Groennings, S., Ibid., 28,303 (1956).
LITERATURE CITED
(1) Deemter, J. J. van, Zuiderweg, F. J., Klinkenberg, A., Chem. Eng. Sci., 5 , 271 (1956). (2) Dimbat, M., Porter, P. E., Stross, F. H., ANAL.CHEX 28, 290 (1956). (3) Eggertsen, F. T., Knight, H. S.,
(4) James, A. T., Martin, A. J. P., Biochem. J . 50, 679 (1952).
(5) Keulemans, A. I. hf., "Gas Chromatography", p. 113, Reinhold, Sew York, 1957. (6) Martin, A. J. P., Synge, R. L. hl., Ibid., 35, 532 (1943).
(7) Pierotti, G. J., Deal, C. H., Derr, E. L., Porter, P. E., J . Am. Chem. SOC.78, 2989 (1956). (8) Porter, P. E., Deal, C. H., Stross, F. H., Ibid., 78, 2999 (1956). RECEIVEDfor review April 4, 1957. Accepted July 22, 1957.
Determination of Five- to Seven-Carbon Saturates by Gas Chromatography F. T.
EGGERTSEN and SIGURD GROENNINGS
Shell Development Co., Emeryville, Calif,
'
b The saturates portion o f a reformer charge stock, containing chiefly Ce and C, compounds, can b e analyzed for all but one of its 25 Cs to C7 components b y gas chromatography employing three separate columns. In general the results agree with those obtained b y an American Petroleum Institute cooperative testing of the same sample b y other means (mass and infrared spectrometry, refractivity intercept, and catalytic dehydrogenation); the average deviation is about 0.3%. The average accuracy i s about 0.170 as established from analyses of a synthetic blend, simulating the CS to C7's in the actual sample. Certain C i s present in the latter cause some interference, but this can b e corrected for b y mass spectrometry of a few cuts. The procedure, with or without spectrometry, i s relatively fast, requiring 12 to 16 hours.
S
hydrocarbons in a petroleum distillate can be determined by various methods, including mass and infrared spectrometry, refractivity intercept, and catalytic dehydrogenation. To ascertain just what these methods will do and how much effort is involved in obtaining various degrees of information, the American Petroleum Institute's Committee on Analytical Research recently carried out an extensive cooperative test program with a reformer charge stock containing essentially C6 and C7 saturates (9). The results from the individual laboratories in some cases shorv appreciable deviations; yet the average values must be considered good, eren though the accuracy is not stated. However, the procedures for the detailed analysis are rather time-consuming, principally because the physical measurements must be carried out on numerous sharply distilled fractions. In view of the rapid strides made with gas chromatography (GC), it seemed that a simpler method could be devised ATURATED
20
ANALYTICAL CHEMISTRY
gram, obtained from Johns-Manville; 14- t o 48- or 30- t o 60-mesh granules. PELLETEX.A furnace black (Godfrey Cabot Co., Boston, Mass.) of about 24 square meters per gram surface area; 14- to 48-mesh granules. Supported Liquids. SQUALANE.A Cs&z paraffin (2,6,1OJ15,19,23-hexamethyltetracosane) of molecular weight 423 and boiling point 210' C, a t 1 mm., obtained by hydrogenation of squalene (an acyclic isoprenoid from shark liver oil) over a platinum catalyst a t 200 to 1000 pounds per square inch (11). Its high boiling point, good thermal stability, and low viscosity make it an especially attractive liquid of the paraffin type. 2,2'-OXYDIPROPIONITRILE, obtained from American Cyanamid Co.; boiling point 120" C. at 1 mm. Hydrocarbon Test Samples. A P I COOPERATIVE TESTSAMPLE. A 170" to 220' F. (72' to 124' C. by ASTM D-86 distillation) straight run catalytic reformer charge stock from California crude, was supplied by the California Research Corp., San Francisco, EXPERIMENTAL Calif. The saturates, 91.3y0 by weight, as determined by ASTM D-939 (9), were Apperatus. The assembly deisolated by liquid phase chromatography scribed previously, with helium as in a specially designed large scale FIA carrier gas and thermal conductivity (fluorescent indicator adsorption) colcells for detection, was employed (6). umn ('7). From a charge of 10 ml., Except in a few experiments when there was obtained 99 =t 1% of the fractions were t o be collected for saturates, which must, therefore, be mass spectrometric analysis, the eluted considered representative of the satuhydrocarbons were oxidized by passage rates in the sample. over hot copper oxide as they emerged SYNTHETICBLEEDS. These were from the column and were thus deprepared from Cs to C7 saturated hydrotected as carbon dioxide. The comcarbons of minimum 997, purity, obbustion train consisted of an 8 X 1/4 inch tained from Phillips Petroleum Co., column of copper oxide (J. T. Baker Bartlesrille, Okla., or from the American Chemical Co., wire form) at 650' C. and Petroleum Institute. The hydrocara 1 foot X, inch column of calcium bons concerned are listed according to sulfate (Drierite) at room temperature boiling points in Table I with abbrevito remove water of combustion. The ations used in the chromatograms. thermal conductivity cells were kept in Column Preparation. The supa liquid bath a t ambient temperature. ported liquids were added t o the Samples were added directly to the solid carrier as solutions in petroleum column inlet through a serum cap. ether (for squalane) or in ethyl alcohol The inlet section, n-hich was packed (for oxydipropionitrile) . The solvent with glass wool, was heated electrically was evaporated slorvly on a hot plate to about 100' C. to achieve rapid or steam bath while stirring, and finally evaporation of the sample during chargin an oven for 1 hour a t 110' C. The ing. Solid Supports. C-22 FIREBRICK.dry material was packed with the aid of an electric vibrator into 50 feet of I/,A Celite-base material having a surinch outside diameter copper tubing by face area of about 3 square meters per
to furnish the same amount of information. A gas chromatographic method for complete analysis of Cg and Ce saturates, employing a liquid-modified solid adsorbent as the column packing, has been reported by the authors (6); and further study of naphthene-paraffin selectivity yielded two other distinctive types of column packing (4). By combining information obtained from separate determinations with these columns a satisfactory detailed saturates analysis through 0, appeared feasible. The test sample chosen was the same material used in the cooperative program mentioned above. Mass spectrometry was utilized t o study interferences by Cs hydrocarbons, which overlap the C?'s to some extent. Also, a combined gas chromatographicmass spectrometric procedure was developed as an alternative to one employing gas chromatography alone.
folding it into U-shape and adding the packing to both ends. The column mas then wound into a coil on a 13/s-inch pipe as mandrel. The percentage of liquid on the carrier is expressed as grams per 100 grams of dry solid support. Samples for Mass Spectrometric Analysis. Fractions for mass spectronietric examination were collected by passing the effluent stream froni the column through a small, liquid nitrogen-cooled glass tube with a sintered-glass plate for contact. Columns and Frocedure. With respect to naphthene-paraffin selectivity, the columns del-eloped in previous work ( 4 ) fall int’o three general classes: relatively nonselective, neutral, or “boiling point” columns which contain a nonpolar liquid on an inactive solid; polar columns which greatly retard naphtheries relative to paraffins, and wliich contain highly polar liquids on an inactive solid; liquid-modified solid adsorbent’ columns n-hich retard paraffins relative to naphthenes, and which contain just sufficient liquid on an active solid to prcvent tailing. The most effective columns of each type found thus far and used in this study are listed in Table 11, with other data pertaining to the procedure. Column 1 separates the sample into the greatest number of individual components, and is, therefore, the master colunin. With a mixture containing about equal amounts of the 26 conimonly encountered C j to C: saturates plus iso-octane this column yields 19 individual compounds, more than obtained with any of numerous packings tested. Other packings tried contained the following liquids on C-22 firebrick and/or Pelletex: Cetane, a mineral oil (Onclina 133), a silicone (General Electric Co. SF-96), diisodecylphthalatc, ethylene g l ~ ~ oand l , Triol 230 (Union Carbide Cheniicals (20.). Although none of these appeared as promising as column 1, it is probable that equivalent or better packings can be developed by proper choice of liquids and conditions. The separations obtained ITitli column I are illustrated in Figure 1. The components not separated by this column are determined using columns 2 and 3, except 3-ethylpentane. Some of the conditions given in Table I1 were selected more or less arbitrarily, while others are thought to be about optimum on the basis of numerous tests with various packings under many conditions. The column inlet pressures were chosen arbitrarily a t 400 to 600 mm., ~vhich fixed the helium flow rates through the 50-foot l:/,-inch columns a t 30 to 60 ml. per minute. Flow rates could be varied in this range with little effect on resolution. Higher flow rates were not tried.
Table 1.
The 26 Commonly Encountered Cg to C, Saturated Hydrocarbons (Plus 1so-octane)n
B.P., “C. 27.9 36.1
Compounds 2-Methylbutane(isopentane)
n-Pentane Cyclopentane 2,2-Diniethylhutane 2,3-Dimethylbutane 2-Methylpentane 3-Methylpentane n-Hexane hlethylcyclopen tane 2,Z-Dimethylpentane 2,4-Dimethylpentane Cyclohexane 2,2,3-Trimethvlbutanei .~ trintane) 3;QiDimethylpentane
49.3
49.7 58 0 60.3 63.3 68.7 71.8 i9.2 80.5 80.7 80.9 86.1 87.8 89.8 90.1
1,l-Dimethylcyclopentane
2,3-Dimethylpentane 2-Me thglhexane
90.8
1,cis-3-Dimethylcyclopentane
91.7 91.9 91.9
l,truns-3-Dimethylcyclopentane 1,truns-2-Dimethylcyclopentane
3-Methylhexane 3-Ethylpentane n-Heptane 2 2 4-Trimethylpentane(iso-octane)
93.5
1~c~s-2-Dimethylcyclopentane
Methvlcvclohexanc Ethyky Elopentane a Listed according to boiling poirIt. Table
Column No. 1
2 3
II.
98.4 99.2 99.5 100.9 103.5
Abbreviated Nomenclature 2-MB n-Pent CP 2,2-DMB 2,3-D M B 2-bIP
Carbon Number 5 5 5 6
6
6 6 6 6
3-NP
n-Hex RICP 2,2-D;\.IP 2,4-DhIP CH 2,2,3-T ?\.IB 3,3-DMP 1,l-DhlCP 2,3-DMP 2-MH 1,cis-3-DMCP l,t~-3-DhfCP l,tr-2-DMCP 3-MH 3-EP n-Hept 2,2,4-TMP 1,cis-2-DhXCP MCH ECP
7
7 6 7 7 7 7 7 7 7 7
7
7 7 8 7 7 7
Columns and Conditions
Column length. 50 feet X 1/4 inch 10 to 15 mg. Sample size. Inlet Helium Mesh Pressure, Flow, Packing Size Mm (>.\tm.) 3\11./Minute 3% squalane on C-22 firebrick 14 to 48 400 65 40Yp 2,2’-oxydipropionitrile on C-22 fire30 brick 30 to 60 600 1.5% squalane on Pelle400 . 32 tex 14 to 48
The other variables had a marked influence on resolution. In general, separations were better the lower the temperature, no doubt because volatility ratios are more favorable a t lower temperatures. However, below room temperature the peaks were sometimes broad and/or distorted. The peak resolution invariably improved as the column was lengthened, but 50 feet seemed to be about the maximum practical length. Rather coarse granular packing was used to permit adequate helium flow through the long columns. The amount of supported liquid in the packing is critical. With column 1, squalane on C-22 firebrick, the optimum is 3%. More or less than this amount changes the relative emergence times of certain compounds, paraffins as n-ell as naphthenes, with a net loss in number of individuals resolved. This is evidence that 3% squalane is not enough solvent to obviate adsorptive effects of the supporting solid, which in this case are desirable. One should never use more solvent than necessary,
Column Temp., O C. 25
25 75
because then a higher temperature is required to avoid unreasonably long emergence times; higher temperatures give poorer separation of close-neighboring peaks. Column 2 contains oxydipropionitrile on C-22 firebrick, and because this is a poorer solvent than squalane, 40% liquid can be used at 25’ C. without excessive emergence times. I n this caSe a large amount of liquid is desirable for maximum retention of naphthenes versus paraffins ( 4 ) , which is the outstanding feature of this column. Column 3 contains 1.5% squalane on Pelletex an amount selected because it is the minimum required to prevent tailing ( 5 ) . K i t h this small amount of squalane the packing has the desired property of retaining paraffins relative to naphthenes (as with bare solid adsorbents), but with more solvent it approaches the type represented by column 1, The resolution is in general better with smaller samples. Ten to 15 mg. was small enough for good resolution and yet large enough to ensure repreVOL. 30, NO. 1, JANUARY 1958
21
sentativeness of sample and t o permit determination of minor components. Larger samples tend to cause peak broadening of the major components with a corresponding loss in resolution of neighboring peaks. I n most of the experiments the hydrocarbons were converted t o carbon dioxide prior to detection by thermal conductivity (8). The effect of any difference in thermal conductivity of the various hydrocarbons is eliminated and the peak areas can be unequivocally expressed as weight per cent of the sample. Furthermore, the detector
Table 111.
Compounds 2-hIB
C6 n-Pent (2,2-DMB
C6 { 3-MP
C7 13-EP
MCH
response is increased about threefold without observable decrease in separating efficiency; this is of particular advantage for the determination o€ minor constituents. I n the calculations a small correction was made for the higher carbon content of the naphthenes, which, in the Cs to C, range, is about 27, greater than for the paraffins; no correction was made for the small variations in carbon content among the paraffins (less than 1%). When auts were to be collected for mass spectrometry, combustion was of course omitted.
Duplicate chromatograms were obtained with each of the three colunins, and the individual peak areas (planimeter) mere expressed as percentage of total area. The data thus obtained for the individual hydrocarbons were combined and normalized to 100%. The areas for partially separated peaks or shoulders were defined by dropping a perpendicular from the minimum, or inflection point, to the base line. Such estimated areas, even of faintly discernible shoulders, n-ere apparently fairly reliable. The apparent number of theoretical
Analysis of C6 to C7 Saturates in a California Reformer Charge Stock, W t . Column 1. 3'30 squalane on C-22 firebrick, 25' C. Column 2. 40% 2,2'-oxydipropionitrile on (2-22 firebrick, 25' C. Column 3. 1.5% squalane on Pelletex, 75" C.
Saturates from Reformer Charge Stock (API Sample) Synthetic Mixture GC Alone GC LIS4 API- __ GC Alone C0.4R Found Deviation Found Deviation Known Found Error 0 0 ... 0 1 ... 0 1 0.0 0.0
+
0.2 0.3 -0.1 ~ 0 . 6 -4.8 -4.2 8.0 11.4 7.3 -0.3 -0.6 -0.2 1.8 -1.8 -3.2 3.5 2.7 6.5 -4.4 -0.4 8.3 1.4
0 2 0 1 0 1 0 7 4 6 4 1 8 2 11 6 7 2 0 3 0 7 0 0 1 3 1 4
3 4 3 6 4
0 4
:::aic
1
-0.1 -0.3
6 8 70 4
-0.7 -0.6 +1.4 +0.4
f
7 0 14 1
13,3
1.0 4.5 8.5 90.9
1 2
0.0 +O. 1 0.0 +O. 1 -0.2 -0.1 -0.4 +0.2 -0.1 0.0 +O. 1 -0.2 -0.5 -0.4 -0.1
0 3 0 5 0 1 0 4 5 3 4 5 9 3 12 1 7 3 0 4 0 5 0 0 1 3 1 2 3 1 5 0 2 7 7 1 3 6 0 6 7 8 0 9 12 2 0 8
+O. 1
f0.2
0.0 -0.2 +0.5 $0.3 +0.7 $0.7 0.0 +o. 1 -0.1 -0.2 -0.5 -0.6 -0.1 +1.5 0.0 +0.6 -0.8 +0.2 -0.5 -0.5 -1.1 -0.2
0.4 0.5 0.4 O.i 5.3 4.6 9.9 13.1 8.4 0.3 0.7 0.2 2.3 2.2 3.7 3.1 4.1 7.0 4.8 0.4 9.6 1.7 15.2 1.5
0.3 0.4 0.3 0.6 5.5 4.7 9.6 13.4 8.8 0.3 0.7 0.2 2.1 2.2 3.8 3.0 4.0 7.0 5.1
-0 -0 -0 -0 +0 f 0 -0 i-0 +0 0 0 -0 +0 0 +0 -0 -0 0 $0
9.6 1.3 15.3 1.4
0 0 -0 4 $0 1 -0 1
!
1 1
1 1 2 1 3 3
4 0 0 0 1 0 1 1 1 0 3
)
araffins -0.1 12 9 -0.1 12 9 CS apht henes 100 0 Total 100 0 0.39 0.3 3 Av. dev. Mass suectrometrv. * Betterty column 3 , hon-ever. c Includes O.lc&of Cs (2,5-dimethrlcyclohexane) according to MS. Bl subtraction of CP e\; column 1. e BYsubtraction of 1.1-UlICP ex column 3. f Kot determinable bv GC alone in this small quantity. 0 Includes 1 .lpGof C8( I ,1,3-trimethj-lcyclopentane) according to MS.
100.1
99.9
%
Column Employed GC ?VISa GC (BPI sample) Alone
+
1 1 1
1 I* 1 1 1 1 1 1 1 1 3 2
1 1 1 1 '
1 1 1
2+ld 2 2 35 2 2
+
1 1
1 1
1
3
3
0 13
2
0
E\TEXGEUTE T'ME. h O C R S
Figure 1. Gas chromatogram of a synthetic mixture of 26 commonly encountered C g to C7 saturated compounds plus iso-octane
22
ANALYTICAL CHEMISTRY
plates was about 5000 for columns 1 and 2, and 3000 for column 3. These values were calculated for components of 1 mg. or less using the expression (4VE/A)z, where liE is the retention volume or emergence time measured from the injection point to the peak maximum, and A is the extrapolated peak width in the same units. This evpression is equivalent to the number
1
gas chromatography alone, employing conversion to carbon dioxide; and separation by only one column (3% squalane on C-22 firebrick) without conversion to carbon dioxide, combined with mass spectrometric determination of three fractions from this column. With the synthetic sample, which contained no interfering Cg hydrocarbons, only the first procedure was used.
of theoretical plates in the idealized case (9). RESULTS
Table I11 summarizes results obtained for the American Petroleum Institute sample and the synthetic sample. M7ith the first sample, two procedures were used : three-column analysis by
COLUMN 1: 3% SQUALANE ON C - 2 2 FIREBRICK [NEUTRAL) ( 2 . 2 . 3-TMB)
+
[ 3 , 3-DMP)
i
2-MP
Includes C , (Lo 5 hrs)
4
1 3-MP
[
( 2 . 2 , 4-TMP)
3-EP I, tr-2-DMCP
CH
MCH 1. 1. 3 - T M C P
( 2 , 2-DMH)
DMH
2 , 3-
DMR n-Pent
0
L
I
I
I
I
I
I
I
1
I
(3. 3-DMP) "-Hex
~
( 2 . 2 , 3-TMB)
I
. II
4
i
I
I
I
3
2
1
0
COLUMN 2 : 40% p , B'- OXYDIPROPIONITRILE ON C - 2 2 FIREBRICK (POLAR)
3-EP n-Hept MCP
n
I n c l u d e s C I ( t o 3.5 hrs)
c
2 , 5-DMH
4
II
I, cis-2DMCP MCH
1, t r - 3 - D M C P
I , I-DMCP 1, tr-2DMCP
3-MH
CH
"-Pent
Y
2.2-DMP
2.2-
W
L
I
1
1
,
n I
I
I
I
I
I
I
I
I
I
I
COLUMN 3:
fl I
I
3
2
I
I . 5% SQUALANE ON P E L L E T E X ( F U R N A C E BLACK)
(LIQUID-MODIFIED SOLID)
( 2 . 2. 4-TMP)
L
I
I
I
I
o
I
I
I
I
I
I
I
Figure 2.
I
I
I
I
l
2
EMERGENCE
TIME, HOURS
Gas chromatograms of CS to C, saturates in a reformer charge stock VOL 30, NO. 1, JANUARY 1958
23
Chromatograms of the American Petroleum Institute sample are given in Figure 2. The sequence of emergence is given for the commonly encountered CS to C, saturates and for the lowest boiling CS saturates. Parentheses indicate the absence of compounds; their emergence times are denoted by arrows. These were determined by tests with individual compounds and with various known mixtures. Iso-octane (2,2,4-trimethylpentane), which boils in the C7 range, can be determined with column 1. Some of the C i s boiling above iso-octane, noalso tably 1,1,3-trimethyI-cyclopentane, overlap the C?’s, and this interference is greater with the more polar column 2. With column 3 there was very little if any overlap of Cis, the separation being essentially by carbon number. Here isooctane, the lowest boiling CS (99.2” C ) emerges just after ethylcyclopentane (103.5’ C.), the last C? to emerge; interference from 1,1,3-trimethylcyclopentane (104.9’ C.), although not expected, is a possibility which was not investigated. However, the result for total CS by column 3, 12.9% (=kl.O), agrees R-ell with the reported value of 13.0%; the figure 12.9 was used in normalizing the values. Column 1 gave a lower result for total CS hydrocarbons than column 3 (10.2%). This is partly due to overlap with Cj’s, amounting to about 1.2y0;some of the CSpeaks from column 1 were low and broad because of the long emergence times (up to 5 hours), and hence were not amenable to measurement. The results for the three-column analysis show that 17 individual components were determined by column 1, five more by column 2, and only two by column 3, although the latter gave total Cis. The effectiveness of column 2 is due to its property of retarding C7 naphthenes relative to C, paraffins, a separation which was not achieved by column 1. Column 2 selectively retards dimethyl-substituted saturates which have adjacent methyl groups. For example, 2,3-dimethylbutane (boilingpoint, 58.0’ C.) emerges after 2-methylpentane (60.3” C.), whereas the reverse is true with column 1; similarly 2,3dimethylpentane and 1, trans-2-dimethylcyclopentane are retarded relative to their near-boiling isomers. -411 components were determined directly-that is, as single peaks-except 2-methylhexane and 1, trans-3-diniethylcyclopentane, which were determined by combining the results from two columns (Table 111). 3-Ethylpentane, a minor component, which emerged from all columns with tlvo or more other components, could not be estimated in the small amount present (0.4%). Interference from Cg’S caused slightly high results for methylcyclohexane, ethylcyclopentane, and two of the dimethyl24
ANALYTICAL
CHEMISTRY
cyclopentanes. By mass spectrometric analysis of the methylcyclohexane band from column 1, 1.1% of 1,1,3-trimethylcyclopentane (based on the whole sample) was found. Similarly, 0.1% ot 2,5-dimethylhexane was found in the ethylcyclopentane band, and 0.4% of 2,Sdimethylhexane in the first two peaks of the dimethylcyclopentanes from column 2. Thus the results for these particular C7’sare somewhat high, though the interference with ethyl cyclopentane is considered negligible. Neither 2,2- nor 2,bdimethylhexane was found in these cuts. I n Table I11 the API-COAR vaIues are the published grand average of values for the sample from 12 laboratories of the American Petroleum Institute Committee for Analytical Research (Q), obtained by means other than gas chromatography; these values have been converted from volume per cent of the \Thole sample (including aromatics) to weight per cent of the saturates portion. [Their designation of the cis- and trans-l,3-dimethylcyclopentanes was reversed to conform to present American Petroleum Institute n‘omenclature (IO).] Three fractions, containing unresolved components, were analyzed by mass spectrometry. The chromatographic determinations were made in triplicate, but the fractions from only one of these were analyzed mass spectrometrically. Only the third cut contained an appreciable amount of CS hydrocarbons-namely, 1.1% of 1,Ij3trimethylcyclopentane; a correction for this was made in the calculations. I n these analyses, which were made without conversion to carbon dioxide, the peak areas were assumed proportional to weight per cent. Relatively low results were obtained for the components of higher molecular weight, and the average deviation was 0.39%, somewhat higher than for the threecolumn analysis by gas chromatography alone. If it is assumed that peak area is a measure of mole per cent, which is then converted to weight per cent, the average deviation is somewhat better, 0.32%. In connection with interpretation of peak areas, Fredericks and Brooks (6) found a good correlation of area with weight per cent for the Cz t o C5 hydrocarbons. On the other hand, calibrations reported for several C5 to C7 hydrocarbons indicate rather rside variations in weight sensitivity (3). Browning and Watts (1) have reported a method by which weight sensitivities can be approximated from the thermal conductivities of the pure compounds. HoIJ-ever, in view of present uncertainties. it appears that for best accuracy one should use experimental calibration factors, or, alternatively, oxidize to carbon diouide.
CONCLUSION
’4blend of 25 C5t o C j saturates, made to simulate that portion of a reformer charge stock, can be analyzed for all components (except 3-ethylpentane) by gas chromatography using three different types of columns. The average accuracy was 0.13% and the maximum error was 0.4% (basis whole sample). With two columns 22 components can be accounted for. V i t h one column 17 compounds can be accounted for and the balance can be resolved by mass spectrometry of three cuts taken from this column. I n an actual distillate some CS saturates are present. These interfere in the three-column analysis and render the values for certain (2,’s somewhat high, probably by about 10% of the values. I n another scheme, utilizing the information from one column plus mass spectrometric data pertaining to three cuts from this column, a correction is made for the interfering Cs’s. Interference from Cs’s could possibly be eliminated by collecting the C6 to C, saturates from the “carbon number” column 3 and rerunning this portion in columns 1 and 2 ; however, this scheme has not been tried. I n the analysis of the American Petroleum Institute sample by gas chromatography alone the accuracy may be presumed to be comparable with that of the synthetic blend, except for three components with which Cs’s interfere, Kithin the average accuracy of the gas chromatographic method, the results provide a means for estimating the accuracy of the grand average API-COAR results. Excluding the data for methylcyclohexane and the 1,3-dimethylcyclopentanes,the average deviation between ilPI-COAR’s and values reported here is 0.22%. Thus the API-COAR results appear to be good. For a complete analysis of the saturates in a reformer charge stock gas chromatography is probably more reliable, on the whole, than other available methods. It is certainly the most expeditious, because spectrometry, the most versatile and reliable of the other methods, requires a lengthy fractional distillation prior to analysis, whereas the three-column analysis can be made in 12 to 16 hours of elapsed time. The time requirement for one-column analysis, employing mass spectrometry, is about the same.
ACKNOWLEDGMENT
The authors wish to express their appreciation to P. A. ITadsTvorth, Jr., for carrying out the mass spectrometric work.
LITERATURE CITED
(1) Browning, L. C., Watts, J. O., ANAL. CHEM.Z9,24(1957). (2) Deemter, J. J. van, Zuiderweg, F. J., Klinkenberg, A,, Chem. Eng. Sci., 5, 271 (1956). (3) Dimhat, M., Porter, P. E., StrOES, F. H., ANAL. CHEM. 28, 290 (1956). (4) Eggertsen, F. T., Knight, H. S., Ibid., 30, 15 (1958).
(5) Eggertsen, F. T., Knight, H. S., Groennings, S.. Ibid., 28, 303
(1956). (6)
Fredericks, E. M., Brooks, F. R.,
Ibid., 28, 297 (1956). (7) Knight, E. S., Groennings, S., Ibid., 28, 1949 (1956). (8) Martin, A. E., Smart, J., Natwe 175, 422 (1955). (9) Martin, C. C., Knrta, S. S., Jr.,
others, ANAL. CHEM. 28,
490 (1956). (10) Rossini, F. D., Li, Xunn, Science 122, 513 (1955). (11) sax,K, J,, stress, F, H., A ~ CHEM.29, 1700 (1957).
RECEIVED for review April 4, 1957. Accepted July 22, 1957. Division of Refining, 22nd Meeting, Amencan Petroleum Institute, Philadelphia, Pa., May 1957.
Exploratory Studies of High Tempercri Gas-Liquid Ghromccltography JAMES L. OGILVIE, M. C. SIMMONS, aind G. P. HINDS, Jr. IHouston Research Laborc!tory, Shell Oil CcL, P. 0. Box 2527, Houston I , Tex. L A ----?:-,,:A rcJu~-,,yv,urhr-mmatography system has been designed which is capable of operoting up to about 400" C. The preheater, separation column, and detector are all cast in an aluminum block and heated electrically. The stationary liquid employed is petroleum-derived asphaltenes, which a r e thermally stable and 'sufficiently nonvolatile to operate in the range from 300' to 400' C. This technique has been used for the determination of n-paraffin distribution in waxes. The results obtained compare favorably with independent moss spectrometric analyses, and indicate that gas-liquid chromatography can be a useful technique for the analysis of higher boiling petroleum fractions.
APPARATUS
Figure 1 shows a schematic flow diagram of the apparatus. The helium enters the preheater coil after passing through a series of regulating devices, proceeds through one passage of the double-pass thermal conductivity cell, and enters the column through an intermediate tee which also serves a s a sample inlet. The exit end of the chromatographic column is attached t o the other passage of the conductivity cell. From this cell passage the gas is vented to the atmosphere.
G
is a useful and growing field in both organic and andytical chemistry (1 4 ). However, in consideration of the upper operating temperatures of most stationary liquid phases and detector systems, the limit imposed by most investigators has been placed a t compounds boiling approximately at 250' C. The possibility of extending the range of gasliquid chromatography t o substances of higher molecular weight has been intriguing. High temperature gas-liquid chromatography would be of interest both as a complete analytical procedure in itself and as a separation tool for other analytical methods. Higher molecular weight separations have been made possible in this laborstory by the use of asphaltenes as the stationary liquid phase and bare-wire thermal conductivity filaments in the detector cell. The apparatus is actually of more or less conventional design, with specific pains being taken t o a 5 sure satisfactory operation at elevated temperature. As-LIQmn CHROMATOGRAPHY
final form of a cylinder 10 inches in diameter and 12 inches high. The top of the conductivity cell was flush with the top of the cylinder and one end of the sample inlet tee protruded from the block. Resistance wire was evenly wound around the circumference of the block to provide beat to the unit. The temperature was controlled by means of a Powerstat. The operating temperature of the instrument was measured by means of a thermocouple placed in a hole drilled next to the conductivity cell. The finished assembly is illustrated in Figure 2. Because of the requirements of low volatility and thermal stability at high temperatures, the choice of a stationary phase was limited. It seemed that a petroleum-based fraction of high molecular weight would satisfy these requirements. The fraction decided upon was derived from the propane deasphalting of West Texas residue. The asphalt from this separation was pentane deoiled, and only the ether-soluble portion of the resulting asphaltenes was used. Although this fraction was chosen arbitrarily, it is assumed that other asphalt fractions would give similar separations.
Figure 1. Gas flow diagram of high temperature gos-liquid chromatographic apparatus
The chromatographic column was prepared by packing 23 feet of stainless steel tubing s/8 inch in outside diameter with 20- t o 40-mesh C-22 firebrick (Johns-Manville). The tubing was vibrated during the introduction of the firebrick, which had been previously screened, water-washed, and dried. The packed column was then helically coiled; one end was attached to the thermal conductivity cell and the other end t o the sample inlet tee. The preheater coil was then fastened in position and these parts of the apparatus were cast in an aluminum block which had the
Figure 2. assembly
Top of
oluminum block
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