FTIR vibrational spectroscopy is a technique which is widely used in characterising defects in semiconducting crystals. The approach has usually been to measure the line strengths of the localised vibrational modes (LVMs) in a series of samples in order to identify the defect centres from which they arise. With improvements in instrumental resolution and detectivity, the emphasis is now on analysing the structure and measuring the widths of LVM-lines. The width of these lines is limited to ~1 cm-1 for samples at room temperature but on cooling to below 100K some lines will be considerably sharper. The width is then limited by both the isotopic distribution of the nearest neighbours of the impurity atom and the internal strain present in the sample. It is this relationship which we shall examine here, generating internal strain by fast particle irradiations in samples which have a very small intrinsic internal strain. Consider carbon impurity in GaAs situated on the As lattice site; its four next nearest neighbours will be Ga atoms. Naturally occurring Ga has 60% of isotope of mass 69 and 40% of mass 71, so different line positions will result from the various combinations of isotopes of the nearest neighbours of the C atom. This structure, first observed by Theis et al.1 for the CAs line at 583 cm-1 has been satisfactorily fitted using a simple model2. If it is assumed that lattice strain is a measure of crystal quality, then the extent to which the anticipated structure is present on a line, such as the CAs LVM, can be used as a measure of the quality of the crystal. Thus, provided all measurements are made at the same temperature and spectroscopic resolution, it is possible to assess crystal quality in terms of an equivalent dose of radiation on a crystal of low internal strain which would produce the same,degradation of line structure. It has been shown previously 3,4 that after electron irradiation of carbon doped samples there is a reduction in the integrated absorption from CAs acceptors due to the capture of an arsenic interstitial and a corresponding growth of lines on either side of the CAS line due to the carbon complex formed. The irradiation induced lines, previously designated C(1)-lines are at 577 and 606 cm-1. Although the samples used for the previous measurements contained much more carbon than the present sample, they were of inferior quality, with the result that some of the structure which might otherwise be present was broadened out due to the intrinsic internal strain.