Application of pyrolysis-gas chromatography to the study of thermal alteration of chlorophyll-a — effect of clay minerals

Abstract

The effect of clay minerals (montmorillonite and kaolinite) on the thermal alteration of chlorophyll-a (Ch-a) was examined by means of pyrolysis-gas chromatrography. When preliminarily heated at 150 °C for a period ranging from 2 h to 504 h in the presence of montmorillonite or kaolinite, Ch-a produced a considerable amount of material which was insoluble in a 9:1 mixture of iso-octane/ethanol (I/E) but soluble in 3:1 benzene/methanol (B/M). The yield of I/E insoluble-B/M soluble fraction (B/M extract) increased with increased preheating time. Upon pyrolysis, the B/M extract produced isoprenoid hydrocarbons (ISOPs) of carbon number 20 or less. With increased preheating time, the yields of ISOPs of carbon number 20 (C20-ISOPs) dramatically decreased, compared to those of ISOPs of smaller carbon numbers (S-ISOPs). The S-ISOPs were mostly identical to those produced upon pyrolysis of kerogens (insoluble organic matter in sedimentary rocks). The trend in the decrease in the yields of C20-ISOPs with preheating time is similar to that observed for Ch-a preheated in the absence of clay minerals. Clay minerals seem to promote the formation of a structure which preferentially produces 1-pristene (2,6,10,14-tetramethyl-1-pentadecene, Pr-1), the most predominant compound in the ISOPs from kerogens, upon pyrolysis.