Anal. Chem. 1989, 61, 1348-1353
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Stationary Phase Solvation in Capillary Supercritical Fluid Chromatography Clement R. Yonker* and Richard D. Smith Chemical Methods a n d Separations Group, Chemical Sciences Department, Pacific Northwest Laboratory (Operated by Battelle Memorial Institute), Richland, Washington 99352
Mass spectrometric tracer pulse chromatography was used to study stationary phase solvation in Capillary supercritical fluid chromatography. A 5 % phenyl poly(methyisiioxane) (SE-54) stationary phase was studied by using various binary fluid mixtures of 2-propanol in supercritical CO,. Sorption isotherms were determlned as a function of density (pressure), temperature, and concentration of the binary modifier in the fluid. Results demonstrate that the amount of 2propanol sorbed in the bonded polymer decreased with increasing fluid density (Le., pressure). Heats of sorption of 2-propanol for transfer from the fluid phase into the stationary phase were determined.
INTRODUCTION The interaction of supercritical fluids with a coated liquid stationary phase has been known since 1966, when it was described by Sie et al. in their pioneering studies of solute retention at near-critical and supercritical conditions with COz ( I ) . Such effects are important in determining chromatographic retention and are crucial to understanding the details of the separation process in supercritical fluid chromatography (SFC). The solvation of the polymer poly(methy1 methacrylate) by COz a t selected temperatures as a function of pressure has been reported by Wissinger and Paulaitis ( 2 ) . Novotny and co-workers (3)have reported on the swelling of nonextractable polymer films in capillary SFC using supercritical butane and COz. The physicochemical effects of swelling on retention and chromatographic performance were discussed. Recently, Lochmuller and Mink ( 4 ) have described the adsorption isotherms for ethyl acetate, which was used as a modifier in supercritical COz on a packed column containing unmodified silica. Selim and Strubinger ( 5 ) have reported the partition behavior of supercritical n-pentane in SE-54 (phenyl poly(methylsi1oxane)) and SE-30 (poly(methy1siloxane)) in capillary SFC using the mass spectrometric tracer pulse chromatography technique. In spite of these initial reports there is still a fundamental gap in the understanding of stationary phase solvation by a supercritical fluid and the role of solvation on the retention mechanism in both capillary and packed column SFC. Mass spectrometric tracer pulse chromatography (MSTPC) is a simple, rapid technique for determining phase equilibria at high temperatures and pressures. As described initially by Peterson et al. (6),tracer pulse chromatography does not require any deductions from the shapes of diffuse concentration boundaries (7) or from asymmetric peaks (8,9). In tracer pulse chromatography the adsorption isotherm of a single component in the mobile phase is obtained from the net retention volume of the tracer component at several pressures. The only special requirement is the use of mass spectrometric detection of the isotopic tracer. MSTPC has
* Author to whom correspondence should be addressed.
been used by Parcher and co-workers (10-15) to study the adsorption isotherms of selected components from the gas phase adsorbed onto solid surfaces, sampling adsorbents and liquid phases. Parcher et al. (15)were able to study gas-solid chromatography as a function of surface coverage by various volatile modifiers to determine the effect of the modifier on solute retention as a function of the modifier isotherm. In this article we report results obtained by using MSTPC applied to capillary SFC with binary fluid solutions of 2propanol/COz. This system was chosen due to the common use of 2-propanol as a fluid modifier for SFC and the fact that the solvent properties of the binary fluid have been extensively studied by the solvatochromic technique. Sorption isotherms for 2-propanol were determined as a function of density (pressure), temperature, and concentration of 2-propanol in COz. Heats of sorption were determined for 2-propanol a t constant density. These results can lead to a better understanding of the retention process in SFC and a more fundamental comprehension of solvation of the bonded stationary phase when supercritical fluid solutions are used as the mobile phase.
THEORY Tracer pulse chromatography is a technique that involves the measurement of the retention time of a group of tracer molecules and not the concentration pulse perturbing the system from the introduction of the tracer sample. Some of the inherent assumptions involved in MSTPC are as follows: (a) The pressure of the isotopic tracer is very low compared to that of the modifier along the column. (b) The pressure drop along the capillary column is small. (c) The chromatographic system is at local equilibrium; that is, the partitioning process is both rapid and reversible. (d) The partition coefficient of the isotope tracer and the nonlabeled modifier are identical. Therefore, the net retention volume (V,) of the tracer component can be used to determine the concentration of the solute (i) in the stationary phase as a function of solute concentration in the mobile phase (Ci) and the volume of the stationary phase (V,) (6, 16):
(ci)
v,
= CiV,/Ci
(1)
From chromatographic retention theory NisP =
CiVs
and
where NisPis the moles of solute (i) in the stationary phase, the moles of solute (i) in the mobile phase, and V, is the volume of the mobile phase. For a highly compressible solvent system, such as a supercritical fluid, one can relate the compressibility (2)to the physical state of the system:
NimPis
2 = PV,,,/N,$T
(3)
where P is the pressure of the fluid, Nmpis the total number
0003-2700/89/0361-1348$01.50/0 1989 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 61, NO. 13, JULY 1, 1989
of moles of fluid solution in the column, R is the gas constant, and Tis the system temperature. For an ideal gas 2 = 1;using a supercritical fluid solution 2 must be determined. The compressibility can be calculated from an appropriate equation of state that describes the physicochemical phase behavior of the solution. For this work, a two-parameter cubic equation of state was used to calculate 2 as a function of pressure, temperature, and mole fraction for the binary supercritical fluid. The Peng-Robinson equation of state (P-R EOS) was chosen because of its greater accuracy near the critical point of the fluid, using relatively simple mixing rules (17). Substituting eq 3 into eq 2b and rearranging, one obtains
Ci = NimPP/NmpZRT= YiP/ZRT
I
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* I
0 110°C
(4)
where Yi is the mole fraction of the solute molecule to be studied in the mobile phase. Upon further substitution of eq 2a and 4 into eq 1and solving for NtP, one finds that the moles of solute in the stationary phase are related to
0.00 100
200
400
300
500
Pressure (atm)
(5)
B
which is similar to the equation reported by Parcher (12) except in the case of supercritical fluids, 2 # 1. The independent parameters of P, Yi,and T can be varied to determine their effect on N?' or N;P/ WL (where WL is the weight of the polymeric stationary phase present in the capillary column). Therefore, from simple chromatographic experiments using an isotopically labeled solute (in this case, C3D70D) in MSTPC, one can determine the amount of solute reversibly partitioned into the stationary phase by varying the system pressure or temperature.
EXPERIMENTAL SECTION SFC/MS System. The SFC/mass spectrometry (MS) system has been described in detail (18). The interface between the capillary column restrictor and the MS ion source involved a high flow rate interface (HFR) and is described elsewhere (19). The capillary column effluent expands into an independently pumped chamber before the standard chemical ionization source. This allows one to work with high flow rates in the chromatographic system using splitless injections. Materials and Column. The 2-propanol used in the binary fluid mixtures was obtained from Burdick and Jackson (Muskenburg, MI) and was used without further purification. SFC grade COPwas obtained from Scott Specialty Gases (Plumsteadville, PA). The binary fluid mixtures of 2-propanol/COz were mixed directly in the high-pressure syringe pump. This procedure involved first adding a known volumn of 2-propanol into the pump, to which was then added a known weight of COz from a lecture bottle (filled from a Copcylinder). The binary solution is then quickly mixed in the pump, and the concentration (mole fraction) of 2-propanol calculated. Deuterated 2-propanol (C2D,0D) was obtained from Cambridge Isotope Laboratories (Woburn, WA) and was mixed with n-hexane for injection. The concentration of the deuterated 2-propanol was approximately 5.0 X M, of which 0.2 p L was injected on the column. This amount of deuterated 2-propanol injected was much less than that of the natural 2-propanol in the mobile phase. The capillary column used was prepared in our laboratory and was 30 m long, having a 100 pm i.d. The column was coated by using a pentane solution of the SE-54 stationary phase and was cross-linked twice with azo-tert-butane (Alfa Products, Danvers, MA), which prevents extraction by the supercritical fluid mobile phase. The stationary phase had a calculated film thickness of approximately 0.10 pm. Distribution Isotherms. The MSTPC technique used in this study is based upon mass spectrometric detection of the peak due to the isotopically labeled pulse of solvent modifier as it elutes from the capillary column. Adsorption data can be determined from simple chromatographic experiments that measure the retention time of the tracer pulse. As the amount of partitioning of the solvent modifier into the stationary phase increases, then retention also increases. The sorption data for 2-propanol in the bound SE-54 polymeric stationary phase were determined at
0.00 0.2
0.3
0.4
0.5
0.6
0.7
0.8
Density (gm/cm3) Figure 1. Weight of IPA (2-propanol)absorbed per weight of stationary phase in the capillary column versus pressure (A) and versus density (B) for a constant mole fraction of 0.0258 2-propanol in C 0 2 at (0) 110, (W) 120, (0) 130, and (0)140 O C . temperatures and pressures above the critical temperature and critical pressure for the binary mixtures of 2-propanol/COp (20). The MSTPC experiments involved the injection of argon (to determine the true volume of the capillary column for a nonretained component) followed immediately by an injection of the deuterated 2-propanol in n-hexane. The mass spectrometer was operated in the electron-impact mode and used single ion monitoring to determine the retention time of argon and of C3D,0D in the presence of 2-propanol. From the net retention time of the deuterated 2-propanol and the geometric volume of the capillary column, the net retention volume for the isotope was calculated. Using eq 5, knowing the pressure, temperature, and mole fraction of modifier and calculating the compressibility of the binary fluid solution, one can calculate the number of moles of 2-propanol sorbed into the bonded polymeric stationary phase.
RESULTS AND DISCUSSION Tracer pulse chromatography, particularly MSTPC, is a technique from which important physicochemical information about the role of fluid pressure, temperature, and modifier composition in stationary phase solvation in SFC can be obtained. The results for the sorption of 2-propanol either onto or into the bonded polymeric stationary phase for a select mole fraction of modifier in COz as a function of pressure and density a t various temperatures are shown in parts a and b of Figure 1, respectively. At each temperature studied, the amount of 2-propanol found in the stationary phase decreases as the pressure increases. The amount of 2-propanol associated with the bonded stationary phase also decreases with increasing temperature at constant pressure and constant mole fraction of binary modifier. As density increases for the four
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ANALYTICAL CHEMISTRY, VOL. 61, NO. 13, JULY 1, 1989
Table I. Weight Percent of 2-Propanol Partitioned into the Stationary Phase as a Function of Density (Pressure) for a Constant Mole Fraction of 0.0258 temp, "C 110
120
130
140
density, g/cm3 pressure, atm 0.323 0.457 0.546 0.626 0.282 0.416 0.503 0.589 0.651 0.704 0.263 0.385 0.480 0.557 0.619 0.674 0.245 0.358 0.446 0.525 0.589 0.645
150 203 248 300 144 201 245 300 350 402 144 200 251 301 350 403 143 199 248 300 350 403
Table 11. Weight Percent of 2-Propanol Partitioned into the Stationary Phase as a Function of Density (Pressure) for a Constant Mole Fraction of 0.0371
wt % of 2-propanol" 22.2 12.6 11.2 8.9 19.0 14.5 12.4 10.8 8.4 7.8 16.4 13.9 11.9 10.4 9.5 8.0 14.3 13.7 12.7 11.4 9.8 9.3
wt % of 2-propanol a t 0.0258 mol fraction is 3.5 wt %.
different temperatures, the amounts of 2-propanol sorbed into the stationary phase converge (within experimental error) as seen in Figure l b . The interesting question of whether the 2-propanol is adsorbed onto the surface of the bound polymer or whether it is absorbed into the polymer can be ascertained from the simple calculation of the surface area of the capillary column and the number of moles of 2-propanol needed to form a monolayer on this surface. The surface area of 2-propanol was calculated from the molar volume of the molecule. In all cases studied for this report, the amount of 2-propanol sorbed with the SE-54 stationary phase was a t least 2 orders of magnitude greater than that needed for monolayer coverage. Therefore, one could conclude that 2-propanol was partitioning into the bound polymeric phase, solvating the polymer rather than adsorbing onto the surface. These data are consistent with data reported with pure polymers (2) and other chromatographic systems where the swelling factors are quite appreciable ( I , 3). The data shown in Figure 1 are also consistent with that reported by Selim and Strubinger for supercritical pentane on SE-30 and SE-54 as a function of pressure (5, 21). The decrease in the amount of 2-propanol partitioned into the stationary phase with pressure can be explained by examining the change in weight percent of the binary modifier in the bound polymer as compared to the bulk weight percent of 2-propanol in the mobile phase. A 0.0258 mol fraction of 2-propanol in COz corresponds to 3.5 wt % of 2-propanol in COP The weight percent of 2-propanol sorbed into the stationary phase is easier to calculate than the mole fraction value because of the unknown molecular weight of the polymer after cross-linking. The estimated weight of 2-propanol in the stationary phase could be high because of the unknown weight of COz also sorbed into the polymer. An additional source of uncertainty is related to the absolute amount of polymeric phase in the capillary, which we believe contributes
