Classification of Microheterogeneity in Solid Samples using µXRF
John L. Molloy, John R. Sieber
Micro X-ray Fluorescence (µXRF) has been used to nondestructively investigate elemental heterogeneity by constructing two-dimensional maps of elemental concentrations in reference materials. µXRF allows probing of sample sizes in the microgram and nanogram range, well below the 100 milligram level currently explored in reference materials by NIST. Sophisticated methods of analysis, such as principal component analysis (PCA), show promise in describing complex data patterns arising from various samples. PCA can identify if nugget effects exist within a material, a case where an element is enriched in small, isolated areas of the sample. Each element must be treated according to its level of heterogeneity within the sample as determined by comparison of mapping data to a PCA model. The model is built based on the variation of analyte signal at a single location within the sample. Examples of this analysis methodology are shown for several different reference materials including SRM 2702 Inorganics in Marine Sediment and SRM 2703 Sediment for Solid Sampling Analytical Techniques to show how PCA treatment can be used to identify which elements exhibit nugget effects within the ng and µg mass range. Additionally, a method of calculating the minimum recommended mass for solid samples is suggested. PCA methods are iteratively used on X ray maps from which adjacent data points have been averaged, simulating an increasing X ray spot size. This is repeated until the mass sampled in a map is indistinguishable from data taken at a single location suggesting no nugget effects can be detected. This methodology is demonstrated for SRM 1577c Bovine Liver to estimate a mass as low as 370 µg can be used without measurable nugget effects.