Detection of Impurities in Organic Crystals by High-Accuracy Terahertz Absorption Spectroscopy.

Quantitative detection of impurities in organic crystals was demonstrated by accurately measuring absorption frequencies using a continuous wave gallium phosphide terahertz spectrometer. THz spectra of l-asparagine monohydrate doped with l-aspartic acid at 0.05 to 12.5 wt % were obtained at 10 K. The three lowest frequency absorption peaks were baseline-resolved, allowing them to be examined independently. Using a least-squares curve fitting technique, impurities were detected at levels as low as 500 ppm. The sensitivity and detection limits of the technique depended strongly on the nature of both the host and the impurities. The projected limit of detection using the current system, given optimal materials, was estimated to be 51.7 ppm. In addition to quantitative assessments, impurities may also be identified by comparing frequency shifts of multiple absorptions.