The spectrum of a polystyrene film of suitable thickness, normally between 0.03 mm and 0.05 mm, is recorded. This includes maxima at the following frequencies, expressed as wavenumbers in cm-1: 3027, 2851, 2924, 1944, 1871, 1802, 1601, 1583, 1181, 1154, 1069, 1028, 907, 699. Acceptable tolerances are ± 8 cm-1 for the range 4000-2000 cm-1 and ± 4 cm-1 for the range 2000-600 cm-1.
The difference between the percentage transmittance of the absorption minimum at 2870 cm-1 and that of the absorption maximum at 2851 cm-1 should be greater than 18 and the difference between the percentage transmittance of the absorption minimum at 1589 cm-1 and that of the absorption maximum at 1583 cm-1 should be greater than 12.