SEMIQUANTITATIVE FT-IR MICROANALYSIS LIMITS - EVIDENCE FROM SYNTHETIC HYDROCARBON FLUID INCLUSIONS IN SYLVITE

Infrared microspectrometry is an essential technique for the in situ analysis of individual fluorescent hydrocarbon inclusions. In order to calibrate this technique alkane and benzene inclusions have been synthesized in sylvite crystals below 100°C at atmospheric pressure. We show that the infrared spectra obtained on pure alkane synthetic inclusions differ from equivalent composition standards. The n-alkane and cyclohexane inclusions exhibit an intense spectral deformation characterized by a broadening, a flattening, and a waving of the peak. This spectral deformation takes place in the C-H stretching and bending infrared regions and affects the CH2 CH3 peak area ratio measurements. No spectral deformation occurs for benzene inclusions in the C-H stretching region. Raman analysis confirms that no change in composition occurs during trapping. When the dilution of alkanes in a solvent increases, the infrared spectral deformation observed for the inclusions disappears. This dilution effect is not observed for pure alkane standards. It is possible to approximate the alkyl chain carbon number (X) which is between the pure n-alkane standard value and the synthetic inclusion value: (AREA [∑CH2]/AREA [∑CH3] + 0.1)/0.27 < X < (AREA [∑CH2]/AREA [∑CH3] - 0.8)/0.09. These results can be applied to the natural environments taking into account the limits of the FT-IR (Fourier Transform-Infra Red) microanalysis of fluid inclusions. © 1990.