Estimation of vapor pressures for nonpolar organic compounds by capillary gas chromatography

Abstract

The accuracy of a method based on capillary gas chromatography (GC) for estimating vapor pressures of nonpolar compounds was tested by comparing vapor pressures measured by GC (P/sub GC/) with literature values for the subcooled liquid (P/sub L/). Two 1.0 m long fused silica columns were used, a poly(dimethylsiloxane) bonded phase column (BP-1) and a WCOT hydrocarbon phase column (Apolane-87). GC measurements were carried out by using n-C/sub 18/ and n-C/sub 20/ hydrocarbons as vapor pressure reference compounds. Plots of log P/sub GC/ vs. log P/sub L/ were made for 24 organochlorines and polycyclic aromatic hydrocarbons to establish a correlation between measured and literature values. Systematic errors between P/sub GC/ and P/sub L/ were observed whose direction and magnitude depended on the vapor pressure range and the column used for measurements. On the BP-1 column P/sub GC/was too low by a factor of 2.3 at P/sub L/ - 10/sup -1/ torr, and too high by a factor of 3.5 at P/sub L/ = 10/sup -7/ torr. The Apolane-87 column showed more nearly ideal behavior. On this column P/sub GC/ underestimated P/sub L/ by a factor of 1.9 at P/sub L/ = 10/sup -1/ torr and the error decreased for lowermore » P/sub L/. Corrections for these systematic errors were made by relating P/sub GC/ to P/sub L/ through regression equations. The GC method was applied to 30 PCB isomerids containing one to seven chlorines. The average vapor pressure (P/sub L/) decreased by a factor of 4.5 for each chlorine added to the biphenyl core, although large differences in vapor pressure were measured for isomers of a particular chlorination level. Within each chlorination level isomers having the greater number of ortho chlorines had the higher vapor pressures.« less