A REVIEW: ORGANIC GEOCHEMICAL STUDIES BY LASER PYROLYSIS MASS–SPECTROMETRY AND HOW THEY COMPARE WITH FLASH PYROLYSIS MASS–SPECTROMETRY STUDIES

Abstract

Various methods of laser pyrolysis have been used by researchers to investigate at a molecular level the chemistry of dispersed organic matter. An understanding of molecular composition is important for evaluations of oil generation potential and maturation levels of source rocks. Laser-based pyrolysis-mass spectrometry studies as applied to petroleum source rocks and coals are reviewed. Experiments undertaken to date involve three main techniques: (i) laser ionisation-mass spectrometry; (ii) laser pyrolysis-gas chromatography-mass spectrometry; and (iii) laser desorption-electron im-pact-mass spectrometry. These techniques have had variable degrees of success for detecting structurally significant hydrocarbon products. Typically assessed through comparison with conventional flash pyrolysis data, the relative merits of the different techniques are described.

The most powerful analysis is obtained through the combined use of chromatographic separation and mass spectrometry. The emergence of the laser pyrolysis-gas chromatography-mass spectrometry technique offers enormous potential for various geochemical applications. Laser pyrolysis in conjunction with a microscope (micropyrolysis) allows analysis of specific organic entities, thereby avoiding the masking of individual properties through bulk analysis. However, laser micropyrolysis-gas chromatography-mass spectrometry has not been widely accepted because of the complexities inherent in interfacing the required hardware devices and the high cost of laser instruments compared with conventional pyrolysers. Chromatography also adds cost with regard to both instrumentation and analysis time. Unfortunately, the ability of laser ionisation to provide insights into molecular structure is severely limited by a predominance of carbon cluster ions from highly carbonaceous materials.

In contrast, the laser desorption-electron impact ionisation routine yields large ion concentrations of aliphatic and aromatic products. Sufficient information has been obtained with this method to distinguish source rocks and coals of different organic composition.