Precious metals assay by means of microbeam XRF technology

There have been increasing demands for a means whereby the transactors and dealers in jewelry can accurately and efficiently appraise the precious metal contents on the premises. These values of such articles cannot be obtained accurately or nondestructively by conventional methods, such as specific gravity measurement, touchstone method, fire assay method, atomic absorption spectrometry and the like. A new method of microbeam X-ray fluorescence analysis (Microbeam XRF) has been developed to analyze the type and quantity of precious metal elements. In this method a beam of X-ray generated from a low powered X-ray tube is collimated by a collimator with a diameter of 0.1-5 mm. An object to be analyzed is positioned by moving a three-dimensional sample stage till the sample can be observed clearly with optical microscope. The X-ray beam will coincide with the viewing point on the surface of the sample. The X-rays produced are detected in a high efficient proportional detector. Its spectrum, whose scattering background is lower than that with the broad beam method, is recorded and analyzed by a multichannel analyzer. The intensities of selected peaks corresponding to target elements are then determined and the relative compositions of the elements are also calculated by a fundamental parametric method. Two sets of standards (No. SYS-01) were measured by the instrument. It showed that the precision of gold measurement was 0.1% and the correlative coefficient between the measurement results and the given values of the standards was better than 99.998%. A piece of gold necklace was also checked step by step. The microbeam XRF is a reliable and accurate method for precious metals jewelry assay.