DISCUSSION
Satisfactory separations of CG to Cll n-paraffins required a highly selective, thermally stable, liquid phase, partial fractionation of the ra-paraffins before making individual separations, an optimum molecular sieve temperature, and minimum interfwences from reaction products in the olefin absorber. The liquid phase inust retain aromatics longer than the highest boiling saturates and olefinti-typically CI1C l r i n gasoline, without itself undergoing thermal decomposition a t the required temperature of a t least 110' C. C E F satisfies these requirements. Well known liquid phases such as &@'thiodipropionitrile (TDPN ) and 1,2,3-tris(2-cyanoethoxy) propane have adequate selectivity, but they yield decomposition products that are absorbed by the sieve and interfere in the n-paraffin determinations. A comparison of the selectivities of C E F and TDPN a t 120' C. is shown in Table V. 'The selectivity of C E F is about one full carbon number higher than that of TDPN; Clx olefin and CU n-paraffin elute well before benzene, and most of the ClS olefin elutes just before benzene. Complete resolution of n-paraffins requires partial separation of the lowerboiling compounds on iiilicone rubber, as the trap warms gradiially from liquid nitrogen temperature :io room temperature. The rest of the separation takes
place on CEF. C E F by itself is not adequate. A sieve temperature of about 165" C. is the highest permissible if n-pentane is to be retained by the sieve while saturates and olefins elute from the C E F column. When the saturates are predominantly isoparaffinic, some Clo-C1l isoparaffins adsorb on the molecular sieve, and n-decane and n-undecane peaks subsequently appear on the shoulders of isoparaffin peaks (Figure 2). However, separations still permit estimates a t concentration levels near 0.1%. If n-pentane is not to be determined, the molecular sieve is kept between 210' to 220' C., at which temperature isoparaffin interferences are substantially eliminated. The mercuric perchlorate absorber should be freshly prepared and kept just warm enough (60' to 65' C.) to prevent condensation of higher boiling components. At higher temperatures, efficiency decreases rapidly, and side reactions with non-olefinic compounds may occur. The absorber is by-passed during backflushing to prevent any reaction products from eluting into the molecular sieve column. Without the by-pass, unknown peaks sometimes occur which mask some of the n-paraffin peaks. This flexible method can be readily adapted to various types of samples boiling within the gasoline range. If
only hydrocarbon-type compositions are of interest, the molecular sieve section can be bypassed. If the sample is olefin-free or contains only traces of olefins, the absorber can be omitted. I n routine operation, a complete analysis requires about an hour and a half-one half to one third as long as molecular sieve methods in which saturates must first be isolated by techniques other than ~ L Sc~hromatogmpliy. LITERATURE CITED
, Whitham, B. T., International Gas Chromatography Symposium, East Lansing, Mich., June 13-16, 1961. (2) Am. SOC. Testing Materials, "ASTM Standards, 1961," Method D-1319-61T, Part 7, p. 702. (3) Defives, D., Buzon, P., Chim. Ind. (1) Adlard, E. R
(Paris) 86, 64 (1961). (4) Eggertsen, F. T., Croennings, S., ANAL.CHEW33,1147 (1961). (5) Frisone, G. J., N u t w e 193, 370
(1962). (6) Frisone, G. J., private communication, Research Laboratories, Rohm & Ham Co., Philadelphia, Pa., March 1962. (7) Kvitkovskii, L. N., Grushetskaya, E. V., Khim. i Tekhnol. Topliva i Masel 7, 61 (1962). (8) Martin, R. L., ANAL. CHEW34, 896 (1962). (9) Standard Oil Company (Ind.), patent applied for. RECEIVEDfor review May 29, 1963. Accepted August 7,1963.
Assignment of cis-trans Configuration to Mono-olefin Pairs by Gas Chromatography R. A. HIVELY The Goodyear Tire anid Rubber Co., Research Division, Akron 76, Ohio
b Fourteen pairs of C4-Ce cis-trans mono-olefin isomers were studied on various substrates by gas chromotography. The retention time of the cis form relative to that of the frans isomer on a highly polar substrate was always greater thon the same ratio on a nonpolar column for the dialkylethylenes. Usiqg the present nomenclature, two trialkylethylene pairs deviate from this rule. It seems probable that the rule is correct for trialkylethylenes also, and that the two pairs need to be renamed. A method has been devised for assigning the geometrical configurations of cis-frans mono-olefins. It was applied to a tetrasubstituted ethylene.
F
inception of gas chromatography to the present, attempts have been made to correlate structure with retention p:ir.nIneters. The reROM THE
lation between the logarithm of the retention time, or relative retention time, and the number of carbon atoms in the molecule was reported by James and Martin (9, 10) and by Ray ($7). A number of investigators (9, 11, 16, $0) have pointed out the advantages of plotting either the retention time or the logarithm of the retention time on a nonpolar column against the same type of data from a polar one. Brown (1) has advocated the use of three substrates -a nonpolar, an electron donor, and an electron acceptor. Miwa et al. (18) and James (8) have shown that the logarithm of the retention time of a fatty acid methyl ester was shifted by the addition of a given functional group. The numerical value of the shift was independent of the chain length of the molecule provided that no interaction with other functional groups occurred. Knights
and Thomas (12) have proposed an additive effect of functional groups on hydrocarbon skeletons. They have found that the position and the nature of the constituent affect the retention volume. Landowne and Lipsky (76) have shown that the separation factors on capillary columns vary with temperature, and further, that this variation has a relationship to the structure. Merritt and Walsh (17) have found that when appropriate liquid phases are employed, the ratio of retention volumes within a homologous series is constant. Different constants are found for different functional groups. Evans and Smith (3-6, E?)have calculated an effective molecular weight from retention parametrrs. The difference between the effective molecular weight and the true molecular weight was correlated with the structure. KOvats et nl. (13, 14, $3) have proposed a VOL. 35, NO. 12, NOVEMBER 1963
0
1921
Table 1.
Ratio of Retention Time of cis Isomer Relative to trans Isomer - ~~Retention time _ _ of the cis isomer relative to the &i?.s i_somer.-16-Meter S1/,-Meter 2'/~-Me!er 21/z-Meter 2'/z-hfeter 21/2-hletcr 6003r-
ODPN,. 35" c. 1.23 1.17 1.06 1.15 0.95 1.21 1.46 1.11 1.02 1.00 1.00
2-Butene 2-Pentene 4-Methyl-Zpentene 3-Hexene 3-Methyl-2-pentene 2-Hexene 4,4-Dimethyl-2-pentene 4-Methyl-2-hexene 3,4-Dimethyl-2-pentene 3-Methyl-3-hexene 3-Methyl-2-hexene 1.19 3-He~tene 2-He;tene 1.19 3,4-Dimethyl-3-hexene ... a ODPN = p,jY-ox di ropionitrile. DMS = dimethyhFo1ane. TCP = tricresyl hosphate. * DNDP = di-n-&cylphthalate. 0 MO = mineral oil.
retention index which Smith has related to his work. It seemed logical to expect a relationship in the elution behavior of cis-trans isomers. Sccordingly, relative retention times of geometric isomers were obtained on a variety of substrates. The study has revealed that a structural correlation does exist. All isomer pairs which were studied fit a generalized rule except two; and it s e e m probable that these two pairs are not exceptions to the rule, but that their designations should be reversed. EXPERIMENTAL
Apparatus. Three different commercially available chromatographs were used; a Barber-Colman Model 20, a Perkin-Elmer Model 154C, and a Burrell Kromo-Tog Model K-2. The columns were divided into three categories. All 21/z-meter columns were Burrell Cat. No. 347-01-25 glass columns which were packed with 29% of substrate on Chromosorb P. The 16-meter column was constructed of l/r-inch 0.d. aluminum tubing which was filled with a mixture of 25% of &@'-oxydipropionitrile by weight on Chromosorb P. The 600-foot stainless steel capillary of 0.01-inch i.d. was coated with squalane. Reagents. The materials which were used to pack the columns were: dimethylsulfolane from Shell Chemical Corp. ; tricresylphosphate from Harwick Standard Chemical Co.; di-ndecylphthalate, @,P'-oxydipropionitrile, and squalane from Eastman Kodak Co.; mineral oil from Purity Laboratories, Los Angeles, Calif. ; and Chromosorb P, 30- to 60-mesh, from Fisher Scientific
co.
The C4 olefins were purchased from The Matheson Co. The rest of the hydrocarbons were obtained from the American Petroleum Institute and from K. W. Greenlee at Ohio State University. 1922
ANALYTICAL CHEMISTRY
ODPN,O
DMS
27" 6.
27°C.
TCP e 27" 6. 1.15
DNDPId 27" C.
1.24 1.18
1.18 1.09
1.06 1.16
0.97 1.05
0.91
1.13
0.89 1.11
0.89f 1.079
1.079
1.279 1.00* 0 .94h 1.05h
0.94
0.98
0.90
1.22
1.45
1.33 1.00 0.96 1.01
1.11
1.02
0.99
1.06
1.11
1.06 0.93
27" C. 1.13 1.04 0.92f
1.02f
1.23 0.96 0.90f
hIO,e
0.99r 0 .8gh 1.07n
1.23* 0.98:
0.92'
1.05'
6~0-F't.
squalane, 27'C.
squalane, 49'C.
1.12
1.12 1.05
1.04 0.93 0.99 0.89 1.07 1.22 0.98 0.91 1.08
0.94 1.00
0.90
1.07
1.23 0.99 0.93 1.07
Column temperature of 28' C. Column temperature of 36" C. Column temperature of 45' C. i Column temperature of 54' C.
f 0
Procedure. The 21/2-meter columns were used on the Burrell Kromo-Tog with a 40 ml. per minute flow rate of helium a t the temperature indicated in Table I. A temperature of 35' C. and a column head pressure of 15 lb. were employed on the 16-meter column in the Perkin-Elmer instrument. The 600-foot capillary was installed in the Barber-Colman with a head pressure of 50 p.s.i.g. of argon and a 200-to-1 split. The temperature was maintained a t 27' C. The sample size was adjusted to keep all peaks on scale a t the maximum instrument sensitivity. After making dead volume corrections, the retention time of the cis isomer was obtained relative to the trans form for each pair, When the separation was poor, the desired ratio could be obtained by first measuring a retention time for each isomer relative to some standard. A special procedure was required to obtain the dead volume for the capillary column with the argon detector. A standard l/r-inch aluminum tube of 50foot length was packed with 25% of squalane and 75% of Chromosorb P by weight. The column was conditioned in an instrument which was equipped with a thermal conductivity detector. A mixture of air, ethane, propane, and butane was chromatographed on the packed column; and the relationship of the peaks t o one another was estab-
lished. A blend of Cz through CS n-alkanes was run on the squalane capillary which was maintained a t the same temperature as the packed column. By using the retention times of ethane and butane, a retention time was calculated for the air peak. The retention times of all of the peaks were then corrected for the dead volume, and the relative retention times were calculated in the usual way. Two internal standards, whose relative retention times had been established, were used in order to avoid the necessity of estimating the retention time of the air peak with each run. RESULTS AND DISCUSSION
Equations were derived for calculating the retention time of sir in systems where ionization detection was used. Let T A = retention time of air Tsl = retention time of standard 1 Tsr = retention time of standard 2 T x = retention time of unknown Rs, = retention time of standard 1 relative t o a given compound Rsn = retention time of standard 2 relative t o the same given compound Rx = retention time of unknown relative t o the same given standard.
Then the usual expression for obtaining relative retention times was written : Table II. Estimation of Elution Time on Squalane Capillary Using Argon Detector Elution time of
air peak in minutea
Column temp.
27" C. 49O
c.
67" C.
Using ethane Usin propane and butane a n 3 butane as standards aa standards 26.12 25.13 26.84 26.84 27.86 27.86
The retention times were made relative to standard 1. Then Rsl was - in Equation 1, reequal to 1, and RSZ RSI duced to Rsz. Equation 1 was solved for T A to obtain the expression:
Table 111.
Trialkylethylene Data after Changing Nomenclature of 3-Methyl-3-Hexene and 3-Methyl-2-Hexene Pairs Retention h i e of t,he cis isomer relative t,o the tra,ns isomer --
-------
16-Meter ODPN, 35O
3-Methyl-2-pentene 3,4-Dimethyl-2-penteno 3-Methyl-3-hexene 3-Methyl-2-hexene Column temperatu-e of Column temperature of Column temperature of Column ?emperaftre uf
21I2-Rleter 21/2-Meter ODPK, DMS, 27" C. 27" C. 0.90 0.94 1.02 0.96 0.99 1.01 0.96 0.99
c.
0.95 I .02 1.00 1.00
Then and k(lix
- RBI)
(5)
uliere 12 was the prcportionality constant. It has the same value in $:quations 4 and 5 because the column parameters were the same in each case. To make sure this was the case, the three compounds were run trimultaneously a t isothermal conditions. Equation 4 was divided by Equation 5 and solved for Rx to get: Rs
0. 95c
0.9%
21/2-Meter MO,
27" C. 0.89" 0 . 92d 0.95" U .93"
600-Ft. squalane, 27" c. 0.89 0.91
0.93 0.92
600-Ft.
squalane, 49O c. 0.90 0.93 0.93 0.93
54' C.
1
=
21/2-Meter DPiDP, 27' C. 0.89" 0.94"
28' C. 36" C. 4.5" C.
By using a packed column with a thermal conductivity detector, values for Rsl and Rszwere eetablished. When the same substrate was employed in the capillary column as in the packed column, the reasonable assumption was made that Rsl and Rsl were the same on the capillary column as on the packed column when nonpolar compounds such as a homologous qeries of n-paraffins n ere used. Values fo Tsi and TsZwere obtained on the capillary column, and this permitted the calculation of the retention time for the air peak with ionization detectors. An estimation of the elution time for the air peak was made on the 600-foot .?qualane capillary column using the relative retention times of ethane, propane, and butane which nere obtained on a 50-foot packed squalane column. The elution time of the air peak calculated from the ethane and butane peaks was identical with the value calculated from the propane and butane peaks as shown in Table 11. But on any given chromatographic column :
Tx - TSl
21/2-R.leter TCP, 27' C. 0.89 0.90" 0 . 93b 0.91*
=
It ma' not necessary then t o calculate the elution time of tlie air peak when two internal standards with known relative retention times we-e added. Equation 6 was used in tkis study to estimate the relative reten tion times needed for the calculations.
The retention time of the cis olefin relative to the trans isomer is shown in Table I for a variety of columns and operating conditions for several monoolefin pairs. In general, the ratios which are given in Table I decrease in the order p,/3'oxydipropionitrile > dimethylsulfolane > tricresylphosphate > di-n-decylphthalate > mineral oil or squalane. There is some deviation, hut a general trend can be seen. However, if the two substrates with the greatest differences in polarity are chosen, one may write: Rxc
Rsc > -RST
(7)
where R = corrected retention time in all cases and the subscripts are: N C = cis form on p,B'-oxydipropionitrile
AT
=
SC ST
=
=
trans form on @,p'-oxydipropionitrile cis form on squalane trans form on squalane
It is not necessary to make the determinations on the two columns at exactly the same temperature. Data obtained on the nitrile and squalane columns a t two temperatures were essentially the same. Variations of more than 40' to 50" C., however, might cause some difficulty where the shift is not large as with the tri- arid tetraalkylethylenes. Equation 7 was valid for the nine dialkylethylenes, but some of the trialkylethylenes, did not obey the equation m-hen the present nomenclature was used. There beems to be no question of the geometry assignment in the dialkylethylenes. Stereospecific syntheses and infrared assignments are accepted methods of identification, However, the structural configuration of the trialkylethylenes is not so straightforward. Differences may esist between the cis and trans forms in the infrared, but as yet these differences cannot be used to assign the structure with certainty. The first isomeric trialkylethylene pair, 3methyl-2-pentene, has had the structure unambiguously assigned only recently (8). The 3,4-dimethyl-2-pentenes were also prepared and named (19). Creen-
Table IV. Ratio of Retention Time of cis Isomer Relative to trans Isomer for Methyl Esters of Fatty Acids"
Retention time of cis isomer relative t o trans isomer Polyethylene glycol Apiezon adipate 1, at at Methyl ester 180" C. 197' C. Ag-Hexadecenoic acid 1.00 0.97 Ag-Octadecenoic acid 1.00 0.96 a Computed from relative retention data published by A. T. James (8). lee and Wiley ( 7 ) have suggested that mono-olefins of the 3-methyl-2alkene type be named in such a way that the lower boiling isomer has the cis designation and the higher boiling isomer the trans. Shortly after this, the American Petroleum Institute reversed the names of the cis-trans pairs for 3-methyl2-pentene and 3,4dimethyl->pentene. These two olefin pairs obeyed Equation 7 when the revised nomenclature was used. If the higher boiling 3-methyl3-hexene and 3-methyl-2-hexene are designated as the trans forms, then the gas chromatographic data for the four trialkylethylenes would be that of Table Ill. Thus, all trialkylethylenes would obey both the boiling point rule and the gas chromatographic rule expressed in Equation 7 . It seems that the designations of the 3-methyl-3hexenes and the 3-methyl-Zhexenes should be reversed so that the trans form has the higher boiling point. Only one tetraalkylethylene pair was available for study. If the rule still applies to these compounds-and it seems reasonable to expect that it should -then the high boiling isomer of the 3,4-dimethyl-3-hexene pair has the cis configuration. Compounds other than mono-olefins have not been thoroughly examined. However, one might expect the rule t o apply to substituted olefins where the substituent is sufficiently removed from the double bond to prevent interaction VOL. 35, NO. 12, NOVEMBER 1963
1923
between the two groups. An illustration can be taken from the results reported by James (8) where he listed the relative retention times of the methyl esters of several fatty acids. Data for the cis-tram isomers of Ae-hexadecenoic and AQ-octadecenoicacids were given on Apiezon L and on polyethyleneglycol adipate columns. As shown in Table IV, Equation 7 is obeyed if the polyethyleneglycol adipate column is considered more polar. ACKNOWLEDGMENT
The author is indebted to Kenneth W. Greenlee of Ohio State Gniversity for special samples and for his encouragement. Roger E. Hinton of the Goodyear Research Laboratory spent 8 considerable amount of time in obtaining the retention date.
LITERATURE CITED
(1) Brown, Ian, Nature 188, 1021 (1960). (2) Cornforth, J. W., Cornforth, R. H., Mathew. K. K., J . Chem. Soc.. 1959. p. 112. (3) Evans, M. B., Smith, J. F., J . Chromatog. 5 , 300 (1961). (4) Zbid., 6, 293 (1961). (5) Zbid., 8 , 303 (1962). (6) Evans, M. B., Smith, J. F., Nature 190, 905 (1961). (7) Greenlee, K. W., Wiley, V. G., J . Org. Chem. 27, 2304 (1962). (8) James, A. T., J . C”hromutog. 2, 552 ( 1959). (9) James, A. T., Martin, A. J. P., Analyst 77, 915 (1952). (10) James. A. T.. Martin, A. J. P.. ’ Biochem.‘J. (London) 50, 679 (1952). ’ (11) Jame8, A. T., Martin, A. J. P., Smith, G. H., Ibid., 52, 238 (1952). (12) Knights, B. A., Thomas, G. Ii., Nature 194, 833 (1962). (13) Kovata, E., Helv. Chim. Acta 41, 1915 (1958).
(14) Kovati, E., 2. Anal. Chem. 181, 351 (1961). (15) Landowne, R. A., Lipsky, S. R., Biochim. Biophys. Acta 47, 589 (1961). (16) Lewis, J. S., Patton, H. W., Kaye, W. I., ANAL.CHEM.28, 1370 (1956). (17) Merritt, C., Jr., Walsh, J. T., Zbid., 34, YO3 (1962). (18) Miwa, T. K., Mikolajczak, K. L., Earle, F. R., Wolff, I. A., Zbid., 32, 1739 (1960). (19) Onesta, R., Castelfranchi, G., Chim. Ind. (Milan) 42, 735 (1960). (20) Fierotti, G. J., Deal, C. H., Uerr, E. L., Porter, P. E., J . Am. Chem. SOC. 78, 2989 (1956). (21) Ray, N. II., J . Appl. Chem. 4, 21 (1954). (22) Smith, J. E., Chem. &- Znd. (London), (,,lp63 1024. e rli, E., Kovats, E., Helv. Chin. Acta 42, 2709 (1959). RECEIVEDfor review April 22, 1963. Accepted August 7, 1963. Presented at Eleventh Detroit Anachem Conference, Detroit, hfich., October 1963.
Determination of Atmospheric Pollutants in the Part-Per-BiI Iion Range by Gas Chromatography A Simple Trapping System for Use with Flame Ionization Detecfors THOMAS A. BELLAR, MARY F. BROWN, and JOHN E. SIGSBY, Jr. Division of Air Pollution, Robert A. raft Sanitary Engineering Center, Cincinnati 26, Ohio
b A simple procedure is described for the determination of atmospheric pollutants at concentrations as low as 0.1 p.p.b. Hydrocarbons in the part-perbillion range in 1 0 O - ~ m .gas ~ samples can easily be determined. A simple trapping system provides for high collection efficiencies over a wide range of flow rates. Most samples can be handled by this system in less than 10 minutes. This collection system is used in combination with analysis by flame ionization gas chromatography. Data are shown for analyses for atmospheric olefins and paraffins derived from automobile exhaust. The general utility of the method is evaluated and described.
W
ITH THE ADVENT of the flame
ionization detector, direct analysis of the more abundant hydrocarbons in the at,mosphere is possible (2-4). The majority of the hydrocarbon contaminants, however, especially the reactive species, are present in concentrations well below the detection limit of even this detector. To extend the range of flame ionization detectors, a concentrating method similar to those used for thermistor analysis was sought. Various reported methods of concentrating were in1924
ANALYTICAL
CHEMISTRY
vestigated, but the results of these investigations were unsatisfactory. Reported trapping efficiencies vary between 80 and 100% (1, 5). In these methods the sample is generally pulled through the trap by a pump. The exhaust from the pump is measured for the volume of the air that has passed through the trap less the contaminants, COz,and water condensed by the trap. By this general method large volumes are concentrated for thermistor detector analyses. Such large volumes are not necessary for the flame ionization detector because of its greater sensitivity. Therefore a method was developed to concentrate a smaller volume (25 to 300 cc.) of polluted air. In this method a gas-sampling valve is used to inject accurately a calibrated volume into a trap. A slow flow of carrier gas (helium) is used to flush the sample from the calibrated volume into the trap. EXPERIMENTAL
Apparatus. P & E flame ionization detector with 0 to 1 mv. recorder. The sample trap (Figure 1) consists of a 10-inch section of No. 316 stainless steel tubing (3/16-inch o.d., 0.016-inch wall thickness). A piece of heavy-wall No. 316 stainless steel tubing (‘/*-inch 0.d.) is silver-soldered into each end of the 10-inch tube. The 10-inch section is
then packed with 10% Carbowax No. 1540 on 50- to 60-mesh firebrick. After both ends are plugged with glass wool, the trap is bent into the shape of a “U” to fit into a 1-liter Dewar flask. Toggle valves ( V , and V,) are then attached to both ends of the trap with swagelock fitt)ings, and the trap is installed on the chromatograph. For further protection a No. 316 stainless steel filter with a mean pore opening of 35 microns can be placed in the swagelock fitting to ensure that no glass wool or firebrick becomes lodged in the toggle valves. Before use the trap is conditioned by heating to approximately 150° C. while purging with helium for about an hour. Procedure. The two main valves (Vl and V,) are standard two-position, six-port, gas-sampling valves. V2 is the gas-sampling valve of the gas chromatograph. VI is used to inject the calibrated volume of sample into the trap. These valves have two Dossible positions : the fill position, a t which the calibrated volume or the trap is filled; and the inject position, at which calibrated volume or the trap is placed in the carrier gas stream. To start the trapping procedure VI and V 2 are placed in the fill position. A diaphragm pump is used t o pull a continuous sample through the calibrated volume; thus it is possible to monitor a changing system continuously.
