Anal. Chem. 1988, 6 0 , 802-806
802
Ultraviolet-Absorption Detector for Capillary Supercritical Fluid Chromatography with Compressible Mobile Phases Steven M. Fields,' K a r i n E. Markides, a n d Milton L. Lee* Department of Chemistry, Brigham Young University, Provo, Utah 84602
Varlatlons In the rehactlve index of low critkal temperature, highly compresolble mobHe phases wwe observed whkh are not seen In caplllaryeell uitravlolet-abeorpth detectlon wlth high crllcal temperature, low compresslbUfty supercrltlcal mobile phases In capHary supercrltlcal fluld chromatography (SFC). Changes In base line apparently resuklng from densly-related refractlve Index changes were reduced to acceptable levels by cooling the detector cell. The system Is senslUve and useful In dudles of mixed mobile phases In capillary SFC. An 8.9 mol % mixture of P-propanol or nitromethane In CO, produced dgnmcant decreases kr reterttlon of p6lar and nonpolar polycycllc aromatic compounds. The P-propanol/CO, moMle phase effected the elution of ovalene at moderate temperature and pressure.
A major emphasis of current research in capillary supercritical fluid chromatography (SFC) is concerned with the use of mixed mobile phases to expand the analytical capabilities of SFC to more polar and higher molecular weight solutes than possible with single fluid mobile phases such as COz. The mixed mobile phases that have been studied are primarily polar organic liquids in COP Low percentages (less than 1 mol 70)of modifiers do not appear to cause any significant change in solute retention (1). Higher percentages (up to 20 mol %) have been shown to produce significant retention changes (2). Since mobile phase flow rates and solute quantities are low in capillary SFC, it is desirable to analyze the entire effluent. However, the high percentages of organic modifiers anticipated in mixed mobile phase studies preclude the use of a flame ionization detector due to high background levels and to base-line changes during pressure or density programming. Ultraviolet-absorption provides a simple and inexpensive detection system for use with mixed mobile phases. Capillary SFC analysis creates stringent demands on allowable UV-absorption cell volume8 (3),so an optical cell was developed (4) based on fused silica capillary tubing available for gas chromatography. Highly compressible mobile phases such as COP create a problem in capillary UV-absorption detection that is not present for high critical temperature, low compressibility mobile phases such as n-pentane. The compressibility of C02produces significant density changes in the cell during pressure or density programming which leads to refractive index changes and significant base-line drift. In this paper, causes of the base-line drift and the modification of the existing cell configuration are described. The sensitivity, linearity, and noise level of the new configuration were evaluated. The utility of the detector is shown with chromatograms of a complex polycyclic aromatic hydrocarbon mixture using C02, 2-propanol/COz, and nitromethane/C02 mobile phases. The modification of the solvating strength of the mobile phase, resulting in retention changes of polar and nonpolar compounds, was studied.
* To whom correspondence should be addressed.
Current address: Analytical Research, Ciby-Geigy, Ltd., CH-
4002
Basel. Switzerland.
EXPERIMENTAL SECTION Apparatus. The chromatographiccolumn used in this study was a 10-m length of 50 pm i.d. fused silica tubing (Polymicro Technologies, Phoenix, AZ) deactivated with poly(methy1hydrosiloxane) (Petrarch Systems, Bristol, PA) (5), statically coated (6)with a 0.25-pm film of 100% methylpolysiloxane (SE-33, Applied Science, State College, PA), and cross-linked with azotert-butane (7). The chromatographic system consisted of a syringe pump modified for preasure control (8) (Varian 8500, Varian Instruments, Walnut Creek, CA, or ISCO Model 314, ISCO, Lincoln, NB), a gas chromatograph oven (HP-5710A,Hewlett Packard, Avondale, PA), a 0.2-pL sample loop injection valve (Valco Instruments, Houston, TX), a capillary SFC injection splitter (8),and a UVabsorption detector (UVIS-203,Chiratech Scientific Instruments, Fort Collins, CO; now produced by Linear Instruments, Reno, NV). A rise time of 1.0 s on the UV instrument was used in all instances,except for sensitivity measurements. The pump cylinder was not cooled when mixed fluids were used. The UV-cell mask was constructed of commonly available injector razor blades glued to small pieces of glass. The mask, cell, and alignment guide were held in place with either tape or a small metal spring clip. The cell guide was constructed of acrylic so as to align the cell in the optical axis as close to the diode as possible. The entire cell assembly and detector were positioned above the chromatographicoven such that the distance between oven and cell (ca. 8 cm) was minimized. Pressure restriction was provided by a short length of 150 pm i.d. platinum-iridium tubing crimped at the end. Reagents. The gaseous mobile phases were SFC grade COP, 8.9 mol % 2-propanol(HPLC grade),and 8.9 mol % nitromethane (HPLC grade) in SFC grade COz,SFC grade NH,, and 17.5 mol % NH3in CHFB(Scott SpecialtyGases, Plumsteadville,PA). The mole percentages listed above refer to the liquid-phase mole percent of the cylinders. The n-pentane mobile phase was spectrograde (EM Scientific, Cherry Hill, NJ). Experimentswith an NH3mobile phase were performed with a separate but similar chromatographic system modified for NH, compatibility (9). RESULTS AND DISCUSSION The capillary UV-absorption detection configuration developed for microbore liquid chromatography (10) and subsequently modified for capillary SFC ( 4 ) involves passing the light perpendicular to the axis of the column, through the cylindrical sample cell. This configuration is shown in Figure 1, where a 50 pm i.d. column was inserted into the cell to just below the light path, eliminating any connecting tubing or tortuous flow paths. The consequence of this cell geometry for low critical temperature, highly compressible fluids are discussed below. Peaden and Lee (3) derived an equation to estimate allowable detector volumes based on capillary column diameter and length, plate height, capacity factor, and resolution loss. Peak compression due to analysis in the liquid state can be accounted for by the accompanying change in density, and the equation becomes r 11/2
where V , is the detector volume, d, the internal column diameter, L the column length, h the theoretical plate height,
0003-2700/8S/0360-0802$01.50/0 0 1988 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 60, NO. 8. APRIL 15, 1988
A
803
- Restrictor
I
I-
BUIt connecto,
-250
HLJ Flgue 1. to scale).
pm i.d. Fused Silica
Figure 2. Schematic diagram of the UV-cell mount (not to scale). 50 prn ~d Column
Schematlc diagram of the capllhly UV-absorpHon cell (not
Table I. Capillary Column SFC Detector Volume RequirementsD column L,m V,pL 10 30
0.2 g mL-'
0.7 g mL-'
k
V,, nL
Ldlmm
Vd,nL
Ld,mm
20
2
60
5 2 5
26 54 47 94
0.5 1.1 1.0 1.9
52 117 89 118
1.2 2.4 2.1 4.1
'Conditions: 250 pm i.d. cell at 20 "C;50 p m i.d. column; C02 mobile Dhase at 50 OC in the column. M8the fractional resolution loss, k the capacity factor, and column and detector densities, respectively. It was of interest to apply eq 1 to a COz mobile phase and account for the effect of low and high density. Calculated detector volumes, and corresponding lengths of 250 fim i.d. cell tubing, leading to a 1%resolution loea based on measured efficiencieswith a COz mobile phase a t low and high densities (1I ) , and accounting for compression from supercritical to liquid state, are listed in Tahle I. The values for different column lengths are directly cornparahle since the plate height values used were obtained a t k = 2; the values for k = 5 are hypothetical because the same plate height was used in the calculations, and a more retained solute would likely have a different plate height. At low densEty and high efficiency, a 0.5-mm length of 250 m i.d. tubing produces a 1% resolution loss, while at higher density and lower efficiency, a 4-mm length produces the same loss. The detector used in this study allowed positioning the cell ahout 2 mm from the photodiode surface. The cell was easily mounted and aligned by using the simple and inexpensive design shown in Figure 2. Alternatively, an optical axis adjustahle cell mount (9) was used. The detector focuses the incoming light to a 750-fim diameter image which required use of a mask (200 pm width) in front of the 250 fim i.d. capillary cell. All chromatograms and sensitivity data were obtained by using the mount shown in Figure 2. Base-line studies were performed with either mount. The salient features of the optical characteristics of cylindrical cells are summarized here. Refraction of a light ray is a result of the ray transversing, at an oblique angle, a boundary between two media of different refractive indexes, simply described hy Snell's law. In the case of capillary cell detectors, the media boundary where a change can oeeur is that between the cell wall and the mobile phase. The concentric cell wall ahout the mobile phase complicates the geometry of the cell, whether used in liquid chromatography (LC)or SFC. Changes in the refractive index of the mobile phase produce a different angle of refraction for a given ray. Considering an incident pe and pd the
light beam composed of many rays, the new angles of refraction alter the totd refractive characteristics of the cell such that more or leea light may be detected by the collection optics, resulting in a change in signal. Vindevogel et al. (12)addressed this problem in LC where the refractive index changes arise from composition gradients of liquids of differing refractive indexes. Using a very simplified model and assuming perfectly collimated light, they predicted the capillary cell to act as a lens that focused the light a t about a one column diameter distance past the cell. Also, if the width of the incident beam was greater than the column diameter, then there would be problems due to reflection and refraction through the cell walls. In SFC, premure or density programming produces density variations, and, therehy, refractive index changes in the mobile phase, expressed by the Lorent.-Lorentz relationship (n2 - l ) M R= (2) (n2 + Z)P where R is the molar refraction, n is the refractive index, M is the molecular weight, and p is the density. For COz, the molar refraction is nearly constant (13, 14). T o a first approximation, the refractive index for COz is linearly proportional to density (the slope of refractive index v8 density curve increases hy only 10% over the density range 0.2-1.0 g mL-'). The optical factors discussed above were not initially considered in our research into mixed mobile phases. However, i t became immediately apparent that there was significant difference in using COzmohile phase as opposed to n-pentane which had been the mobile phase in other UV-detector studies. A rapidly falling hase line was observed when the COZ pressure was increased. The baseline behavior of several other fluids was investigated; a hsseline drop was also ohserved with an NzO mobile phase, a much smaller base-line drop was observed with an NH,mobile phase, and a 17.5 mol % mixture of NH, in CHF, (critical temperature 33 "C) increased the magnitude of the drop relative to pure NH,. In contrast, a slight base-line rise was observed with an n-pentane mobile phase, possibly due to the concentration of UV-absorbing impurities increasing as the pressure and flow increased. These results indicated that the rate of base-line drop was inversely proportional to the mobile phase critical temperature. Hence, higher critical temperature fluids, generally liquids at room temperature (e.g., n-pentane), cool significantly below their critical temperature before reaching the optical cell, which leads to low compressibility. While NH3 is not a liquid at room temperature, it has a high critical temperature and cools to a state of lower compressibility than mobile phases such as COz or NzO. The more compressible a fluid is in the cell, the larger will he the density and refractive index changes. The cell area temperature was measured to he approximately 34 "C, which was due to heat from the diode stabilizing resistor and heat conduction through the detector c h i s from the deuterium lamp. Further experiments revealed that the
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ANALYTICAL CHEMISTRY, VOL. 60, NO. 8, APRIL 15, 1988
rate of C 0 2 base-line drop was proportional to the cell area temperature for the range of 20-50 OC. This arises from larger density changes at higher temperatures. When the cell area and oven were regulated to the same temperature, and the mobile phase density was linearly programmed, the base-line drop was very close to linear. Since density and refractive index are linearly related to a first approximation, the cause of the falling base line seemed to be due to changes in the mobile phase density in the cell. As noted above, the detector used in this study focused the incident light to a small spot. The possibility of the entire cell geometry and optics being such that the direction of base line changes could be influenced by cell position relative to the focal point was investigated. Specifically, whether converging or diverging incident light resulted in different cell optical characteristics was studied. The cell was first placed in front of and then past the 254-nm focal point, and in both cases a base-line drop was observed with increasing pressure. The results indicate that the cell position relative to the focal point is not a factor causing the base-line changes, except as related to total throughput. The image at the focal point is that of the fiber optic bundle used in the internal optics of the detector in this study. The light at this point is not homogeneously diverging or converging, so the possibility could not be appropriately tested. Other possible causes of the falling base line were also investigated. Critical point opalescence occurs in a small pressure-temperature region near the critical point of a fluid. The cell area and oven were maintained a t 31 "C while the pressure was increased through the critical point. No absorption was observed, contrary to what would be expected from opalescence. The flow rate was decreased to a very low rate and, under the same conditions, opalescence could not be induced. These results indicated that critical point opalescence was not present in the system. Scattering induced from turbulent flow, as a possible consequence of the expansion from the 50 pm i.d. column to the 250 pm i.d. cell, was investigated. The column was withdrawn to several centimeters below the light path while flow was maintained. The resulting base-line profile was identical with that obtained with the column immediately below the light path. These results indicated that flow expansion induced turbulence was not the cause of the base-line changes. When the cell area temperature was regulated at about 20 "C, the base-line drop was acceptable and the system could be used with density or pressure programming. If it is desirable to pressure or density program the mobile phase and keep the cell at supercritical conditions, then some form of collection optics which directs all refracted light to the detector photodiode appears necessary. Base line compensation programs could be used, but they require a blank run for each different set of operating conditions, and they may decrease sensitivity. The optical behavior of C02 in typical liquid chromatography UV-absorption cells may be related to the refractive index phenomenon observed in this study. These cells are cylindrical with light being transmitted lengthwise through flat windows on the ends of the cells. A falling base line with increasing pressure was reported by Schneider and co-workers (1.516) with a C 0 2mobile phase. Experiments with such an LC cell made for the detector in this study also produced a falling base line with increasing pressure. The linearity of the detector was determined by measuring the peak area vs peak mass of injected solute. The cell area temperature was 35 "C. An earlier evaluation of split injection in capillary SFC (8) showed nonlinear peak area vs split ratio behavior; the 20:l C 0 2split ratio used in this study was estimated to result in about 40% of the injected sample mass
!c
1
k
Time (rninl 0 Pres (am)
60
30 I
I
I
1
80
120
160
200
Figure 3. SFC UVabsorption chromatogram of a coal tar extract with linear pressure programming.
being transferred to the column. The detector gave linear response for 1.6-200 ng introduced on the column. The detector has a built-in set of digital filters, but the sensitivity evaluation was performed with the detector rise time set to zero. This provided a comparison of the raw detector noise to the noise level after the signal was conditioned with an external, electronic low-pass filter prior to analog-to-digital conversion. The signal-to-noise level at 1.6 ng was about 1 0 1 before filtering but increased to 401 after electronic filtering. AU. The noise level with external filtering was about 5 X From these data, the minimum detectable amount (chrysene at 269 nm) with the present system was estimated to be about 30 pg at a signal-to-noise ratio of 3:1, about 5 times more sensitive than our previously reported system (8). Direct injection without split could lower the minimum detectable limit by at least a factor of 2 or more (9). Another factor should be noted in the discussion of minimum detectable limits with this system; the 1.6-ng value was obtained with the detector operating at 0.005 absorbance units full scale (AUFS). This setting is more sensitive than the usual setting used (0.02-0.05 AUFS). Consequently, operation of the detector near its maximum sensitivity would also make any base-line changes more significant. A pressure-programmed analysis of a coal tar extract with the cooled cell (20 "C) is shown in Figure 3 with the C 0 2 mobile phase at 80 OC. The magnitude of the base-line drop was acceptable, and the peak widths indicated no significant detector cell dead volume was present and that the decrease in temperature in the cell area, relative to the oven temperature, produced no significant peak-broadening. Detection was at 254 nm throughout the program, although the detector is capable of changing wavelengths during a chromatographic run if desired. An "on-the-fly" spectrum of pyrene was taken during an isobaric chromatographic analysis and compared to a liquid solvent spectrum (17) (Figure 4). Absorption values were taken at 2-nm intervals, which resulted in a 12-s scan time. The total peak width was about 60 s, so the absorption values in the SFC scan were dependent on the
ANALYTICAL CHEMISTRY, VOL. 80, NO. 8, APRIL 15, 1988
805
Table 11. Effects of Polar Modifiers in COz on Retention'
kb
solute
COZ (0.32)
carbazole
2.2 4.4 9.1
pyrene 4-hydroxypyrene
a
2PA'fCOZ (0.43) CHSN02fCOZ (0.54) 0.2 0.3
0.4 0.6 0.8
COZ
2PAfCOZ
CH3NOZfC02
2.0 2.1
1.4 1.2
1.4
"Conditions: mole percent modifier in COz at 80 "C, 125 atm for 2-propanol and 135 atm for CH3NOz,column described in text. Numbers in parentheses are fluid densities (g mL-') calculated from the Peng-Robinson equation of state. 2PA, 2-propanol. B
A
r
220
260
300
340
220
260
I
A
300
340
Wavelength (nm)
Flguro 4. Comparison of UV-absorptlonspectra of pyrene: (A) SFC peak, CO, mobile phase at 80 "C and 125 atm, 2-nm step scan, 34 "C cell area; (B) cyclohexane solvent, conventional cuvette, ambient temperature.
changing concentration of the solute. If spectra were to be taken during an SFC analysis, the spectral scan rate would have to be increased or, alternatively, the system could be changed to a photodiode array detector. Before a mixed fluid can be used as a mobile phase, the critical parameters must be known. The critical points of mixed mobile phases were calculated by the methods of Cheuh and Prausnitz (18)for critical temperatures and Kreglewski and Kay (19)for critical pressures, with parameters given by Reid et al. (20). The critical temperatures and pressures of the 2-propanol and nitromethane mixed fluids were calculated for mole fractions of 0 to 0.2, using the C 0 2constants listed in ref 20. Conditions for retention measurementswere selected such that the temperatures and pressures were above estimated critical values. The base-line characteristics of the system with a mixed mobile phase were evaluated with an 8.9 mol % 2propanol/C02 mixture. In light of the behavior of pure COz, the effect of pressure programming on base-line changes was studied. When a mixed mobile phase was used, the refractive properties of the fluid in the cell were changed. The base-line changes of the mixed mobile phase with increasing pressure were less than the changes for pure C02 This behavior would be expected considering that the mixed mobile phase ought to be less compressible than pure C02, since the modifier is normally a liquid. Lower compressibility should lead to smaller changes in density, refractive index, and base line with increased pressure. High levels of noise were observed for both 2-propanol/C02 and nitromethane/COz mobile phases below certain pressures. These pressures were higher for a noncooled cell than for a cooled cell. The noise was most likely a result of phase immiscibility in the cell. This phenomenon is being investigated as a possible method to determine critical parameters and phase boundaries. Nonpolar and polar PAC test solutes were used to test the solvating strengths of the pure and mixed mobile phases. These data are listed in Table 11, and they show dramatic retention changes, similar to the changes reported by Yonker and Smith (2). Interestingly, 4-hydroxypyrene displayed the largest absolute retention change with the 2-propanol/COz
I
B
0.03 AU
IL
Time (min)
Pres ( a m )
0
L 20
w 80
100
I
0
Time ( w i n )
1
Figure 5. SFC UV-absorptlon chromatograms of a coal tar extract: (A) 8.9 mol % 2-propanoilC0, mobile phase (80 "C) with linear pressure programming; (B) 8.9 mol % CH3N02/C02mobile phase (80 "C) Isobaric at 135 atm; detection at 254 nm, 20 OC cell area.
mobile phase which may have been due in part to the possible hydrogen bonding character between it and the 2-propanol. Yonker and Smith (21)have shown excellent correlation between supercritical fluid density data to values predicted with the Peng-Robinson equation of state for a 6.5 mol % 2propanol/C02 mixture at 70.1 "C. The mixing procedure (22, 23) accounted for difficulties in composition calculation arising from fluid transfer to the pump from the mixing vessel. Estimates of density values from the Peng-Robinson equation of state, using interaction parameters regressed from binary vapor-liquid equilibria data (24),for conditions in this study, are therefore reasonably accurate. However, the applicability of the procedure to the nitromethane/C02 mobile phase may be subject to error. The same coal tar sample as analyzed in Figure 3 with the neat COz mobile phase was analyzed with the mixed mobile phases. With the 2-propanol/C02 mobile phase (Figure 5A), the sample elutes with very little separation due to possible operation at subcritical conditions (24).The nitromethane/COPmobile phase was operated just above the critical point. All components of the sample eluted very rapidly but with better separation than in Figure 5A. The retention data and chromatograms of the coal tar sample indicate that very high molecular weight compounds may be amenable to SFC analysis in reasonable analysis times with mixed mobile phases. Ovalene, a high molecular weight (398.5 g mol-') and nonvolatile polycyclic aromatic compound, was eluted at 205 atm and 120 "C (0.5 g mL-') with the 8.9 mol % 2propanol/COz mobile phase in about 15 min (Figure 6). The
808
ANALYTICAL CHEMISTRY, VOL. 60, NO. 8, APRIL 15, 1988
mi 1;
I
0 0015 AU
1.
I
!
. '
I
m
o
Tlme ( m r \
15
Flgure 6. SFC UV-absorption chromatogram of ovalene at 337 nm, in a saturated solution of warm 1-methylnaphthalene and 1,2,4-trichlorobenzene (1: 1).
low absorptivity (25)and solubility in the injection solution necessitated a very sensitive absorbance setting of 0.005 AUFS. The cell area was also not cooled (34 "C). The ,A, appeared to be a t about 337 nm from spectra taken during a chromatographic run, a shift of more than 10 nm from a published value of A, (25). At the published value of 349 nm, no peak was detected.
ACKNOWLEDGMENT The authors thank Bob Wright, Pacific Northwest Laboratories, for preliminary calculation of the P-propanol/C02 critical parameters, Jacob Kuei for performing the NHB experiments, and Klaus Anton, Ciba-Geigy, Ltd., for molar refractivity data. The mixed mobile phase fluids were provided by Robert Denyszyn, Scott Specialty Gases. Registry No. COB,124-38-9;carbazole, 86-74-8;pyrene, 12900-0;Chydroxypyrene,31700-39-7;ovalene, 190-26-1;naphthalene, 91-20-3; phenanthrene, 85-01-8; chrysene, 218-01-9; benzo[a]pyrene, 50-32-8;2-propanol, 67-63-0; nitromethane, 75-52-5.
LITERATURE CITED (1) Wright, B. W.; Smlth, R. D. J. Chromatogr. 1988,355, 367-373. (2) Yonker, C. R.; Smith, R. D. J. Chromatogr. 1986,361, 25-32.
Peaden, P. A.; Lee, M. L. J. Chromatogr. 1983,259. 1-16. FjeMsted, J. C.; Jackson, W. P.; Peaden, P. A.; Lee, M. L. J. Chromatogr. Scl. 1983,2 1 , 222-225. Woolley, C. L.; Kong. R. C.; Richter, B. E.; Lee, M. L. HRC CC, J. High Resolot. Chromatogr . Chromafogr. Commun 1984, 6,329-332. Kong, R. C.; Flelds, S.M.; Jackson, W. P.; Lee, M. L. J. Chromatogr. 1984,2 8 , 105-116. Wright, B. W.; Peaden, P. A.; Lee, M. L.; Stark, T. J. J. Chromatogr. 1982,248, 17-34. Fjeldsted, J. C. Ph.D. Dissertatlon. Brigham Young University, Provo, UT, 1985. Kuei, J. C.; Markides, K. E.; Lee, M. L. HRC CC, J. High Resolut. Chromatcgr. Chromatogr. Commun. 1987, 10, 257-262. Yang, F. A. HRC CC J. High Resolot. Chromatogr. Chromatogr. Commun. 198194 , 83-85. FieMs, S.M.; Lee, M. L. J. Chromatogr. 1985, 349, 305-316. Vindevogel, S.;Schuddlnck, G.; Dewaele, C.; Verzele, M. Proceedlngs of the Eighth International Symposium on Capillary Chromatography, Riva del Garda, Italy, May 19-21, 1987; pp 1175-1184. Beggerow. G. NumericalData and Functional Relatbnships In Science and Technology, Group I V : Mscrosmplc and Technical Properties of Matter; Schafer, K.. Ed.; Springer-Verlag: Berlin, 1960; Vol. 4, pp 263-267. Leach, R. A.; Harrls, J. M. Anal. Chem. 1984,5 6 , 1461-1487. Wllsch, A.; Schneider, G. M. J. Chromatogr. 1988, 357, 239-252. Linnemann, K. H.;Wiisch, A.; Schneider, G. M. J. Chromatogr. 1986, 369, 39-48. Spectral Atlas Of Po&cyc/ic Aromatic Compounds, Karcher, W., et ai.. Eds.; Reidel: Dordrecht, Holland, 1985; p 92. Cheuh, P. L.; Prausnltz, J. M. AIChEJ. 1967, 1 , 1099-1107. Kreglewski, A.; Kay, W. B. J. Phys. Chem. 1989. 73, 3359-3366. ReM, R. C.; Prausnitz, J. M.; Sherwocd, T. K. Properties of Gases and LlquMs, 3rd ed.; McGraw4ill: New York, 1977; Chapter 5.7. Yonker, C. R.; Smith, R. D. Anal. Chem. 1987,59, 727-731. Yonker, C. R., personal communication, Richland, WA, 1986. Yonker, C. R.; McMlnn, D. G.; Wright, B. W.; Smith, R. D. J. Chromatogr. 1987. 396, 19-29. Radosz, M. J. Chem. Eng. Data 1988,31, 43-45. Ckr, E. Po~cyc/lcHydrocarbons; Academic: London, 1964; Vol. 2, p 103.
.
RECEIVED for review January 27,1987. Accepted December 14, 1987. This work was supported by the Gas Research Institute, Contract No. 5084-260-1129. Partial support was also provided through a grant from the Dow Chemical Co., Midland, MI. Any opinions, findings, conclusions, or recommendations expressed herein are those of the authors and do not necessarily reflect the views of GRI or Dow Chemical.
