TuO-05


INTERFACING A LASER MICROPROBE TO A GAS CHROMATOGRAPH - MASS SPECTROMETER

P.F. Greenwood1, S.C. George1, M.A. Wilson2 and K.J. Hall3

1. Australian Petroleum Cooperative Research Centre, CSIRO Division of Petroleum Resources, PO Box 136, N. Ryde, NSW 2113
2. Dept. of Chemistry, Uni. Technology Sydney., PO Box 123, Broadway, NSW 2007
3. GC2 Chromatography, Millbrook Bus. Centre, Floats Rd, Manchester M23 9YJ, Eng.


Chemical and petrographical analysis represent two important and complimentary geochemical methods for appraising heterogenous organic matter. Chemical analysis is typically performed on bulk samples and the discerned properties together with the measured petrographical composition often provide valuable information about the sample under investigation. However, definitive inferences are not always possible, particularly from aberrant samples. A much better appraisal of complex organic matter would be possible if each distinct entity could be chemically characterised.

An instrument which interfaces a laser microprobe with a gas chromatograph mass spectrometer (GC-MS) has recently been developed in our laboratories. The laser microprobe allows selectivity over which samples or parts of samples are to be pyrolysed and consists of a high powered laser (Laser Electronics Nd:YAG) interfaced with a microscope (Olympus BH2). The laser beam is focussed by the microscope objectives to spot sizes in the range 20 - 200 µm.

The sample is housed in a pyrolysis chamber and a purpose built gas inlet system has been carefully designed to maximise pyrolysate transfer from the pyrolysis chamber to the GC column. Particular features of the inlet system include a manually controlled 2-way valve which switches between high carrier flows (> 100 ml/min) for efficient removal of pyrolysates from the pyrolysis chamber and a lower flow (~ 2 ml/min) for acceptable GC resolution, a rotary pump and regulated vent valve, a regulated purge valve, liquid nitrogen cold traps and heating jackets to heat all hardware components to contact the pyrolysates.

The analyser is a HP-5890 gas chromatograph interfaced to a VG Autospec-UltimaQ mass spectrometer. This state of the art instrument offers very high sensitivity and mass resolution.

Data will be presented from a range of geochemical materials including a Sydney Basin torbanite, synthetically prepared and naturally occurring oil bearing fluid inclusions. The predominant products from these samples are straight chain aliphatic and alkylaromatic hydrocarbons in good agreement with GC-MS data from more conventional analytical methods. The results demonstrate that it is now possible to perform analytical pyrolysis at a microscopic level.