Raman spectroscopy alkyl stationary phases

To determine the role of stationary phase structure in the retention process, it is of primary importance to understand its structure in various solvent environments. Studies of the interphase region using deuterium NMR have shown that it is the mobile phase composition that determines the structure of the stationary phase (d). Results have shown that water does not associate strongly wdth the alkyl chains. However, acetonitrile can associate strongly, even at low mole fractions because of the microheterogeneous environments that exist for the binary mixture. Previous in situ studies of the alkyl stationary phases using surface enhanced Raman spectroscopy have also concluded that little conformational change occurs with the addition of a polar solvent to the interface (7). 

Ho, M. and Pemberton, J.E., Alkyl chain conformation of octadecylsilane stationary phases by Raman spectroscopy. 1. Temperature dependence. Anal. Chem., 70, 4915,... 

This lack of solvent-induced monolayer disorder was also observed by Pemberton and coworkers (7). Pemberton and coworkers used Raman spectroscopy to examine Cis alkylsiloxane monolayers on a SiQ/Ag substrate and found little disruption of the alkyl chains in contact with acetonitrile, as well as water. The results of Carr and Harris (19), however, showed that a Cis chromatographic stationary phase assumes a collapsed structure in water. The stationary phase can evolve from this collapsed state with the addition of an organic modifier, such as acetonitrile, to the aqueous solvent. The modifier intercalates into the monolayer, causing an increase in volume, polarity, and alkyl chain order. 

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