The recoveries of 95 to 1007, of theoretical include instrumental variations and the possibility of impurities in the commercial samples used as standards. Because of the unusual difficulties in maintaining constant operating conditions during a series of runs a t reduced prcssurr. frequent calibrations or internal standards (6) should be employed. h routine method devised for the analysis of similar aromatic sulfonic acid mistures gavr results with a two sigma variation of 5% relative for the major component when a peak area method with internal standardization vr-as employed.
ACKNOWLEDGMENT
The author is indebted to IV. A. Gregory for his helpful discussions on the conrcrsion reactions, and t o G. J. Wallace, who assisted in the experimental work. Certain samples of sulfonic acids used in this study were generously supplied by J. E. Callen of the Proctcr & Gamble Co., Cincinnati, Ohio. LITERATURE CITED
(1) Bayer, E., "Gas Chromatography 1958," D. H. Desty, ed., p. 333, Academic Press, New York, 1958. (2) Bosshard, H. H., Mory, R., Schmid,
>I Zollinger, ., H., Helv. Chem. Acta 42,
1658 (1959). (3) Farbwerke Hoechst Aktiensgesellschaft, Belgian Patent 553,871 (Jan. 15) 1957). (4) Gregory, W..L, U. S. Patent 2,888,486 (May 26, 1959). (5) James, A. T , Martin, A. J. P., Brit. hled. Bull. 10, 170 (1954),.( (6) Keulemans, h. I. M., Gas Chromatography," 2nd ed., p. 34, Reinhold, Kew York, 1959. (7) hfcInnes, -4.G., Ball, D. H., Cooper, F. P.. Bishoo. C. T., J. Chromatoq. 1 , 556 (1958)
RECEIVED for review March 31, 1960. Accepted July 15, 1960. Division of Analytical Chemistry, 137th Meeting, ACS, Cleveland, Ohio, April 1960.
Applications of Carbowax 400 in Gas Chromatography for Extreme Aromatic Selectivity LARRY RANDALL DURRETT' Shell Oil
Co.,Houston Refinery
laboratory, Housfon, Tex.
b Carbowax 400 has proved very useful as the stationary liquid in gas chromatography for the selective retention of aromatic hydrocarbons relative to paraffinic hydrocarbons. This selectivity results in benzene, boiling at 80" C., emerging with CIO paraffins boiling a t approximately 170" C. A gas chromatographic method has been developed for the determination of the aromatic content of aviation gasoline 'taking advantage of the selective retention of aromatics by Carbowax 400. All paraffinic hydrocarbons in aviation gasoline are eluted prior to the elution of benzene. This method requires approximately 45 minutes. A further application of gas chromatog ra phy utilizing Ca rbowa x 400 as the stationary liquid has been developed for the determination of hydrocarbon impurities in high-purity petroleum benzene and toluene. This determination also requires approximately 45 minutes.
I
s GAS CHROMATOGRAPHY, aroiiiatic hydrocarbons can be retained selectively relative to paraffinic hydrocarbons to varying degrees, deprnding upon the nature of the material utilized as the stationary liquid. Carbowax 400, a polyethylene glycol \Tith an awrage molrcular weight of approsi1 Present address, Shell Oil Co., Houston Research Laboratory, P.O. Box 2527, Houston, Tex.
niately 400 and available from Union Carbide Chemicals Co., has proved very useful for the sclective retention of aromatics relative to paraffins. ;in evaluation of some polyglycols as stationary liquids in gas rhromatography has been published by Adlard ( I ) . n'hitham (9) has described the use of polyrthylrne glycols as stationary liquids for gas chromatographic solvent nnalyses. Two application:: of gas chromatography of particular inter& to the petroleum industry have been developed which t'ake advantage of the aromatic select,ivity of Carbowas 400. The gas chromatographic mrthod for the determination of the aromatic content, of aviation gasoline: described herein, is superior to thc commonly used methods: silica gel adsorption ( 2 ) and fluorescent indicator adsorption (FId) (6). The f0rmc.r .%ST11 method is sufficiently accurate but is rat'hcr li'ngthy, while the lattcr dS'l3I mrtliod is rclntively short but lws : i ( u r a t ( ' . Thc gas c,lirom:ito~iapliicrnc~thotlrombinrs both *]iced and awurary. Thc gas c,hroin3togl.alihi(' m r t h o d for the detrrmination of 11)-tlrocarbon impurities in high-purity p['trolruni bcnzonc and t o l u ( w i> supcrior to t h r Kattwinke91 rwgcmt t w t (3). n-hicah is uscd (,urr(mtly. -A roii.ac.tivc indes method rcprcwnting an i m p r o w m m t in sensitivity o w r thc .A. has b w n describd by A and Lipkin (10). Fabrizio et al. ( 7 ) recently published a gas chromatographic method for detrrmining trace
impurities in petroleum benzene and toluene which is still more sensitive than the refractive index method. The method described herein is comparable in sensitivity to the method of Fabrizio and coworkers, but has the advantage of determining the amount of hydrocarbon impurities in both petroleum benzene and toluene using a single stationary phase. APPARATUS
G a s Chromatographic Instrument.
.I laboratory-fabricated gas chromatographic unit was used in this work. The unit consisted of a n oil bath with heating element a n d thermoregulator for controlling the bath temperature, regulator valves for carrier gas flom control, a conventional dual-pass thermal conductivity cell with necessary bridge circuitry. attenuator, power supply, and a 0- to 2-mv. Brown recorder. Heliuni was used as thc carrier gas in all caws. Chromatographic Column. The resolving column was a coiled 10-foot length of copper tubing n i t h a n outer diameter of 1 inch and packed with 28.5Yc \wight of Carbowax 400 on 30to 60-mesh firebrick. T h e firebrick was pretreated by washing with aqua wgia and t h r n neutralizing with repcated dilute sodium hydroxide washings. The aqua regia treatment of the firebrick significantly decreased thc prcwure drop across the column with no apparcint effect on resolution. The etatioiiary phase was prepared by dissolving 80 grams of Carbowas 400 in approsiniately 400 ml. of acetone. This misturc was slurried into 200 grams VOL. 32, NO. 1 1 , OCTOBER 1960
1393
I BO
d 160
--
!
140
n tY
%
p
0 -XYLENE
,I:,/;
120
IADOEDI
100 80
,
60 I
2
o BENZENE
d so
5 IO 20 FINAL RETENTION TIME. MINUTES
100
" _ L
Fisure 1 .
Aromatic selectivity of Carbowax
(100" C.)
400
_._-
0
Figure 2.
of 30- to 60-mesh firebrick in a large evaporating dish, making certain that all particles were wetted. The major portion of acetone was allowed to evaporate over a steam bath with continuous stirring. The material was heated under an infrared lamp until dry to the touch and then dried in a vacuuni oven a t 70" C. overnight. The packing \vas finally rescreened to 30- to 60mesh to remove any fine material which may have been produced during the coating operation. EXPERIMENTAL
The effect of Carbowax 400 on paraffinic and aromatic hydrocarbons was determined by injecting blends of pure hydrocarbons into the Lolumn which was held a t 100" C. A carrier gas flow rate of 80 ml. per minutci, measured a t atmospheric pressure a t the column exit, was employed. These data are depicted graphically in Figure 1, which shows a semilogarithmic plot of the boiling points of the hydrocarbons us. their respective retention times. The data show that the hydrocarbons emerge according to boiling point within a homologous series; however, Carbowax 400 has very little affinity for paraffinic hydrocarbons while exhibiting a pronounced aromatic sdectivity. This aromatic selectivity results in benzene, boiling a t 80" C., emerging with Ci0 paraffins boiling at approximately 170" C., a boiling point-elution inversion of 90" C. N o attempts were made to determine the maximum operating tem-
I.
perature for Carbowas 400; however, the original column has been held a t 100" C. for approximately 18 months without signs of deterioration. Determination of Aromatic Hydrocarbons in Aviation Gasoline. With t h e column temperature regulated at 100" C. and t h e flow rate adjusted t o 80 ml. per minute, 0.003 ml. of t h e sample containing a n added internal standard is injected into t h e column with a Hamilton microsyringe S o . 701. T h e last component t o emerge from the column 25 minutes after t h e time of sample injection is t h e internal standard, o-xylene, which is added to the aviation gasoline sample in known concentration to eliminate the necessity of a constant volume charge to the gas chromatograph. This compound was chosen as the internal standard because no Cs aromatics were found in the aviation gasoline samples. If Cs aromatics were present in the sample, another compound would have to be used as the internal standard. A chromatogram of a typical aviation gasoline sample containing a n added internal standard is shown in Figure 2. The entire determination, including calculations, requires approximatcly 45 minutes. The areas under the aromatic peaks are measured and the concentration of each is calculated by comparing its peak area with the peak area of the internal standard as illustratcd below:
GLC Repeatability and Accuracy Data for Aromatics in Aviation Gasoline yo Volume Benzene ' Volume Toluene - 7 Added" Found Deviation Added Found Deviation 1 10 0 99 -0 11 1 99 2 06 +O 07 1 02 -0 08 2 03 +0 04 1 42 1 50 +o 08 4 98 4 88 -0 10 4 92 -0 06 1 46 +o 04 1 98 1 99 +o 01 7 97 8 05 +O 08 2 04 +O 06 8 09 +o 12 Av. dev. 0 06 Av dev 0 08 Includes 0.42% volume benzene in base stock as determined by GLC.
Table
1394
ANALYTICAL CHEMISTRY
10 IS TIME, MINUTES
20
25- -
Aromatics in aviation gasoline
or rearranging and normalizing to loo%, %VA =
% V S T DX P A A X S A X 100 PASTDX (100 - %VSTD)
where '%VA %VSTD
PAA
=
=
=
P.4srD =
SA =
volume per cent of aromatic of unknown concentration volume per cent of added internal standard peak area of aromatic of unknown concentration peak area of internal standard the sensitivity of aromatic relative to that of internal standard
Results. The standard deviations ( 4 ) and accuracies of this method of measuring the aromatic content of aviation gasoline were determined by analyzing aviation gasoline blends containing known volumes of benzene and toluene. The standard deviations calculated from results of 10 determinations of a synthetic aviation gasoline blend containing 1.42% volume benzene and 4.98% volume toluene were found to be 0.02% volume benzene, 0.05% volume toluene, and 0.06% volume total aromatics. Results of duplicate determinations of several synthetic aviation gasoline blends of known aromatic content are shown in Table I. A comparison of gas chromatographic and FIA determinations of synthetic aviation gasoline blends containing 2 to 10% volume total aromatics is given in Table 11. These data show that the gas chromatographic method is more accurate than the FIA method for total aromatics although the time requirements for the two methods are comparable. The data further show that the standard deviation and accuracy of the gas chromatographic method are equivalent
BENZENE
\ I
IO
TiME, MiNUTES
Figure 3.
Right.
yo Volume iiromatics
-
Added. 2 07 3 41 5 38 7 97 9 95 a
GLC I 97 3 58 50 i 85 9 78 Av. dev.
Devia-
FIA
tion
2 17 4 12 ii 12 8 17 10
-0.10
1 0 $0 -0 -0 0
4 2
0 5 5
14
Deviation
t O 33
+ O 79 + O 62 +O 53 + O 55 0 56
Includes O.42Yc volume benzene in
babe stock as determined
ti!
GLC.
to the other currently used silica gel adsorpt'ion method (Z), which has a standard deviation of O.OS'l, volume total aromatics and is accurate to +0.27c volume. The gas chromatographic method, however, requires only 45 minutes per determination while about 6 hours are required by the silica pel adsorption method. Determination of Hydrocarbon Impurities in Petroleum Benzene and Toluene. As can be seen in Figure 1, benzene is retained in C a r b o n a s 400 until after n-nonane is e l u t d . It thus appeared t h a t this stationary liquid would also be very effective for determining hydrocarbon impurities in high-purity petroleum benzene and toluene. Initial chromatograms of petroleum benzene and toluene showed that no n-nonane was present in either aromatic; consequently, n-nonane was choscn as a n internal standard. The conditions cmployed for these determinations are a column temperature of 100" C. and a f l o ~rate of 40 ml. per minute. A >ample size of 0.03 ml. is used for both benzene and toluene samples. Coniplctc chromatograms are ohtaincd in
M
25
Hydrocarbon impurities
Left.
Table 11. Comparison of GLC and FIA Methods for Aromatics in Aviation Gasolines
IS TIME, MINUTES
In petroleum toluene
In petroleum benzene
approximately 30 minut'es. Typical chromatograms depicting the hydrocarbon impurities in benzene and toluene produced from straight-run fractions of Louisiana, and East and West. Texas crudes by catalytic reforming and extractive distillation are shown in Figure 3. Only the impurities actually present arc shown; the internal standard has not b t n i addrd. The ~ o l u n i cpiwcbntagcs ~ uf the impurities in bcmzcmx :id t o l u i w are calculated. as tlcsc4bcd a b o w . by comparing the p w k areas of the paraffinic and aromatic impurities to that of the added internal standard, n-nonane. Since the paraffinic impuritirs are not rrsolved by Carbowas 400, an average sensitivity is calculated for the total paraffins relative to the internal standard. This average sensitivity calculation is based on the individual paraffins and their relative amounts, either determined directly on the sample as described below for benzene or from analyses of the stream from which the aromatic is extracted. =In altcrnate procedure, which would require no knowledge of the individual paraffinic. impurities or t'hcir relative amounts! would be t o osidizca the hydrocarbons over copper oside a t 650" C. prior to detection, remove the water produced, and detect the hydrocarbons as carbon dioxide (6). This procedure gives per cent weight carbon directly by comparing the peak areas of the impurities to that of the internal standard, and since the carbon contents of Cg to Clo paraffins are not appreciably different, per cent weight carbon is essentially equal to per cent weight hydrocarbon. The lower limits of detectability for the paraffinic impurities and for benzene in toluene were determined t o be O . O l ~ o volume with synthetic blends. The lower limit for the toluene impurity in bcnzene was determined to be 0.027, volume. The
limits of dctcctability could be lowered by a t lrast a factor of 10 by decreasing the recorder range (0 to 1 mv.), increasing t h r cell cwrent, and increasing the volumc CJf sample charged to the chromatograph. Identification of Paraffinic Impurities in Petroleum Benzene. The paraffinic, inil~uritirs in petroleum bmzenr wrro identified by charging the h e n z c w iample t o a %-foot column packed x i t h 307, weight of 3,3'-osyciipi,oi,ioiitrile on 30- to BOmrsh fircabrick st 40' C. T h e merits of 3,3'-oxytli1)iopionitrile as a stationai'y liquid for the gas chromatographic separation of hydrocarbons h a w been dcscribcd by l'ennry ( 8 ) . The inipurities were idcntified from retention time data as n-hcbxane. 2,2-dimethylpentane 2,4-dimcthylpentane, 2-niethylhesane, 2,3-dimethylpentaneJ methylcyclopentanr, 1.1-dimethylcyclopentane 1.3-dimethylcyclopentanes,and cyclohexane. The 3:3'-oxydipropionitrile column, which is even more selectiw for aromatics than is Carbowas 400, is not used routinely for determining the impurities in benzene because some 5 hours are required for the elution of benzene. The toluene impurity in benzene could not be detrcted with this column a t the conditions en-iployed for this pliaw of the iirestigation. F o attempt,< were made t o identify individual paraffinic. impuritics in petroleum toluene. Results. The standard deviations ( 4 ) and accuracies of the gas chromatographic determination of hydrocarbon impurit,ies in benzene and toluene iscre determined by analyzing berizrne and toluene samples containing known volumes of impurities. The impurities present in the benzene and toluene base stocks were determined previously. The standard tlc,viations for paraffinic! and tolwnc impiuitics in benzene 'ivercl fount1
+
+
VOL. 32, NO. 1 1 , OCTOBER 1960
1395
Table 111.
GLC Repeatability and Accuracy Data for Hydrocarbon Impurities in Benzene
yo Volume Paraffins
% Volume Toluene Found Deviation Addedu Found Deviation 0.24 +0.01 0.11 0.13 +o, 02 0.00 0.13 +0.02 0.23 0.38 0.35 -0.03 0.24 0.23 -0.01 0.38 0.00 0.24 0.00 0.59 0.57 -0.02 0.32 0.31 -0.01 0.58 -0,Ol 0.33 $0.01 0.82 0.84 0.02 0.41 0.39 -0.02 0.86 + O , 04 0.39 -0.02 Av. dev. 0.016 Av. dev. 0.014 o Includes 0.03% volume paraffins and 0.02% volume toluene in benzene base stock as determined by GLC. Addeda 0.23
+
Table IV.
GLC Repeatability and Accuracy Data for Hydrocarbon Impurities in To1uene
yoVolume Benzene Deviation Added0 Found Deviation i-0.01 0.11 0.12 +0.01 0.00 $0.02 0.11 0.42 0.44 +0.02 0.21 0.21 0.00 0.44 +o. 02 0.22 +0.01 0.30 0.32 $0.02 0.62 0.59 -0.03 0.61 -0.01 0.32 0.02 Av. dev. 0.018 Av. dev. 0.010 Includes 0.02% volume paraffins and 0.01% volume benzene in the toluene base stock as determined by GLC. % Volume Paraffins
Addeda 0.22
Found 0.23 0.24
+
to be 0.014 and 0.01670 volume, respectively. The standard deviations for paraffinic and benzene impurities in toluene were found to be 0.014 and 0.012% volume, respectively. Results of 10 determinations were used to calculate the standard deviations cited. Results of duplicate determinations of several benzene and toluene samples to which 0.33 to 1.23y0 volume impurities have
been added are given in Tables I11 and
IV. Determinations of paraffinic impurities in high purity aromatics by the ASTM Kattwinkel reagent test are systematically low by 0.6% volume due to absorption of paraffins in acid and for this reason concentrations below 0.6% volume cannot be detected (10). From this fact and from the precision,
accuracy, and sensitivity data presented above, the gas chromatographic method is concluded to be superior to the ASTM Kattwinkel reagent test for determining hydrocarbon impurities in high purity benzene and toluene. ACKNOWLEDGMENT
The author expresses his appreciation to R. G. Eveld for his assistance in the preparation of the paper and to A. H. Cherry, M. C. Simmons, and M . Nager for critical reviews thereof. LITERATURE CITED
(1) Adlard, E. R., “Vapor Phase Chromatography,” pp. 98-114, D. H. Desty, ed., Academic Press, New York, 1957. (2) Am. Soc. Testing Materials, Philade!phia, Pa., “Aromatic Hydrocarbons in Olefin-Free Gasolines by Silica Gel Adsorption,” D 936-55, 1955. (3) I b i d . , “ASTM Standards on Benzene, Toluene, Xylene, and Solvent Naphtha,” D 851-47, 1947. 14) Zbid.. ‘IASTM Standards on Petro‘id., “Hydrocarbon Types
D 1319-581’. 1658 (6) Eggertsen: F. T., Groenninge, S., ANAL.CHEM.30,20 (1958). ( 7 ) Fabrizio, F. A., King, R. W.,Cerato, C. C.. Loveland. J. W., Ibid., 31, 2060 (1959). (8) Tenney, H. l l , , Ibid., 30, 2 (1958). (9) Whitham, B. T., ‘‘Vapor Phase Chromatography,” pp. 395-410, D. H. Desty, ed., Academic Press, Sew York, 1957. (10) Wood, J. C. S., Martin, C. C., Lipkin, M. R., ANAL. CHEM.30, 1530 (1958). RECEIVED for review March 14, 1960. Accepted May 23, 1960. Division of Analytical Chemistry, 137th Meeting, ACS, Cleveland, Ohio, April 1960.
END OF SYMPOSIUM
Factors Affecting the Use of Gas-Liquid Chromatography for the Separation of Large Samples The Column Dimensions W. J. d e WET Central Tobacco Research Station, Kroondal, Transvaal, South Africa VICTOR PRETORIUS Department o f Physical Chemistry, University of Pretoria, South Africa
b Theoretical and experimental evidence is put forward to show that for a given column efficiency the maximum sample volume increases as either the column diameter or the column length is increased. 1396
ANALYTICAL CHEMISTRY
I
T H . ~ Sbeen
shown (8) that the efficiency of a gas-liquid chromatography column decreases as the sample volume is increased and that the rate of decrease is affected by the method used for introducing the sample, b y the
amount of liquid in the stationary phase, and by the distribution coefficient of the solute. I n this paper the study is extended to include the effect of the dimensions of the column. An equation relating the column
