Carr Research Group

Education: B.S., Peking University, 1998

Honors and Membership: BP Amoco Travel Grant, 2002; Beaker and Bunsen Award, 2002; American Chemical Society; Minnesota Chromatography Forum

Teaching Experience: Instructed undergraduates in analytical laboratory courses; Tutored graduate level material characterization course

Research Project Description:

A Critical Comparison of Reversed-Phase and Ion-Exchange Interactions on Polybutadiene Coated Zirconia and Octadecyl Bonded Silica Phases

Octadecyl bonded silica-based (ODS) columns are by far the most popular stationary phases used in reversed phase liquid chromatography (RPLC) for the separations of basic compounds. The two major modes of interactions of such analytes on silica-based columns are hydrophobic and ion-exchange (Coulombic) interactions. The hydrocarbon of the bonded chains and the ionized residual silanol groups account for the hydrophobic and ion-exchange interactions, respectively.
Polybutadiene coated zirconia (PBD-ZrO2) is a new type of stationary phase characterized by its excellent chemical and thermal stability, as well as high efficiency. When hard Lewis bases, such as phosphate, are added to the eluent, they are adsorbed on the Lewis acid sites on zirconia's surface and cause the surface to become highly negatively charged. Under these conditions, the stationary phase acquires both reversed-phase and ion-exchange characteristics, qualitatively similar to silica-based phases, but quantitatively quite different.
Although the influence of hydrophobic and Coulombic interactions on the chromatography of cationic species is qualitatively well recognized, the quantitative relationship between hydrophobic and ion-exchange interactions remains unclear. The retention mechanisms on ODS and PBD-ZrO2 phases were studied with a specific set of judiciously selected basic compounds, namely a homologous series of p-alkylbenzylamines (R-C6H4-CH2-NH2). This series was chosen so that the carbon chain in the R group could be varied without having any steric effect on the interaction of the protonated (> 95% under the pH conditions of the study) amino group with anionic surface sites. Other homolog series (C6H4-(CH2 )n-NH2 ) did not behave in this fashion. The basic and alkyl functional groups on the molecules allowed a quantitative study of the effect of number of methylene units and the eluent salt concentration on the retention behavior. Various quantitative models of mixed-mode retention mechanism on the two types of phases were compared and evaluated. The one-site model in which hydrophobic and Coulombic interactions combine has been disproved. Furthermore, the model equations developed allow us to, for the first time, quantify the relative amount of hydrophobic and Coulombic contributions to retention of any solute on any stationary phase. The data on both ODS and PBD-ZrO2 point to a multi-site model. The results clearly show that while ion-exchange plays an important but minor role (1-10%) on retention of bases on ODS phases, it is by far the dominant factor (> 90% in some cases) in retention on PBD-ZrO2. The results also explain the very high sensitivity of PBD-ZrO2 phases compared to silica based phases to the structure and type (primary, etc.) of the amine solute.

Publications:

1. Yue Hu, Xiqin Yang, and Peter W. Carr, "Mixed-Mode Reversed-Phase and Cation-Exchange Separations of Basic Analytes on Polybutadiene-Coated Zirconia", Journal of Chromatography, February, 2002, in press.

2. Xiqin Yang, Jun Dai, and Peter W. Carr, "An Analysis and Critical Comparison of the Reversed-Phase and Ion-Exchange Contributions to Retention on Polybutadiene Coated Zirconia and Octadecyl Silane Bonded Silica Phases", Journal of Chromatography, A, submitted, November, 2002.

3. Jun Dai, Xiqin Yang, and Peter W. Carr, "Comparison of the Chromatography of Octadecyl Bonded Silica and Polybutadiene Coated Zirconia Phases Based on a Variegated Set of Cationic Drugs", in preparation.

Presentations:

1. Xiqin Yang, Yue Hu and Peter W. Carr, "Optimizing the Separations of Basic Compounds in Mixed-Mode Separations by Understanding the Role of Lewis Acidity of Zirconia Based Phases". The Pittsburgh Conference, New Orleans, LA, March 21, 2002, Oral Presentation.

2. Xiqin Yang, Jun Dai and Peter W. Carr, "A Comparative Study of the Retention Characteristics of Basic Analytes on Polybutadiene-Coated Zirconia and Octadecyl-Bonded Silica Phases", Minnesota Chromatography Forum, Minneapolis, MN, May 16, 2002, Oral Presentation.

3. Jun Dai, Xiqin Yang and Peter W. Carr, "Hydrophobic and Ion-Exchange Interactions in RPLC Stationary Phases", Minnesota Chromatography Forum, Minneapolis, MN, May 16, 2002, Oral Presentation.

4. Xiqin Yang, Jun Dai and Peter W. Carr "Optimizing the Separations of Basic Compounds in Mixed-Mode Chromatography by Understanding the Role of Lewis Acidity of Zirconia Based Phases", HPLC 2002, Montreal, Canada, June 2-7, 2002, Poster Presentation.

5. Xiqin Yang, Jun Dai and Peter W. Carr, "A Comparative Study of the Retention Characteristics of Basic Analytes on Polybutadiene-Coated Zirconia and Octadecyl-Bonded Silica Phases", Oral Presentation submitted to Eastern Analytical Symposium, Somerset, NJ, November 18-21, 2002

6. Jun Dai, Xiqin Yang, and Peter W. Carr, "Mixed-Mode Retention Mechanism of Cationic Compounds on Octadecyl Bonded Silica-Based Reversed Phase", Eastern Analytical Symposium, Somerset, NJ, November 18-21, 2002, Poster Presentation.

7. Xiqin Yang, Jun Dai, and Peter W. Carr, "A Critical Comparison of Reversed-Phase and Ion-Exchange Interactions on Polybutadiene Coated Zirconia and Octadecyl Bonded Silica Phases". The Pittsburgh Conference, Orlando, FL, March 9-14, 2003, accepted as Oral Presentation.

8. Jun Dai, Xiqin Yang, and Peter W. Carr, "Study of Amine Modifier on the Retention of Basic Compounds on Octadecyl Bonded Silica-Based Phase and Polybutadiene Coated Zirconia Phase". The Pittsburgh Conference, Orlando, FL, March 9-14, 2003, accepted as Oral Presentation.

9. Xiqin Yang, Jun Dai, and Peter W. Carr "A Comparative Study of the Retention Characteristics of Basic Analytes on Polybutadiene-Coated Zirconia and Octadecyl-Bonded Silica Phases", 27th International Symposium on High Performance Liquid Phase Separations and Related Techniques, Nice, France, June 15-19, 2003, submitted.

Email: yang@chem.umn.edu

Address: 321 Smith Hall

Phone: (612) 624-5870

Modified by Hao Luo --- hluo@chem.umn.edu