Dr. S. Arzhantsev (Center for Drug Evaluation and Research, US Food and Drug Administration, St. Louis) and his colleagues have published some very interesting research. The research group is engaged in the determination of toxic metals in pharmaceutical materials using hand-held XRF spectrometers. It is extremely important with respect to toxic metal contamination to establish a reliable technique for classifying a large number of samples. As the procedure is basically a kind of pattern recognition, the problem that needs to be overcome is finding a suitable filter for signals and noises in XRF spectra. The research group chose a continuous Wavelet transform, which is an extension of short-time Fourier transform (STFT) and is capable of constructing a time-frequency representation of a signal that offers very good time and frequency localization. In the paper, they discussed the comparison of the signal-to-noise ratios at the energies of the elements of interest obtained by wavelet filtering and those obtained by the conventional empirical method. The results were evaluated in a collaborative study that involved 5 different hand-held XRF spectrometers used by multiple analysts in 6 separate laboratories across the United States, leading to more than 1200 measurements. The detection limits estimated for arsenic, lead, mercury, and chromium were 8, 14, 20, and 150 μg/g, respectively. For more information, see the paper, "Rapid Limit Tests for Metal Impurities in Pharmaceutical Materials by X-ray Fluorescence Spectroscopy Using Wavelet Transform Filtering", S. Arzhantsev et al., Anal. Chem., 83, 1061 (2011).
