1242
Anal. Chem. 1891, 63,1242-1250
loading; otherwise the system is similar to previously described DS/IC instruments (14). Good collection efficiency and very low detection limits have also been observed in this case. We therefore believe that the general technique has considerable potential for the measurement of trace atmospheric gases; especially since it allows the use of virtually any liquid to be used as the scrubbing medium. ACKNOWLEDGMENT We wish to thank Candace H. Haigler, Department of Biological Sciences, TTU, for photomicrography and Kavin Morris for expert machining. Daniel P. Y. Chang, University of California, Davis, is acknowledged for pointing out to us the importance of oscillatory films in multiphase flow. Registry No. Zonyl FSN,57534-41-5;H202,7722-84-1; SO2, 7446-09-5. LITERATURE CITED Herlng, S. V.; Lawson. D. R.; Allegrlnl, 1.; Febo. A.; Possanzlnl, M.; Sickles, J. E., 11; Anlauf, K. G.; Wlebe, A.; Appel, B. R.; John, W. A t ” . Envhon. 1988, 22, 1519-1539. All, 2.; Paul Thomas, C. L.; Alder, J. F. Analyst (London) 1989, 774,
759-769.
Cheng, Y.-S. I n A t Sampllng Instruments, 7th ed.;Hering, S. V., Ed.; American Conference of Governmental Industrial Hygienlsts: Cincinnati, OH, 1989; pp 405-419. Braman, R. S.; Shelley, T. J.; McClenny, W. A. Anal. Chem. 1982, 54, 358-364. Sianlna, J.; Keuken, M. P.; Schoonebeek, C. A. M. Anal. Chem. 1987, 5 9 , 2764-2766. Keuken, M. P.; Wayers-Ijpelaan, A.; Mols, J. J.; otjes. R. P.; Slanlna, J. A W . E n v k o n . 1 9 8 9 , 2 3 , 2177-2185. Lanaford, A. 0.: Goldan. P. D.; Fehsenfeld, F. C. J. Atmos. Chem. 1983, 7 , 137-152. Klockow, D.; Nlessner, R.; Malejczyk, M.; Klendl, H.; Vom Berg, B.; Keuken, M. P.; Wayers-Ypelian, A.; Sianina, J. Atoms. Envkon, 1989, -23. - 1131-1138. . .- . . . - -. BOS,R. J . Alr Poll. Control Assoc. 1980. 30, 1222-1224. Dasgupta, P. K. Atmos. €nnvkon. 1984, 78. 1593-1599. Dasgupta, P. K.; Dong, S.; Hwang. H.; Yang, H.C.; Genfa, 2.; Atmos. Envlron. 1988, 22, 949-984.
.
(12) Genfa, 2.; Dasgupta, P. K.; Dong, S. fnvkon. Scl. Techno/. 1989, 23, 1467- 1474. (13) Lindgren, P. F.; Dasgupta, P. K. Anal. Chem. 1989, 67, 19-24. (14) VeEeia, 2 . ; Dasgupta, P. K. fnvkon Scl. Technd. 1991, 2 5 , 255-260. (15) Lawson, D. R.; Winer, A. M.; Biermann, H. W.; Tuazon, E. C.; Mackay, 0.I.; Schiff, H. I.; Kok, G. L.; Dasgupta, P. K.; Fung, K. AerosdScl. TechnOl. 1990, 72, 84-78. (16) Kok. G. L.; Schanot, A. J.; Lindgren, P. F.; Dasgupta. P. K.; kgg, D.; Hobbs, P. V.; Boatman, J. F. Atmos. Envlron. 1990. 2 4 A .
1903- 1908. (17) Fendlnger, N. J.; Glotfelty, D. E. Envkon. Sci. Technol. 1988, 22, 1289-1293. (18) Keuken, M. P.; Schoonebeck, C. A. M.; Wensveenlouter, A.; Slanlna, J. A t ” . Envkon. 1988, 22, 2541-2548. (19) Yunghans, R. S.; Munroe, W. A. Technlcon Symposia. 1965: Automatlon In AnalyNcel Chemlstry; Mediad Inc.: New York, 1966 pp 279-284. (20) Lazrus, A. L.; Kok, G. L.; Llnd, J. A,; Oitlln, S. N.; Helkes. B. G.;Shetter, R. E. Anal. Chem. 1986, 58, 594-597. (21) Genfa, 2.; Dasgupta, P. K.; Cheng, Y.-S. A m . Envkon., in press. 122) . . Mercer. T. T.: Tlllerv. M. I.: Chow, H. I.Am. Ind. Wg. .- Assn. J . 1968, 29, 66-78. (23) Kieber, R. J.; Mopper, K. Envlron. Scl. Technol. 1990, 2 4 ,
-
1477-1 . .. . .A8 . 1..
(24) Scott. R. P. W. I n Smew Bore Liquid Chrometogephy Columns: Thek properties and Uses; Scott, R. P. W.. Ed.; Wiley: New York, 1984. (25) Haddad, P. R.; Heckenberg, A. L. J. Chromatogr. 1984, 299, 301-305. 128) . . Bucholz. A. E.: VerDlouah. C. I.;Smith, J. L. J. Womfatog - Scl. 1982, 2 0 , 499-501.’ (27) Jupllle, 311-3113T.; Burge, D.; Togami, D. Chromatogrephia 1982, 76,
-
- .- - .- .
(28) American Chemical Society Committee for Environmental Improvement. Anal. Chem. 1980. 5 2 , 2242-2249. (29) Gormley, P. G.; Kennedy, M. PrOcRoy. I f . Aced., Sect. A 1949, 5 2 , 163-169. (30) Chandhok, A,; Voorhles. N.; McCready, M. J. AIChE J. 1990, 36, 1259-1262. (31) Warneck, P. Chemistry of the Natural Atmosphere; Academic Press: New York, 1988; p 520.
RECEIVED for review November 19, 1990. Accepted March 11, 1991. This work was supported by the Electric Power Research Institute through R P 1630-55, the US. Environmental Protection Agency through Grant R-81592801-0,and the State of Texas Advanced Research Program.
Unified Open Tubular Column Gas and Supercritical Fluid Chromatography Ilona L.Davies*J and F r a n k J. Yang* Dionex Corporation, 1228 Titan Way, Sunnyvale, California 94086
Unled gas chromatography (GC) and supercritical fluld chromatography (SFC) are descrlbed. Moblle phase Is sup piled to a 100 pm 1.d. open tubular fused-slllca column Installed In a chromatographlc oven. One end of the column k directly connected to a rotary valve Injector, and the other end Is butttonnected to a frll restrlctor Inserted In the fiamelonlzatlon detector (FID). The GC mobile phase conrkts of e l t k high-pressure hellum (at tank pressures of 158 atm) on carbon dioxide at 40 atm. Supercrttlcal fluid carbon dloxlde Is usod for SFC. The theoretical aspects of thls technique are flrst Investigated wtth respect to mobllaphase characterlstlcs, optlmum llnear velocltles, and peak capacltks. Applkatlon of sequentlal GC and SFC Is demonstrated for the analyds of a gasdlne/crude oil mixture and a sample of clear nali lacquer. lPresent addrees: Hitachi Instruments, Inc., 3940 N o r t h First St., 5 a n Jose, CA 95134. *Present address: FFFractionation Inc., 1270 W. 2320 S., Suite D, Salt Lake City, UT 84119. 0003-2700/91/0363-1242$02.50/0
INTRODUCTION Gas chromatography (GC) is always the chromatographic technique of choice because it provides fast and efficient separations. However, not all solutes are thermally stable and/or volatile enough for GC and in these cases supercritical fluid chromatography (SFC) is the preferred technique. When neither GC nor SFC are applicable, then liquid chromatography (LC) should be used to analyze the sample of interest. Complex samples, such as plant extracts, perfumes, foodstuffs, and petroleum products often contain a wide variety of solute types and exhibit a wide boiling point range. These solutes may be either volatile or nonvolatile, thermally stable or labile. In such cases, a sequential combination of GC, SFC, and LC may be desirable in order to fully resolve the sample components in one chromatographic run. Several research groups have demonstrated that unified fluid chromatography is achievable by using a single chromatographic system (1-14). In unified chromatography, GC, SFC, and LC may be selected sequentially by changing the @ 1991 American Chemlcal Society
ANALYTICAL CHEMISTRY, VOL. 63. NO. 13, JULY 1, 1091
-.
,dr
PKU*AlICbLLY ACTIVATED TVO-WAY SIX-PORT V A L V E
....
1243
.r
m HEUUMGC COZQC I COZSFC
INJEClW VALVE
!e# -
COLUMN
He CYLINDER
-
0
0
Flgwe 1.
2
4
6
Schematlc dlagram of the unified GC and SFC system.
8
10
12
14
16
u (cmls)
F b w r 3. Experlmental van
Deemter pbts of plate helght ( H ) versus the average linear veloclty of the mobile phase (u). Experimental condltlons: 10 m X 100 pm i.d. open tubular column, 0.25 pm d, polymethylsiloxane, 50 pm 1.d. frlt restrictor; 158 atm/40 "C helium GC (k' = 4.30 for heptane), 40 atm/40 "C carbon dioxide GC (k' = 1.30 for heptane),and 120 atm/lOO "C carbon dbxlde SFC (k'= 1.45 for hexadecane).
0.4
i-
2.0 1.5
11
i
HEUUMGC COZSFC COZGC
1
8, I
/
-0.8 I E 2.6 2.7 2.8 2.9 3.0 3.1 3.2 3.3 2 1.0 'ir
( 1 m x 10-3 (1IK)
Flgue 2. Van? Hoff pkts of b g k'versus l/Tfor 158 atm hellum GC and 40 atm carbon dbxlde GC. Experimental condkbw 10 m X 100 pm 1.d. open tubular column, 0.25 pm d , polymethylslloxane, 50 pm 1.d. frlt restrlctor; k'measwed for heptane versus methane at 40,60, 80, and 100 "C (u = 1-2 cm s-').
1 0.5
0.0 0
10
20
30
40
50
60
u 2 (cm/s)
pressure and temperature of the chromatographic column. The mobile phase may be a single fluid, such as carbon dioxide; alternatively, different fluids may be used, such as helium (for GC), carbon dioxide (for SFC), and acetonitrile/ water (for LC). Naturally, a single fluid is most desirable for unified chromatography. However, the use of a single chemical as mobile phase for GC, SFC, and LC is limited in that it may not be the optimum choice for each individual mode, particularly in dealing with samples of wide polarity ranges. The goal of this research was to demonstrate that unified open tubular column GC and SFC could be carried out in one chromafagraphic syatem and also to inveatigate the theoretical aspects of using helium and carbon dioxide mobile phases in this system. Pentoney et al. (14)described the separation of both volatile and nonvolatile compounds on a single open tubular column by the following procedure: After the sample was injected into the column, the column was installed into a gas chromatograph to elute the volatile components with a helium mobile phase; then the column was transferred into a supercritical fluid chromatograph, connected to a restrictor, and run under SFC conditions with a carbon dioxide mobile phase to elute the nonvolatile material. The difficulty of combining low-pressure GC with high-pressure SFC is that both need to use the same injector and restrictor in order to be used in a common system. Normal low-pressure GC operates at pressures of 1-3 atm, whereas commercial SFC operates at 80-400 atm; a restrictor, which is suitable for SFC conditions, would be unacceptable for normal GC pressurea and flow rates. In the 19608, Knox and Giddings (15,16) recognized the potential for high-pressureGC (20-2000 atm). Because helium
Flgure 4. Calculation of experimental diffusion coefficients (H,,,): W versus u2 plots for 158 atm helium GC,40 atm carbon dioxide GC, and 120 atm carbon dioxide SFC. Experimental conditions are the same as for Figure 3.
lacks solvating power, its pressure had little effect on chromatographic migration but other gases (such as carbon dioxide and ammonia) showed substantial solute migrations at high pressures (18,19). The technique of SFC grew from these initial observations. This report describes the work we have done to provide a simple technique for carrying out sequential GC and SFC separations in one chromatographic run and to identify operating conditions and common instrumentation requirements for the application. In particular, the choice of high-pressure helium GC, or 40 atm carbon dioxide GC, was investigated for the elution of low-molecular-masscomponents prior to the SFC separation.
(In,
THEORY Compressed Gases as Mobile Phases. The solvating power of a gas depends on both a 'state effect" and a 'chemical effect" (18,20,21). The state effect is determined by the degree of compression (or molecular closeness) and temperature. The chemical effect describes the polarity, acid-base properties, and other chemical properties of the gas that are related to its binary interaction parameters. Helium is a poor solvent because it is a small monatomic molecule with a correspondinglytight electron cloud (unlike xenon, which has a large electron cloud and is a superior solvent to carbon dioxide (22)).
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ANALYTICAL CHEMISTRY, VOL. 63,NO. 13, JULY 1, 1991
Table I. Predicted and Experimental Diffusion Coefficient Valuesn
mobile phase
solute
P, atm
T,K
k’
helium carbon dioxide carbon dioxide
heptane heptane hexadecane
158 40 120
313 313 373
4.30 1.30 1.45
D’,(F-S-
G),cm2/s D,(F-S-G),
cm2/s
1.67 x 10-3 1.23 X lo4 3.10 X lo-‘
0.2645 0.0616 0.0531
D m (W-C)
cmz/s 2.73 2.04
X X
lV3 lo-’
D,(expt), cm2/s 1.24 X lVa 5.80 X lO-‘ 2.40 X lo-’
oD’,(F-S-G) is the binary diffusion coefficient for solutes in gases at 1 atm, calculated from the FullerSchettler-Giddinge equation. D,(F-S-G) is D’,,,(F-S-G) corrected for pressure by the Takahashi correlation (28).D,(WC) is the binary diffusion coefficient for solutes in liquids, calculated from the Wilke-Chang equation. D,(expt) is calculated from the slope of the Hu versus 12graph in Figure 4.
/
6o
co2 SFC W.C)
1
50
COZGC
c
COZSFC
s
c
0
U
4
40
30
0 Y
2
20
CO20C(Fg4)
0
2
6
4
8
10
12
14
10
16
u (cmls)
5. Theoreticel van Deemtsr plots of plate heiaM (H)versusthe average linear velodty of the moMle phase (u). Redlcted theoretical data were calarletedfrom etlher the F u k r - a n g s (F-S-G) equation or the Wllke-Chang (W-C) equation. 1207
\
100 80
B
C02GC HaGC
0
2
20 0
4
6
8
10
12
10
12
14
16
u (cmls)
Flgwr 7. Plots of N’versus u using 40 atm carbon dioxide QC and 120 atm carbon dioxice SFC, for similar k’ values. Experimental conditions are as for Figure 3.
H = (2D,jfl/u
40
2
8
theoretically predicted for a unified GC and SFC system. For open tubular columns, column efficiency may be characterized in terms of the theoretical relationship between the column plate height (H) and the mobile-phase average linear velocity (u),as described by the Golay equation modified by Giddings:
60
0
6
4
14
16
u (cmls)
Flgwo 6. Plots of peek capacity (N’) versus u for n-heptane using 158 atm kllum W and 40 atm carbon dioxide GC. Experimental conditions are as for Figure 3.
Chromatography with dense gas helium is referred to here as high-pressure GC, even though the helium is in its supercritical region: whereas that using dense gas carbon dioxide is referred to as either SFC or GC, depending on whether the operating pressure is above or below the critical pressure, respectively. (Carbon dioxide has a critical pressure (P,) of 72.9 atm and a critical temperature (T,) of 31.3 “C; helium has a Po of 2.26 atm and a T,of -267.9 “C.) Golay Equation. The unification of GC and SFC in one system needs to be studied in terms of the optimum mobile-phase flow rates for each condition. High pressures affect the optimum linear velocity of the mobile phase because the solute diffusion coefficient in the mobile phase is inversely proportional to pressure (21). The boundary conditions of high-pressure GC and SFC in terms of linear flow velocities (and thus column inlet pressure and column outlet restriction) that will result in the highest separation efficiencies can be
+ [d,2f(1 + 6k’ + 11k’2)~/96D,(1 + k’I2j] + (2k’d&)/3(1 + k’)2D, (1)
where d, is the column internal diameter, df is the stationary-phase film thickness, k’ = ( t R - t M ) / t M , where t~ is the solute retention time and t~ is the unretained peak time for the column, D, and D, are the solute diffusion coefficients (measured at 1 atm pressure) in the mobile phase and in the stationary phase, respectively, and j and f a r e pressure-dependent factors: j = 3 ( P - 1 ) / 2 ( F - 1) (2) f = 9 ( P - 1 ) ( P - l ) / 8 ( F - 1)2
(3)
where P = Pdet/POutleb Maximum column efficiency is obtained at the minimum plate height, H,,, at which point the linear velocity is at its optimum (uopt):
Hmin= [(d,f)/2][(1
+ 6k’+ llk’2)/3(1 + k’)2]1/2
uOpt= [(8D,j)/dc][3(1
(4)
+ k’)2/(1 + 6k‘+ llk’2)]1/2(5)
For the same column, Hmhis influenced mainly by the solute capacity factor, k’, and the presence factor, f. However, uwtis influenced by the solute capacity factor, k’,the j term (which is inversely proportional to the pressure drop across the column), and the solute diffusion coefficient in the mobile phase, D,. Unlike low-pressure normal GC, high-pressure GC is
ANALYTICAL CHEMISTRY, VOL. 63, NO. 13, JULY 1, 1991
0
m
40
IO
,
IO
m
m
1245
1m
lb
tg MI*
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Flguro 8. Temperature-programmed gas chromatogram of the gas-
oilne/crude oli mixture using the 158 atm helium GC mobile phase. Experimental conditions: 10 m X 100 pm 1.d. open tubular column, 0.25 pm d , polymethyisibxane, 50 pm i.d. frit restrictor; oven temperature-programmed from 40 to 280 OC at 3 O C mh-‘; u = 1.95 cm s-’ for helium at 158 atm/40 OC. characterized by a relatively low optimum linear velocity; at high preasurea, the value of j is smaller and, more importantly, higher gas viscosity significantly decreases solute diffusion coefficients. However, this is true only under conditions of laminar flow. Giddings and co-workers (23-25) showed that turbulent flow in high-pressure GC could be achieved above certain Reynolds numbers. When turbulent flow was achieved, the plate height was drastically reduced. Turbulence was not encountered in this work because a back-pressure reatrictor was used to control the flow rate of the mobile phase and to ensure that depressurization occurred mainly at the restrictor frit. Without restriction at the end of the column, tank pressure helium would be forced through the column at such high flow rates that turbulence may occur. Solute diffusion coefficients may be calculated from the Golay equation, or they may be determined from experimental data plotted as graphs of Hu versus u2 (26): Hu = 20, jf [d,2f(l + 6k‘+ llk’2)~2]/96D,(1 k’I2j (6)
+
+
D, can be calculated from either the y intercept or from the slope of the graph. Solute diffusion coefficients in gases may be theoretically predicted from the Fuller-Schettler-Giddings equation (27-29) and in liquids from the Wilkdhang equation (28-30). Van’t Hoff Equation. The slope of a Van’t Hoff plot of log k‘versus 1 / T can be used to study the physical nature of the mobile phase (31). Carbon dioxide could be used as a gas, liquid, or supercritical fluid in chromatography, depending on its physical conditions. Carbon dioxide that is below its critical pressure but above ita critical temperature should behave as a gas, and a linear region of positive slope in the Van’t Hoff plot corresponding to GC behavior is expected. A region of negative slope at high pressures in the Van’t Hoff plot corresponds to SFC-like carbon dioxide behavior. The Van’t Hoff equation is given as follows (32): In K = AG8/RT
(7)
The partition coefficient, K, is related to the capacity factor, k’, by the phase ratio, 8:
& 0
I 120
20
TIME (MINI PRESSURE ( A T M )
300
120
Flguro 8. Pressure-programmed supercritical fluid chromatogram of the gasoHnelcrude oil mixture using a carbon dioxide mobile phase and the same restrictor as in Figure 8. Experimental condkbns: same as for Figure 8 except oven temperature held isothermal at 100 OC; carbon dioxide pressureprogrammed from 120 to 350 atm at 7 atm min-’; u = 1.17 cm s-’ for carbon dioxide at 120 atmllO0 OC.
K=k@
(8)
In k’ = -AG8/RT - In /3
(9)
therefore where R is the gas constant, T i s the temperature in K, and AG8 is the free energy change associated with the partitioning of a solute between the stationary phase and an inert mobile phase. In GC, the free energy change is due to the free energy of solution of the solute in the stationary phase; however, in SFC the solute is also solvated by the mobile phase, resulting in a reduction of the net free energy change of the partition process. Thus k’is reduced by switching from GC to SFC at a given temperature. Peak Capacity. The advantage of unified GC and SFC can also be realized by comparing the obtainable GC/SFC peak capacity with that of GC or SFC alone. Peak capacity (N’) is defined aa the number of resolved peaks that can be eluted from a column between k’= 0 and a given k’value (33): N’= 1 + [In (1 k’)]/ In [ ( r ~ ’ / ~ /+2 )11 - In [(n1I2/2)- 11 (10)
+
where n is the number of theoretical plates, determined by dividing the column length by the plate height value. For a unified GC and SFC system, column peak capacity could reach a theoretical limit as a sum of GC and SFC alone, if the initial zone spreading before the GC and SFC run can be reduced.
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ANALYTICAL CHEMISTRY. VOL.
63,NO. 13, JULY 1, 1991
GASOLINE
I
I
Gc
20
€a
io
40 I 120
a lh lbo
150
la
im
hold
hold
TIME IMINI
TEMP (*Cl
I
PRESSURE ( A T M I Cop
Flguo 10. Unlfied gas chromatogram and supercritlcai fluid chromatogram of the gaso#ne/crude oil mixture using sequential helium QC/carbon dioxide SFC mobile phases and the same restrictor as in Figure 8. Experimental conditions: same as for Figure 8, except mobile phase no. 1 was helium at 158 atm (u = 1.95 cm s-' for helium at 158 atm/40 OC), temperature-pmgrammed 40 to 150 OC at 3 OC min-' and then cooled to 100 OC; then mobile phase no. 2 was carbon dioxide at 100 O C , pressure-programmed from 120 to 350 atm at 7 atm min-' (u = 1.17 cm
s-' for carbon dioxide at
0
10
0
40
120 atm/100
m 1m
"C).
m
,m
l l Y l lMlNl
2@ Iold T E M P 1.C1
Flguro 11. Temperature-progammd gas chromatogram of the gasoline/crude oil mixture using the 40 atm carbon dioxide mobile phase. Experimental comons: same as for Figure 8 except for the different mobile phase and a different restrictor which gave u = 2.08 cm s-' for carbon dioxide at 40 atm/4O OC.
Scientific was mounted directly above their heated GC injector block, which served only as a controllable heating zone at the column inlet. The injector valve was cooled to 10 "C with a circulatorycooling bath, while the heated zone directly below the injector was held at 150 "C. Timecontrolled direct injection was used to control the volume of sample delivered to the column. A 10 m X 100 pm i.d. (375 pm 0.d.) open tubular fused-silica column coated with 0.25-pm polymethylsiloxane stationary phase (SB-methyl-100, Lee Scientific) was directly connected to the rotary valve injector by a ferrule, nut, and a short piece of PEEK tubing (Upchurch, Oak Harbor, WA).The column was threaded through the GC injector into the oven, and the column end was connected to a 50 pm i.d. frit restrictor (Lee Scientific) by a butt-connector (SGE, Austin, TX). The frit restrictor was cut to a suitable length and inserted into the FID,which was held at 350 OC. The choice of mobile phase was selected at an air-actuated tweway six-portvalve (Valco). The SFC mobile phase was carbon dioxide delivered by the system syringe pump; the GC mobile phase was either high-pressure (158atm) helium delivered directly (unregulated) from the gas cylinder, or low-pressure (40 atm) carbon dioxide delivered from another syringe pump. (Alternately, 40 atm of carbon dioxide could be supplied from a regulated gas cylinder with no dip tube). Procedure. Experimental van Deemter plots were generated by cutting back the restrictor to obtain different average mobile-phase linear velocities, u. The plate height, H, was calculated from experimental data with the following equation: H = (L/5.54)(Wh/tdZ
EXPERIMENTAL SECTION Apparatus. A schematic diagram of the GC and SFC system is shown in Figure 1. This system consisted of a Lee Scientific 600 series oven, pump, and controller (Lee Scientific, Salt Lake City, UT). Only the injector and the means of mobile-phase selection were modified for this work: The Valco 200-nL SFC injector (Valco Instruments, Houston, TX) supplied by Lee
(11)
where W, is the solute peak width at half-height and L is the column length. For GC, H was determined for heptane (C,), with k' = 4.30 in helium at 158 atml4O OC and k ' = 1.30 in carbon dioxide at 40 atml4O OC. For SFC, H was calculated for hexadecane (Cia) in isooctane, at k' = 1.45 at 120 atm/lOo "C. Van't Hoff plots of log k'versus 1/T were obtained for both helium at 158atm (u = 1.46-1.74 cm s-l), and for carbon dioxide at 40 atm (u = 1.16-1.56 cm s-l) by measuring the k'values of
ANALYTICAL CHEMISTRY, VOL. 63,NO. 13, JULY 1, 1991
1247
1)and a sample of commercial clear nail lacquer (application 2) were individually analyzed by GC, SFC, and by unified GC and SFC.
RESULTS AND DISCUSSION Mobile-Phase Characteristics. If one column and re-
-IO
I 120
20
120
40
TIME ( M I N )
3bo
PRESSURE (ATM)
Fburo 12. Presswegrogrammed supercrltical fluid chromatogram of the gaaso#ne/crucbOR mixtve uslng a carbon dioxide mobile phase and the same restrictor as in Flgure 1 1. Experimental conditions: same as for Figure 9 except that u = 5.19 cm s-' for carbon dbxide at 120
atm/100 "C.
heptane (C,) versus methane at 40,60,80,and 100 "C. Different restrictors were used for helium and carbon dioxide. Application 1 (Gaeoline/Oil). For GC analyses, the oven was temperature-programmedfrom 40 to 280 "C at 3 "C min-'; for SFC analyses, the oven temperature was held isothermal at 100 O C while the carbon dioxide mobile phase was pressureprogrammed from 120 to 350 atm at 7 atm mi&. For unified GC and SFC analyses, the oven was temperature-programmed from 40 to 150 O C under gaseous mobile phase at constant pressure and then cooled to 100 "C before switching to supercritical fluid carbon dioxide at 120 atm and pressure-programmedto 350 atm at constant temperature. Application 2 (Nail Lacquer). For GC analyses, the oven was temperature-programmed from 35 to 250 "C at 5 "C min-'; for SFC analyses, the oven temperature was held isothermal at 100 OC while the carbon dioxide mobile phase was pressureprogrammed from 100 to 350 atm at 8 atm min-'. For unified GC and SFC analyses, the oven was temperature-programmed from 35 to 100 O C at 5 "C m i d under a gaseous mobile phase at constant pressure and then switched to carbon dioxide at 100 atm and pressure-programmed to 350 atm at 8 atm min-' at constant temperature. Practical applications of sequential GC/SFC were run by using linear velocities of 1.95 cm s-l for helium GC at 158 atm/40 OC (corresponding to 1.17 cm s-l for carbon dioxide SFC at 120 atm/100 "C with the same restrictor) and 2.08 cm s-l for carbon dioxide GC at 40 atm/4O O C (corresponding to 5.19 cm s-l for carbon dioxide SFC at 120 atm/l00 OC with the same restrictor). A gasoline/light crude oil/heavy crude oil mixture (application
strictor are to be used for both GC and SFC, normal lowpreseure GC (1-3 a b ) will not work due to the flow restriction imposed. The GC separation needs to be carried out at pressures similar to those used in SFC (80-400atm) in order to keep the restrictor in place and to minimize any pressure surges during the transition from one mode to the other. Firstly, we needed to investigate the chromatographic retention characteristics of solute molecules in carbon dioxide a t 40-atm inlet pressure and a t various temperatures above T, in order to c o n f m that these conditions are truly GC-like. Figure 2 shows experimentally derived graphs of log k'versus 1/T (K)between 40 and 100 OC for carbon dioxide at 40 atm and for helium at 158 atm. (The density of carbon dioxide under these conditions is between 0.0638 and 0.0859 g mL-'.) The positive slopes of the two graphs demonstrate the GC-like behavior of both carbon dioxide and helium under these conditions. According to the carbon dioxide phase diagram, the liquid-gas transition temperature for carbon dioxide at P
