The infrared transmission spectra of some hydrogen halide salts of methamphetamine are observed to display significant variations dependent upon the alkali halide matrix in which the salt is dispersed for examination. These effects appear to arise from a chemical exchange of halide ions between the methamphetamine salt and the matrix material, which can complicate the interpretation of the spectra. A strong absorption in the hydrogen-stretching region of the infrared spectrum is found to be particularly sensitive to the anion present in the salt, and changes in the appearance of this absorption band can be revealing of any chemistry between the salt and the alkali halide matrix. The analyst should be aware of this phenomenon in the forensic identification of methamphetamine salts, as well as its possible occurrence with other amine drug substances.
