INFLUENCE OF TEMPERATURE ON DEAD VOLUME OF ODS COLUMNS AND ON N-ALKANE RETENTION IN HIGH-PERFORMANCE LIQUID-CHROMATOGRAPHY

Dead volume of ODS columns as determined from the homologous series of n-alkanes with n-pentane or methanol eluent decreases with increasing temperature. This decrease which occurs in addition to the influence of thermal expansion of the eluent, is thought to be caused by thermal expansion of the stationary phase. Dead volume decreases about 2% per 40 degrees C with n-pentane eluent and 5% with methanol eluent. No such decrease is found with a silica column. In addition, there appears to be a slight dependence of exclusion coefficients on temperature. With n-pentane eluent, net retention of n-alkanes on ODS is very low (on the order of 1% of retention with methanol eluent). Solutes smaller than eluent molecules (butane to methane) exhibit higher retention times than expected from larger solutes (hexane and higher). With methanol eluent, plots of log capacity factor (a) versus carbon number for various temperatures and (b) versus inverse temperature for various carbon numbers show common intersection points.