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.
Figure 1: Spectra of polystyrene films supplied by instrument manufacturers. The transmission at 4000 cm-1 varies between 71% and 85% because of differences in scattering. Interference fringes are clearly visible in one spectrum
and are present at lower levels in two others.
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Instrument manufacturers supply polystyrene films for routine checks of their spectrometers. The films have a matte finish
that reduces the amplitude of interference fringes but also increases scattering. They vary significantly in thickness, scattering
properties, and even chemical composition. Spectra of a number of such films are seen in Figure 1. The differences are unimportant
for wavenumber accuracy tests because the tolerances in the tests, typically ±1 cm-1, are much greater than the variations seen between different commercial polystyrene films. If required for compliance, calibrated
polystyrene wavenumber standards are available from sources such as the National Institute of Standards and Technology (NIST)
(5). However, the resolution tests are more problematic because they are sensitive to both the thickness and the scattering
properties of the films, as well as any residual fringes. A practical consequence of this is that instruments may fail resolution
checks simply because of an unsuitable choice of test film.
Experimental
Spectra were measured on a PerkinElmer Frontier FT-IR spectrometer. The resolution was 4 cm-1 except where indicated, using medium Norton-Beer apodization. Eight different films were obtained from three instrument
manufacturers. The thickness of some of the films was quoted as 0.038 mm and from relative band intensities the thickness
of all films was estimated to be between 0.03 and 0.038 mm. In the spectra of some films there is a band at 964 cm-1 that is associated with the presence of butadiene.
