DEPENDENCE OF GAS-CHROMATOGRAPHIC RETENTION DATA OF HYDROCARBONS ON THE FILM THICKNESS OF THE POLYDIMETHYLSILOXANE STATIONARY-PHASE

Kovats indices of cyclic and aromatic hydrocarbons, separated by capil lary gas liquid chromatography on polydimethylsiloxane capillary colum ns, were found to increase with increasing stationary phase film thick ness. This effect is explained in terms of adsorption of the stationar y phase on the active sites of the inner surface of the capillary colu mn. Since the number of active sites is limited, the overall polarity of the polydimethylsiloxane stationary phase is better defined in colu mns with thick stationary phase films. Interlaboratory reproducibility of retention indices of cyclic and aromatic hydrocarbons is therefore also better when using capillary columns with thick films of the poly dimethylsiloxane stationary phase. Retention data obtained on such thi ck-film columns are influenced by adsorption of the stationary phase o n the column walls to a lesser extent than in columns with thinner fil ms.