BINARY MOBILE PHASES FOR SUPERCRITICAL-FLUID CHROMATOGRAPHY WITH HELIUM MICROWAVE-INDUCED PLASMA DETECTION

Microwave-induced plasma atomic emission detection is examined for sup ercritical fluid chromatography detection with binary methanol/carbon dioxide mobile phases. Spectra are obtained and compared with pure hel ium plasmas, plasmas with added carbon dioxide, and added binary mobil e phases. Trends of molecular emission intensities and species were ex amined as functions of methanol/carbon dioxide composition ratios and supercritical fluid pressures. Adding carbon dioxide to the pure heliu m plasma reduced and/or eliminated molecular bands from OH and N2+ whi le producing intense bands of CN, C2, NH, and N2. The addition of meth anol to the carbon dioxide ''doped'' plasma produced intense OH emissi on while depressing the intensities of the other aforementioned molecu lar bands. Emission from OH increased significantly with additional me thanol. As the pressure of the methanol/carbon dioxide mobile phase wa s increased, the general trend was that molecular band intensities ten ded to decrease. The effect was attributed to plasma cooling. The plas ma easily tolerated the mixed mobile phases despite the added spectral complications of these molecular emissions. The advantages of the bin ary mobile phases for chromatography are illustrated with separations of moderately high molecular weight chlorinated organic compounds. Ret ention times are reduced and detectabilities are in the range of 1.4-1 .9 ng/s for p-chlorobiphenyl, 2,4'-dichlorobenzophenone and hexachloro benzene.