Several pharmacopeias propose two tests for the resolution of Fourier transform infrared (FT-IR) spectrometers using films of polystyrene. These tests are updated versions of tests that were originally developed for dispersive instruments with the result that one of the tests is unsuitable for an FT spectrometer. In some cases, the tests are defined in absorbance, making the results dependent on sample thickness. In others, the tests are in transmission so that the results depend on the scattering properties of the sample used. Because the requirements for these properties are not defined with sufficient precision in the pharmacopeias, instruments can fail tests simply because of using an unsuitable film. Here, we illustrate these problems and suggest improvements to the tests.
Polystyrene films have long been used for checking the general performance of infrared (IR) spectrometers and specific tests for wavenumber accuracy and resolution based on such films are enshrined in various pharmacopoeias (1–4). The purpose of this article is to draw attention to some limitations of the tests for resolution proposed in several pharmacopoeias.