ANALYTICAL CHEMISTRY, VOL. 50, NO. 6, MAY 1978
749
Change in Resolution of Reverse Phase Liguid Chromatographic Columns with Temperature W. A. S a n e r , * J. R. Jadamec, and R. W. Sager U.S. Coast Guard Research and Development Center, Avery Point, Groton, Connecticut 06340
Improvements in liquid chromatographic separations usually observed at elevated column operating temperatures (60 “C) were not observed for 5-pm RP-8 and RP-18 packed columns: in fact, a substantial loss in efficiency occurred. Conversely, increased efficiency was obtained when a 10-pm RP-18 reverse phase column was operated at 60 OC. The loss in efficiency for the 5-pm reverse phase packings at 60 OC is most likely attributable to a decrease in diffusitivity within the stationary phase, causing reduced mass transfer.
T h e recent development of smaller diameter particles for liquid chromatographic packings has increased resolution by decreasing channeling, and increasing the mass transfer between moving and stationary phases. T h e particle size reduction is especially important for totally porous particles since the intra-particle pathways occupied by the “stagnant” mobile phase are drastically shortened, causing improved diffusion within the stationary phase. A second means of increasing chromatographic resolution is by heating the liquid chromatographic column, Le., decreasing t h e mobile phase viscosity and increasing the mass transfer in both the “stagnant” and “moving” mobile phases. T h e use of elevated column temperatures, however, is infrequently utilized as a means of improving resolution since t h e net improvement is small relative to the improvement gained by particle size reduction. Additionally, in some cases. the separation of labile substances precludes the use of high column temperatures. T h e use of elevated column temperature (60 “C)was investigated as a means of increasing resolution for complicated mixtures of petroleum derived aromatics. The following report describes t h e chromatographic behavior of three different reverse phase column packings at room temperature and a t 60 “C.An unexpected loss in resolution was observed a t the elevated temperature for 5-pm diameter particles with both C8and CI8 reverse phase columns. EXPERIMENTAL Apparatus. The liquid chromatograph. syringes, column heater, detectors, and recorders were described previously (1). Three different, prepacked reverse phase analytical columns were utilized: Waters Associates 10-pm pBondapak CI8 (0.39 X 30 cm) column; Altex (0.46 X 25 cm) column packed with 5-Fm LiChrosorb (E Merck) RP-8; and ES Industries Chromegabond CI8 (0.46 X 15 cm) column packed with 5-pm LiChrosorb RP-18. All analytical columns were used in conjunction with the same Whatman guard column (0.21 X 7 cm) packed with Co: Pel1 ODS (25-35 Atm Reeve Angel reverse phase CIS). All the above column packings were silanized; hence their thermal and chemical stabilities are comparable. Reagents. Deionized, glass distilled water and spectroquality methanol (MCB MX 475) were used as the mobile phase after filtering through 0.22-pm pore hlillipore filters (GSU‘P04700) and 0.5-pm pore Fluoropore filters (FHUP04700), respectively. Sample Preparation. Petroleum oils were extracted with acidified methanol by a method described earlier (2). The extracts used in the present study were not concentrated before injection. Separation Procedure. All samples were chromatographed using a linear gradient profile from 50% methanol/water to 10070 methanol in 50 min at flow rates of 1 mL/min or 0.5 mL/min, This paper not subject to U.S. Copyright
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