Anal. Chem. 1998, 70, 3619-3628
Comparison of the Retention Characteristics of Aromatic and Aliphatic Reversed Phases for HPLC Using Linear Solvation Energy Relationships Jianhong Zhao and Peter W. Carr*
Department of Chemistry, Smith and Kolthoff Hall, University of Minnesota, 207 Pleasant Street, SE, Minneapolis, Minnesota 55455
The similarities and differences in retention characteristics of aromatic and aliphatic phases have been elucidated by the use of linear solvation energy relationships (LSERs). Three aromatic phases and three aliphatic phases were investigated in a series of mobile phases. The results of LSERs on a polymer-based aromatic phase, poly(styrenedivinylbenzene) resin (PRP-1) are very different from those on either silica- or zirconia-based aromatic and aliphatic phases. Retention on all aromatic and aliphatic phases except PRP-1 is dominated by two factors: the solute size and hydrogen bond acceptor basicity. On the other hand, in addition to these two major contributions, retention on PRP-1 is markedly influenced by the solute hydrogen bond donor acidity. We believe that PRP-1 exhibits a more adsorption-like retention mechanism than do the other phases. With the inorganic oxide-based phases, the aromatic phases are less retentive than the aliphatic phases but show a larger dependence on molecular polarizability. The enhanced polarizability of aromatic phases is the likely cause of some differences in their chromatographic selectivity relative to the aliphatic phases. Reversed-phase liquid chromatography (RPLC) is the most commonly used mode of liquid chromatography. However, the retention mechanism in RPLC is still incompletely understood. A great deal of research using linear solvation energy relationships (LSERs) has been carried out to better understand the intermolecular processes in RPLC stationary phases. Tan et al.1,2 focused on different octadecyl- and octyl-bonded silica-based phases (C18SiO2 and C8-SiO2, respectively) while Li and Carr.3 compared polybutadiene-coated zirconia-based phases (PBD-ZrO2) with C18-SiO2 phases. Abraham et al.4 characterized poly(styrenedivinylbenzene) polymer and immobilized artificial membrane stationary phases. The purpose of the present work is to systematically study LSERs on different aromatic and aliphatic phases on different base materials. The aromatic phases used in this study are a phenyl-bonded silica-based phase (Ph-SiO2), a (1) Tan, L. C.; Carr, P. W.; Abraham, M. H. J. Chromatogr., A 1996, 752, 1. (2) Tan, L. C.; Carr, P. W. J. Chromatogr., A 1998, 799, 1. (3) Li, J.; Carr, P. W. Anal. Chim. Acta 1996, 334, 239. (4) Abraham, M. H.; Chadha, H. S.; Leitao, R. A. E.; Mitchell, R. C.; Lambert, W. J.; Kaliszan, R.; Nasal, A.; Haber, P. J. Chromatogr., A 1997, 766, 35. S0003-2700(98)00173-5 CCC: $15.00 Published on Web 08/06/1998
© 1998 American Chemical Society
polystyrene-coated zirconia-based phase (PS-ZrO2), and a poly(styrene-divinylbenzene) resin (PRP-1) while the aliphatic phases are C8-SiO2, C18-SiO2, and PBD-ZrO2. We attempt to assess the chemical origin of the differences in retention characteristics between aromatic and aliphatic phases, as well as the differences between inorganic oxide-based and purely polymer-based phases. Finally, we want to determine whether there is any difference in the selectivities of PBD-ZrO2 and PS-ZrO2 phases. Such studies will let us explore the practical applications of PS-ZrO2. LSERs were first developed by Kamlet, Taft and their coworkers in the mid-1970s.5,6 Based on this model, the free energy of retention can be correlated with various fundamental molecular properties. In RPLC, one includes cavity formation/dispersive interactions, dipolarity/polarizability interactions, and hydrogenbonding interactions in the LSER. The most recent LSER equation for RPLC as developed by Abraham4 relates retention to the solute’s properties as follows:
log k′ ) log k′0 + vV2 + sπ2* + aΣR2H + bΣβ2H + rR2 (1) where log k′0 is the regression intercept. The solute molecular descriptors are V2, π2*, ΣR2H, Σβ2H, and R2, where V2 is the solute’s molecular volume computed according to McGowan;7,8 π2* is its the dipolarity/polarizability, ΣR2H and Σβ2H are the solute’s overall hydrogen bond acidity and basicity, respectively, and R2 is its excess molar refraction. The coefficients v, s, a, b, and r are related to the chemical nature of the mobile and stationary phases. Here, v, s, a, and b are nonzero when there are differences in the mobile- and stationary-phase cohesiveness/dispersiveness, dipolarity/polarizability, hydrogen bond acceptor (HBA) basicities, and hydrogen bond donor (HBD) acidities, respectively. The rR2 term compensates for the inadequacy of lumping the dipolarity/ polarizability into a single parameter (π2*) and reflects the tendency of the system (mobile/stationary phases) to interact with solutes through π- and n-electron pairs.9 For a given mobile-/ stationary-phase pair, the sign and magnitude of the coefficients indicate the direction and relative strength of different kinds of solute/stationary- and solute/mobile-phase interactions affecting (5) Kamlet, M. J.; Taft, R. W. J. Am. Chem. Soc. 1976, 98, 377. (6) Kamlet, M. J.; Taft, R. W. J. Am. Chem. Soc. 1976, 98, 2886. (7) McGowan, J. C. J. Chem. Technol. Biotechnol. 1984, 34A, 38. (8) Abraham, M. H.; McGowan, J. C. Chromatographia 1987, 23, 243. (9) Abraham, M. H. Chem. Soc. Rev. 1993, 22, 73.
Analytical Chemistry, Vol. 70, No. 17, September 1, 1998 3619
retention. Recent LSER studies on C8-SiO2, C18-ZrO2 and PBD-ZrO21-3 indicate that on all these phases the solute’s size (V2) and its HBA basicity (Σβ2H) are the two major contributions to the retention; and the solute’s dipolarity/polarizability (π2*) and its HBD acidity (ΣR2H) are much less important. In this work, we obtained retention data for a single carefully chosen set of solutes in a series of mobile phases for each stationary phase. First, we compared the LSER coefficients for the different stationary phases in a fixed mobile phase. Since the mobile phase is fixed, the variables corresponding to the mobilephase properties are constant and hence the differences in the LSER coefficients reflect different properties of the stationary phases as they exist in equilibrium with the mobile phase. Second, for several of the stationary phases, we examined the changes in the LSER coefficients as the mobile-phase composition was changed. The retention factor of a solute is related to the mobile-phase composition by the following approximate linear relationship, widely used in RPLC,10
log k′ ) log k′w -Sφ
(2)
Table 1. Solutes and Solute Descriptorsa no.
solutes
V2
π2*
ΣR2H
Σβ2H
R2
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
benzene toluene ethylbenzene p-xylene propylbenzene butylbenzene naphthalene p-dichlorobenzene bromobenzene nitrobenzene p-nitrotoluene anisole benzonitrile p-nitrobenzyl chloride methylbenzoate acetophenone benzophenone 3-phenylpropanol benzyl alcohol N-benzylformamide phenol p-chlorophenol
0.7164 0.8573 0.9982 0.9982 1.1391 1.2800 1.0854 0.9612 0.8914 0.8906 1.0315 0.9160 0.8711 1.1539 1.0726 1.0139 1.4808 1.1978 0.9160 1.1137 0.7751 0.8975
0.52 0.52 0.51 0.52 0.50 0.51 0.92 0.75 0.73 1.11 1.11 0.75 1.11 1.34 0.85 1.01 1.50 0.90 0.87 1.80 0.89 1.08
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0.30 0.33 0.40 0.60 0.67
0.14 0.14 0.15 0.16 0.15 0.15 0.20 0.02 0.09 0.28 0.28 0.29 0.33 0.40 0.46 0.48 0.50 0.67 0.56 0.63 0.30 0.20
0.610 0.601 0.613 0.613 0.604 0.600 1.340 0.825 0.882 0.871 0.870 0.708 0.742 1.080 0.733 0.818 1.447 0.821 0.803 0.990 0.805 0.915
a
where k′w is the extrapolated retention factor of a solute when the mobile phase is pure water and S is related to the susceptibility of retention to changes as the volume fraction (φ) of organic modifier in the mobile phase is varied. The combination of eqs 1 and 2 results in a roughly linear relationship between the LSER coefficients and the mobile-phase composition; moreover, the slope of a LSER coefficient (e.g., v or b) vs φ should be similar for stationary phases that experience similar retention mechanisms. EXPERIMENTAL SECTION Instruments. A Hewlett-Packard 1100 liquid chromatograph equipped with a vacuum degasser, a quaternary pump, an autosampler, a thermostated-column compartment, a variablewavelength UV detector, and a computer-based HP ChemStation was used to carry out all chromatographic measurements. Reagents. All reagents employed for the synthesis of the stationary phases made in this laboratory were reagent grade or better. Diethoxymethylvinylsilane, chloromethylstyrene, and dicumyl peroxide were purchased from Aldrich (Aldrich Chemical Co., Milwaukee, WI); toluene was from Fisher (Fisher Scientific, Inc., Fair Lawn, NJ 07410). The organic solvent for the chromatographic measurements was ChromAR HPLC grade acetonitrile purchased from Mallinckrodt (Mallinckrodt Chemical Co., Paris, KY). HPLC water was produced in this laboratory by treatment of house deionized water with a Barnsted Nanopure deionizing system which was equipped with an organic-free cartridge and a 0.2-µm filter. The HPLC water was boiled to remove carbon dioxide before use with zirconia columns. The 22 solutes listed in Table 1 were carefully selected from the 87 test solutes in ref 1. These solutes representatively cover a very wide range of solvatochromic parameter values (V2, π2*, ΣR2H, Σβ2H, R2) and give LSER coefficients nearly identical to those obtained with the full 87-solute set.1 All solutes were commercially available and were dissolved in acetonitrile/water or acetonitrile at a concentration of 1-2 mg/mL depending on their solubilities. (10) Jandera, P.; Chura´cˇek, J. J. Chromatogr. 1974, 91, 207.
3620 Analytical Chemistry, Vol. 70, No. 17, September 1, 1998
Values of V2, π2*, ΣR2H, Σβ2H, and R2 were obtained from refs 7-9.
The dead time markers acetone and uracil were obtained from Mallinckrodt and Aldrich, respectively. Analytical Columns. Four different RPLC stationary phases were examined in this work. A 15 × 4.6 mm i.d. Alltima Phenyl column with a particle size of 5 µm was a gift from Alltech (Alltech Associate, Inc., Deerfield, IL 60015), while a Hamilton PRP-1 (polystyrene-divinylbenzene) column purchased from Chrom Tech (Chrom Tech, Inc., Apple Valley, MN 55124) had the same dimensions but a different particle size (10 µm). Polybutadienecoated zirconia particles (PBD-ZrO2, batch 2-61) were provided by ZirChrom (ZirChrom Separation, Inc., Anoka, MN 55303). The carbon content of the PBD-ZrO2 is about 3% and the particle size is about 3 µm.11 A 50 × 4.6 mm i.d. PBD-ZrO2 column was packed using an upward stirred slurry method.12 PS-ZrO2 was prepared as follows. A solution of 1 mL of diethoxymethylvinylsilane, 5 mL of chloromethylstyrene and 0.15 g of dicumyl peroxide in 30 mL of toluene was refluxed for 4 h. The copolymer mixture was then added to a slurry of 5.0 g of bare zirconia (PICA-7, made in this laboratory by the polymerization-induced colloid aggregation method) in 100 mL of toluene which was previously sonicated under vacuum for 10 min. Note that the bare zirconia is used without drying so that the water adsorbed on the surface of zirconia will initiate the silanization. After refluxing for 3 h, the suspension was filtered, washed with hot toluene to remove the unattached polymer, and dried in a vacuum oven at 80 °C overnight. The coated particles were then heated at 160 °C for 5 h to bring about cross-linking between the chloromethyl and phenyl groups of the copolymer chains. Residual un-cross-linked copolymers were removed by overnight Soxhlet extraction of the particles in refluxing toluene. The carbon content of the resulting polystyrene-coated zirconia (PS-ZrO2) was 2.7%, based on elemental analysis (Micro-Analysis, Inc., (11) Mao, Y. Ph.D., University of Minnesota, Minneapolis, in preparation. (12) Sun, L.; McCormick, A.; Carr, P. W. J. Chromatogr., A 1994, 658, 465.
Table 2. Log k′ Values in Acetonitrile/Water for Four Stationary Phases Studieda PS-ZrO2
Ph-SiO2
PBD-ZrO2b
PRP-1
no.
solutes
50/50c
40/60c
30/70c
20/80c
50/50c
40/60c
80/20c
70/30c
60/40c
50/50c
50/50c
30/60c
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
benzene toluene ethylbenzene p-xylene propylbenzene butylbenzene naphthalene p-dichlorobenzene bromobenzene nitrobenzene p-nitrotoluene anisole benzonitrile p-nitrobenzyl chloride methyl benzoate acetophenone benzophenone 3-phenylpropanol benzyl alcohol N-benzylformamide phenol p-chlorophenol
-0.282 -0.126 0.013 0.020 0.156 0.304 0.172 0.127 0.014 -0.268 -0.116 -0.262 -0.432 -0.025 -0.314 -0.483 0.106 -0.547 -0.847 -0.948 -0.633 -0.225
-0.032 0.156 0.322 0.324 0.496 0.681 0.499 0.442 0.311 -0.004 0.179 -0.003 -0.186 0.302 -0.051 -0.249 0.441 -0.284 -0.634 -0.708 -0.405 0.061
0.231 0.482 0.718 0.714 0.968 1.225 0.942 0.876 0.680 0.282 0.525 0.272 0.072 0.694 0.249 0.009 0.886 0.020 -0.405 -0.442 -0.159 0.401
0.509 0.829 1.108 1.103 1.434 1.744d 1.418 1.311 1.063 0.621 0.934 0.612 0.385 1.148 0.632 0.344 1.406 0.416 -0.132 -0.112 0.104 0.795
0.219 0.329 0.443 0.434 0.560 0.673 0.451 0.445 0.370 0.183 0.287 0.215 0.100 0.337 0.169 0.045 0.420 -0.042 -0.219 -0.305 -0.110 0.055
0.435 0.583 0.739 0.726 0.899 1.055 0.757 0.747 0.643 0.406 0.546 0.438 0.301 0.627 0.394 0.235 0.729 0.171 -0.074 -0.150 0.058 0.289
0.182 0.313 0.423 0.449 0.549 0.688 0.613 0.585 0.466 0.044 0.171 0.153 -0.081 0.131 0.087 -0.090 0.423 -0.366 -0.511 -0.728 -0.502 -0.334
0.383 0.536 0.668 0.693 0.818 0.979 0.856 0.831 0.692 0.248 0.394 0.356 0.106 0.379 0.276 0.076 0.658 -0.215 -0.381 -0.603 -0.334 -0.141
0.587 0.763 0.920 0.946 1.088d 1.269d 1.107 1.084 0.927 0.454 0.624 0.561 0.295 0.633 0.471 0.248 0.906 0.050 -0.247 -0.466 -0.164 0.058
0.818 1.026 1.171 1.191 1.354 1.561d 1.372 1.316 1.197 0.691 0.887 0.798 0.508 0.930 0.697 0.444 1.201 0.139 -0.102 -0.323 0.014 0.285
-0.292 -0.123 0.082 0.097 0.300 0.534 0.096 0.228 0.028 -0.524 -0.344 -0.395 -0.726 -0.383 -0.505 -0.756 -0.154 -0.818 -1.152 -1.311 -0.982 -0.583
0.278 0.558 0.842 0.846 1.159 1.473 0.875 0.989 0.747 0.053 0.298 0.192 -0.191 0.365 0.077 -0.244 0.618 -0.236 -0.677 -0.750 -0.456 0.050
d
a Log k′ values are averages of triplicate determination of retention data. b Obtained from ref 11. c Denotes acetonitrile/water composition. Extrapolating from log k′ of other alkylbenzenes using the linear relationship between log k′ and carbon number.
Wilmington, DE 19808). Physical and chromatographic characterization of the PS-ZrO2 will be detailed elsewhere. The particle size was about 2.5 µm. PS-ZrO2 was packed into a 50 × 4.6 mm (i.d.) stainless steel column. Chromatographic Conditions. All chromatographic measurements were made at 30 °C ((0.05 °C) at a flow rate of 1 mL/ min for the 5-cm PS-ZrO2 and PBD-ZrO2 columns and 1.5 mL/ min for the 15-cm Ph-SiO2 and PRP-1 columns. The dead time was determined using acetone for PBD-ZrO2 and PS-ZrO2 and uracil for Ph-SiO2 and PRP-1. The UV detector was set at 254 nm since all solutes were aromatic compounds. The injection volume was 1 µL. RESULTS AND DISCUSSION LSER Study of Retention Data. In Table 2, we reported the log k′ values for the 22 solutes on the four different phases (PSZrO2, PBD-ZrO2, Ph-SiO2, PRP-1) in a series of acetonitrile/ water compositions, based on triplicate determinations of the retention time. The average standard deviation of all the measurements of the retention factor was less than 1%. The log k′ values on the C8-SiO2 and C18-SiO2 columns were presented previously.1,2 The LSER coefficients are summarized in Table 3. The resulting values were obtained from multivariable linear regression of log k′ against the solute descriptors, V2, π2*, ΣR2H, Σβ2H, and R2. The variance-covariance matrix of these descriptors is shown in Table 4; it indicates that the covariances among the variables are weak. As we expected, π2* and R2, π2* and Σβ2H are somewhat correlated. Despite the fact that there is a high proportion of very polar and HB-active solutes, which are usually the hardest solutes to fit in the data set, the results are quite acceptable. In general, the fits get better when the data set has a high proportion of nonpolar solutes with low π2*, ΣR2H, and Σβ2H. The goodness of
fit of all the equations can be examined from several perspectives. First, the correlation coefficient (r) for each equation is good; all are better than 0.98. Second, the average residuals (sd) are less than or equal to 0.10. Third, the plots of experimental log k′ against calculated log k′ (Figure 1) for all four phases in 50/50 acetonitrile/water measured in this study show no serious outliers. Among these four phases, PS-ZrO2 is the most poorly fit. However, as shown in Figure 2, the most deviant points were the same in all four mobile phases. The reason for the poor fit on PS-ZrO2 is obviously not random experimental error; but in any case, the residuals are small and do not affect our comparisons and conclusions. Figure 3 shows the normalized residual plots for the five different stationary phases. The solute p-nitrobenzyl chloride clearly deviates in the same direction on all phases, indicating that one or more of this solute’s parameters may not be correct. Significance of the LSER Coefficients and Mobile-Phase Effects. In Figure 4, we compare the LSER coefficients (v, s, a, b, r) of the six different stationary phases in 50/50 acetonitrile/ water mobile phase. The standard deviations of the coefficients are indicated by the error bars. This figure shows that C8-SiO2, C18-SiO2, and PBD-ZrO2 are very similar while the aromatic phases are different. Ph-SiO2 has the lowest hydrophobicity, and PRP-1 has an unusually high a coefficient in agreement with Abraham.4 Figure 5 shows the dependence of the LSER coefficients on mobile-phase composition for five stationary phases. The v Coefficient. As mentioned above, the v coefficient reflects the difference in the mobile- and stationary-phase cohesiveness/ dispersiveness complementary to the solute’s size.1 When we compared the v coefficients of the phases in 50/50 acetonitrile/ water (Table 3), we noticed that the three aliphatic phases (PBDZrO2, C18-SiO2, C8-SiO2) have the largest and most comparable v values (1.58 ( 0.06, 1.62 ( 0.05, and 1.47( 0.03, respectively), Analytical Chemistry, Vol. 70, No. 17, September 1, 1998
3621
Table 3. Coefficients of LSER Equationsa
phase
mobile phase (acetonitrile/ water)
log k′0
v
s
a
b
r
sdd
re
80/20 70/30 60/40 50/50 50/50 40/60 30/70 20/80 50/50 40/60 50/50 30/70 50/50 50/50 40/60 30/70 20/80
-0.35 ( 0.03f -0.23 ( 0.03 -0.11 ( 0.04 0.03 ( 0.07 -0.95 ( 0.11 -0.84 ( 0.12 -0.85 ( 0.12 -0.85 ( 0.14 -0.22 ( 0.06 -0.17 ( 0.07 -1.07 ( 0.05 -0.86 ( 0.08 -0.23 ( 0.05 -0.28 ( 0.03 -0.24 ( 0.04 -0.27 ( 0.04 -0.29 ( 0.05
0.92 ( 0.03 1.07 ( 0.04 1.23 ( 0.05 1.35 ( 0.08 1.09 ( 0.13 1.32 ( 0.14 1.79 ( 0.14 2.21 ( 0.16 0.83 ( 0.07 1.16 ( 0.08 1.58 ( 0.06 2.23 ( 0.09 1.62 ( 0.05 1.47 ( 0.03 1.84 ( 0.04 2.35 ( 0.04 2.77 ( 0.05
-0.38 ( 0.03 -0.38 ( 0.03 -0.39 ( 0.03 -0.39 ( 0.06 -0.20 ( 0.09 -0.20 ( 0.10 -0.22 ( 0.10 -0.24 ( 0.12 -0.15 ( 0.05 -0.17 ( 0.06 -0.42 ( 0.04 -0.45 ( 0.07 -0.32 ( 0.03 -0.25 ( 0.03 -0.27 ( 0.03 -0.25 ( 0.03 -0.23 ( 0.04
-0.81 ( 0.04 -0.84 ( 0.03 -0.88 ( 0.03 -0.94 ( 0.06 -0.27 ( 0.09 -0.27 ( 0.10 -0.22 ( 0.10 -0.22 ( 0.12 -0.34 ( 0.05 -0.36 ( 0.06 -0.40 ( 0.04 -0.36 ( 0.07 -0.54 ( 0.04 -0.41 ( 0.04 -0.42 ( 0.04 -0.47 ( 0.04 -0.43 ( 0.05
-1.31 ( 0.04 -1.52 ( 0.04 -1.72 ( 0.05 -1.89 ( 0.08 -1.48 ( 0.14 -1.67 ( 0.15 -2.06 ( 0.15 -2.29 ( 0.10 -0.99 ( 0.07 -1.26 ( 0.09 -2.01 ( 0.07 -2.58 ( 0.10 -1.77 ( 0.06 -1.71 ( 0.04 -2.09 ( 0.04 -2.50 ( 0.05 -2.68 ( 0.05
0.44 ( 0.03 0.43 ( 0.04 0.44 ( 0.05 0.48 ( 0.07 0.34 ( 0.12 0.34 ( 0.13 0.36 ( 0.14 0.41 ( 0.16 0.09 ( 0.07 0.09 ( 0.08 0.17 ( 0.06 0.15 ( 0.09 0g 0g 0g 0g 0g
0.02 0.02 0.03 0.05 0.08 0.08 0.09 0.10 0.04 0.05 0.04 0.06 0.03 0.06 0.06 0.07 0.07
0.999 0.999 0.999 0.997 0.980 0.981 0.986 0.985 0.990 0.990 0.998 0.997 0.998 0.995 0.996 0.996 0.995
PRP-1
PS-ZrO2
Ph-SiO2 PBD-ZrO2 C18-SiO2b C8-SiO2c
a Regression results of log k′ against the solute descriptors, V , π *, ΣR H, Σβ H, and R . b Regression results of log k′ of 22 solutes in ref 1, which 2 2 2 2 2 were measured at 25 °C. c Obtained from ref 2, which were measured at 25 °C. d Average residual of the fit. e Correlation coefficient. f Standard deviation. g Not statistically significant.
Table 4. Variance-Covariance of Solute Descriptors
V2 π2* ΣR2H Σβ2H R2
V2
π2*
ΣR2H
Σβ2H
R2
1 0.346 -0.200 0.381 0.484
1 0.304 0.657 0.718
1 0.327 0.091
1 0.350
1
but the three aromatic phases (PS-ZrO2, PRP-1, Ph-SiO2) have considerably smaller and distinguishable v coefficients (1.09 ( 0.13, 0.83 ( 0.07, and 1.35 ( 0.08, respectively). We rationalize these differences as being due to the opposing effects of the net unfavorable cavity formation arising mostly in the mobile phase and the net favorable dispersive interactions which take place predominantly in the stationary phase.1,13,14 The free energy of transfer of a methylene group (∆G°CH2) from the gas phase to bulk hexadecane or toluene is very large and negative (-634 13 and -697 cal/mol,15 respectively); on the other hand, ∆G°CH2 from the gas phase to water is only +159 cal/mol. This clearly indicates that dispersive interactions between the solute and the nonpolar stationary phase have a major effect on the retention of nonpolar moieties. As stated in the work of Tan et al., the aqueous/organic mobile phase is far more cohesive than is the bonded stationary phase.1 This is, of course, the origin of the hydrophobic effect and the solvophobic model of RPLC.16 However, at a fixed mobile-phase composition (e.g., 50/50 acetonitrile/water), the variations in the v coefficients among these six stationary phases are due to the differences in the cohesiveness (also termed the cohesive energy (13) Carr, P. W.; Li, J.; Dallas, A. J.; Eikens, D. I.; Tan, L. C. J. Chromatogr., A 1993, 656, 113. (14) Abraham, M. H.; Whiting, G. S.; Fuchs, R.; Chambers, E. J. J. Chem. Soc., Perkin Trans. 2 1990, 291. (15) Eikens, D. I. Ph.D., University of Minnesota, Minneapolis, 1993. (16) Horvath, C.; Melander, W.; Molnar, I. J. Chromatogr. 1976, 125, 129.
3622 Analytical Chemistry, Vol. 70, No. 17, September 1, 1998
Figure 1. Experimental vs calculated log k′ for four different stationary phases in 50/50 acetonitrile/water: (a) PRP-1; (b) PhSiO2; (c) PS-ZrO2; (d) PBD-ZrO2. Symbols: (0) nonpolar solutes; (b) polar but non-HB donors; (O) polar and HB donors.
density17) and dispersion interactions of the aromatic and aliphatic stationary phases. In general, aromatic liquids are more cohesive than aliphatic liquids. For instance, benzene’s solubility parameter (δH2) is 81 cal/mL and octadecane’s δH2 is 66 cal/mL.18 In addition, in the case of the inorganic oxide phases, the highly polarizable phenyl groups of the aromatic phases might allow more of the organic components (acetonitrile) in the mobile phase to sorb than (17) Hildebrand, J. H.; Prausnitz, J. M. Regular and Related Solutions; Van Norstrand: New York, 1970. (18) Riddick, J. A.; Bunger, W. B.; Sakano, T. K. Organic Solvents, 4th ed.; WileyInterscience: New York, 1986.
Figure 2. Experimental vs calculated log k′ for PS-ZrO2 in four mobile-phase compositions. Symbols: (O) 50/50 acetonitrile/water; (0) 40/60 acetonitrile/water; (4) 30/70 acetonitrile/water; (3) 20/80 acetonitrile/water. Filled symbols denote the most deviant solutes.
do the aliphatic groups. The net result of the greater cohesiveness of the aromatic phases leads to smaller v coefficients for the aromatic phases as compared to the aliphatic phases. The reason for the slightly larger v coefficient for the PRP-1 phase as compared to the other two aromatic phases is not clear. We suspect that the retention mechanism on PRP-1 is different from that on the other phases. There are two proposed extreme retention mechanisms in RPLC:19-26 the partitioning mechanism, in which a solute molecule is fully embedded within the stationary phase, and the adsorption process, in which the solute is only in surface contact with the stationary phase and is not fully embedded. On the basis of the work of Tan, the retention is partitioninglike for both the monomeric and polymeric bonded phases whose chains are eight carbons or more in length.27 Because the surface of PRP-1 is more solidlike, we suspect that solute molecules are more likely to be sorbed to the surface instead of being embedded into the polymer network, and thus, the retention mechanism is more adsorption-like. However, the presence of a substantial fraction of micropores in PS-DVB sorbents such as PRP-128,29 may be the cause of the slow diffusion of solutes;30 this process may simulate embedding and it is more partition-like. We cannot (19) Dorsey, J. G.; Dill, K. A. Chem. Rev. 1989, 89, 331. (20) Martire, D. E.; Boehm, R. E. J. Phys. Chem. 1983, 87, 1045. (21) Scott, R. P. W.; Simpson, C. F. Frarday Symp. Chem. Soc. 1980, 15, 69. (22) Snyder, L. R. Principles of Adsorption Chromatography; Marcel Dekker: New York, 1968. (23) Melander, W. R.; Horvath, C. In High Performance Liquid Chromatography: Advances and Perspectives; Horvath, C., Ed.; Academic Press: New York, 1980; Vol. 2, p 113. (24) Jaroniec, M.; Martire, D. E.; Borowko, M. Adv. Colloid Interface Sci. 1985, 22, 177. (25) Everett, D. H. In Adsorption from Solution; Ottewill, R. H., Rochester, C. H., Smith, A. L., Eds.; Academic Press: New York, 1983. (26) Hammers, W. E.; Meurs, G. J.; DeLingny, C. L. J. Chromatogr. 1982, 246, 169. (27) Tan, L. Ph. D., University of Minnesota, Minneapolis, 1994. (28) Nevejans, F.; Verzele, M. Chromatographia 1985, 20, 173. (29) Nevejans, F.; Verzele, M. J. Chromatogr. 1987, 406, 325. (30) Li, J.; Cantwell, F. F. J. Chromatogr., A 1996, 726, 37-44.
offer a more definite explanation without further investigations of the PRP-1 phase. Dispersive interactions, also called the London interactions, are related to the refractive index of a material. The lower the refractive index of a substance, the weaker are the dispersive interactions. On the basis of the refractive indices of water (1.333), octadecane (1.44), and benzene (1.5011),18 aqueous mobile phases are much less dispersive than are organic stationary phases; furthermore, among the organic stationary phases, aliphatic stationary phases are less dispersive than aromatic stationary phases. These differences in the exoergic dispersive effects between the mobile and stationary phases should lead to a more positive v coefficient for the aromatic phases. However, the v coefficients for the aromatic phases are experimentally smaller. Given the high degree of miscibility of toluene and acetonitrile in comparison to the very low miscibility of hexane and acetonitrile, we believe that the smaller ν coefficients of the two aromatic phases (Ph-SiO2 and PS-ZrO2) result from a very high sorption of acetonitrile from the mobile phase. As shown in Figure 5, for all five stationary phases, the v coefficient decreases approximately linearly as the volume fraction of acetonitrile in the mobile phase is increased. This reflects the fact that acetonitrile is much less cohesive than water and hence less energy is required to form a cavity in an acetonitrile-richer mobile phase.2 However, the slopes of the v coefficient vs the volume fraction of acetonitrile (φacetonitrile) for the different phases are definitely not the same. The slope of v vs φacetonitrile for the PRP-1 phase is strikingly small; it is less than half of the slope of any other stationary phases. Since v is one of the two important LSER coefficients, this indicates that at least over the range in mobile-phase composition studied here on the PRP-1 phase a change in mobile-phase composition has a considerably smaller effect on the retention of most solutes than on the other phases. We believe that the retention process of solutes on the PRP-1 phase is more adsorption-like than on the other phases. In comparison to a partition process in an adsorption mechanism fewer solute/mobile-phase interactions are exchanged for solute/ stationary-phase interactions upon solute transfer from the mobile to the stationary phase.19 Consequently, the difference in cohesivity/dispersivity between the stationary and mobile phases in an adsorption process is smaller than that in a partition process and thus changes in mobile-phase compositions will have less effect on the difference in these interactions for the PRP-1 phase than for the other phases. In later work, we will show that S (see eq 2) for nonpolar solutes is smaller for PRP-1 phases than for the other phases studied here. In addition, if we compare the intercepts of plots of v vs φacetonitrile at φacetonitrile ) 1, which are the v coefficients in pure water, we observe the lowest v value for the PRP-1 phase. In contrast extrapolation to pure acetonitrile shows that the PRP-1 phase has the largest value of v of all five stationary phases. This indicates that in pure water the solvophobic selectivity of PRP-1 is much smaller than that of the other phases, but it is much larger in pure organic phase. Water is a very poor solvent for polystyrene, but acetonitrile is a better solvent for polystyrene; the Flory χ parameters are 30.1 and 1.02 for water and acetonitrile, respectively.31 In water-rich mobile phases, where the polymer is unswollen, rigid, and solidlike, the retention process is more likely Analytical Chemistry, Vol. 70, No. 17, September 1, 1998
3623
Figure 3. Normalized residual distribution for the LSER analysis of five different stationary phases in 50/50 acetonitrile/water: (a) C18-SiO2; (b) PBD-ZrO2; (c) Ph-SiO2; (d) PRP-1; (e) PS-ZrO2.
to be an adsorption-like process and the solute less retained; on the other hand, in acetonitrile-rich mobile phases, where the polymer is probably more swollen and more liquidlike, the retention process may be more partition-like and the solute more retained. The phenomenon that retention process changes from adsorption to partition as the organic content in the mobile phase is increased is opposite to what is observed on conventional silicabonded phases.27 However, we must point out that the data of PRP-1 we obtained only cover a narrow range of composition in relatively organic-rich eluents. Data not given here indicate that log k′ is a very nonlinear function of φ. The slope S of eq 2 increases in water-rich mobile phases. The b Coefficient. The b coefficient represents the difference in HBD acidity of the mobile and stationary phases. Obviously (31) Grulke, E. A. In Polymer Handbook; Brandrup, J., Immergut., E. H., Eds.; Wiley: New York, 1989; pp VII/519.
3624 Analytical Chemistry, Vol. 70, No. 17, September 1, 1998
neither alkyl nor phenyl groups have any inherent HBD acidity (R ) 0); therefore, the acidity of the stationary phase can only arise from sorbed mobile-phase components and from accessible silanol or zircanol groups on the support surface.27 The aqueous mobile phase is highly acidic since water is a very strong HB acid (Rwater ) 1.17). Among these aromatic phases, PRP-1 has the most negative b coefficient. This is probably due to the very small amount of water sorbed on its surface. Among the inorganic oxide-based phases, the aromatic phases have less negative b coefficients than do the aliphatic phases. For example, in 50/50 acetonitrile/water, bPS-ZrO2 is -1.48 but bPBD-ZrO2 is -2.01; and bPh-SiO2 is -0.99 but bC18-SiO2 is -1.77 (see Table 3). This indicates that the aromatic phases have higher HBD acidities than do the aliphatic phases. There are at least two possibilities for this: (1) the aromatic phases sorb more acidic mobile phase than do the aliphatic phases; (2) the bonding or coating density of the aromatic
Table 5. Contribution of Each Interaction Term to Retentiona percent variance due to solute termb phase c
C8-SiO2 C18-SiO2c PBD-ZrO2 Ph-SiO2 PS-ZrO2 PRP-1
vV2
sπ2*
aΣR2H
bΣβ2H
rR2
40 40 32 33 32 20
4 5 9 4 4 7
6 6 3 8 3 14
48 45 57 52 65 44
0 0 1 1 5 4
a Based on retention data in 50/50 acetonitrile/water. b Percent variance due to term x in the LSER is computed as 100Cx2rx2/σlog k′2, where x denotes the solute descriptor, Cx, the corresponding LSER coefficient, σx2 and σlog k′2, the variance for x and log k′, respectively. c Obtained from ref 1.
Table 6. Ratios of LSER Coefficientsa
Figure 4. Comparison of the v, s, a, b, and r coefficients for six different stationary phases in 50/50 acetonitrile/water: (a) C18-SiO2; (b) PBD-ZrO2; (c) C8-SiO2; (d) PRP-1; (e) PS-ZrO2; (f) Ph-SiO2.
phase
s/v
a/v
b/v
r/v
C8-SiO2 C18-SiO2 PBD-ZrO2 Ph-SiO2 PS-ZrO2 PRP-1
-0.17 ( 0.02 -0.20 ( 0.02 -0.27 ( 0.03 -0.18 ( 0.06 -0.18 ( 0.09 -0.29 ( 0.05
-0.28 ( 0.03 -0.33 ( 0.03 -0.25 ( 0.03 -0.41 ( 0.07 -0.25 ( 0.09 -0.70 ( 0.06
-1.16 ( 0.04 -1.11 ( 0.05 -1.26 ( 0.07 -1.19 ( 0.13 -1.36 ( 0.21 -1.40 ( 0.10
0 0 0.11 ( 0.04 0.11 ( 0.08 0.31 ( 0.12 0.36 ( 0.06
a
Figure 5. LSER coefficients vs the volume fraction of acetonitrile for five different stationary phases. Symbols: (O) C18-SiO2; (b) PBD-ZrO2; (0) Ph-SiO2; (9) PS-ZrO2; (4) PRP-1.
phases is lower and thus the surface hydroxyl groups are more accessible. We also notice that the zirconia-based phases have larger negative b coefficients than do the analogous silica-based phases. For example, for the aliphatic phases, bPBD-ZrO2 is -2.01 while bC18-SiO2 is -1.77; and for the aromatic phases, bPS-ZrO2 is -1.48 while bPh-SiO2 is -0.99. While we suspect that this is due
Based on retention data in 50/50 acetonitrile/water.
to differences in the acidities of silanol and zircanol groups, the amount of water sorbed on the two surfaces may be different.3 For the five phases shown in Figure 5b, the b coefficient, which is negative, becomes more positive as the volume fraction of acetonitrile in the eluent is increased. This results because the highly acidic water (Rwater ) 1.17) is replaced with the much less acidic acetonitrile (Racetonitrile ) 0.19). The same trend is observed in measurements of the bulk mobile-phase solvatochromic parameters.32-34 The acidity of the stationary phase decreases as the volume fraction of acetonitrile in the mobile phase is increased because less water is sorbed to the stationary phase.35,36 The variations in the b coefficients for C8-SiO2, Ph-SiO2, PBDZrO2, and PS-ZrO2 are quite similar while the change in b for the PRP-1 phase is much smaller. As stated above, the PRP-1 phase is only slightly solvated by the mobile phase, especially by water, and thus its acidity does not change much upon increasing the amount of water in the mobile phase. The a Coefficient. Although the v and b values of the PRP-1 phase (see Figure 4) and its dependence on the mobile-phase composition (see Figure 5) are quite different from the other phases, the most interesting and significant difference is the much more negative value of the a coefficient compared to the other five phases.4 This is most clearly indicated in Table 6 when we compare the six phases in terms of the relative LSER coefficient in a common mobile phase. The HBA basicities (β) of the mobile and stationary phases are the complementary properties to the (32) Park, J. H.; Jang, M. D.; Kim, D. S.; Carr, P. W. J. Chromatogr. 1990, 513, 107. (33) Cheong, W. J.; Carr, P. W. Anal. Chem. 1988, 60, 820. (34) Park, J. H.; Dallas, A. J.; Chau, P.; Carr, P. W. J. Chromatogr. 1994, 677, 1. (35) Yonker, C. R.; Zwier, T. A.; Burke, M. F. J. Chromatogr. 1982, 241, 257. (36) Yonker, C. R.; Zwier, T. A.; Burke, M. F. J. Chromatogr. 1982, 241, 269.
Analytical Chemistry, Vol. 70, No. 17, September 1, 1998
3625
solute’s HBD acidity. The 50/50 acetonitrile/water mixture is only moderately basic37 (β ) 0.68). Alkyl chains and phenyl groups have low basicities (β ≈ 0.1). Thus, the HBA basicity of the stationary phase arises mainly from sorbed mobile phase and partially from the hydroxyl groups on the surface of the silica and zirconia. The large negative a indicates that the basicity of the PRP-1 phase is small due to the minimal amount of sorbed mobile phase on the polymer surface. In addition, we observe that the other aromatic phases (PS-ZrO2 and Ph-SiO2) have smaller absolute values of the a coefficient as compared to the aliphatic phases. This may originate in the following: (1) the higher basicity of the aromatic ring (βbenzene ) 0.12) compared to the basicity of an aliphatic chain (βcylcohexane ) 0); (2) the greater solubility of mobile phase in the aromatic phases; and (3) the higher accessibility of the surface hydroxyl groups on the aromatic phases. Except for the PRP-1 phase, the a coefficient is virtually independent of the mobile-phase composition (see Figure 5c). The basicity of water (β ) 0.47) is roughly equal to that of acetonitrile (β ) 0.40). As a consequence, an increase in water content does not extensively alter either the HBA basicity of the mobile phase or the stationary phase and hence it has little effect on the a coefficient. However, the a coefficient of the PRP-1 phase increases slightly as the amount of acetonitrile in the mobile phase is increased; we do not have an explanation. The s Coefficient. All six phases have small negative s coefficients (see Figure 4), which indicates that the differences in the dipolarity/polarizability between the mobile and stationary phases are quite small; the negative sign tells us that dipolar solutes only slightly prefer the mobile to the stationary phase. Both components of the mobile phase, water (πwater* ) 1.17) and acetonitrile (πacetonitrile* ) 0.75), are highly dipolar substances; in the stationary phase, the dipolar interactions are attributed mainly to the sorbed mobile-phase components. Both aromatic phases (Ph-ZiO2, PS-ZrO2) have less negative s coefficients than do the aliphatic analogues because aromatic phases are more polarizable than aliphatic phases; the π* of benzene is 0.52 while the π* of cyclohexane is zero. The PRP-1 phase has a larger s coefficient as compared to the other two aromatic phases. This is probably caused by the same process that leads to the larger b and a coefficients of PRP-1: the surface of PRP-1 is not as well solvated as those of the other two aromatic phases. As shown in Figure 5d, for all five phases, the s coefficient is essentially independent of the mobile-phase composition. The lack of dependence of the s coefficient on φacetonitrile for silica- and zirconia-based phases can be explained by the mechanism proposed by Tan et al. in a study of C8-SiO2 phases2: as the water content of the mobile phase is increased, the bonded chain collapses and sorbed solvent molecules accumulate in the relatively polar interface region instead of in the dense, nonpolar phase.20,38,39 Because we believe that retention on PRP-1 involves a more adsorption-like retention process, the lack of variation in s coefficient with changes in the mobile-phase composition cannot be explained. At this time we can offer no hypothesis to explain this observation. (37) Dallas, A. J. Ph. D., University of Minnesota, Minneapolis, 1995. (38) Scott, R. P. W.; Kucera, P. J. Chromatogr. 1977, 142, 213. (39) Bohmer, M. R.; Koopal, L. K.; Tijssen, R. J. Phys. Chem. 1991, 95, 6285.
3626 Analytical Chemistry, Vol. 70, No. 17, September 1, 1998
The r Coefficient. The r coefficient is a correction factor to the dipolar/polarizability term (the s coefficient) and reflects the tendency of the system to interact with the solute through π- and n-electron pairs.9 In contrast to the negative s coefficients, the r coefficients are either nearly zero or positive. Silica-based columns have r coefficients close to zero; the r coefficient of PBD-ZrO2 is slightly positive while PS-ZrO2 and PRP-1 have relatively large and positive r coefficients. A positive r coefficient implies that a stationary phase has more π-π interactions with the solute than does the mobile phase (Rwater ) 0.0 and Racetonitrile ) 0.237). This is particularly true for aromatic phases such as PS-ZrO2 and PRP-1 due to the presence of phenyl groups (Rbenzene ) 0.610 and Rn-octane ) 0.0). However, Ph-SiO2 does not behave this way, and the reason is not yet clear. For PBD-ZrO2, the polymer contains some residual olefinic unsaturation (Rbutadiene ) 0.320), but we doubt that the π-π interaction ability is significant. Figure 5e shows the lack of dependence of the r coefficient on the mobile-phase composition. The mobile phase has little ability to interact by a π-π process with solutes and thus it has no effect on the r coefficient. In general, our LSER results and interpretations suggest that on the PRP-1 phase a solute undergoes a more adsorption-like retention process. However, we cannot conclude that it is a pure adsorption process because of the complexity of the micropore structure of the polymer matrix. In addition, we observe that the LSER coefficients of aromatic phases differ somewhat from those of aliphatic phases. Nevertheless, the effects of mobile-phase composition on LSER coefficients are quite similar for both aromatic phases (PS-ZrO2 and Ph-SiO2) and aliphatic phases (PBD-ZrO2 and C8-SiO2). This indicates that the chemical properties of different stationary phases depend on the mobilephase composition in a similar manner. Comparison of Aromatic and Aliphatic Phases. Contributions from Different Interactions. Table 5 compares the percent variance of log k′ accounted for by each interaction term in the LSER. We immediately see some differences between these six stationary phases. First, both zirconia-based and silica-based phases show very comparable and large contributions from the vV2 and bΣβ2H terms. On the other hand, the polymer-based phase (PRP-1) has distinctively smaller contributions from these two terms; instead, it shows a markedly increased contribution from the aΣR2H term. In this regard it is nearly unique among RPLC phases. Our observation of a high a coefficient for PRP-1 agrees with Abraham’s work.4 Second, all silica-based phases (C8-SiO2, C18-SiO2, Ph-SiO2) have very similar contributions from the sπ2*, aΣR2H, and rR2 terms. Interestingly, zirconia-based columns (PBD-ZrO2, PS-ZrO2) also have similar contributions from aΣR2H; even though sπ2* and rR2 are different for PBD-ZrO2 and PS-ZrO2, the sums of these two terms (sπ2*, rR2) for both columns are amazingly close to one another. Since the rR2 term acts to compensate for polarizability contributions not present in the sπ2* term, it might be reasonable to combine rR2 and sπ2*. Third, the rR2 terms for C8-SiO2, C18-SiO2, Ph-SiO2, and PBDZrO2 are negligible; however, those for PS-ZrO2 and PRP-1 are rather significant and similar. This implies that these two aromatic phases have very similar polarizability and they might have very similar selectivity toward highly polarizable solutes, such as polyaromatic hydrocarbons (PAHs). The percent variances in
Table 7. Analysis of K-K Plotsa,b C8-SiO2
C18-SiO2
phases
slopec
sdd
re
C8-SiO2 C18-SiO2 PBD-ZrO2 Ph-SiO2 PS-ZrO2 PRP-1
1.15 1.21 0.63 0.81 1.29
0.01 0.03 0.02 0.05 0.06
1.00 0.99 0.99 0.97 0.98
PBD-ZrO2
Ph-SiO2
PS-ZrO2
slope
sd
r
slope
sd
r
slope
sd
r
slope
sd
r
0.87
0.01
1.00
0.83 0.95
0.03 0.03
0.99 0.99
1.05 0.55 0.70 1.12
0.03 0.02 0.05 0.05
0.99 0.99 0.96 0.98
1.59 1.82 1.96
0.02 0.02 0.02
0.99 0.99 0.98
0.51 0.66 1.05
0.02 0.04 0.06
0.98 0.96 0.97
1.23 1.43 1.52 0.77
0.05 0.05 0.04 0.06
0.97 0.96 0.96 0.98
1.30 2.06
0.06 0.07
0.98 0.99
1.52
0.09
0.97
a A κ-κ plot is a plot of log k′ of one phase vs log k′ of another phase. b Based on retention data in 50/50 acetonitrile/water. c Slope of the κ-κ plot of other phase vs C8-SiO2. d Standard deviation of the slope. e Correlation coefficient of the fit.
Figure 6. Comparison of LSER coefficient ratios, s/v, a/v, b/v, and r/v for six different stationary phases in 50/50 acetonitrile/water: (a) C18-SiO2; (b) PBD-ZrO2; (c) C8-SiO2; (d) PRP-1; (e) PS-ZrO2; (f) Ph-SiO2.
Table 5 do not add up to exactly 100, and this is probably because LSER theory does not include all possible interactions in the retention process. Ratios of LSER Coefficients. It is evident (see Table 3) that the v coefficient for a given stationary phase can be easily adjusted over a wide range by changing the composition of the mobile phase. Different stationary phases will exhibit different chromatographic selectivities if they have different ratios of LSER coefficients. Note that this important assertion is the subject of work in progress. Thus, to compare the selectivities of the six stationary phases, we compare them in terms of the s/v, a/v, b/v, and r/v ratios (Table 6) in a common mobile phase and not in terms of their absolute v, s, a, b, and r values. Figure 6 shows the ratios of LSER coefficients for the six stationary phases. The s/v ratio is quite similar for all phases while the r/v ratio distinguishes the PS-ZrO2 and PRP-1 phases from the other phases. The dipolarity/polarizability interactions between the solute and the stationary phase are complicated; the solute can interact with the sorbed mobile phase, the phenyl groups on the stationary phases, the silanol groups, or the zircanol groups. However, the similarity in the r/v ratio between PSZrO2 and PRP-1 once again strongly indicates that these two phases are more comparable in terms of polarizability.
PRP-1 is distinguished from the other phases by its large a/v ratio while the a/v ratios of other phases are quite similar and small. The polar silica and zirconia substrates help their bonded or coated phases to be better water sorbents; in contrast the nonpolar polymer base repels water. Thus PRP-1 is a poor water sorbent. Consequently, we observed a large a/v ratio for the PRP-1 phase. The magnitude of the b/v ratio increases slightly in the order C18-SiO2 < C8- SiO2 < Ph-SiO2 < PBD-ZrO2 < PS-ZrO2 = PRP-1. This indicates the hydrogen-bonding acidity of the stationary phases increases in the same order. The two aromatic phases (PS-ZrO2, PRP-1) are nearly indistinguishable in terms of b/v. Energetics of Retention (κ-κ Plots). Plots of log k′ vs log k′ for two phases as the solute is varied (called κ-κ plots),40 were used to compare the energetics of retention for the aromatic and aliphatic phases. Table 7 lists the slopes, standard deviations (sd), and correlation coefficients of the κ-κ plots for each column pair in 50/50 acetonitrile/water mobile phase. As stated by Horvath,40 a linear correlation with unit slope indicates a homoenergetic retention process. This means that the two phases have identical intrinsic thermodynamic behavior. Good linear correlation with a slope other than unity suggests that the physicochemical bases for retention on the two phases are similar. This is termed homeoenergetic retention. A poor correlation means that the two columns are heteroenergetic and have different retention mechanisms. If we consider that a correlation coefficient of 0.99 is very good and e0.98 is relatively poor, we can observe some differences and similarities in these six phases. Both C8-SiO2 and PBDZrO2 are only homoenergetic with C18-SiO2 and homeoenergetic with all the other phases and with one another. In addition to homoenergeticity with the aliphatic phases C8-SiO2 and PBDZrO2, C18-SiO2 is homeoenergetic with the three aromatic phases. All three aromatic phases are homeoenergetic with each other and with the three aliphatic phases. We are only able to compare the κ-κ plots in the 50/50 acetonitrile/water mobile phase where the measurement of retention data for all six phases is achievable. Unfortunately, as shown in Figure 5a, in this volume fraction of acetonitrile the v coefficient for the PRP-1 phase is much closer to C8-SiO2 and PBD-ZrO2 than to the other two aromatic phases and hence the slopes of log k′ of the PRP-1 phase vs three aliphatic phases (1.29, 1.12, and 1.05) are much closer to 1 than those vs PS-ZrO2 and Ph-SiO2 (2.06 and 1.52, respectively). Therefore, the results obtained from the κ-κ plots in 50/50 acetonitrile/water seem to (40) Melander, W.; Stoveken, J.; Horvath, C. J. Chromatogr. 1980, 199, 35.
Analytical Chemistry, Vol. 70, No. 17, September 1, 1998
3627
contradict our interpretation of the adsorption-like mechanism of the PRP-1 phase. If we could compare κ-κ plots in the 20/80 acetonitrile/water mobile phase where the v coefficient for PRP-1 is very different from all the other phases (Figure 5a), we would have drawn the same conclusion, i.e., that retention on the PRP-1 phase favors an adsorption-like mechanism. However, the retention data are experimentally inaccessible for PRP-1 at this high composition of aqueous component in the mobile phase; the retention time is too long to be measured. In summary, the three aliphatic phases (C8-SiO2, C18-SiO2, PBD-ZrO2) are rather similar in terms of selectivity; on the other hand, the three aromatic phases (Ph-SiO2, PS-ZrO2, PRP-1) are different from the aliphatic phases and from one another. PRP-1 has rather different balance of intermolecular interaction than do the other phases. PS-ZrO2 is more comparable in terms of molecular polarizability to PRP-1 than either Ph-SiO2 or PBDZrO2. CONCLUSIONS Linear solvation energy relationships are an important and successful method for exploring differences in the retention characteristics and selectivity of different aromatic and aliphatic phases. Polymer-based aromatic phases (e.g., PRP-1) have significantly different properties than do either aromatic or aliphatic inorganic oxide phases. In addition to the solute size and HBA basicity, which are the major retention governing factors for other stationary phases, the retention of a solute on the PRP-1 phase is also markedly controlled by its HBD acidity. The analysis of LSER coefficients and their dependences on mobile-phase compositions suggests that PRP-1 probably exhibits a more adsorption-like retention mechanism. We also observed differences between the aromatic and aliphatic phases. In general, solutes are less retained on the
3628 Analytical Chemistry, Vol. 70, No. 17, September 1, 1998
aromatic phases than on the aliphatic phases; the aromatic phases have distinctively smaller LSER coefficients except for the r coefficient, which is larger for the aromatic phases. The r coefficient and the attendent greater polarizability interactions between solutes and stationary phase could be one of the principal causes of the differences in selectivity between aromatic and aliphatic phases. This conclusion has considerable practical implications. The close similarity in selectivities among all aliphatic phases is the source of both simplifications and increased difficulties in method development.41 Aromatic phases such as PRP-1, Ph-SiO2, and PS-ZrO2 are practically useful due to their different chromatographic selectivity compared to aliphatic phases, such as C18-SiO2 and PBD-ZrO2. On the basis of this, we predicted that the selectivity among a variety of PAHs would be different on aromatic phases than on aliphatic phases. However, a follow-up experiment on the separation of 16 PAHs on PBDZrO2 and PS-ZrO2 showed very similar selectivity for both phases. The results did not support our prediction and indicated that selectivity is a more complex issue than encompassed by LSER theory, which only allows us to look globally at selectivity issues but disguises the important but more subtle issue of molecular geometry. ACKNOWLEDGMENT The authors acknowledge the financial support of the National Institute of Health (Grant GM-54585).
Received for review February 13, 1998. Accepted May 29, 1998. AC980173V (41) Snyder, L. R. J. Chromatogr., B 1997, 689, 105.
