24
December 2010
Recent Developments in Type C Stationary Phases: Exploiting the Versatility of Silica Hydride Materials
by Joseph J. Pesek*, Department of Chemistry, San Jose State University, San Jose, CA 95192, USA email:
pesek@sjsu.edu • Tel: 1-408-924-4950 Fax: 1-408-924-4945
Maria Matyska, Microsolv Technology Corporation, One Industrial Way West, Eatontown, NJ 07724, USA Corresponding author*
TYPE C™ silica is a relatively new chromatographic material that has been finding ever-increasing use in the last few years. The properties exhibited by these stationary phases are often significantly different than the ordinary silica used for most commercial products. While all TYPE C phases can be utilized in the reversed-phase, organic normal phase and aqueous normal phase modes, there are some unique capabilities within each retention mode that have resulted in innovative method development strategies with great success. Some of the more challenging separation problems involve polar compounds; two approaches for the analysis of hydrophilic compounds are described in this report.
Introduction
TYPE C silica, based on a surface of Si-H, was introduced many years ago. However, it has only been recently that some of the unique chromatographic features of this material have been discovered and exploited in solving challenging separation problems. This report focuses on the capabilities of these stationary phases for the separation of hydrophilic materials in two modes: aqueous normal phase (ANP) that utilizes high organic content mobile phases and in reversed-phase using high aqueous content mobile phases. For descriptions about the chromatographic properties of TYPE C in the organic normal phase, earlier reports have provided examples of separations utilizing this separation mechanism [1,2]
.
The chromatographic retention and separation of polar compounds continues to be a challenging analytical problem. The versatility and ruggedness of reversed-phase chromatography for separations based on hydrophobic interactions has not been matched by any single method for hydrophilic species. A number of approaches have been developed for polar compound retention but many are limited in their applicability or have other serious
Figure 1. Analysis of urea in a 100% aqueous mobile phase on a Cogent Bidentate C18 column. Mobile Phase: 100% DI water (isocratic). Column: 4.6 x 150 mm. Flow rate: 0.5 mL/min. Detection at 210 nm. Sample: 1 mg/mL. Injection volume: 10 µL.
drawbacks. For example polar compounds can be derivatized to make them amenable to RP methods, but this is often time- consuming or not very reproducible. Ion- exchange can be used for some polar compounds, provided they have a permanent charge, but is not applicable to neutral polar compounds like carbohydrates and is also not compatible with mass spectrometry, the most rapidly expanding method of detection. Making polar compounds neutral by the use of extremely high pH mobile phase and more recently, hydrophilic interaction liquid
chromatography (HILIC) have been introduced as a means of analyzing polar compounds. However, many labs report that HILIC methods have poor reproducibility and systems equilibrate slowly when gradient elution is used. Also, many of the analytical schemes developed are not compatible with MS detection. Some of the problems reported for HILIC are likely related to the retention mechanism of these materials; generally regarded to be the formation of a water layer near the surface of the stationary phases so that polar molecules partition between it and the more organic-rich
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