HEXAMETHYLDISILOXANE AS A MOBILE-PHASE COMPONENT FOR NORMAL-PHASE LIQUID-CHROMATOGRAPHY

Hexamethyldisiloxane (HMDX) has been evaluated as a modifier solvent f or the normal phase separation of substituted naphthalene compounds as well as aromatics such as nitrobenzene, acetophenone, benzonitrile, a nd aniline. It is found to be a weaker eluting solvent than n-butyl et her and other commonly used normal phase solvents filling a niche in t he eluotropic series. HMDX exhibited an unusual retention factor (k') profile as a function of mobile phase composition in hexane. In a rang e from 0 to 40 %, the k' values remained quite constant with increasin g HMDX composition. As the concentration of HMDX progressed to 100 %, the k' values actually increased in a generally linear fashion. This t rend in k' is due to longer solute retention not just a change in the retention time of the unretained component. Although slight silanizati on of the silica column may occur with initial use of HMDX, retention of these analytes on a commercial C-1 silica column is much lower with no positive k' slope with increasing HMDX. The chromatographic data p rovide evidence that HMDX is likely adsorbed to the silica surface and a solute partitioning mechanism may be operative.