negative in-phase component while the quadrature component is positive. The reverse situation of a negative quadrature component with a positive in-phase faradaic admittance has been reported for the reduction of concentrated Ni(I1) solutions (9). The latter is probably caused by a parallel reaction pathway involving polymeric species, although so far no quantitative interpretation has emerged. In the presence of a negative charge transfer resistance, both in-phase and quadrature components of the faradaic ad(9) R. de Levie and A. A . Husovsky, J. Electroanal. Chem., 20, 181 (1969).
mittance are negative (10). Perhaps such negative values of Y,’ and/or Y,” are, after all, not quite as “unusual” as the title of this communication suggests. RECEIVED for review September 8, 1970. Accepted March 3, 1971. Work supported by the National Science Foundation (Grant G P 8575), the Air Force Office of Scientific Research (OAR, USAF, Grant 68-1344), and the Office of Naval Research. (10) Ibid.,22, 29 (1969).
Gas Chromatographic Analysis of Samples Containing Both Volatile and Nonvolatile Organic Components L. E. Philyaw,’ A. E. Krc, and M. J. O’Neal2 Houston Research Laboratory, Shell Oil Company, P. 0. Box 100, Deer Park, Texas 77536 TEMPERATURE AND PRESSURE PROGRAMMING are widely used to increase the range of applications of gas chromatography (1-3). These techniques make gas chromatography applicable to a wide variety of mixtures containing components with a broad range of boiling points. However, these techniques do not provide a means of direct analysis of samples which contain nonvolatile residue. Thus, in general, gas chromatography is limited in applications to materials like crude oils containing asphalt, various biological mixtures, and mixtures containing inorganic residue like shale and coal. Some indication of the amount of the nonvolatile portion of such samples can be obtained by the use of a known amount of an internal standard ( 2 ) . However, this approach is often not very accurate, and it is frequently impossible to find standards which are not interfered with by the sample components (as in crude oils which contain a continuum of components as a function of boiling point). These considerations led to an investigation of the possible separation and selective determination of nonvolatile organic constituents directly in the gas chromatographic equipment. EXPERIMENTAL
The apparatus developed for the purpose of determining nonvolatile residue during a gas chromatographic analysis is shown in Figure 1. The equipment basically consists of a programmed temperature column with the effluent passing into a furnace containing a copper oxide tube to convert the organic components to carbon dioxide so as to eliminate response factors in thermal conductivity detection (4). The system is arranged so that the column can be backflushed to remove any low volatility material that vaporizes but does not Present address, Celanese Chemical Co., P. 0. Box 58009, Houston, Tex. 77058. * To whom correspondence should be addressed. (1) J. Griffiths, D.
James, and C. Phillips, Analyst, 77, 897 (1952). (2) F. T. Eggertsen, S. Groennings, and J. J. Holst, ANAL. CHEM., 32,904 (1960). (3) A . Zlatkis, D. C. Fenimore, L. S. Ettre, and J. E. Purcell, J . Gas Cliromatogr., 3,75 (1965). (4) M. C. Simmons, L. Taylor, and M. Nager, ANAL. CHEM., 32, 731 (1960).
pass completely through the column and as a means for raising the injection port temperature and introducing oxygen in order to oxidize nonvolatile residue remaining in the injection port. The zone into which the sample is injected can be heated to 700 “C for oxidation of the nonvolatile residue. The outer portion of the injection port is water-cooled to prevent decomposition of the silicone rubber injection port septum. A slipstream of preheated carrier gas (helium) introduced immediately inward from the water jacket prevents back condensation of sample in the cooled area. It is necessary to use an oxygen scrubber (tube packed with copper wire) to remove the excess oxygen during the backflush/ residue-combustion operation so that the excess oxygen does not interfere with the COzpeak due to the residue oxidation. In making an analysis, the injection port furnace is set at a temperature high enough to vaporize the volatile components, but below the temperature at which pyrolysis occurs. In the case of crude oils and similar petroleum fractions, an injection port temperature of about 350 O C is satisfactory. The volatile portion is chromatographed using programmed temperature in the usual manner up to the maximum permissible column temperature (300-350 “C for the heavier silicone column packings). The carrier gas flow through the column, combustion tube furnace, and injection port is reversed by means of the backflush valve. The material backflushed from the column passes through the injection port into the backflush combustion tube furnace where it is converted to carbon dioxide and water and passes into the drier/detector system. The direction of flow through the drier/detector is unaltered from the forward flow conditions. After backflushing the column, the injection port temperature is raised to 700 “C over a period of approximately three minutes to vaporize and/or thermally decompose any heavy residue left in the injection port. This step removes most, but not all, of the nonvolatile residue in samples like crude oil. Any coke or other very heavy material remaining in the injection port is recovered by introducing oxygen into the injection port by mixing it with the helium slipstream. Excess oxygen remaining after oxidation is removed by the copper scrubber which is also located in the combustion furnace. The elution of the nonvolatile residue in three separate portions is useful in characterizing certain types of samples since some are easily vaporized with little coke formation and others form a large fraction of coke. This approach also serves to prevent overloading the detector. Some heavy ANALYTICAL CHEMISTRY, VOL. 43, NO. 6, MAY 1971
787
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Figure 2. Calibration of retention time/boiling point relationship using n-alkanes 788
ANALYTICAL CHEMISTRY, VOL. 43, NO. 6, MAY 1971
Figure 3. Residue peaks obtained by GLC analysis of Heidelberg-Eucutta-Yellow Creek (Mississippi) Crude oil crude oils and asphaltic fractions contain 50% or more nonvolatile residue. Such materials can easily cause overloading and nonlinear detector response if the residue peak is eluted in a single peak. This procedure for residue oxidation not only provides a quantitative measure of nonvolatile residue but it also provides for its removal from the system, thus rendering it clean for subsequent analyses. After an analysis has been completed, the copper-wire oxygen scrubber tube must be regenerated with hydrogen. Regeneration of the copper oxide oxidation zones is required less frequently but generally should be carried out after 3 or 4 analyses. Regeneration of the copper-wire oxygen scrubber is carried out as follows (referring to Figure 1): the helium carrier flow is stopped, the helium vent valve is closed, the backflush valve is switched to the closed position, and the regeneration valve set to connect the scrubber to the hydrogen vent-line. Hydrogen is introduced for a few minutes by
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Figure 4. Comparative TBP analysis of Baxterville (Mississippi) Crude by distillation and GLC means of a quick-connect fitting (this fitting has a double-end shutoff when disconnected so it also serves as a valve). After regeneration, the quick-connect hydrogen line is removed and the excess hydrogen is flushed from the system with helium flowing through the backflush valve. The regeneration valve is reset to its normal position and the helium vent line opened in preparation for the next analysis. The copper oxide oxidation tubes are regenerated as follows : helium flow is stopped and the helium vent line is closed, the backflush valve is closed, and the regenerate valve switched to the regenerate position. The air valve is opened and air is permitted to flow through the oxidation tubes, the column, and the injection port to the air vent. In order to prevent oxidation of the stationary phase on the column, it is preferable that this operation be carried out with the column cooled. After a few minutes regeneration, the backflush valve is set to the backflush position and helium flow started to flush air out of the system. The regenerate valve is set to its normal position and the helium vent valve opened so that the system is ready for analysis. A very useful application of this technique is the simulation of precision distillation on materials such as crude oils. A convenient means of integrating the gas chromatographic data for simulation of distillation is the use of a voltage-to-frequency converter (such as the Vidar Model 260-02-Unipolar) feeding into a counter with a preset controller (such as the Computer Measurements Company Model 786C) which actuates a printing counter (such as the ITT General Controls Model CPA04A1029) on equal increments of area. Thus retention time (and therefore equivalent boiling point) can be recorded automatically for equal area increments of the chromatogram. This is equivalent to recording temperature in a distillation at equal increments of distillate (say each 1 taken overhead). Similar computation, of course, can be made using a dedicated computer system.
RESULTS AND DISCUSSION
The application of this technique to the simulation of precision distillation on complex materials such as crude oils ( 5 ) can be accomplished by the use of blends of n-alkanes to provide a calibration to convert retention time to true-boiling point (Figure 2). The elution of the nonvolatile residue in three separate peaks (backflushing, pyrolysis of the residue, and coke oxidation) is illustrated in Figure 3 as applied to a sample of crude oil. A comparison of the results of a simulated distillation by the gas chromatographic technique with that obtained by precision distillation (30 plate Oldershawtype column to about 700 O F followed by vacuum Engler type distillation on the residue) is shown in Figure 4 for a sample of Baxterville (Mississippi) crude oil. Only about half of this crude oil is volatile at the 900 O F end point and the two techniques compare very favorably. Table I shows a comparison of replicate GLC determinations and the value obtained by distillation on the residue content on three typical crude oils from different sources. Table I1 shows a comparision of GLC and distillation residue determinations on a group of heavy crude oils. In this case, two values for GLC are given indicating the amount recovered before and after the injection of oxygen into the injection port at 700 OC. These data indicate satisfactory agreement between the two techniques and thus provide a relatively rapid technique for the assay of crude oils. RECEIVED for review December 2, 1970. Accepted January 25,1971.
(5) L. E. Philyaw and M. J. O'Neal, U. S. Patent No. 3,527,567, Analytical Distillation by Gas Chromatography, Sept. 8, 1970.
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