problems. First, certain bonded phases change in nature as well as in the amount of stationary phase; both of these effects result in irreproducible separations. Second, the degradation products from the bonded phase can contaminate a fraction isolated during a separation; such adulteration is of special consideration in preparative chromatography.We have developed two new classes of bonded phases that result in significantly increased stability of HPLC column packings during use. The first class uses bifunctional (or bidentate-type) silanes, which contain one reactive site on each of two silicon atoms of the silane. These two silicon atoms are connected by a bridging group, such as -Oor -(CH2)"-The type of bridging group can be varied, to change the spacing between the silicon atoms of the silane for the most favorable reaction with the SiOH groups on the surface of silica supports. Bonded-phase packings with methyl, vinyl, phenyl, isopropyl, and tert-butyl functional groups show similar properties-but greater stability at low pH-than their corresponding dimethylsilyl derivatives.A second class of more stable bonded-phase materials uses monofunctional silanes, which contain one or two bulky groups (e.g., isopropyl or tert-butyl) on the silicon atom of the silane. These bulky groups provide steric protection to the Si-O-Si bond on the surface of the silica supports against hydrolysis at low pH, while still providing equivalent retention and column efficiency, compared to those of conventional monofunctional silane bonded-phase packings.ACKNOWLEDGMENT Special thanks are given to R. F. Carver for performing the solid-state NMR experiments. We also thank G. R. Wooler and J. B. Marshall for their work in performing many of the syntheses and chromatographic experiments.
Abstract. The influence of forces on the particles other than the applied cross flow is studied in hollow fiber flow field-flow fractionation (HF'). Such forces can be of either attractive or repelling in nature. It is shown by injecting polystyrene latex beads (0.091 pm in diameter) at different cross flows and by using eluents with different ionic strengths, that there will be disturbances in the concentration profile of the sample irrespective of the type of interacting forces. The retention time and peak broadening were either increased or decreased compared to the theoretical value, depending on the nature of the dominating interaction force. It is, nevertheless, possible to miuimize the total interaction energy between particles and between the particles and the channel wall by choosing a suitable ionic strength.
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