CLASSIFICATION OF MICROHETEROGENEITY IN REFERENCE MATERIALS USING µXRF
John L. Molloy, John R. Sieber, and Rolf L. Zeisler
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. Several examples will be presented to highlight why this problem is of interest in reference materials.
Existing research performed by other authors suggests that complex data analysis techniques like Monte Carlo modeling can be used in conjunction with µXRF to estimate elemental heterogeneity on a microscale. Equally 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 statistically treated according to its level of heterogeneity within the sample. Samples must be separated into elements with nugget effects, heterogeneous elements that do not exhibit nuggets, and homogeneous elements, with a different statistical treatment used in each case. Data from the first group conforms to a wide variety of distributions only adequately described by nonparametric statistical methods, while the second two groups can be adequately summarized using more common statistical methods, which assume normally distributed data. The intended result of these different treatments is a standard method for calculating the minimum recommended sample size of each reference material. However, the complexity of many reference materials and tendency of higher atomic number elements to affect the signals of low atomic number elements makes modeling such systems nontrivial.
In addition to µXRF, Instrumental Neutron Activation Analysis (INAA) is able to measure small samples of material in the range of 1 mg to 100 mg. INAA gives complimentary information to mXRF because the more penetrating radiation is able to thoroughly probe thicker samples containing many heavy elements. Because its sources of uncertainty are very well characterized, INAA can also identify inconsistencies such as sample changes from preparation for analysis, and it offers a different picture of each sample, giving more information on which to build a descriptive model of a reference material. INAA bridges the gap between current, bulk analysis methods and methods developed using µXRF, confirming models constructed of heterogeneous samples.
Many modern chemical analysis techniques are able to analyze very small sample sizes, an advantage which can be particularly important for expensive or difficult to obtain samples. Reference materials must be properly used for calibration or validation of such techniques to assure that accurate results are obtained. NIST makes many different reference materials, each coming with a certificate that identifies constituents of the material, their concentrations, and the minimum recommended mass of the material. However, no method currently exists to classify the heterogeneity of samples smaller than about 100 mg. Considering the amount of effort applied to furnishing accurate values for reference materials, it seems prudent to examine if these values are still correct at the sample sizes used by microanalysis techniques. In this way NIST’s reference materials can have expanded applicability and wider use.
Author: John Molloy
Mentor: John Sieber
Division: Analytical Chemistry (839), CSTL
Building and room: 227, Rm A347
Mail Stop: 8391
Membership in Sigma XI: Neither author nor mentor