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.

Resolution Tests

Resolution tests are prescribed in some pharmacopoeias to facilitate comparisons with published reference spectra. These tests measure the depth of the trough between pairs of incompletely resolved peaks. This makes the results dependent on the lineshape, not just on the width at half height of isolated bands, which is the basis for published instrument specifications. The lineshapes of spectrometers from different manufacturers may differ for the same nominal resolution because of different choices of default apodization function. Although these differences are not significant for the purpose of identifying materials, they can make the difference between passing and failing a resolution test. For example, at 4 cm^-1 resolution the band at 1601 cm^-1 has a width at half height of approximately 10 cm^-1. Changing the apodization from medium Norton-Beer to strong Norton-Beer increases this by less than 0.3 cm^-1 but reduces the depth of the trough at 1583 cm^-1 by 12%.

Figure 2: Regions of polystyrene spectrum indicating the points used for resolution tests.

Figure 3: Polystyrene at 4 and 8 cm^-1 resolution.

Table I: Tests for 4 cm^-1 resolution

Figure 4: Spectra of two polystyrene films with different thicknesses from the same instrument manufacturer.

Table II: Trough depths in percent transmission at 2850 cm^-1 for 11 polystyrene films before and after baseline subtraction

Possible Improvements

It was necessary to test the resolution of dispersive spectrometers at different frequencies because they used multiple gratings and varying slit widths. However, this is unnecessary with FT spectrometers because a single test is sufficient.

Tests in absorbance are very sensitive to sample thickness. It would be preferable to specify a minimum film thickness as the current specification of "about 0.035 mm" falls between commercial products at 0.0318 and 0.0381 mm. Our experience is that films thicker than 0.035 mm do not cause problems. It is rather difficult for a user to measure the thickness of a film that does not show interference fringes, so it is certainly advisable to use only films of known thickness.

Tests in transmission are sensitive to scattering as well as to sample thickness. The effects of scattering can be eliminated by applying a baseline subtraction that forces the transmission values to 100% T at 4000 and 2050 cm^-1. Alternatively, suitable films could be identified by specifying a minimum value such as 80% T for the transmission at 4000 cm^-1.

Summary

Although a polystyrene film is useful for routine checking of FT-IR spectrometer performance, variations in film thickness and scattering can cause large variations in the results for the resolution tests described in pharmacopoeias. The test using the peak at 2850 cm^-1 is insensitive to resolution but extremely sensitive to film thickness. It should not be used for FT-IR spectrometers.

R.A. Hoult, B. Perston, and R.A. Spragg are with PerkinElmer LAS in Seer Green, UK.

References

(1) European Pharmacopoeia 6.0 Chapter 2.2.24 – Absorption Spectrophotometry, Infrared (The European Pharmacopoeia, Strasbourg, France, 2007).

(2) Indian Pharmacopoeia 2010, Chapter 2.4.6 – Infrared Absorption Spectrophotometry (Indian Pharmacopoeia Commission [IPC], Raj Nagar, Ghaziabad, India, 2010).

(3) Japanese Pharmacopoeia 15th edition 2008, Methods of Analysis – 1.7 Spectrophotometry in the infrared region (Japanese Pharmacopoeia, Japan, 2008).

(4) Chinese Pharmacopoeia 2005, Volume 1, Appendix V C (The Pharmacopoeia of the People's Republic of China [PPRC], 2005).

(5) Standard Reference Material 1921b (National Institute of Standards and Technology [NIST], Gaithersburg, Maryland, 1995).

(6) The Dow Chemical Company.