A NEEDLE IN THE X-RAY HAYSTACK: DETECTION LIMITS IN POWDER X-RAY DIFFRACTION OF GEOLOGIC MATERIALS

Powder X-ray diffraction (XRD) is a powerful forensic tool for the identification and characterization of geological materials. It is suited for bulk samples, is non-destructive, and less subjective than other methods (e.g., PLM or TEM). A perceived limitation of XRD is its relatively high detection limits best summarized by Saferstein (2001): “The technique is suitable for identifying the major constituents of a mixture, but it often fails to detect the presence of substances comprising less than 5 percent of a mixture.” In studies performed in the X-ray Diffraction Laboratory at the University of Idaho we have demonstrated detection limits as low as 0.05% (500 ppm) for the determination of amphibole in samples of vermiculite and chrysotile. Both studies were performed on a Siemens D5000 XRD. The first study (Sanchez and Gunter, Am. Min., 2006) involved the determination of amphibole content in expanded vermiculites from Libby Montana. Five samples known to have originated from Libby were compared to a vermiculite sample with no detectable amphibole. The non-Libby sample was spiked with 100, 500, 1000, 2500, 5000, 7500, and 10,000 ppm of an amphibole from Libby to determine the detection limit. All samples were prepared as back-filled powder mounts. Two scans were performed: one long scan from 2° to 45° at 9sec/step, and a second short scan from 9.5° to 11.5° at 180sec/step specifically over the 2θ region containing the 110 amphibole peak. The 110 amphibole peak was undetectable below 1000 ppm in the spiked non-Libby sample thus defining the detection limit. The five Libby vermiculite samples had detectable amphibole ranging from 1171 ppm to 9218 ppm. A second investigation (Gunter, et al., Can. Min., 2007) determined detection limits of amphibole in chrysotile ore. Chrysotile ore with no detectable amphibole was spiked with tremolite at five levels (100, 500, 1000, 5000, and 10,000 ppm), homogenized, and run as back-filled powder mounts. 2θ scans were performed to detect the 110 and 310 tremolite peaks. The 110 peak is observed in the 500 ppm spiked sample and possibly in the 100 ppm spiked sample. The results of both studies indicate that the detection limits for XRD are lower than commonly thought. These results should be of some interest to the Forensic community.