Trace element determinations using a 15-keV synchrotron X-ray microprobe.

At the Synchrotron Radiation Source (SRS), Daresbury, U.K., a synchrotron microprobe was constructed, in order to create an instrument capable of analyzing at the ppm or in favorite cases sub-ppm level with a lateral resolution of 10 x 15 microns2. In order to span a wide range of elements to be analyzed, a beam energy of 15 keV was chosen. Focusing and monochromation of the white beam was done in one single step with a high-precision ellipsoidally concave curved Si(111) crystal. Sufficient flux of X-rays in a narrow energy band is available in the spot to measure trace elements at the femtogram level. Measurements on standard materials, reference standard materials, and biological samples showed the lower relative minimum detection limits and higher sensitivity for the higher Z-elements obtainable with micro-SXRF (synchrotron X-ray fluorescence) as compared with microtechniques using ion accelerators. Moreover, the much lower energy deposited in the specimen represents a major argument to prefer X-rays to ions for the analysis of radiation-sensitive samples.