm. borosilicate glass test tube. The temperature of the bath decreases about 10" C. durinp the heating period.
Gas chromatography is used to complete the analysis (10). The accuracy of the technique was determined by the use of prepared mixtures of acetic, propionic, n-butyric, and n-valeric acids. Weighed amounts of the acids were diluted t o 1 liter with water. illiquots were taken, the p H was adjusted to 8.5 by addition of 0.1N potassium hydroxide, and the solutions were evaporated on a steam cone. The potassium salts were dried and treated with a n equal weight of potassium ethyl sulfate. RESULTS AND DISCUSSION
The optimum temperature and heating time for the transesterification reaction were determined by experiment. Temperatures lower than 275" C. gave slow formation of ethyl esters. Interchange mas rapid a t 300" C. A heating period of 10 seconds was required to get reproducible extent of exchange. A 1 to 1 molar ratio of potassium salt to potassium ethyl sulfate was necessary to give maximum exchange with no side reaction. The pure potassium ethyl sulfate undergoes a thermal decomposition a t 300' C. to produce ethylene and ethyl alcohol (8); this interferes with the estimation of acetic acid as ethyl acetate. When a 1 to 1 molar mixture of a potassium salt and potassium ethyl sulfate is heated, only traces of ethylene and ethyl alcohol are produced. The ester exchange is very much faster than the thermal decomposition of potassium ethyl sulfate. The degree of formation of ethyl esters is increased by using a 1 to 2 molar ratio of potassium salt to potassium ethyl sulfate I n samples containing no acetic acid, it may be advantageous to use a 1 to 2 molar ratio to get greater ester formation and to use the ethylene and ethyl alcohol generated to sweep VOL. 32,
NO. 3, MARCH 1960
0
333
higher boiling esters from the capillary tube. A critical sample size of the 1 to 1 molar mixture of potassium salt and potassium ethyl sulfate was found. Samples weighing lcss than 3.5 mg. gave erratic degrees of ester formation. I n the runs with loner drgrees of ester formation, the apparent bulk of the sample expanded only slightly on heating. In samples with high degrees of ester formation, the apparent bulk was increased threefold. When samples of 4.5 f 0.4 mg. were used, the degree of ester formation was uniform and reproducible within the sample weighing error. The retention times and uncorrected retention volumes for ethyl esters formed from the potassium salts by re-
Table II.
action with potassium ethyl sulfate are tabulated in Table I. The values are averages of three determinations. The precision on retention times was =k15 seconds. Several pairs of esters were not separated on the LAC column: butyrate and methacrylate, crotonate and isocaproate, and isobutyrate and trimethyl acetate. Resolution was much better with the silicone-stearic acid column; all 14 esters studied were separated by this column. The transesterification reaction is general for potassium salts of saturated or unsaturated monobasic, aliphatic carboxylic acids. The range of application is limited by the lower degree of volatility of the ethyl esters of higher boiling acids. Salts of acids higher than
Transesterification between Potassium Salts of Acids and Potassium Ethyl Sulfate
Ester Formed Ethyl acetate Ethyl propionate Ethyl n-butyrate
300" C. for 10 seconds Average Peak Area, Sq. Average Retention Vol., Mm./prnole, 1 Mv. Uncorrected, Ml. Full Scale Injected Exchanged Injected Exchanged 26 10 648 173 181 328 339 2325 656 2170 602 623 622
Average Exchange,
%
25 35 36
Table 111. Accuracy of Flash Exchange Gas Chromatography Method for Determination of Mixtures of Volatile Carboxylic Acids as Ethyl Esters Amount of Acid Found, hlg. Parent Acid Iinovn 1 2 3 ilverage
Acetic Propionic n-Butyric n-Valeric Table IV.
5.5 5.1 5.9 6.5
5.6 5.0 6.7 8.7
5.7 5.0 58 7.3
4.4 3.9 5.8 5.1
5.2 4.6 6.1 7.0
Retention Times for Carbonyl Compounds Regenerated from 2,4-DNPH by Exchange with a-Ketoglutaric Acid
10-foot Carbowax, flow rate 32 ml./min., theoretical plates = 1150 (propionaldehyde) Column Retention Temperature, Time, Remarks "C. Min. Carbonyl Compound Aldehydes ... Polymerizes ? 90 Formaldehyde 4.5 90 Acetaldehyde 6.8 90 Propionaldehyde 7.7 90 Isobutyraldehyde 10.4 90 n-Butyralde hyde 13.1 90 Isovaleraldehyde 17.0 90 n-Valeraldehyde 12.0 2- Alethyl-1-butanal 90 9.2 90 Acrolein 26.9, 6 . 9 90, 150 Crotonaldehyde 12.2 150 2,4Pentadienal 12.7 150 2-Hexenal ... Decomposes 150 Methional >32, >20 210" on LAC 150, 210 Benzaldehyde Acetone Butanone 2-Pentanone 3-Pentanone 3-Methyl-2-butanone 2-Hexanone 4-Methyl-3-penten-3-one Cyclohexanone Biacetyl(bis)
334
ANALYTICAL CHEMISTRY
Ketones 90 90
90, 150
90, 150
90 90, 150 150 150 90
7.6 11.7 17.8, 4.6 19.1, 5.3 14.0 30.0. 6 . 6 12.3 21.6
>25
Not regenerated
Gpresent affect the mcthod by acting as sample diluents, producing low responses at longer retention times. The method of choice for analysis of Cs to C20acids is the separation of mrthyl esters on capillary columns with an ionization detector (6). The reaction does not have a stearic hindrance limitation; ethyl trimethyl acetate was formed readily. The only dibasic acid considered was carbonic acid. Heating a 1 to 2 molar mixture of potassium carbonate and potassium ethyl sulfate gave only ethylene and ethyl alcohol. An estimate of the degree of ester formation was obtained by comparing peak areas of esters formed in the flash exchange with injection of the same esters. These data are shown in Table 11. There was good agreement in retention volumes between the two methods of sample introduction. The average extent of regeneration for the three compounds used was 32%. The utility of the method for semiquantitative analysis is illustrated by the data in Table 111. These results were obtained by measuring peak areas for each compound in triplicate samples. Temperatures lower than 250" C. give smaller amounts of regenerated carbonyl in the 2,CDNPH-a-ketoglutaric acid reaction. A competitive thermal decomposition of the 2,4-DNPH takes place at temperatures above 250" C., manifested by the appearance of large post air peaks. This decomposition took place a t less than 250" C. in soft glass capillary tubes. The heating time and temperature required for maximum exchange nith minimum decomposition vary with the composition of the sample and with the wall thickness of the capillary tubes. The optimum conditions for a given sample were found by a series of experiments a t bath temperatures of 245O, 250°, 255", and 260" C. for heating times of 8, 10, 12, and 15 seconds. The best ratio of 2,4-DNPH to aketoglutaric acid was determined in tests where the ratio was varied from 1 to 1 up to 1 t o 10 molar. For saturated aldehydes and ketones, there was no significant difference among the 1 t o 2, 1 t o 3, and 1 to 4 ratios. The 1 t o 10 ratio gave a poor degree of regeneration due to holdup of the volatile portion in the larger melt volume. With acrolein2,4-DNPH zone, the 1 to 3 ratio was much poorer in degree of exchange than the 1 to 5 ratio and the latter was selected for general use. Assuming an average molecular weight of 260 for the 2 , 4 D N P H , the 1 to 5 ratio is about 1 to 3 on a weight basis. Heating pure a-ketoglutaric acid at 250" C for 10 seconds gave no change in the base line of the recorder for a 30minute period. A 1 to 5 molar mixture of 2,4dinitrophenylhydrazone and aketoglutaric acid under the same condi-
AIR 7
I /
90
'
80
70
60
50
40
30
20
0
IO
2 mv. Full Scale Figure 1.
Flash exchange
of 2,4-dinitrophenylhydrazones
tions showed some post air response. The pen was almost back to base line by 4 minutes where the first peak (acetaldehyde on Carbowax column) appeared. The retention times for aldehydes and ketones regenerated from 2,4-DNPH (Table IV) are generally the averages of three determinations. The initial time was taken as the start of the heating period. The exchange reaction takes place smoothly with monoaldehydes and monoketones in the Cz to Ce range. Formaldehyde is either not regenerated or is polymerized under the conditions of the exchange reaction. The conjugate unsaturated aldehydes and ketones are regenerated to a lower degree than the saturated compounds. This is shown by the appearance of smaller peaks from the unsaturated carbonyl derivatives. For example, acrolein has a peak area of 360 sq. mm. per pmole compared to 1520 for acetaldehyde on a 1-mv. full scale recording. Injected samples of acrolein and acetaldehyde give approximately the same peak area per micromole values. Aromatic aldehydes (benzaldehyde) and polycarbonyl compounds (biacetyl) are not regenerated sufficiently t o give a recorded response. The method has definite advantages over paper chromatography. The difficultly separable (by paper chromatography) pair of butanone and nbutyraldehyde (as 2,4-DNPH) are cleanly resolved by gas chromatography as the free carbonyl compounds. The retention times of 10.4 minutes for n-butyraldehyde and 11.7 for butanone produce tn o distinct peaks when a mixture of the two corresponding 2,4-DNPH is exchanged. A similar resolution is obtained n-ith the pentanones and isovaleraldehyde. Mixtures containing several components are separated readily. KO evidence was found for the intcmction of regenerated
compounds under the rather rigorous conditions of the flash exchange reaction. Figure 1 shows a recording of the response from the products regenerated from a mixture of the 2,4-DNPH of acetaldehyde (0.38 mg.), propionaldehyde (0.36), acetone (0.21), and nbutyraldehyde (0.34) Rith 3.8 mg. of uketoglutaric acid. I n every case examined, the retention times of the material regenerated from the derivative by the exchange reaction were the same, within experimental error, as those of the pure parent compound. As a n added proof of the validity of the method, the exit gas from the exchange reaction of propionaldehyde-2,CDNPH with &-ketoglutaric acid was passed into a 2,4-dinitrophenylhydrazine trap. The solid which precipitated was collected by centrifugation, washed with 2N hydrochloric acid, with water, and dried. The infrared spectrum of this material was identical to that of a purified sample of propionaldehyde-2,4-DNPH. In addition to confirming the identity of the regenerated carbonyl compound, this technique can be used to characterize unknown materials. As a further demonstration of the actual regeneration of the parent carbonyl compounds from 2,4-DNPH by the flash exchange, the retention volumes of three injected compounds were com-
Table V.
Comparison
pared m ith the regeneration of the same compounds from their derivatives in the exchange reaction, These data are shown in Table V. The initial times for the two methods are not identical because of the different n a y of introducing the sample. The values for the retention volumes are in good agreement. Comparison of the peak areas of the injected and regenerated sample gives a measure of the extent of the regeneration in the flash exchange; the average extent of regeneration was 56%. A final demonstration of the utility of the technique was based on work in these laboratories on the volatile carbonyl compounds of fresh onions. Carson and Wong have fractionated the 2,4D N P H from onions by silica gel chromatography and fractional crystallization and have identified acetaldehyde, propionaldehyde, acetone, n-butyraldehyde, and butanone-2,4-DNPH by crystallographic examination ( 2 ) . A sample of the onion-2,4-DKPH provided by Carson and Rong was run in flash exchange technique and the same five carbonyl compounds were found. This technique is useful for the analysis of the highly volatile aldehydes, ketones, and acids of vegetables with the exception of formaldehyde and biacetyl. Fortunately, excellent, specific methods for determining formaldehyde ( I ) and biacetyl (If, 14) are available. The application of this technique to the determination of flavor components of vegetables is also reported (12). The method could be refined to a point where it might serve as a quantitative technique. Observations made during the development of the technique to its present state indicate the following modifications would increase the accuracy : use of analytical microbalance for sample weighing, use of uniform wall thickness capillary tubes, use of sizing screens to provide uniform particle size, more precise temperature and time control in flash heating, and standardized packing of the sample in the capillary tube. ACKNOWLEDGMENT
The author is indebted to John F. Carson for stimulating discussions. The 2,4-DNPH of 2-hexen-1-a1 and 2,4pentadien-1-a1 were provided by E. L.
of Injected and Exchange Regenerated Carbonyl
Compounds [!%foot LAC 446, 90' C., flow rate (He) 46 ml. per min., theoretical plates = 452 (propionaldehyde)]
Carbonyl Compound Acetone Propionaldehyde n-But yraldehyde
RegenRetention Volumes Peak AreaE, Sq. blm./pmole eration, Injected Exchanged Injected Exchanged % 120 147 207
129 161 216
2264 2250 2616
1290 1268 1464
VOL. 32, NO. 3, MARCH 1960
57 56 56
335
‘‘
Pippen and Masahide Konaka, and Others by The infrared determinations made G. F. Bailey and E. L. Gong.
w’
LITERATURE CITED
(1) Bricker, C. E., Johnson, H. R., IND.
ENG.CHEM.,ANAL.ED. 17,400 (1945).
( 2 ) Carson, J. F., Jr., Wong, F. F.,
Western Regional Research Laboratory, Albany, Calif., private communication, May 1959. (3) Evans, P. N., Albertson, J. hl., J . Am. Chem. SOC.39, 456 (1917). (4) Keeney, M., ANAL. CHEM.29, 1489 (1957). (5) Lipsky, S. R., Lovelock, J. E., Land-
owne, R. A., J . Am. Chern. SOC.81, 1010 (1959). (6) Little, A. D., Inc., “Flavor Research and Food Acceptance,” pp. 290, 317, Reinhold, New York, 1958. (7) Meyer, H., Monalsh. Chem. 15, 164 (1894). (8) Nef, J., Ann. Chem. Liebigs 318, 40 (1901). (9) “Organic Syntheses,” Vol. 23, p. 27, Wiley, New York, 1946. (10) Phillips, C., “Gas Chromatography,” Academic Press, New York, 1956. (11) Ralls, J. W., J . Agr. Food Chem. 7, 505 (1959). (12) Ibid., 8, in press (1960). (13) Rodd, E. H., “Chemistry of the Carbon Compound,” Vol. IA, p. 583, Elsevier, Il’ew York, 1951.
(14) Westerfeld, R. W.,Jr., J . Bid. Chem. 161, 495 (1945). RECEIVEDfor review April 20, 1958. Accepted November 2, 1959. A laboratory of the Western Utilization Research and Development Division, Agricultural Research Service, U. S. Department of Agriculture. The author, as collaborator, is employed by the National Canners Association with which this work was conducted cooperatively. Presented in part before the Division of Agricultural and Food Chemistry, 135th Meeting, ACS, Boston, Mass., April 1959. Mention of specific products does not imply endorsement by the Department of Agriculture over others of a similar nature not mentioned.
Gas Chromatographic Analysis of Olefinic Naphthas in the Three- to Six-Carbon Range With the Aid of a Subtraction Technique RONALD L. MARTIN Research and Development Department, Standard Oil
b Analysis of olefinic naphthas for individual components is exceedingly difficult because of the many closeboiling compounds present. A new gas chromatographic technique, involving successive runs in the presence and absence of olefins, greatly simplifies the problem. In one run, olefins are removed with a short absorber containing silica gel coated with sulfuric acid following the chromatographic column; saturated hydrocarbons pass quantitatively through the sulfuric acid absorber and are determined directly. The other run is made without the absorber, and the olefins appearing in the same peaks with saturates are determined by difference. Three typical olefinic naphthas demonstrate wide variation in components of three to six carbons.
N
containing unsaturated as well as saturated hydrocarbons are common in the petroleum industry. Determination of individual components in such olefinic naphthas is exceedingly difficult because of the many closeboiling compounds present. Distillation techniques (3, IO) were valuable when there was no other approach, but are time-consuming and difficult. Conventional gas chromatographic techniques have been of little value beyond the five-carbon region for the analysis of olefinic naphthas. Polar and phases stationary phases (f,6,8,12) 336
APHTHAS
ANALYTICAL CHEMISTRY
Co. (Indiana),
Whiting, Ind.
containing silver nitrate (a, IS) have been used to retain olefins for longer times than saturated hydrocarbons of like boiling point; however, above five carbons, the retarded olefins are usually eluted with saturated hydrocarbons of higher boiling point. The only reported method for determining olefins through six carbons (7) requires that saturated hydrocarbons be absent. A different approach was used in a recently reported method for gaseous olefins and saturates through five carbons (6);olefins were removed by an absorption precolumn, and saturates were determined before and after hydrogenation. The absorption column was about 5 feet long and contained silver sulfate and sulfuric acid on kieselguhr. Disadvantages of this method are that all olefins with the same carbon skeleton are lumped together, and determination above five carbons is almost impossible because of the extreme tailing from the absorption column. A somewhat similar approach has been used in developing a simple technique that aids in the analysis of olefinic naphthas for components of three to six or more carbons. The technique is based on the removal of olefins by a n exceptionally short absorber containing silica gel coated with sulfuric acid following the chromatographic column. The olefins are absorbed as a result of the addition reaction of sulfuric acid to olefins. Saturated hydrocarbons pass through quantitatively and are deter-
mined by difference from a chromatogram obtained with them present. Analyses of three typical naphthas for components of three to six carbons show widely different isomer distributions. EXPERIMENTAL
Conventional gas chromatographic equipment was employed except for a gas density balance of new and simplified design (11), developed in these laboratories. With this detector, calibration of output was not necessary. The chromatographic column was made of copper tubing and packed with causticwashed firebrick of 35 t o 48 mesh, carrying 10% isoyuinoline as the stationary phase. Nitrogen was ased as the eluting gas. Absorber. The absorbing mixture was prepared by shaking together 3 parts by weight of concentrated sulfuric acid and 2 parts of Davison Grade TO silica gel of 60 to 200 mesh. By completely covering the surface of the silica gel with t h e large amount of acid, adsorption on the silica gel and the resulting tailing of peaks was eliminated. An absorber 20 mm. long and 4 mm. in diameter, operated a t 20’ to 50’ C., quantitatively removed mono-olefins, diolefins, cyclo-olefins, and acetylenes through at least eight carbons, except for ethylene and acetylene. Little ethylene or acetylene is present in olefinic naphthas, but if they must be determined, a n absorber containing sulfuric acid saturated with silver sulfate can be used. Although sulfuric acid react* ~ t h
