2384
Anal. Chem. 1986, 58, 2384-2388
Hydrocarbon Group Type Analyzer System for the Rapid Determination of Saturates, Olefins, and Aromatics in Hydrocarbon Distillate Products Paul C. Hayes, Jr.,* and Steven D. Anderson
AFWALIPOSF, Aero Propulsion Laboratory, Aeronautical Systems Division, Air Force Systems Command, United States Air Force, Wright-Patterson Air Force Base, Ohio 45433-6563
A dlelectrlc constant (DC) detector for high-performance Ilquld chromatography (HPLC) is the basis of a rapld and accurate hydrocarbon group type analysis. Thls novel method can determine saturates, olefins, and total aromatlcs In hydrocarbon llqulds wlth dlstlllation end polnts (ASTM D 2887) of at least 400 OC. The HPLC separatlon is achieved by using a single, "olefin-selective" column, a backflush valve, and Freon 123 as the moblle phase. The DC detector ensures a genuine unlformlty of response (less than 2.5% relatlve standard deviation) for each hydrocarbon group type independent of the carbon number distrlbutlon of the sample. Unlty response factors are suffklent. On the bask of complex solutlons of hydrocarbon standards, the accuracy for each structural group type is wlthln 1% absolute. The chromatographic limit of detection is less than 0.5 voi % (800 ng) for the last eluting hydrocarbon group, Le., the olefins.
Liquid chromatography has helped to characterize the group composition of crude oils and hydrocarbon products since the beginning of this century. Specifically, the fluorescent indicator adsorption (FIA) method, ASTM D 1319 ( I ) , has served for over 30 years as the official method of the petroleum industry for measuring the paraffinic, olefinic, and aromatic content of motor gasolines and jet fuels. Despite its widespread use, the FIA has numerous limitations as detailed by Suatoni et al. (Z), Ettre et al. (31,and Norris and Rawdon (4). The modern petrochemical analyst faces a monumental task. A myriad of hydrocarbon products derived from varying sources of crudes and process streams must be analyzed rapidly and accurately and in sufficient detail. In response, a host of high-performance liquid chromatographic (HPLC) approaches to separating hydrocarbon group types have emerged. However, nearly all suffer a common handicap, i.e., deriving accurate response factors, in a timely manner, that are at once applicable to a wide range of distillate products. Drushel remarked ( 5 ) that given significant changes in the hydrocarbon distribution within a certain group type for a series of samples, the quantitative accuracy provided by some of these HPLC techniques could be in serious error. The root of the problem lies in the fact that the response to hydrocarbons of most routinely used HPLC detectors varies markedly with carbon number. The ideal detector for a truly versatile and accurate hydrocarbon group type analysis is one that is sensitive to hydrocarbon species and demonstrates a response that is independent of carbon number. Recent presentations and publications from this laboratory (6-10) (available on request) demonstrate the merits of a dielectric constant (DC) detector as an integral part of a hydrocarbon group type analyzer system. Presented herein is a versatile and accurate determination of saturates, olefins, and total aromatics in a myriad of hydrocarbon distillate products using high-performance
liquid chromatography with dielectric constant detection, Le., HPLC-DC. EXPERIMENTAL SECTION Apparatus. A generic version of a hydrocarbon group type analyzer system is displayed in Figure 1. A Varian Model 4200 liquid chromatograph was used to perform all the analyses in this study. The system has syringe-type pumps and is equipped with a Varian Model 8000 autosampler, a Valco six-port injection valve fitted with a 10-pL sample loop, a Valco six-port backflushing valve, and an Optichrom Model 430 dielectric constant detector (Applied Automation, Inc.). The electrometer of the DC detector was set at an amplification range of 1.0 and at an attenuation of 1000 mV full scale. Both the sample cell and the reference cell require flowing environments. However, the flow rates need not be matched. The reference cell flow rate was measured to be approximately 0.1 mL/min. The column flow rate was 1.0 mL/min, but could be increased to significantly reduce the analysis time with minimal deterioration to the separations. All analyses were performed under ambient conditions. The DC detectok is not commerically available as a laboratory unit. However, Bio-Rad (M. Gray, Research Products Group, Richmond, CA) is pursuing the reintroduction of the detector to the market. Olefin-Selective Column. An experimental, “olefin-selective” HPLC column was used for the separations. The column was 150 X 4.6 mm i.d. and contained 5-pm silica particles that had been bonded to a strong cation exchange stationary phase. The silver form of the ion exchange column was prepared by in situ flushing with aqueous silver nitrate. The lifetime of the column is at least 6 weeks but probably not much longer than 2 to 3 months for all the applications examined in this study. The column is removed and regenerated when the resolution obtained between the saturates and the aromatics has fallen to approximately 95%. Even then, however, olefins are still quantitatively separated from the other hydrocarbon groups. Versions of the olefin-selectivecolumn are now under evaluation by two HPLC column manufacturers, Le., silica-based (L. Metts, Whatman, Inc., Clifton, NJ) and resin-based (M. Gray, Bio-Rad, Richmond, CA). A silica guard column (Bio-Rad) preceded the analytical column to adsorb polar heteroatomic compounds present in the injected samples. The total system back pressure was less than 600 psig. Reagents. Freon 123, i.e., 2,2-dichloro-l,l,l-trifluoroethane (Halocarbon Products Corp.),was used for the mobile phase. The mobile phase was filtered to remove any particulates above 0.45 pm in size. Samples were prepared for analysis by diluting them -1:40 in Freon 123. This was accomplished by using a micropipetter to deliver approximately 35 pL of sample into an HPLC autosampler vial and diluting with mobile phase (- 1500 pL total solution). This dilution factor ensured that sample concentrations were well within the linear dynamic range of the detector (8). Procedure. The combination of Freon 123 with the olefinselective HPLC column effected a separation of hydrocarbon distillate products into three chromatographic peaks. Just what hydrocarbon species eluted in what peak was ascertained from injections of over 100 pure reference standards (Wiley Organics, Inc.). Chromatographic runs of complex standard mixtures of known hydrocarbon group type composition indicated that unity response factors were sufficientto give accurate analytical results.
This article not subject to US. Copyright. Published 1986 by the American Chemical Society
ANALYTICAL CHEMISTRY. VOL. 58. NO. 12, OCTOBER 1986
2385
saturates
F b r e 1. Hy&ocarbon group type analyzer system with flow reversal.
A diluted sample of the hydrocarbon distillate was then injected into the chromatographicsystem via a 1@wLsample valve. After the saturates and aromatics eluted, the olefins were hack flushed as a single peak. The actual time for back flushing can he varied from sample to sample depending on the complexity of the aromatic hydrocarbon envelope. To facilitate the analysis of a wide range of distillate products, the predetermined time for hack flushingwas set at 200 s at a flow rate of 1.0 mL/min. The elution of any three-ring polynuclear aromatic hydrocarbons would have occurred prior to this back flushing time. Under these conditions, the chromatographic run was complete in 8 min. The back flushing valve was then returned to ita original p i t i o n , and after a 2-min reequilihration time, another sample was injected. Realistically, the back flushing valve need not be rotated back to its original position prior to injection of another sample. Consequently, the sample turnaround time could he shortened from 10 to 8 min. Quantitation was accomplished on a Hewlett-Packard 3357 laboratory automation system. Perpendicular base line drops displayed in the chromatograms were automatically determined by the computer without operator intervention.
RESULTS AND DISCUSSION Detector. In general, the DC detector measures small changes in the dielectric constant of a liquid stream eluting from an HPLC column. The capacitance of two nearly identical parallel-plate capacitors is monitored. One capacitor (reference cell) has simply pure mobile phase flowing through it. The other capacitor (sample cell) has the HPLC column eluants passing through it. T h e difference in capacitance is converted to an analog output signal for data collection and reduction. Basically, the DC detector responds to a change in a hulk property (dielectric constant) of the mohile phase. Recent publications (6-8) describe the requirements of the optimum mobile phase for a successful hydrocarbon group type analysis using dielectric constant detection. Freon 123 has both a relatively low solvent strength and a sufficiently high dielectric constant to meet those criteria (9, IO). When the DC detector is used in unison with a mobile phase of high dielectric constant, the detector performs as a universal hydrocarbon analyzer. In this mode, the detector has high sensitivity for hydrocarbon species and responds independently of the carbon-number distribution of the sample type or the hydrocarbon group type. Qualitative Analysis. T o identify the different hydrocarbon group types separated in a chromatographic run, the elution window for each group had to be mapped. Over 100 hydrocarbon reference standards were chromatographed in the system described in the previous section. In general, for a given hydrocarbon group type, molecules with a higher degree of alkyl substitution and/or a longer carbon backbone eluted earlier from the olefin-selective column than simpler molecules. Figure 2 illustrates the elution profiles found for each hydrocarbon group type. This figure is actually a composite of three HPLC-DC chromatograms stacked on top of each other. The saturates elute in a relatively narrow hand with the smaller molecules, Le., C5s, eluting last. For this study, the earliest eluting aromatic hydrocarbon examined was a C, alkylbenzene, i.e., n-octylbenzene. If alkylbenzenes
i s ~1 = 0
2
3
4
RETENTION
5
6
7
8
TIME (mid
Flgure 2. Hydrocarbon group type elution windows: aromatics. (C) olefins.
(A)
saturates. (0)
of considerably larger size were present in a distillate product, overlap might ocw between the small saturated hydrocarbons (Css) and the beginning of the aromatic envelope. The last aromatic compound eluted in this study was anthracene. Representative C1 through C3 alkylnaphthalenes and alkyldiphenyls, and three-ring polynuclear aromatic hydrocarbons examined in this study all eluted prior to anthracene. After the elution time of anthracene had elapsed, the back flushing valve was rotated, and the olefins were back flushed as a single peak. Over 3 dozen olefins having carbon numbers between C5 and CI6 were investigated. Internal and terminal monoolefins, conjugated and unconjugated diolefins, cycloolefins, and aromatic cycloolefins all eluted within the window shown in Figure 2. The only olefins that eluted outside the olefin window were alkylstyrenes, which coeluted with anthracene. Until now the discussion has focused solely on the hydrocarbon matrix, ignoring the potential interference from polar additives and impurities. A composite mixture called the “polars mix” was prepared. Approximately equal portions were,taken of over 30 heteroatomic compounds that have a high probability of occurring as polar material in a hydrocarbon distillate product. The heteroatom compounds of the “polars mix” were as follows: oxygen heteroatom: acetone, 2-methoxyethanol, methyl alcohol, p-dioxane, 2,6-dimethylphenol, dibenzyl phthalate; nitrogen heteroatom: 3,5-dimethylpiperidine, 4-methylpiperidine, piperidine, pyrrolidine, 2,4-dimethylaniline, aniline, quinoline, 2,6-dimethylquinoline, carbazole, pyridine, 2-ethylpyridine, 3-picoline, 3-methylindole, indole, 3,4-lutidine, 2,3,6-collidine, 1,2,5-trimethyIpyrrole, n-decylamine, tri-n-butylamine; sulfur heteroatom: ethyl sulfide, methyl disulfide, carbon disulfide, thiophene, phenyl sulfide, 3-methylthiophene. A 1:40 aliquot of this solution was injected into the chromatographic system. A blank run of injected mobile phase showed only a minor base line disturbance near the dead time of the column. Injection of the “polars mix” gave only one additional peak of comparable size. The vast majority of polar compounds are either irreversibly adsorbed on the silica guard column or elute outside the retention windows of the hydrocarbon group types. It is possible, though, that a few polar compounds could elute with or adjacent to a hydrocarbon group. However, polar species usually have high dielectric constants, i.e., close to that of the mobile phase, and the detector, as a result, would he insensitive to their presence. Apparently, a large amount of polar materials in a hydrocarbon distillate product should have no interference in the determination of the hydrocarbon ratios. Quantitative Analysis. T o establish the quantitative accuracy of the hydrocarbon group type analyzer system, a series of complex solutions of known group composition were chromatographed (10). Each mixture was individually prepared to emulate different types of real-world products. Blending solvents were used wherever applicable to increase sample complexity and more closely mimic different product types. The composition of a typical standard solution, i.e.,
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 12. OCTOBER 1986
Table IV. HPLC Repeatability Study"
Table I. Preparation of Standard Mix: "Simulated" Gasoline HC group type
Sample: ASTM D-2 HPLC Gasoline Standard vol 9i
blending stock/pure component
saturates
mean vol % HPLC (FIA)
n
std dev
saturates aromatics olefins
50.7 (51.5) 39.8 (36.7) 9.5 (11.8)
5 5 5
1.0 0.9 0.4
60.0 hydrocracked n-C16 (CG-c16 isomers) petroleum ether (bp 37-55 "C) "isopar C" (Exxon isoparaffinic solvent, bp 98-106 "C)
10.0 30.0 20.0
30.0
aromatics
olefins
HC group type
toluene xylenes (0-,m-, p - ) xylene bottoms (Cs-Clo benzenes)
5.0 5.0 20.0
comDosite mixture of C6s-cl6s
10.0
Table 11. Comparison of Absolute Error for Aromatics in Complex Standard Solutions (vol % ) sample code
known
HPLC
FIA
high olefin mix all groups mix grand mix simulated gasoline high aromatic mix
10.0 25.0 25.0 30.0 60.0
-0.2 -0.1 +1.0 -0.1 -0.2
+6.3 +1.4 +3.0 +3.6 -0.2
known
HPLC
FIA
high aromatic mix all groups mix grand mix simulated gasoline high olefin mix
0.0 5.0 5.0 10.0 30.0
0.0 -0.8 0.0 -0.1 -0.5
+LO
1
2.5 1.9 0.9
2.1 2.1 3.7
saturates
ULCJ aromat i c s
olefins
5
a
1
0
2
3
6
4
7
8
r e t e n t i o n time [min) Figure 3. HPLC-DC profile of an ASTM gasoline standard.
saturates
I
f0.3 +1.6 -1.3 4-4.7
the "Simulated" gasoline mix, is presented in Table I. As indicated, the olefins varied significantly in carbon number. The olefin compounds used were as follows: 2,3-dimethyl-1butene, 3-methyl-2-pentene, 2,4-dimethyl-l,3-~entadiene, 1-heptene, 3-heptene, 3-methyl-l-hexene, 2,4,4-trimethyl-2pentene, 2-ethyl-l-hexene, 2-methyl-l-heptene, 1-octens, 2,5-dimethyl-1,5-hexadiene, 4-nonene (cis and trans), 3,5,5trimethyl-1-hexene, 1-decene, 1-undecene, 1-dodecene, and 1-hexadecene. The "simulated" gasoline mix was analyzed by capillary gas chromatography mass spectrometry and shown to contain over 400 compounds. The exact amount of each compound in each blending solvent is not known. However, mass spectrometric analysis found each solvent to be 99+ wt 9'0 pure relative to the hydrocarbon group type it was meant to represent. The relative amounts of each of the hydrocarbon group types were deliberately varied from one standard solution to the next. Overall, the aromatic contents ranged from 10 to 60 vol % and the olefinic contents from 0 to 30. Comparisons of the absolute error in measuring the aromatics and the olefins are displayed in Tables I1 and 111, respectively. The results clearly indicate superior quantitative accuracy is afforded by the HPLC-DC method. The most significant feature of the quantitative analysis of hydrocarbon group types by HPLC-DC is that unity response factors yield accurate results regardless of the carbon number distribution of a particular group type or sample. The repeatability of the HPLC system was determined over a 2-week period using two different olefin-selective columns, as shown in Table IV. A motor gasoline, i.e., ASTM gasoline standard, was selected for this evaluation. The quantitative repeatability for this sample was excellent.
range
"analyses performed over a 2-week period using two different "olefin selective" HPLC columns.
Table 111. Comparison of Absolute Error for Olefins in Complex Standard Solutions (vol 70) sample code
RSD
%
aromatics 0
1
2
3
4
6
5
7
8
r e t e n t i o n time [minl Figure 4. HPLC-DC profile of an experimental high density kerosene fuel (high aikyldecalin content).
jet
i
saturates
olefins 0
1
2
3 4 5 6 r e t e n t i o n time [minl
7
I
Figure 5. HPLC-DC profile of a diesel fuel no. 2.
The limit of chromatographic detection is crucial in evaluating any new HPLC technique. This limit of detection incorporates the band broadening contributions of the sample injection loop (10 pL), the guard column, the analytical column, all the interconnecting tubing, the back flushing valve, and the dead volume of the detector (-23 pL). For the last hydrocarbon group to elute, Le., the back flushed olefins, the chromatographic limit of detection was found to be less than 0.5 vol % (800 ng). T o demonstrate the versatility of the hydrocarbon group type analyzer system, a wide range of distillate products were evaluated by both HPLC-DC and FIA. The samples included several motor gasolines, jet fuels, cat-cracked naphthas, and
ANALYTICAL CHEMISTRY, VOL. 58,NO. 12, OCTOBER 1986
2387
Table V. Comparative Analyses of Various HC Distillate Products (vol % ) $
70-
1
HC sample code
ASTM gasoline standard
group type
saturates olefins
ASTM gasoline I
aromatics saturates olefins
ASTM gasoline I1
aromatics saturates olefins
gasoline (b)
aromatics saturates olefins
gasoline (c)
aromatics saturates olefins
gasoline (s)
aromatics saturates
cat-cracked naphtha
aromatics saturates
olefins
v o l ? olefins F I R
Figure 7. Correlation of FIA with HPLC-DC for volume percent olefins. a diesel fuel. The comparative analyses of these fuels are listed in Table V. The chromatographic profiles of a selected few of the samples are displayed in Figures 3-5, the rest are available upon request. Serious discrepencies in the results of the two methods occurred for two, almost identical catcracked naphtha samples, as shown in Table V. The processing history and composition of the two samples were known to be very similar. However, the volume fractions of the aromatics and the olefins found by FIA varied widely between the two samples. The HPLC-DC results are, given the history of the samples, far more believable. Correlating the aromatic content measured by FIA vs. HPLC-DC from all the samples, except the naphthas, revealed a significant trend. Figure 6 indicates that the volume fraction of aromatics found by FIA are consistently higher than the results determined by HPLC-DC. The solid line represents the hypothetical case of a perfect 1:l correlation between the two methods. A standardized least-square curve fit of the data gave a Pearson product moment, i.e., a correlation coefficient, of 0.971. Previous published work (8) and unpublished analyses using a different HPLC-DC method substantiate this trend. In particular, over 60 hydrocarbon samples that varied ,in distillation range from kerosene fuels to light pyrolysis fuel oils were comparatively analyzed. The resulting linear correlation coefficient was 0.997. Remarkably, this other HPLC-DC method (8)utilized n-butyl chloride as the mobile phase and three bonded phase HPLC columns of two different chemistries to effect an entirely different kind of class separation. Figure 7 displays the correlation of the olefinic content measured by the two methods. The slope of the standardized least-squares curve fit was 1.02, almost matching that of the 1:l correlation line. However, on a relative basis, the difference in olefins reported by each method is significant. There are two types of distillate products that tend to amplify that disparity. Highly colored samples are historically troublesome for the FIA and olefins as well as aromatics might not be measurable. In addition, for hydrocarbon samples with a low olefinic content (less than 0.5 vol %), the FIA might still report one to two volume fraction of olefins. This was true for the FIA analyses of certain standard reference mixtures and for severely hydrotreated fuel samples that were known to be free of olefins.
FIA(s)
HPLC lab l/lab 2 51.4 9.4 39.2 54.8 5.7 39.5 62.9 13.0 24.1 57.6 11.0 31.4 58.5 7.3 34.2 57.0 6.0 37.0 56.9
43.9/51.5 7.0/11.8 49.1/36.7 49.2156.2 5.116.3 45.7137.5 51.7/61.0 13.8119.3 34.5119.7 52.0 10.0 38.0 55.7 5.7 38.6 54.3 3.6 42.1 44.3
25.2 17.9 53.1
21.2 34.5 38.9
28.2 18.7 84.2 0.0 15.8 84.5 0.7 14.8 78.3 3.9 17.8 74.6
11.4 49.7 80.1179.6 1.312.1 18.6/18.3 82.4182.0 1.111.2 16.5116.8 75.71763 2.214.5 22.1118.7 70.6
0.0 25.4 77.7 2.0 20.3 12.8
1.5 27.9 71.0 0.7 28.3 0.0
12.0 15.2 13.1
0.0 100.0 0.0
2.9 84.0
0.0 100.0
(85-POSF-2107)
olefins aromatics
cat-cracked naphtha
saturates
(85-POSF-2108) olefins
ASTM jet fuel standard
aromatics saturates olefins
ASTM jet fuel I ASTM jet fuel I1 high density jet fuel (85-POSF-2398)
aromatics saturates olefins aromatics saturates olefins aromatics saturates olefins aromatics
diesel fuel $2
light pyrolysis fuel oil
saturates olefins aromatics saturates
(85-POSF-2338) olefins
hydrostabilized fuel oil
aromatics saturates
(85-POSF-2339) olefins
aromatics
A caveat should be introduced here. The FIA results, as with the HPLC-DC results, are from one operator at one laboratory. The first-order correlations of the aromatic and the olefinic contents might not be as evident when the data from several laboratories are included in one plot. Special Application. A highly colored light pyrolysis fuel oil was analyzed by HPLC-DC and FIA both before and after hydrostabilization, Le., samples 85-POSF-2338 and 85POSF-2339, respectively, in Table V. The corresponding HPLC-DC profiles of the two samples are presented in Figure 8. Hydrostabilization is a mild hydrotreating process that converts highly reactive olefinic species to more stable hydrocarbons. FIA determined both samples to be 100% aromatic whereas analysis by HPLC-DC found significant amounts of saturates and olefins also present. Examining the HPLC-DC data more closely reveals additional information about the chemical composition of the samples that could not be ascertained by FIA. After hydrostabilization, there is a significant increase in the the total aromatic content of the
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 12, OCTOBER 1986
content of only one hydrocarbon group type and that was in the aromatics, not the saturates.
[saturates
I\
\LL
olefins
" .
0
1
'B
L
0
2
3
4
5
6
7
8
4
5
6
7
8
arumat i c s
1
2
3
I
r e t e n t i o n time (minl Flgure 8.
HPLC-DC profile of hydrogenation sequence of a light pyrolysis fuel oil: (A) pyrolysis fuel oil charge, (6)hydrostabilized fuel
oil.
sample. In fact, the gain in the aromatic content of the sample is almost quantitatively matched by the loss in the olefinic content. Furthermore, the HPLC-DC profile shows that the increase in the aromatics occurs exclusively in the alkylbenzene section of the aromatic envelope. I t is theorized that the olefinic species present in the untreated sample may have included a large proportion of aromatic olefins. Mild hydrogenation could have saturated the olefinic portion of these species leaving the aromatic section intact. The resultant products would be alkylindans and alkyltetralins and would elute with the alkylbenzenes. The following method was used to verify the possible presence of aromatic olefins in the fuel oil charge. A fixed wavelength (254 nm) ultraviolet HPLC detector was inserted downstream of the DC detector. Selected aromatic olefins, i.e., 1,2- and 1,4-dihydronaphthalenes, were chromatographed with back flushing to confirm their elution within the olefins window. A dramatic response to the aromatic olefins was observed on the tracing from the ultraviolet detector that corresponded in retention time to the single, olefins peak found on the recording from the DC detector. Injections of each of the two fuel oil samples were then made into this HPLC-DC-UV system. A significant olefins peak was observed on the UV tracing for the chromatographic run of the fuel oil charge, but no corresponding peak appeared in the run of the hydrostabilized product. Conjugated diolefins could also have accounted for the sizable olefins peak in the UV tracing of the fuel oil charge. However, hydrogenation of these species should have caused a significant increase in the saturates content for the hydrostabilized fuel oil sample. Recall, though, hydrostabilization caused an increase in the
CONCLUSIONS The hydrocarbon group type analyzer system introduced in this paper is ideal for the simple, accurate, and rapid determination of saturates, olefins, and total aromatics in a wide range of fuel distillates without operator intervention. The quantitative results are directly determined in volume percent. The presence of light hydrocarbons ((3,s and C5s), highly colored species, and/or polar heteroatomic compounds does not affect the hydrocarbon ratios. Use of unity response factors gave analytical results within 1%absolute for complex standard solutions. Any isocratic HPLC system can perform this analysis with minor modification, if any, to accommodate the volatile Freon 123 (boiling point 27 'C). The mobile phase required for this application is rather expensive, i.e., $310/gal, but can be recycled without distillation or special treatment at least 6 times to significantly reduce the actual solvent cost per run. Total automation can be achieved with an autosampler, an electronically controlled back flushing valve, and a computing integrator. The adaptability of this method to on-line process stream analysis may be fairly easy, since the detector is already an integral part of a commercially available process liquid chromatograph (J. Crandall, Applied Automation, Inc.). ACKNOWLEDGMENT The authors extend their appreciation to the following without whose cooperation this work could not have been accomplished: J. Crandall and L. Benningfield, Jr. (Applied Automations, Inc.), for the loan of the DC detector and for technical assistance in its operation and F. Piehl (ASTM D-2, Subcommittee 04, Section C) for supplying certain of the gasoline and jet fuel samples. LITERATURE CITED Annual Book of ASTM Standards, Part 23; American Society for Testing and Materials: Philadelphia, PA, 1980. Suatoni. J. C.; Garber, H. R.; Davis, B. E. J . Chromatogr. Sci. 1975, 13, 367-371. Mlller, R. L.: Enre, L. S.; Johansen. N. G. J . Chromatogr. 1983, 259, 393-4 12. Norris T. A.; Rawdon, M. G. Anal. Chem. 1984, 56. 1767-1769. Drushel, H. V. J . Chromatogr. Sci. 1983, 21, 375-384. Hayes, P. C., Jr.; Anderson, S. D. Presented at the 1985 Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, New Orleans, LA, March 1985. Hayes, P. C.. Jr.; Anderson, S . D. Anal. Chem. 1985. 57, 2094-2098. Hayes, P. C., Jr.; Anderson, S. D., AFWAL-TR-85-2028, AFWAL/ POSF, Wright-Patterson Air Force Base, OH, 1985. Hayes, P. C., Jr.; Anderson, S. D. Presented at the 1986 Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Atlantic City, NJ, March 1986. Hayes, P. C., Jr.; Anderson, S . D. AFWAL-TR-613-2044, AFWALIPOSF, Wright-Patterson Air Force Base, OH, 1986.
RECEIVED for review April 25, 1986. Accepted June 9, 1986. This paper was presented in part at the 1986 Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Atlantic City, NJ, 10-14 March 1986. The mention of a specific product does not constitute official endorsement, condemnation, or approval of that or any other products.
