Table 111. Comparison between Observed and Calculated Linear Program Rates Required to Produce a Given Analysis Time Reduction Factor aa Experimental Calculated a-Experimental program rate program rate value (atm/min) (atm/min) 0.62 0.11 0.11 0.52 0.19 0.20 0.39 0.47 0.46 1.11 1.12 0.27 a Based on C1l retention data, Rf = 0.112. Constant pressure retention time, t . = 9.65 min; Initial inlet pressure for programs, pin = 1.54 atm; Inlet pressure for constant pressure experiments, p a = 1.54 atm; Outlet pressure (constant), p o = 1.0 atm. for a linear pressure program. The constant pressure retention time ts for component m in a column operated at constant inlet pressure p , is, approximately
Because PR = Pin and Equation 11,
+ (b/po) tR, we can write from Equation 10
If the pressure programmed retention time, t R , is written in terms of the constant pressure retention time, tR = at, where a is less than unity, we have
b =
2 PO [(pa - 1) - @in a 2ta
- 111
(1 3)
Equation 13 permits qualitative estimation of the programming rate required to obtain a time reduction factor a,or the a produced by rate b, in terms of the initial program pressure and a previously determined constant-pressure retention time, Equation 13 is valid only to the extent that the approximations Equation 10 and Equation 12 are valid. Furthermore, Equation 13 assumes an approximately linear pressure program. Table I11 shows an evaluation of Equation 13. The observed time reduction factors a for C11 in four pressure programs are substituted into Equation 13, along with the C11 retention time ts for a constant pressure run at P8. Programming rate constants b are then calculated and compared to the actual rate constants which produced the observed a's. The agreement is satisfactory. RECEIVED for review February 12, 1969. Accepted May 19, 1969. This research was conducted under the McDonnell Douglas Independent Research and Development Program. Part of this work was presented at the 157th National ACS Meeting, Minneapolis, Minn., April 1969.
Separation of Asphaltic Materials by Reversed Phase Partition and Adsorption Chromatography Comparison of the Fractions by Infrared Spectrometry R. V. Helm Laramie Petroleum Research Center, Bureau of Mines, Laramie, Wyo. 82070
THESEPARATION of thermally sensitive, high molecular-weight materials requires the development of techniques different from those used in the gas chromatographic separation of thermally stable materials. Gas-liquid chromatography (GLC) has many advantages such as small sample size, speed of separation, and continuous quantitative detection of the samples. However, the use of GLC in the analysis of high molecular-weight materials is restricted by the low volatility and the thermal instability of these materials. Liquid-solid chromatographic techniques are well known and are suitable for the separation of thermally sensitive materials. Silica gel and alumina have been used to separate materials containing hydrocarbons as well as organic sulfur-, nitrogen-, and oxygen-containing compounds. However, some materials are irreversibly adsorbed on silica gel and alumina. A relatively new method of liquid-liquid chromatography appears promising for the separation of polar and reactive materials with a minimum of compositional changes. This method is referred to as reversed-phase partition chromatography (RPPC) and was first described by Howard and Martin 1342
ANALYTICAL CHEMISTRY
(I) in 1950 for the separation of fatty acids. RPPC is a column chromatographic technique using an inactive packing material to support a nonpolar stationary liquid phase on the column. The sample, after introduction on the column, is partitioned by percolation with a polar mobile phase. This procedure removes the more polar materials in the percolate, leaving the less polar materials on the column. The less polar materials are removed from the column by washing with suitable solvents. RPPC has been of great value in the separation of biological materials and inorganic compounds. These separations, however, have been primarily with aqueous systems. A review of the literature on RPPC by Bandami (2) was published in 1965. In 1968, Locke (3) reported the separation of model hydrocarbons on silanized Chromosorb P using squalane as the stationary phase and acetonitrile as the mobile phase. (1) G. A. Howard and A. J. P. Martin, Biuclrem. J . , 56, 532 (1950). (2) R . C. Bandami, Client. Z / i d (Loridmt), 1965, 1211. (3) D. L. Locke, J. Cltrumatugr., 35,24 (1968).
~
~
~~
~
~~~~
Table I. Polar Group and Aromatic Absorbances of RPPC Fractions from Molecular Distillation Fraction 1 of Wilmington Asphalt Absorbance of band at RPPC fraction 3600 cm-l 3480 cm-1 1740 cm-l 1700 cm-1 1655 cm-l 1600 cm-l number (OH) "(1 (C=O) (C=O) (C=O) (aromatic C=C) (C=O) 1 2 5
0.01 0.03 0.03
0.25 1.14 0.50
0.04 0.10 0.10
The following preliminary report describes the use of a nonaqueous RPPC system followed by silica gel chromatography to separate a molecular distillation fraction from a Wilmington (California) asphalt. In addition, the separation of the same fraction on silica gel only is reported; and a comparison of the separated materials is made by infrared spectrometry. Infrared spectrometry has previously been found useful ( 4 , 5 ) in showing differences in chemical composition among high molecular-weight petroleum fractions.
0.05 0.25 0.28
0.02 0.05 0.04
0.08
0.71 0.65
percolated through the column to displace the entrapped air. A 2-gram sample of the molecular distillation fraction or the RPPC residue from this fraction, diluted with an equal volume of heptane, was placed on the column. When the sample had entered the silica gel, elution with heptane was started and the heptane fraction collected until the detector indicated the solute in the effluent stream had dropped to a constant level. Elution of the sample with benzene was then started. When the detector again indicated a constant level of solute, elution with l/l benzene-methanol was started. Methanol was used as the final desorbant.
EXPERIMENTAL
Apparatus. CHROMATOGRAPHIC COLUMN.The chromatographic column used for the RPPC and silica gel separations was 12 mm 0.d. glass tubing with a solvent reservoir at the top of the column. The bottom of the column was fitted with a fritted glass disk to support a packed section 122 cm long. A Teflon (DuPont) stopcock at the bottom of the column was used to control the effluent flow rate at approximately 1/2-ml/min. Water at constant temperature was circulated through a glass jacket to control the column temperature during the RPPC separation. Solute concentration in the column effluent was monitored with a Glowall Continuous Fraction Detector having a hydrogen flame detector and equipped with a strip chart recorder. INFRARED SPECTRA.Infrared spectra were measured with a Perkin-Elmer Model 521 spectrophotometer. Spectra were determined on the recovered fractions in methylene chloride solutions employing solvent compensation. Procedure. RPPC SEPARATION.The chromatographic column was packed with 20-40 mesh Fluoropak 80, an inert fluorocarbon material (silanized celite was unsatisfactory as a column packing because of irreversible adsorption of asphaltic materials on the celite). Cyclohexane saturated with nitromethane at 25 "C was passed through the column to displace the entrapped air and to serve as the stationary phase. Nitromethane saturated with cyclohexane at 25 "C (mobile phase) was percolated through the column until the nitromethane phase appeared in the column effluent. The temperature of the column was maintained at 25 "C to prevent compositional changes in the stationary and mobile phases during the separation. A 2-gram sample of fraction 1 from the molecular distillation of Wilmington (California) asphalt diluted with an equal volume of the stationary phase was then placed on the column. After the sample had entered the packing material, percolation with the mobile phase was resumed; and 26 fractions corresponding to one free volume (interstitial volume) each were collected. Percolation with nitromethane was stopped when the detector indicated that the concentration of the solute in the effluent was constant at less than 1 % of the sample per fraction (fractions 7-26). The RPPC residue remaining in the column was removed by washing the column with cyclohexane and finally with benzene. SILICAGELSEPARATION. The chromatographic column was packed with 28-200 mesh silica gel, and heptane was (4)J. E. Stewart, J. Res. Nut. Bur. Stand., A , 58, 265 (1957). (5)F.J. Linneg and J. E. Stewart, ibid., 59,27 (1957).
RESULTS AND DISCUSSION
Fraction 1 from the molecular distillation of the Wilmington asphalt used in this work has been described previously (6). This fraction had a number average molecular weight of 370 and a boiling point of about 500 OC. When the silica gel separation was preceded by the RPPC separation, the molecular distillation fraction was recovered quantitatively. Silica gel separation of the whole fraction (omitting RPPC) resulted in a 1.5 loss of the sample. After the silica gel runs, the column in which the RPPC residue was chromatographed appeared white and had no dark bands. However, the silica gel column in which the whole fraction was chromatographed had an irreversibly adsorbed dark band extending downward about 12 cm from the top of the column. These results indicate that materials in the whole sample which are irreversibly adsorbed on the silica gel may be removed prior to silica gel chromatography by extraction with nitromethane. Table I gives the infrared data on selected fractions from the RPPC separation of the molecular distillation fraction. Table I1 gives the infrared data on the silica gel separations of both the whole fraction and the residue after extraction of the whole fraction by RPPC. The values shown in the tables are the absorbances at 3600 (OH) and 3480 (NH), the carbonyl ( e o )absorbances at 1740, 1700, and 1655 cm-l, and the aromatic absorbance at 1600 cm-l. The concentration of the fractions in methylene chloride solution varied; therefore, the absorbances reported are not comparable between fractions on a quantitative basis. The data in Table I indicate that materials containing free OH, NH, and carbonyl groups are present, together with aromatic materials in the RPPC fractions extractable with nitromethane. These materials are probably weakly hydrogen bonded or associated in the asphalt fraction and are solubilized by the nitromethane. Previous studies (6) have shown that in weakly associating fractions the broad carbonyl band at 1700 cm-1 observed in asphaltic materials in neat films or carbon tetrachloride solutions is resolved into three bands in methylene chloride solution. The earlier work attributed the 1740 cm-1 band to naphthenic acids in the molecular distillation fraction used in these studies. The (6)R. V. Helm and J. C. Petersen, ANAL.CHEM., 40, 1100 (1968). VOL. 41, NO. 10,AUGUST 1969
0
1343
Table 11. Polar Group and Aromatic Absorbances of Silica Gel Fractions from Wilmington Asphalt Molecular Distillation Fraction 1 and Its RPPC Residue Absorbance of band at Silica gel 3600 cm-1 3480 cm-1 1740 cm-1 1700 cm-' 1655 cm-' 1600 cm-l Wt. per cent of fraction eluted by (OH) (NH) (C=O) (C=O) (C=O) (aromatic C=C) molec. dist.a fr. 1 Heptane RPPC residue 0.00 0.00 0.00 0.00 0.00 0.00 40.3 b b Whole fraction 1 0.00 0.00 b 37.6 Benzene RPPC residue 0.03 0.08 0.00 0.00 0.00 0.26 39.6 Whole fraction 1 0.02 0.22 0.02 0.03 0.00 0.26 47.6 Benzene-methanol E c 0.00 RPPC residue 0.69 0.27d 0.18 4.6 Whole fraction 1 0.01 0.08 0.01 0.43 0.09 0.18 11.6 Methanol c C 0.00 c e 1.4 RPPC residue c E 0.10 Whole fraction 1 0.19 0.04 0.03 1.8 a RPPC residue was 85.9% of rnoIecular distillation fraction 1. Broad band from 1770-1610 cm-1. Broad band from 3600-3200 cm-1. Appeared at 1640 cm-1. e Broad bands.. (I
absence of broad absorption bands at 3600-3200 and 1700 cm-l in the RPPC fractions suggests that the more strongly hydrogen bonded or associated materials are not soluble in nitromethane. The data on the fractions separated on silica gel from the whole sample and the silica gel fractions from the RPPC residue are compared in Table 11. These results show that no materials containing OH, NH, carbonyl functions, or aromatic structures are present in the heptane fraction from the separation of the RPPC residue on silica gel. However, when the whole fraction is separated on silica gel, the heptane fraction contains materials with associated carbonyl groups as indicated by the presence of a broad absorption band in the 1700 cm-1 region. These latter carbonyl-contahing materials may be present because of the loading of active sites on the silica gel by those polar materials that are removed with nitromethane in the RPPC separation. In ihe fractions remwed from silica gel with benzene, the material from the RPPC residue shows no carbonyl absorption but does show free OH absorption at 3600 cm-l which is probably due to phenolic-type materials (7). The fraction from the whole sample also shows materials containing free OH absorbing at 3600 cm-1 and in addition shows free acid carbonyl absorption at 1740 cm-' and other carbonyl groups absorbing at 1700 cm-1. Both fractions contain aromatics absorbing at 1600 cm-1. Materials containing the carbonyl absorption at 1700 cm-1 are found in the RPPC extract (Table I), and their absence in the RPPC residue may account in part for the smaller benzene fraction obtained from the RPPC residue. The benzene-methanol fraction from the RPPC residue shows a broad absorption band in the 3600-3200 cm-' region (7) J. C. Petersen, Fuel, 48,295 (1967).
1344
ANALYTICAL CHEMISTRY
that is probably due to strongly bonded materials containing OH and NH groups. This fraction also shows carbonyl groups absorbing at 1700 and 1655 and aromatics absorbing at 1600 cm-1. However, there is no free acid carbonyl (1740 cm-1) in this fraction. The benzene-methanol fraction from the whole sample also has a band at 3600-3200 cm-1 but in addition has free peaks in the OH and NH regions and free acid carbonyl at 1740 cm-*. This fraction also contains materials with carbonyl groups absorbing at 1700 cm-1 and 1655 cm-1 and aromatics absorbing at 1600 cm-I. The smaller fraction size from the RPPC residue is probably related to the material removed in the RPPC fractions which contain free OH, NH, and acid carbonyl groups. The weight-per cents of the methanol fractions from the silica gel separation of both the RPPC residue and the whole sample are about the same. Both fractions show strongly bonded groups absorbing in the 3600-3200 cm-I region with no discernible free peaks in this region. The fraction from the RPPC residue shows no free acid carbonyl at 1740 cm-l and only broad bands in the 1700 to 1625 cm-l region. The methanol fraction from the whole sample contains materials giving rise to the three carbonyl bands. Similar materials showing these carbonyl bands are probably distributed between the benzene-methanol fraction from the RPPC residue and the RPPC extract. RECEIVED for review April 28, 1969. Accepted June 2, 1969. Presented at the Division of Petroleum Chemistry, 157th Meeting ACS, Minneapolis, Minnesota, April 1969. Reference to specific brand names is made for identification only and does not imply endorsement by the Bureau of Mines. Work was done under a cooperative agreement between the Bureau of Mines, U. S. Department of the Interior, and the University of Wyoming.
