Generally for the EXAFS technique, if the absorber concentration is very low (< 100 ppm), its contribution to the total absorption of the sample becomes negligible. Simply monitoring the photon flux before and after the sample is not sufficient to provide information on the chemical environment of the absorber. An indirect method involving the measurement of the fluorescence from the absorber is required. Under these circumstances, the absorber contribution to the total absorption is proportional to the ratio of the absorber fluorescence and the incoming flux. This approach combines the sensitivity of X-ray fluorescence with the accurate short range structural and chemical information provided by X-ray absorption spectroscopy (XAS).

K-edge EXAFS measurements of transition elements at 25 ppm absorber concentration can be performed at several synchrotron radiation sources by using energy-dispersive solid-state fluorescence detectors. However, in order to obtain good statistics, acquisition times of 1 day or more are normally necessary. Beamline ID26 has already shown that with the flux and stability of third generation sources, appropriate detectors, and by using filters, energy resolution is not so important at that concentration level. Moreover the measurements can be performed with a timescale ranging from seconds to a couple of hours [1].

Nevertheless, the ultimate detection limits can only be achieved by using energy-dispersive detectors. With a 12-element Silicon Drift Detector [2] and appropriate filters (6 absorption lengths of Ni), concentrations as low as 4.4 and 1.2 ppm of Cu (i.e. 69 and 19 µM) in hydrosulfide solutions have been measured at ID26. These concentrations were determined by inductively coupled plasma mass spectrometry (ICP-MS). Figure 32 reproduces the EXAFS data at both concentrations. The 4.4 ppm data set was acquired in about 12 hours elapsed time while the 1.2 ppm data set was acquired in about 8 hours elapsed time. With this remarkable performance ID26 is opening new possibilities in environmental and analytical sciences.

References
[1] V.A. Solé, C. Gauthier, J. Goulon, F. Natali. J. Synchrotron Rad., 6, 174-175 (1999).
[2] E. Moguiline, C. Gauthier, G. Goujon, J. Goulon, M.O. Lampert, P. Dressler, R. Henck. J. Phys. IV France, 7, 339-340 (1997).

Principal Publication and Authors
J.F.W. Mosselmans (a), R.A.D. Pattrick (b), J.M. Charnock (a,b) and V.A. Solé (c), Mineralogical Magazine, 63, 769-772 (1999).

(a) CCLRC Daresbury Laboratory (UK)
(b) Department of Earth Sciences, University of Manchester (UK)
(c) ESRF