|
Referencing NIST Standard SRM 1921A, a comprehensive approach, has been developed for use by all practitioners of infrared spectroscopy, to calibrate their instruments. This approach utilizes a thin film of Poly 1-phenylethylene) Polystyrene rotationally cast, commonly on an Alkyl Halide crystal, with a radial molecular alignment. Historically, drawn polystyrene films have provided reasonable accuracy with respect to band locations documented by NIST. The principal difference between the two methods lies in their respective ways of manufacture. The rotational method exhibits radial stress patterns where drawn films exhibit parallel stress lines. Comparative studies show that a radial film is just as accurate, yet with fewer shortcomings. Greatest attention is fixed on band position, however, energy absorption at critical points is generally ignored. A radial film can be case in an array of precise pathlengths to monitor an instruments Y coordinate efficiency. Drawn films have limitations. All test films are mounted perpendicular to the directional path of an incident beam. When a conventional film, with transverse and longitudinal orientations, is rotated along a 'Z' axis, or the axis which an incident beam travels, results may differ. This is a well-documented phenomenon. When the orientation of a vibrator changes with respect to a fixed energy beam, the moment of dipole of that vibrator also changes. Due to the configuration of a radial film, there is no significant change in orientation upon rotation. This would be valuable information for anyone using infrared for quantitative purposes. Significance of the rotational method lies in accurate pathlength reproducibility in a variety of thicknesses, a unique molecular arrangement, limited control in the absorbance range, and maintained precision in band location. Lastly, it is a cost-effective measure.
|
