Xiqin Yang, Jun Dai, Peter W. Carr* Chemistry Department University of Minnesota

Transcription

1 Comparison of the Reversed-Phase and Ion- Exchange Contributions to Retention on Polybutadiene Coated Zirconia and Octadecyl Silane Bonded Silica Phases Xiqin Yang, Jun Dai, Peter W. Carr* Chemistry Department University of Minnesota

2 Outline Mixed-mode retention on silica and zirconia based stationary phases Reversed-phase vs. ion-exchange interactions Retention models in mixed-mode retention mechanism One-site vs. two-site model Three plots: log k vs. n CH2, log k vs. log [C + ] m, and k vs. 1/[C + ] Relative contribution of ion-exchange retention: k IEX / k Conclusions One-site model is WRONG! Two-site model is more accurate.

3 Mixed-Mode Separation on ODS Phases A + H 3 C CH 3 (CH 2 ) 17 Si CH 3 A + H 3 C O O - OH O O O O Si Si Si CH 3 (CH2)17 Si CH 3 O O Si Bonded C 18 Chains Reversed-Phase () Interactions Ionized Silanol Groups Ion-Exchange (IEX) Interactions Mixed-mode retention mechanism!

4 Mixed-Mode Separation on PBD-ZrO 2 Adsorbed Lewis base anion O - A + A + O - PBD coating O P OH A + A + O - OH O H 2 O OH OH H 2 O H 2 O O Zr O Zr O Zr O Zr O Zr O Zr O Zr O Zr O PBD Coating Reversed-Phase () Moieties Lewis Base Anions Ion-Exchange (IEX) Sites Mixed-mode retention mechanism!

5 Columns ODS columns Ace Inertsil ODS3 Eclipse XDB Zorbax Extend Zorbax SB Zorbax Rx Alltima Ace Alltima PBD-ZrO 2 Ace: 5 µm, 100 Å, 300 m 2 /g, 15.5% C Alltima: 5 µm, 100 Å, 350 m 2 /g, 16% C PBD-ZrO 2 : 4.1 µm, 500 Å, 11 m 2 /g, 2.0% C

6 κ-κ Plot - log k on different columns log k'on Ace R 2 = s.d.=0.153 log k'on PBD-ZrO R 2 = s.d.= log k'on Alltim a log k'on Alltim a Condition: 72 % MeOH, 25 mm phosphate, ph=6, 35 C, 1 ml/min. Solutes: 11 antidepressant drugs Poor correlations in the κ κ plot indicate changes in selectivity PBD-ZrO 2 is really different.

7 One-Site Model for Mixed-Mode Retention ADDITIVE Free Energy ln k ' = φ obs ln( ) + φ: the phase ratio for the single type of site. G RT o + G o IEX RT Reversed-Phase Interactions Ion-Exchange Interactions + Cationic solute Silica Substrate U. D. Neue et al. J. Chromatogr. A, 925(2001), 49-67

8 Two-Site Model for Mixed-Mode Retention Two INDEPENDENT Interaction Sites ' obs ' k = k + k ' IEX Pure Ion-Exchange Interactions Pure Reversed-Phase Interactions Silica Substrate Sokolowski, A.; Wahlund, K. G. J. Chromatogr. 189 (1980), 299 A. Nahum and C. Horvath, J. Chromatogr. 203 (1981), 53-63

9 Ion-Exchange Retention Characteristics log + k ' = Constant log[ C ] IEX m One-site model log k ' = A' + logk' log[ C + ] obs m Two-site model log k ' = log( k' + k' obs IEX ) Slope of log k obs vs. log [C + ] m Retention Mechanism One-site Two-site only 0 0 IEX only -1-1 dominant -1 ~ 0 IEX dominant -1 ~ -1

10 Reversed-Phase Retention Characteristics For a homologous series of solutes: log k' = A + BnCH B: independent of the homologous used 2 (CH 2 ) n CH 3 NH 2 CH 2 (CH 2 ) n CH 3 Alkylbenzenes One-site model p-alkylbenzylamines log k ' = logk' + logk' obs IEX B: independent of the homologous used ' ' ' Two-site model log log( ) k = k + obs k IEX B: NOT independent of the homologous used

11 Effect of Counterion Concentration PBD-ZrO 2, y = x log k' Alltima, y = x log [NH4 + ] Ace, y = x Conditions: 55/45 MeOH/ammonium phosphate buffer, ph = 6.0, T= 35 C Solute: p-butylbenzylamine One-site model is WRONG (slope is NOT 1)! Ion-exchange contribution is not large on ODS phases!

12 Effect of Number of Methylene Units log k'obs Alltima y = 0.29x n CH2 4 6 log k'obs PBD-ZrO 2 y = 0.19x y = 0.30x y = 0.27x n CH2 4 6 : alkylbenzenes; : p-alkylbenzylamines Mobile phase: 55 % MeOH, 25 mm ammonium phosphate, ph 6.0. Other experimental conditions: 1 ml/min; 35 C; 254 nm. One-site model is WRONG Two-site model is more accurate Ion-exchange retention dominates on PBD-ZrO 2

13 k obs = k ' k'obs of p-propylbenzylam ine Two-Site Model ' Constant [C + ] m PBD-ZrO 2 : y =463x +1.4 Alltima: y =11.9x +2.0 Ace: y =1.3x /[N H 4 + ] Intercept 55 % MeOH, ammonium phosphate, ph =6, 1 ml/min, 35 ºC k'iex/k' of p-propylbenzylamine = k ' IEX = k ' k ' 100% 80% 60% 40% 20% 0% Ace Alltima PBD-ZrO 2 15mM 25mM 35mM 50mM [NH 4 + ] Quantitative estimation of the relative contributions of and IEX Much more IEX on PBD-ZrO 2 causes selectivity difference k' ;

14 log k IEX vs. log [NH 4+ ] Alltima log k obs vs. log [NH 4+ ] k ' IEX = k' obs k' log k IEX vs. log [NH 4+ ] 1.8 y = -0.16x y = -0.93x log k'obs log k'iex log[nh 4 + ] log [NH 4 + ] : propylbenzylamine; : butylbenzylamine; : pentylbenzylamine; : hexylbenzylamine Estimation of k IEX by using two-site model is good!

15 Conclusions One-site free energy addition retention model is incomplete Two-site mixed-mode retention model is more correct The dominant of IEX on PBD-ZrO 2 accounts for the dramatic different selectivity towards basic solutes compared to ODS phases. A plot of k vs. 1/[C + ] m is a very useful tool for quantitatively estimating relative contributions of reversed-phase and ion-exchange interactions to the retention of basic solutes.

16 Acknowledgements National Institutes of Health University of Minnesota ZirChrom Separations