This instrument development project was aimed at developing an x-ray spectrometer configuration for the x-ray fluorescence microprobe optimized for the detection of precious metals in geological and biological samples with the objective of improving detection limits and reducing interference from major elements and the scattered beam. The approach used Bragg diffraction from a surface shaped to a log-spiral to focus x-rays of a particular energy onto a solid-state detector. The result is an enhancement of the precious metal lines within the relatively narrow bandpass of the Bragg crystal surface and suppression of interfering elements and detector artefacts, such as tailing and escape peaks arising from intense elemental lines and scattered photons. This combined enhancement resulted in a factor of 3 improvement in the count rates at the optimized energy. Combined with the reduction in background, this results in an order of magnitude improvement of the Au detection limits for samples of geological significance. Copyright © 2007 John Wiley & Sons, Ltd.