Anal. Chem. 1990, 62, 1181-1185
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Composition and Density Effects Using MethanoVCarbon Dioxide in Packed Column Supercritical Fluid Chromatography Terry A. Berger* and Jerome F. Deye
Hewlett-Packard, P.O. Box 900, Route 41 and Starr Road, Avondale, Pennsylvania 19311
The density of mixtures of carbon dloxlde with 0-1 1.5 mol % methanol was measured at 40 O C , between 80 and 200 bar. Chromatograms were run at constant denbut varying compodtlon and at constant composition but varying dendty. For the systems studied, changing fluid compodtlon provldes wider adlustment of solute retentlon than density changes. Plots d log k’vs percent modifier, at constant density, were nonlinear. However, plots of log k’vs mobile phase solvent strength, as meawed using the solvatochromic dye Nile Red, produced a linear relatknship. The change In retentbn vs the pore volume (proportlonal to surface area) was also measured.
INTRODUCTION There has been a recent resurgence of interest in the use of polar modifiers in packed column supercritical fluid chromatography (SFC) but the literature explanation of the role of modifiers is ambiguous and needs clarification. Modifier effects include (1) coverage of “active sites” (i.e. silanol), (2) swelling or modifying the stationary phase, (3) increasing mobile phase density, and/or (4)increasing the solvent strength of the mobile phase. The relative magnitude of each process in a given separation has been the subject of debate. We feel the role of density changes and active sites has been overemphasized while other factors, particularly mobile phase solvent strength, have been underemphasized. Changes in retention due to changes in modifier concentration have been consistently downplayed. A number of papers concluded that modifier addition simply increased the density of the mobile phase and it was this density change that produced the change in solvent strength. Subsequent work (1,2) recognized that solvent strength increased even when the experiments were performed at constant density. However, their data lead one to conclude that the relative magnitude of change, due to composition changes, was much smaller than the density effect. Most of the work using binary mobile phases with capillary columns and a significantfraction of packed column studies used 2-propanol as the polar modifier. However, 2-propanol is of only modest polarity (3-7) (i.e. P’ = 3.9 compared to 0.0 for pentane, 5.1 for methanol or 10.2 for water). Similarly, most of the solutes studied have been of low polarity, such as aliphatic alcohols, ketones, large aromatic hydrocarbons, with or without single substitutions of functional groups, and phenones. Solvatochromic dye studies (49)have measured the change in supercritical fluid solvent strength accompanying both prmure (density) and composition changes. Pure supercritical carbon dioxide (50 “C, 5OOO psi) was found to be slightly more polar than liquid pentane, but its solvent strength decreased rapidly below 1500 psi. The addition of 9.5% methanol increased solvent strength to nearly that of pure tetrahydrofuran (THF) and more than half way to the solvent strength of pure methanol. Solvent strength also ceased to be a function of pressure. Surprisingly, the authors state (8,9) that “... small 0003-2700/90/0362-1181$02.50/0
additions of methanol do not drastically alter the solvent behavior” (see also ref 10, p 371). To the contrary, the first small additions of methanol significantly increased solvent strength (solute-mobile phase interactions), while, at the same time, the impact of pressure (and density) decreased. Recent work (11) in this laboratory, using different dyes, also concluded that small additions of methanol caused large increases in solvent strength and decreased the effect of density. The effect of phase ratio tends to be ignored, while active sites are often blamed for increased retention (10,12-14) on packed columns compared to capillaries. According to manufacturers information, “totally porous” silica particles typically possess a ratio of surface area to void volume ( A / V,) around lo6 cm2/cm3 (100 m2/g). For a 50 pm i.d. capillary, A / V, = 800 cm2/cm3. The packed column, coated with CI8, at 40% efficiency, should have a stationary phase/mobile phase volume ratio (V,/ V,) > 6 X cm3/cm3,while a 50 pm i.d. column with a 0.15 pm film thickness and 80% coverage should have V,/V, < 1 x cm3/cm3. If retention follows a partition mechanism (V,/ V , a k 9, solutes should be retained at least 6 times longer on the hypothetical packed column. Of course, the phases and retention mechanisms could be very different on the two column types. While it is obvious that differences in phase ratio should produce major differences in retention, it is not clear that the larger absolute number of active sites present on packed columns has any major effect in increaseing retention. Active sites are not unique to packed columns. Lee et al. (15) determined that capillary column coating efficiency on smaller diameter (25-100 pm i.d.) columns and various phases ranged from 8070,down to 20%, compared to typical surface coverages no better than 60% on silica packing materials (16-18). Thus, both packed and capillary columns retain significant numbers of active sites, although the relative number of active sites will be larger and the absolute number will be much larger on a packed column. Modifiers do not produce substantially different effects on capillary and packed columns. However, it is routinely stated that modifiers produce much larger (even “drastic” (10)) changes in retention on packed columns compared to capillary columns (10, 19-23) because the modifier covers the large number of active sites on the former. To the contrary, the retention of polycyclic aromatic hydrocarbons (PAH’s) has been shown to decrease by 15-32% on a range of packed columns (12,23) and by 26-28% on capillaries (I,13) when about 2% of 2-propanol or methanol was added to carbon dioxide (at constant pressure). More polar solutes tended to exhibit larger percentage retention shifts on both column types. It is often recommended (24,25)that, in order to retain the use of pure carbon dioxide and the flame ionization detector (FID), the polarity of the stationary phase should be decreased in order to elute more polar solutes. However, if the solutes are substantially more polar than either of the phases, interaction with active sites tends to become the preferred stationary phase-solute interaction and deactivation becomes progressively more important (24, 26). 0 1990 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 62, NO. 11, JUNE 1, 1990 I
Table I. Pressures in Bar Producing Densities of MethanolCarbon Dioxide Mixtures at 40.0 "C
density,
I
I
I
I
I
I
0.836 -
b
1
1
1
1
I
I + .
F =
pressure at mol % MeOH in COz
i0.002 g/cm3
0'
1.2%
2.3%
4.6%
6.9%
9.2%
11.5%
0.650 0.700 0.725 0.750 0.775 0.800 0.825 0.850
103 114 122 133 146 163 184 210
101 112 120 131 144 160 181 207
97.4 108 106 128 139 156 177 203
91.3 101 109 120 132 149 171 199
87.0 95.4 103 114 126 143 166 194
89.8 98.6 105 120 137 160 188
78.1 85.6 92.0 103 114 131 154 184
E
0.65
aCalculatedusing EOS of Bender (31).
We have been interested in the separation of much more polar solutes, which require more polar modifiers to obtain reasonable solubility in the mobile phase and appropriate partition ratios (k9. In the present work we wish to focus on changes in retention caused by the addition of methanol to a carbon dioxide mobile phase. We would like to differentiate solvent strength effects due to density changes from those due to composition. However, there is almost no published information on the density of methanol/carbon dioxide mixtures (2,27). The critical point changes with composition (14) and, near the critical point, calculated densitites using equations of state (EOS) (i.e. ref 28) were found to differ by up to 20% from empirical measurements. The present work was undertaken to extend density vs composition measurements to more polar mobile phases and solutes. EXPERIMENTAL SECTION Instrumentation. Density measurements were made with a modified "U" tube densitometer manufactured by Sodev, Inc., of Sherbrooke, Quebec, Canada, modified to allow high-pressure operation. The "U" tube compartment was evacuated and thermostated. Resistance (temperature) was monitored with a four-lead, calibrated platinum resistance sensor and a HewlettPackard (HP) Model 3456A 61/rdigit digital multimeter. Pressure was monitored with a Robinson-Halpern Model 176AK003 pressure transducer calibrated against a dead weight tester. The period of the "U" tube is proportional to the mass of the tube plus any fluid inside and was monitored with a HP Model 5316 universal counter. Temperature was stable to better than fO.O1 "C and pressure was stable to less than f0.15 bar over the full set of measurements. The densitometer was calibrated against known density values (i.e. ref 29) for water and nitrogen as well as with known values for pure carbon dioxide. Composition accuracy was on the order of
