CALIBRATING RAMAN SPECTROSCOPIC SYSTEMS FOR QUANTITATIVE ANALYSIS OF GEOLOGICAL FLUIDS USING STANDARDS PREPARED IN FUSED SILICA CAPILLARIES
The first type of standard is the most reliable in terms of composition and total pressure, but it is bulky and not convenient for inter-laboratory calibration. The second type of standard is reliable and compact, but currently the total pressure must be less than 4 MPa due to the pressure limit of the cold seal. The third type of standard is small, normally less than 3 cm in length, but the compositions of mixed gases and their total pressures must be estimated after sealing the capsule.
After calibrating our Raman spectroscopic system with some of these standards, we were able to determine, for example, (1) the pressures of CH4 in fluid samples (Lu et al., 2007, GCA, 71, p. 3969), (2) the diffusion coefficient of CH4 in water at room temperature (Lu et al., 2006, Appl. Spectr., 60, p.122), and (3) the solubility of methane hydrate in water (Lu et al., 2008, GCA, 72, p. 412). It should be emphasized that every Raman spectroscopic system needs to be calibrated before it can be used for quantitative analyses of fluid mixtures. Fluid standards prepared in fused silica capillaries are reliable for calibration of Raman systems and small enough that they can be used for inter-laboratory comparisons.