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.
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.
(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.