PREPARATION AND CHARACTERIZATION OF DRIED-RESIDUE CALIBRATION STANDARDS FOR USE IN MICROSAMPLE X-RAY ANALYSIS (MXA), E.C. Anderson, B. Shumway and D.C. Turner, pp. 456-464

Microsample X-ray Analysis (MXA) is one of the most sensitive XRI? methods using conventional energy-dispersive or wavelength-dispersive spectrometers. Detection limits in the parts-per-billion are being routinely achieved using standard laboratory instruments with only minor modifications, In the technique, a droplet of sample-typically 50 microliters-is evaporated onto a thin film substrate and the resulting residue is analyzed for trace elements. Quantification in XKF can be accomplished by comparison to lurown standard reference materials of similar matrix to the sample or by using fundamental parameters methods. Up to now there have been no commercially-available reference materials for MXA analysis. This work describes the preparation and characterization of a set of reference standards for MXA. The standards were prepared as dried residues by evaporation from multi-element standard reference solutions. The mix of elements was chosen to be compatible within each solution and to minimize spectral overlaps. In order to match the matrix of the resulting sample residue, standards were prepared in both ultrapure nitric acid and in an acidified natural water matrix. The trace element concentration ranges from 0 to 1000 ppb. Since the background signal is dependent upon the thin film support, standards were prepared on both APITM and Prolene@ films, Accuracy of the standards was determined by comparison to NIST singleelement SRM solutions. To determine whether individual elements were lost due to volatilization during drying and to check for interferences from the residue matrix, the fluorescent intensity of each element was compared to the theoretical fluorescent yields. An estimate of the standards’ precision was determined by preparation of a large number of replicate sites using manual sample depositions and also with an automated sample deposition robot. Stability of the standard residues was monitored over a period of twenty months.