Zirconia-based columns

Another approach to increase HPLC speed is the use of higher temperatures. The viscosity of a typical mobile phase used in reversed-phase separation decreases as the column temperature is increased. This allows an HPLC system to operate at a higher flow rate without suffering too much from increased back pressure. Zirconia-based packing materials provide excellent physical and chemical stability. They have been used successfully for high-throughput bioanalysis at elevated temperatures.9... 

Although most classical silica-ODS columns are not stable at temperatures much above 60°C (dependent on eluent conditions), columns made from other materials such as polymeric and zirconia-based phases have been used for high temperature work (see earlier text). 

The stability of zirconia and zirconia-based packings also allows the use of these columns at an elevated temperature. The key advantage of this is the reduction in analysis time due to the reduction in the viscosity of the mobile phase. In addition, the much broader temperature range also permits a fine-tuning of the selectivity of a separation compared with silica-based packings. 

Column temperature affects the relative retention of different compounds and elevated temperature permits high-speed chromatography to be conducted.25 Figure 25-28 suggests a systematic procedure for method development in which solvent composition and temperature are the two independent variables.1 For elevated temperature operation, pH should be below 6 to retard dissolution of silica. Alternatively, zirconia-based stationary phases work up to at least 200°C. 

The principal limitation of the use of superheated water has been the thermal instability of conventional ODS-silica-based stationary phases, which are unstable above 70°C or 80°C. Early work concentrated on PS-DVB columns, which were stable up to 220°C. Then zirconia-based PBD and ODS bonded... 

Although endcapped columns offer some advantages, the aqueous instability is still problem. To over come these problems of silica based columns, alternative supports like alumina (AI2O3), zirconica (Zr02), and titania (Ti02) have been developed. Alumina columns are stable in the pH range of 2-12, while zirconia columns extend the range from 0 to 14. These are basic oxides and hence silanol-like interactions are... 

At higher temperatures, zirconium dioxide and titanium dioxide supports gave much greater stability along with polymer-based supports [100,101] based on polystyrene-divinyl benzene (PS-DVB) such as PLRP-S noted in Table 9.5. PS-DVB supports have been reported to give a serious column bleed at 250°C [66]. Polybutadiene (PBD) modified zirconia columns have been used at temperatures up to 300°C and carbon-coated zirconia has been used at temperatures up to 370°C [66]. Applications have included the separation of steroids [73] and herbicides [102].The specific order of column bleed varied depending on the detection method as shown in Table 9.5. 

A few applications have employed conventional packed columns, although recent developments in new thermally stable stationary-phase materials have generated a renewed interest and the temperature stability of the different stationary-phase materials has been reviewed by Claessens and van Straten [43]. The new materials have included stable metal oxide materials, based on zirconia (Figures 18-4 and 18-5) and titania [44, 45] and hybrid phases combining silica and methylene or ethyl bridges [46]. These have been applied in a number of applications to pharmaceutical compounds (Table 18-1). 

Fu, H.J. Feng, Y.Q. Zhang, Q.H. Da, S.L. Zhang, Y.J. High-performance liquid chromatography of some bases and nucleosides on alkylphosphonate-modified magnesia-zirconia column. Anal. Lett. 1999, 32, 2761. 

The base matrices are divided into two groups. The inorganic matrices are based on silica or zirconia, on which the transition metal ion is complexed with the ligand bonded on the matrix. The inorganic sorbents have high column efficiency but low capacity and low chemical stability. The hgand can also be coated prior to, or during, the experiment, on the octadecylated silica (ODS) reversed phase, which presents the most inexpensive but efficient and selective sorbent for CLEC. 

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