Calibration Raman shift scale

Direct calibration of the Raman-shift scale has much to commend it, but this approach also has limitations that cause it to fail in achieving the accuracy required for the quantitative applications. A number of materials have been proposed for use as Raman-shift standards. A recent ASTM study considered eight naphthalene, l,4-bis(2-methyl-styryl)benzene, sulfur, 50/50 toluene-acetonitrile, 4-acetamidophenol, benzonitrile, cyclohexane, and polystyrene [2]. The limitations are several. The shift values are not known with sufficient accuracy. (Of the 25 lines reported for acetamidophenol, only 9 are reported with a standard deviation of less than 0.5 cm For low resolving power, wide spectral coverage systems there are too few resolved lines to provide accurate calibration. Finally, there is the question as to how one incorporates such material for routine calibration of a real-time measurement system. 

The K coefficient of the lubricant Raman vibrational mode (usually the most intense one) is first calibrated in a high pressure hydrostatic cell. The pressure in the contact is determined by measuring the frequency shift of the corresponding Raman band. The time-scale over which a vibrational mode adapts its frequency is several orders of magnitude shorter than Iwth the lubricant transit time in EHD contacts and the time required to calibrate the frequency shift at a given hydrostatic pressure. 

For monitoring the pressure in anvil cells we use the frequncy shift of internal, chemically inert pressure calibrants. For Raman spectroscopic measurements, the most commonly used method is based on the pressure-induced frequency shift of the fluorescence line of a small piece of ruby that is placed in the sample compartment of the cell, next to the sample [1]. For infrared spectroscopic measurements, we have developed a quartz pressure scale [9], a BaSO pressure scale [10], and an HOD pressure scale [11], In the case of the first two techniques, a small amount of powdered quartz or BaSO powder are placed in the sample hole on the gasket, together with the sample under investigation. The infrared spectra of quartz or BaSO, which are relatively simple, are recorded simultaneously with the spectrum of the sample and the pressure on the sample b then determined from the frequency shift of the infrared bands of quartz or BaSO. The HOD pressure scale was developed specifically for aqueous solutions. In this case, the pressure in solution is determined from the frequency shift of the uncoupled O-H stretching band of residual HOD in DjO solutions, or from the uncoupled O-D stretching band of residual HOD in HjO solutions [11]. 

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