Identification of Hydrocarbon Peaks in Gas Chromatography by Sequential Application of Class Reactions ROBERT ROWAN, Jr. Analytical Research Division, Esso Research a d Engineering Co., P. 0. Box 121, liden, N. 1.
b In the identification of components separated by gas chromatography, a knowledge of the compound class greatly increases the effectiveness of the usual aids. By proper application of class reactions, peaks from a complex hydrocarbon mixture can b e identified as n-oleflns, iso-olefins, nparaffins, isoparaffins, naphthenes, or aromatics. The method depends upon the use of two series of class reactions in different sequence. Reactions employed include absorption of oleflns or olefins plus aromatics, hydrogenation of olefins, dehydrogenation of naphthenes, and absorption of straightchain compounds. These operations are carried out rapidly and conveniently in a closed system which is an integral part of the gas chromatographic apparatus. The method has been demonstrated on a synthetic sample and a commercial gasoline. All peaks were identified by hydrocarbon class and over half by name. In many cases hidden peaks were revealed,
T
(individual components) is an important part of the over-all problem of gas chromatographic (GC) analysis of complex mixtures. To take full advantage of recent advances which have resulted in greatly improved sensitivity, resolution, selectivity, and over-all separating power, the ability to identify individual components must be improved. The problem is especially acute in the higher boiling ranges, where mixtures tend to be more complex. In this study the emphasis haa been on the identification problem in connection with natural hydrocarbon mixtures, where only a few important classes of compounds are normally present. Some new techniques for the identification of peaks according to hydrocarbon type have been combined into a general scheme of analysis. The development of the scheme and its successful application to a mixture of practical importance are described. The processes employed aa diagnostic aids are based on chemical or physical absorption and on catalytic hydrogenation and dehydrogenation. These clasa HE IDENTIFICATION OF PEAKS
658
0
ANALYTICAL CHEMISTRY
reactions are as follows: absorption of aromatics and olefins in H&IOl; absorption of normal compounds by Molecular Sieve; removal of olefin by mercuric perchlorate; hydrogenation of aromatics and olefins, or of olefins alone; and dehydrogenation of naphthenes to aromatics. (In this paper, the term “naphthenes” shall mean Ce ring naphthenes unless otherwise specified.) Most of the individual operations used here have been studied by others. Absorption in Ha04 has been used in connection with GC for the analysis of olefinic naphthas, according to Martin (6). The use of Molecular Sieves for the absorption of normal paraffins from a mixture prior to analysis by GC was first reported by Whitman (8) and was also discussed by Brenner et d. ( 1 ) . Kokes (6) was apparently the first to suggest placing a reactor in series with a GC apparatus. The versatility and importance of such arrangements have been discussed by Emmett (8). Analytical hydrogenation aa an integral part of a GC identification procedure, however, is believed to have been used here for the first time, aa has vaporphase dehydrogenation. In a paper which appeared during the couree of this work, Keulemans and Voge (4) describe the use of a GC column connected directly to a small catalytic reactor for the analysis of the products of the dehydrogenation of naphthenes. Although their technique was somewhat different, their findings are of interest in connection with the part of this study dealing with dehydrogenation. These authors suggest the use of catalytic dehydrogenation for distinguishing between C‘ and CI ring naphthenes. The olefin scrubber containing mercuric perchlorate, which waa evaluated here as a part of an identification scheme, waa that of Coulson (8). The present work is similar in principle to that of Thompson et al. (7), who described the identification of organic sulfur compounds by studying the products of reaction over a desulfurization catalyst. Their apparatus and mode of operation were different, however.
The general procedure has been to pass the original sample through the column, then through a reaction zone and on into a cold trap. The products in the cold trap are then returned to the column to see what change has taken place. This procedure is in contrast to another type of operation in which two samples are injected, one of which has undergone the reaction in an apparatus separate from the GC unit. The deckion was made to employ the first type of operation because it is more rapid, and it is possible to segregate and opcrate on individual peaks or upon any chosen part of the sample. This may be valuable to avoid confusion when many peaks change position as the result of a reaction. Furthermore, it ie a micromethod-i.e., at most only milligrams of sample are required. If separate reaction facilities are used, much larger samples must be processed to provide for the problems of handling and transfer. EXPERIMENTAL
Apparatus and Operational Procedures. The apparatus employed was a Perkin-Elmer, Model 164-C, Vapor Fractometer modified as shown in Figure 1 by the addition of fourway valves (V-1, etc.) and connecting lines. An important criterion in the design was to make the equipment aa versatile as possible so that a variety of operations could be carried out in sequence. Because of the ver nature of GC operation, no more t L n a momentary interruption in flow, such as during the turning of a valve, could be permitted. No blind spota in flow channels could be tolerated. Connecting linea were kept short and placed inside the heated zone of the unit, in so far aa poasible, along with the valves. The design of the apparatus is such that a sample can be inserted at the indicated lace (Figure 1) passed through eit!er the catalyst tube or the olefin scrubber or Molecular Sieve columns (interchangeable), and into the cold trap. In Figure 1, the valves are shown in position to pass the sample through the furnace after traversing the partition column and before the cold trap. With the sample in the cold trap after one of the foregoing opera-
tions, V-2 is turned to isolate the trap. After removing the cold bath and warming up the trap, v-3 is turned to divert the carrier gas stream through the trap and sweep its contenta into the partition column. This and similar operations can be repeated as often as desired. By suitable manipulation of valves, other operations can be performed, such as back-flushing the column. The olefin scrubbers and Molecular Sieve column were made for interchan cable connection to the same stanrkard '/,-inch tubing fittings. I t would be more desirable, of course, to have both of these permanently attached, and this could be done by the addition of another valve. In the apparatus shown in Figure 1, all added connecting lines were !/r inch stainless steel or copper tubin lagged with heating tape where expose! outside the heated bone. The valves V-1, etc., were two-position, four-wa cocks of low internal volume in whic the sealing device was a Perbunan 0ring (Circle Seal Go., Pasadena, Calif. Model P1-418, '/dnch), Experience so far has shown that the Perbunan rings eventually lose tension and leak after a period of use at higher temperaturea. After what amounted to several weeks of daily use at 100" to 160" C., including several days a t 185" C.,the rin s on nearly all valves had to be repgced. It is possible to obtain rings made of Viton A, an elastomer intended for use a t -30" to 230" C., although these have not been tried. Operating conditions for almost all runs made in connection with develop ment of these m c t h d s were standardized as follows: column: %meter didecyl phthalate (Perkin-Elmer A) ; temperature: 125" C.; carrier gas pressure: 25 p.s.i.g. (He or H3; flow rates: He, 61 ml. per minute; HI, 125 ml. per minute. Other operating conditions are given in connection with individual analytical studies. In order to remove the ox gen imurity (0.3%) in cylinder hylrogen, a beoxo unit waa employed ( ~ w x o Purifier, Model D, Baker Platinum Works, Newark, N. J.). This device is a metal cylinder containing a noble metal catalyst which converts the oxygen to water. The water waa removed by passage of the gaa through a 15-inch bed of Molecular Sieve 5A. The helium stream waa dried the same way. This did not solve the water problem completely, but it reduped the water concentration in the inlet stream of carrier gas to a level which was not troublesome. The cold bath used for trapping out products waa liquid nitrogen, although in many casea dry ice would have done aa well. Sample sizes ranged from 0.01 to 0.05 ml. The cold trap used for almost all of this work was a single hairpin of I/rinch copper tubing. Although no quantitative measurements were made, recovery appeared quite satisfactory. Losses due to lack of trapping efficiency are believed to have been negligible, particularly if compared with losses from
E
59-1734
Figure 1. Gas chromatography unit with facilities for carrying out class-type reactions in sequence
I I
I
COLD TRAP
HZS04 OR
MOLECULAR SIEVE ABSORBER THERMAL CONDUCTIVITY
I
L
v-5
_ HEATED _ _ _ _ZONE _ _- _INSIDE _ _ _G.C._ _INSTRUMEM ___-(REFERENCE AND DETECTOR)
other causes, and over-all losses were tolerable. More recently, some experiments were done with a trap of the same dimensions except containing 2 inches of column packing in the '/rinch tube (35% silicone oil DC-200on firebrick). I n this cam recoveries based on integrated areas were: isopentane, 100%; cyclohexane, 1020Jo, 94%; benzene, W%. The first method seemed satisthe second method, however, is to be preferred, since, in k%?y theory a t least, there is less likelihood of loas of sample. Absorption in Sulfuric Acid. The quantitative absorption of olefins from a carrier gas stream by HzSO4 is relatively easy, but in the absorption of aromatics i t is difficult to provide adequate contact time and contact surface. The absorber ultimately developed consisted of a 17-inch bed of 10 strands (to, give a ti ht fit) of twisted glass stnng in a 'P;cinch copper tube, containing 2 ml. of concentrated H804. An additional 2 ml. of acid waa successful in restoring activity after it showed sighs of deterioration. The acid was distributed as well as possible through the packing by injection with a long h podermic needle from each end. &e drier (required downstream of the absorber) waa a 2- to Mnch bed of 4A Molecular Sieve with a few granules of Aacarite at the exit end. This scrubber, used a t 54" C., removed olefina (CC to C,, both normal and branched) quantitatively under all conditions. It removed toluene and Ca aromatics when helium waa the carrier gas, but not when hydrogen waa the carrier, probably because of the difTerence in flow rate (61 ml. per minute vu. 125 ml. per minute). Benzene was not completely removed under a n conditions. $here is one disadvantage associated with the use of Ha04 for olefins removal. Immediately after olefins have passed into it, and for some time thereafter (1 or 2 hours), the acid evolves small amounts of light hydrocarbons amounb ing to 3 to 6% of the olefins added.
! I
-I
It is impossible to exclude these from
the collected sample, One can never be sure, therefore, whether these small peaks were present, or to what extent they were present, in the original charge. Normally this causes no trouble, because of the small size of each peak, but for trace analysis it is intolerable. Removal of Olefins with Mercuric Perchlorate. A new type of olefin scrubber was evaluated, principally to see if the HzSO, absorber could be improved upon. The mercuric erchlorate scrubber was described by godson (f?) and used by him for absorption of olefins in maaa spectrometry. The active filling waa repared according to his directions. f n the work described here, a 10-inch bed of the material in a '/(-inch copper tube, at 82" or 100"C., waa used. Molecular Sieve 4A was best for absorbin the large amounts of water evolved rom this scrubber. However, a minor disadvantage is that it will also absorb other species smaller than about 3 angstrom units in cross section. Among the hydrocarbons, this includes all CI and C, molecules as well as ropylene, but not propane or adything bgher The temperature of operation prescribed by Coulson was 100" C. Tests made here showed that in order to avoid loss of part of the paraffins along with olefins, 82" C. was better. A strong peak which appears to be COS was observed when olefins were passed through the scrubber. The perchlorate scrubber absorbed olefins quantitatively, but did not absorb aromatics at all. This is in contrast to H80,which absorbs both, except for benzene. Adsorption on Molecular Sieve, Investigation of the well-known adsorption of n-olefins and n-paraffins waa not undertaken except for a study to find the optimum temperature. A 5-inch bed of this sieve in a '/4-inch tube operated at 93" C.waa satisfactory in most casea, but sufficient allowance had to be made for the fact that the
f
.
VOL 35, NO. 6, M A Y 1961
659
W
z
w
3
c
CARRILI1
Figure 2.
O U D
HELW
c
-
CARRIER OU* HVMIXEN
c
Successive hydrogenation of toluene and dehydrogenation of resulting product
heavier the material, the longer it takes to traverse the sieve. With the heavier fractions, operation at about 165" C. with due allowance for the time factor was better. One of the striking things observed about this operation is the extremely sharp and complete separation of normal from other t y w of compounds. The GC record after a second psss through the abeorber exactly duplicated the record after the first pass. Hydrogenation and Dehydrogenation. The selective hydrogenation of aromatics and of olefins (as clsaaes), either together or separately, turned out to be relatively easy. Succeeaful conditions for analytical dehydrogenations have also been found, although this reaction is relatively much more difficult. What was desired in all cases was an atmospheric-preeeure, vapor-phase reaction, resaonably complete a t very low concentrations of reactant in camer gas, with negligible side reactions and no retention of products by catalyst. None of the critical variables (catalyst, temperature, HSpartial pressure, contact time, catalpt history) was studied more completely than waa necessary to find at least one set of satisfactory operating conditions.
olefins, but did not sflect aromatics. For the hydrogenation of aromatics, the platinum catalyst in a 13-cm. bed Platinum (17.6%) on activated charcoal Platinum (1.4%) on */lpinch A l l 0 8 length waa satisfactory in the temperacontained chlorine ture range of 93" to 315" C. Partial PaYEP ium (6 o) on aebeatos (Ba er conversion even took place a t room Platinum Works) temperature. Palladium (0.6%) on '/rinch alumina peUeb (Baker Platinum Works) Although dehydrogenation is much lese straightforward than hydrogenation, analytical dehydrogenation can be All catalysts were initially treated 1to 2 successfully carried out on naphthenea hours at 370' to 450" C. in a stream of at least as heavy as G. hydrogen. Keulemans and Voge (4) also studied The materials used in hydrogenation naphtbe dehydrogenation of C, to studies were: 1-hexene, Zheptene, 2,2,4thenea over platinum-alumina-halogen trimethyl-%pentenel l-octene, Zoctene, catalyst. Conversion of C& cyclohexbenaene, toluene, xylene, and ethyla n a waa 100% at 360" C. and near bensene. It was desired to be able to 100% at 300" C. Furthermore, they hydrogenate, at will, both olefins and were operating with 1 atm. of )Ir aromatica together or olefins alone, preaent, which would tend to suppress without converting aromatica. In all dehydrogenation. Their data indicate cases, hydrogen was the camer gas. The platinum-on-activateded-chsrcoal also that the eaee of dehydrogenation increasw in the direction C, to CB. catalyst was wholly unsuitable for any This is encouraging, aince it suggests of the operations desired here, where the that C,, etc., cyclohexanes will react amounts of feed material were 80 very wily. d compared with the quantity of The Pt-Al&halogen catalyst was catalyd. In no cate could any of the not only more active than either of the material paeaed through the catalyst be palladium catalysts, but showed less recovered from it. tendency to crack the sample. All At room temperature a 2.1-cm. bed of subsequent work was therefore done fresh, 6% Pdsn-asbestos in a '/cinch with this catalyst, and conditions were tube catalysed the hydrogenation of The catalpta tried mere:
ccabV
+
k
standardized aa follows: bed length, 13 cm. in '/,-inch tube (1.5 gram); temperature, 299' C.; helium flow, 60 to 70 ml. per minute. Figure 2 shows first the hydrogenation of toluene followed by dehydrogenation of the resulting methyl cyclohexane. Both reactions were carried out at the same temperature (293" C.) and p m sure (atmospheric) with the catalyst indicated above. The only difference was the carrier gas; for hydrogenation i t was HSand for dehydrogenation, He. This experiment demonstrates the d e gree of completenese of these reactions and the fact that it is possible to collect a product and return it for further reaction. Successive hydrogenationds hydrogenation such aa is shown in Figure 2 has no practical significance with regard to identification, although it would be an easy and rapid way to label aromatics with tritium or deuterium. The problem of cracking was not completely conquered, although i t waa reduced to what is thought to be a tolerable level. Cracking first appears in the side chain attached to an aromatic nucleus. Of the compounds studied (up to and including C,), ethylbenwne was the most unstable. Upon cracking, ethylbenzene yielded toluene and benzene along with (presumably) light gases which were not detected. Toluene seldom cracked, and benzene cracked under only the most severe conditions. In the general scheme of analysis to be described, it was assumed at the outset that under the mild dehydrogenating conditions used, the cyclopentanes would not undergo dehydrogenation and would therefore be found with the isoparaffins. I n view of the known tendency for cyciopentanes to isomerize over an acid catalyst, however, a limited study waa made of the behavior of these materials. Keulemans and Voge (4), in their extensive study of the dehydrogenation of naphthenes over R similar catalyst, found that in general the cyclohexanes were much more reactive than the cyclopentanes. They concluded that an analytical method for distinguishing between Cs and C6rings could be based on the reaction. They report conversions of cyclopentanes at 300" C. of less than 5%, except for isopropylcyclopentane (7.7%), while conversions of cyclohexanes under the same conditions approached 100%. In these atudies the carrier gas waa hydrogen and the pressure was slightly more than one atmosphere. The present resulta (Table I), although not so extensive, in general confirm those of Keulemans and Voge. The amount of HIpresent was different; they used a full atmosphere of Ht, whereas in this study, usually, no hydrogen waa present. To observe the effect of a emall partial pressure of hydrogen,
AROMATICS, NAPHTHEMS, IWARhFFINS, n- W F F I N S , ISOOLEFINS, n- OLEFINS
Figure 3. Analytical scheme for identification of GC peaks by hydrocarbon type
AROMATICS
ALl
HYOFIOOEN(LTI0N. LOW TEHI!
OCEFINS
AROMATICS ALL W F I N S NAPHTHENES
-
NAPHTHENES AJL PqRAFFlNS
U AJ-L WRAFFiUS NAPHTHENES ALL OLEFINS (AS PARAFFINS)
NAPHTHENES ISOWRAFFINS
n-OLEFlNS(AS
NAPHTHENES [AS AROMATICS) ISWARAFFINS
NAPHTHENES ISOPllWFFlNS ISOOLEFINS (AS ISOPAPAFFINS) NAPHTHENES (AS AROMATICS)
4
ISOPARPCFINS
however, in a few runs about 3 volume % of HSWBB added to the helium. The resulta shown in Table I indicate that there is a great difference in reactivity between CS ring and Cc ring naphthenes 80 far as conversion to aromatics ia concerned, and this can be used to distinguish between them. The optimum temperature appears to be about 300" C., and it is beat to have some hydrogen present, since this suppresses dehydrogenation. The cycle
Table 1.
hexanea are reaotive enough 80 that eome suppreasion of dehydrogenation can be accepted. RESULTS
Scheme of Analyde. The analytical scheme in which the individual operations described heretofore are combined ie shown in Figure 3. By performing successive operations on two separate portions of a mixture in
Dehydrogenation of CC and 6 Naphthenes over Pt-A120rHalogen Catalyst
Naphthene Cyclopentane Methyl-
T:m(-.,
Hio Preaent
299
YeS
299 299
No
310
No
trans-l,%DimethylEthyl-
299
YeS Ye3
n-Propyl-
299 310 299 310
&-l,%Dimethyl-
&-l-Methyl-%thylCyclohexane Methylcyclohexane
299 299
YeS
No
Per Cent Conversion To aromatice To others' 0.3
2.1 2.1 4.2 7.1 7.5 25
No No
16
No No
20
21 17 12.2 36 16
22 22
8.7 3.4
... ... ... ... ...
313 299 299
No Ye8
No
100 94 95
...
310
No
100
...
299 285 274
No No No
97
96 78
2.2
... ... ...
* About 3 volume % in helium. b Does not include cis-trans ieomebtion, which in aome caam may be considerable.
VOL 33, NO. 6, MAY 1961
661
Figure 4. Demonstration of application of Parts A (left) and 6 (right) of GC identification scheme
two different sequences, it is possible to combine the information in such a way as. to deduce the hydrocarbon class to which each peak belongs. A GC run is made, of course, to observe the effect of each individual operation To illustrate the procedure, an explanation of Part A of the scheme will be given. Passage of the mixture through the HSO, scrubber removea aromatics and olefins, leaving only naphthenes and paraffins. The remaining material is then passed through 5A Molecular Sieve, which removea np a r s f i s . The dieappearance of peaks in this step is clear evidence that the peaks are *paraffins. The naphthenes and isoparaffins are then passed through the catalyst under dehydrogenating conditions, which converts C' ring naphthenes to aromatics. The peaks which shift, of coum, belong to naphthenes,
662
ANALYTICAL CHEMISTRY
and the positions of the aromatics peaks created may be helpful in deciding which naphthenes are involved. Some of the peaks which shift may be cyclopentanes which have undergone cistrans isomerisation; these, however, will not become aromatics. The residue is passed through the H8O4scrubber, which removea the aromatics (from naphthenes) and leaves only isoparaffis. At this point there is enough evidence to identify (by class) the naphthenes and isop8rafE.m. This information can then be referred back ta the point prior
to the 5A Molecular Sieve to confirm the identity of the n-paraftin peaks, by subtracting naphthene and jsoparafiin peaks from the record. The procedure for Part B of the scheme is similar to that for Part A (Figure 3). Information is referred from one leg to the other in order to complete the identification of the olefins. The classes of hydrocarbons included in the scheme in ita present form are the most important from the standpoint of hydrocarbon analysis of natural or procesa petroleum mixtures. Certain
classes which may be encountered a t times, however, have not been included. Probably the most important of these are cyclopentane and its homologs. Their behavior under the reaction conditions ucled here is discussed in another part of this paper. Another class not included is the cyclic olefina, which would very likely appear as isoolefina in the scheme. A third clasa is the acetylenes, which are relatively unimportant. No account has been taken of nonhydrocarbon compounds, such as those containing sulfur. The general approach should be applicable in these cases, however, if proper claaa reactions can be found. Analysis of Synthetic Sample. Figure 4 illustrates the application of the foregoing scheme to a synthetic mixture containing at least one each of n-olefin, iso-olefin, n-paraffin, isoparaffin, naphthene, and aromatic. In this ideal case there is no difficulty identifying each peak by hydrocarbon class. Analysis of Commercial Gasoline. It was, of course, necessary t o test the method upon something more difficult than a simple synthetic mixture, preferably a mixture of practical importance. A full-range gasoline was selected, although the choice involved some risk because of the great complexity of the material and because the scheme had not been tested on simple, known mixtures of higher boiling range. The analysis had to be broken into two parts: a aeries of rune at lower temperature (100" C.) to achieve sufficient resolution to identify the lighter constituents, and another series at high temperature (160" C.) 80 that heavier compounds would be eluted in a reasonable time. Even this was a compromise; three temperature ranges would have been better. One advantage was gained by the two-range operation: It was possible to eliminate the dehydrogenation step in the lowtemperature runs by properly choosing the cut point. The cut point was placed just above benzene. The thought here was that only one CI naphthene (cyclohexane) could occur in this range, and this could be looked for as a special caae. AB it happened, the fuel contained no cyclohexane. Conditions for these runs are given in Table 11. For economy of space, only examples of steps in the procedure will be shown. The chromatograms obtained for the lighter fraction are shown, stepmise, in Figure 5. A list of the measured retention ratios corresponding to numbered peaks in these figures is given in Table 11, along with peak identity according to class and, where possible, compound name. These identifications by name were made on the basis of class
Table II,
Peak No..
C W
1
2 3 4 6
6 7 8 9 10 11 12 13
14 16 16 17 18 19
20
21 22
23 24 26 26 27 28
0 IP
+ IP 0) go + &o, 0 0 IP IP
+ IP(0) IP + IP(0) NP + NP(0) 0 0 IP 0
++
iP IP(0) 10 IP(0) IP IP NP IP 0 IP IO A 0 IP
++
G C Analysis of a Commercial Gasoline
Compound LOW-BOILING FRACTION Propane Ieobutane n-Butane Isobutene tram-%Butene %Methylbutane n-Pentane %Methyl-l-butene eM-2-Pentene %Methyl-%butene 2,%Dimethylbutane ZMethylpentane and/or 2,Mimethylbutane %Methylpentme n-Hexane ZMeth 1-1-pentene cis- andrtram-3-hexene 2,4Dimethylpentane 3-Methyl&-2- entene 2,2,3-Trimethyl\utane 3,3-Dimethylpentane or Zmethylhexane 3-Meth lhexane 3-EthyLexane n-Heptane isoparaffin (could be iso-octane) C, olefin Cs isoparaffin Csisoparaffin benzene trace Olefin Cs iaopara5
+
+
Re!. Ratloo 0.16 0.33 0.46
0.63
0.60 0.81 1.00 1.04 1.20
1.31 1.37 1.73
1.96 2.16 2.20 2.44 2.71 2.76 3.05 3.66 3.89 4.38 4.60 4.72 6.27 5.62 6.'94 6.67
HIQH-BOILING FRACTION 0.67 Benzene Ca isoparaffin 0.65 0.69 29 CI isoparaffin 0.88 Co isoparaffin 30 1.00 Toluene 31 1.67 Ethylbenzene 32 1.79 p-, m-Xylene 33 2.13 Cumene or o-xylene 34 2.73 COaromatic 35 2.87 1-Methyl-, 4-ethylbenzene 36 3.15 1 3,bTrimethylbenzene 37 3.22 d, aromatic 38 3.63 1,3,4- and 1,2,3-trimethylbenzene or A 39 sec-butylbenaene 4.35 1,3-Diethylbenzene 40 A 4.53 Cl0aromatic A 41 6.21 42 A Cloor CUaromatic 5.62 A 43 Clo or C11 aromatic 7.25 Aromatic .. A 7.90 A Aromatic .. 8.23 Aromatic A .. 10.1 Aromatic A .. 13.6 Naphthalene A Refer to Figure 5 for low-boiling, Figure 6 for high-boilin fraction. * P = paraffin; o olefin; A = aromatic; N = normal; = ]so; IP(O), NP(O) IP or NP derived from 0. * For low-boiling fraction, n-pentane was used as standard; for high-boiling, toluene Conditions are given below for each: w a ueed. ~ 27 28
..
A IP IP IP A A A A A A A A
-
B
-
Low-boiling High-boiling 100 160 Temperature, C. 61 48 Gae flow, ml./min. For both fractions, the column was 2 meters Perkin-Elmer A (di-ndecyl phthalate) plus 2 metere Ax (dusodecyl phthalate), and the carrier gas was He at 30 p.e.i.
and retention ratio. Many of these compounda, particularly the lighter ones, probably could have been identified by retention ratio alone: many others, however, could not have been. Some of the compounds indicated to be present by these technique8 would not otherwise have been detected, since they were constituents of composite peaks.
Not all the identifications by compound name are completely firm, although the picture fits together fairly well. A disadvantage of the method for this sample was the fact that paraffin counterparts were already present for most of the olefins converted to paraffins by hydrogenation. This made it necessary to depend solely upon change in size of VOL 33, NO. 6, MAY 1961
663
w m o w m scnumfn-
ALL PARUflNS REMAIN
Figure 6. GC analysis of gasoline by sequential class reactioru, original gasoline
Aromatics, Naphthanos. n-Paraffins. Isoparaffins, n-Olefins. Iso.olefins
figure 5. 9 ;GC analysis of gasolinet by sequential class reactions of Parts A (left) and B (right) of lower-boiling fraction
the paraffin peak for evidence of the olefin when the latter waa not initially resolved. More resolution would help here. Experience with this sample call^ attention to a minor weakness in the scheme of analysis. If the mixture is complex, it is difficult to tell whether an olefin is normal or branched unless one can identify it by name. Upon hydrogenation, olefins usually change position in the chromatogram, and if several peaks dissppear and several new ones appear, the problem is to tell which belongs to which. Add to thii the complication of having peaks appear which coincide with peaks already present (which may be hard to detect), and the problem becomes too difficult. In theory, of course, it is only necessary to subtract the b o l e f i n peaks from the total olefin peaks to find the normal olefins, and in favorable this can be done. Figure 6 shows nearly the entire chromatogram at the higher operating temperature (160" C.). Only the major peaks are numbered, the numbers being consecutive with those of Figure 5. Table I1 lid retention values, claes identities, and some compound identities. Almost all major peaks in this fraction were aromatic, with only one d peak (a trace) which seemed clearly naphthenic. In general, although the absence of m y substantial quantity of naphthenee was established, this gasoline waa not
+ Figure 7.
Alternate scheme of identification
a very fortunate choice for demonstrating the operability of the d e h y d r o g m tion part of the procedure. Although the emphasis here has been on the qualitative aapecta of the analysis, quantitative values estimated from peak area measurements are quite consietent with the ASTM distillation and hydrocarbon-type analysis mults. If all operations were completely auccessful the first time, the analysis (not counting interpretation) of the heavier gaaoline fraction could be done in one day. The lighter fraction would require about half M long.
mixture according to hydrocarbon class, and the scheme has been shown to be operable on mixtures of practical importance. Although the method waa used sucCe88fully on a gasoline, it cannot be claimed that the present apparatus is wholly suitable for the heaviest materials in gasoline. Higher temperatures of operation would be much better. Equipping the four-way valves with high-temperature O-rings (Won A), and using a silicone, or equivalent, substrate should allow operation at 200'
c0Nc1usK)Ns
that one can trap constituents from the exit gas stream and return them to the column With only slight and entirely tolerable loss of material or peak sharp
A number of useful diSgnOatic rea^tions and operations have bean studied. Details of methods have been worked out for performing these operetions sa an integral part and extension of a GC analysis. A scheme has been devised for arrying out a number of thw operations in sequence on the same a m p l e to identify components of a
to 2 2 5 O C. A significant finding of this work is
neea. The analytical scheme tried here has been shown to be faulty in one respect: It ie diEcult or impossible to dietinguish between normal and branched olefins. Aa a r d t of experience gained, a
modified scheme has been devised which should overcome this difficulty. In addition, the new sequence of operations yields the same information with one less operation, which will save time. The new scheme is shown in Figure 7. It haa not yet been tested thoroughly, although each component operation is known to be satisfactory. Future Work. While the scheme and its constituent techniques are believed, a t present, to be practical research tools, there are still directions in which fruitful work can be done. The area of greatest potential usefulness, application to high-boiling
mixtures, is yet to be properly explored. In this connection, operation with capillary columns appears mpecialy attractive. Trapping of the column effluent is not possible in this case, but trapping and operating on the portion of injected sample which issues from the sample splitter appears feasible.
(3) Emmett, P.H. Divieion of Petroleum Chemiitp A&, 136th Meeting, Boston, pril 1969. (4) Keulemans, A. I. M., Voge, H. H., J . Phys. chem. 63,416-80 (1969). (5) Kokea, R. J., Tobm, H. Jr., Emmett P. H., J . Am. C h a . &. 77, 686d
LITERATURE ClTED
(8) Whitman, B. T., Nature 182, 391
N., Cie linski, E. W., Coates, $. J., J . &romalog. 3, BO (1960). (2) Coulson D. M., ANAL. CHEM.31, 906 (1959).
RECEIVED for review November 17, 1960. Accepted January 30, 1961. Division of Analytical Chemistry, 137th Meeting, ACS, Cleveland, Ohio, April 1960.
(1956). (6) Martin, R. L., ANAL. CHEM. 32, 330-86 (1960).
n, C. J. Coleman, H. J., ‘ 7 b 2 m r C . .. Rsll.. k. T.. Zbid..~.32. 424 (igwj.
(1958).
(1) Brenner
Analysis of Gas-Liquid Chromatog rams by a Punched Card Technique ROBERT K. TANDY, FRANK T. LINDGREN, WALTER H. MARTIN, and ROBERT D. WILLS Donner laboratory of Medical Physics and the Lawrence Radiation laboratory, University of California, Berkeley, Calif.
b A
method is described whereby gas-liquid chromatograms may be analyzed using a punched card technique. Although the application presented involves analysis of fatty acid methyl esters in which a beta particle ionization detector is used, with minor revisions this method has potential applications to all gas-liquid chromatographic work. The advantages of this technique are: elimination of nearly all manual arithmetic calculations, equivalent or greater accuracy to existing manual techniques, and ease of data manipulation and storage.
D
the last decade one of the most promising and exciting developments for the analysis of volatile organic compounds of biological interest has been the technique of gasliquid chromatography ( 1 ) . An important application of this technique is in the study of long-chain fatty acids and their relationship to both normal and abnormal lipide metabolism. However, one of the technical difficulties encountered in this and other applications of gas-liquid chromatography (GLC) is the complicated nature of the data. I n the fatty acid studies, for example, there are about 100 biologically occurring fatty acids. In a typical gas-liquid chromatographic analysis approximately 25 or more of these fatty acids are frequently resolved on each chromatogram. Tabulation and comparison of such extensive data are very tedious manual tasks. Usually, the amount of each fatty acid component has been quantitated by measuring URINO
under each peak on the chromatogram is calculated by multiplying the product of the elution time and pcak height by a first order correction function based upon the peak height. Thus, for a given chromatogram, the mass of each component is calculated together with the total mass of all chromatographic components present. For convenience, the mass per cent of each component aa well as its retention time (relative to methyl stearate) is also calculated. An additional calculation correcting each chromatographic component on the basis of its relative retention time completes the program.
the area under each chromatographic peak through integration, planimetry, and triangulation (2). If carried out manually, these methods are laborious and unfortunately subject to frequent human error. To avoid the above-mentioned technical difficulties, the authors have developed a technique for analysis of gss-liquid chromatograms using punched cards together with an a p propriate computer (IBM 650). In essence, each chromatogram consisting of a complicated sequence of peaks is reduced to a small deck of IBM punched cards (one for each chromatographic component). The basic date placed on these input IBM punched cards consist of the elution time and peak height of each chromatographic component. From these data, a measure of the area
Figure 1. Relationship between triangulated peak width and retention time
-E
EXPERIMENTAL
For a given fatty acid methyl ester component on a gas-liquid chromato-
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