The incoherently and coherently scattered X-ray intensities with the corresponding scatter factors have been used previously define in a sample matrix the light elements (Z<12) constituents which can not be a s sessed via their fluorescent radiation in conventional XRF. Recently, a more complete sample definition has been achieved based on the backscattered X-rays, by using two excitation modes. However, all quantitative EDXRF methods, based on the scattered radiation from the sample relied hitherto uniquely on the use of monochromatic excitation radiation.

In the spectroscopic analysis of composite elements by x-ray fluorescence (XRF), it is the fundamental feature of this method that the background x-ray intensity is lower than that with electron excitation. However, the background x-rays of this method, which consist of Thomson (Rayleigh) and Compton scattered x-rays from the primary radiation, impair the analytical performance at the ppm level. In order toinvestigate the intensity of the background x-rays precisely,the study was conducted in two parts. The first part compared the measured and theoretically calculated x-ray intensities for Rh Kα and Rh Kβ peaks from various materials. The second part examined the determination of low concentrations of lead,arsenic and colonium in steel samples. The variation in the background x-ray intensities of the analyzed elements was found to be caused by the variation of the major elements and a correction equation for it is derived.

Automation of X-ray fluorescence equipment has proliferated during the last 10 years. However, the focus has been on data collection and data massage. Data presentation has been limited to a few rudimentary formats. These formats ‘ generally involve printing the results in some arbitrary fashion, followed by manual transfer to the client.

This paper describes a system (using rudimentary artificial intelligence techniques) that automates data presentation. It handles data from 14 different methods; evaluates the data against the requirements of the method (i.e. mass balance, detection and reporting limits, and matrix interferences). Further, a data report is generated in a consistent format, including the reporting of significant figures. Additionally, an exception report is printed when the measured results are outside the applicable range of the method or violate quality assurance constraints. In such cases, alternative methods are recommended in the exception report.

Statistical Process Control (SPC) methods have become extremely popular in quality control and process improvement in the manufacturing environment, first in Japan and more recently in the U.S. They are gradually being introduced into the research and development setting as well, where they offer some advantages over more traditional statistical approaches. The greatest advantage of the SPC approach is that it ensures that the statistical description generated is valid, i.e., that the data on which it is based incorporate only random error. This is verified both initially and on a continuing basis. This means that the results provide valid predictions of future performance, not just a description of an historical set of replicate samples. SPC statistical evaluations of analytical results and instrument performance require only simple calculations and are easily applied in a consistent, valid manner by statistically unsophisticated analysts. Besides the numerical statistical analysis tools, the SPC approach provides other problem-solving techniques for improvement of the entire analysis process. These aspects of the SPC approach will be described and illustrated with examples from our x-ray fluorescence analysis laboratory.

Quantitative XRF analyses almost always depend on measurements of x-ray emission intensities of standards of known composition. The usual approach is a careful initial standardization with multiple standards lhat are checked for accuracy, stability, homogeneity, and internal consistency. This is followed by limited periodic restandardization or recalibration with one or two of the initial standards or a reference sample. The frequency and protocol of these periodic restandardizations tend to be rather arbitrary; for example adjustment on the basis of the average of a triplicate measurement of a standard at the beginning of the work day. There are certainly a large number of other possible restandardization strategies. They range all they way from no restandardization at all, through single or n replicate restandardization measurements before analysis of each group of M unknowns, to cumulative or moving averages, again with variable replicate size, frequency, and now averaging range. This paper examines the outcomes of some of these restandardization strategies applied to synthesized, realistic XRF analysis data which incorporate both abrupt and gradual changes in instrument response. SPC (Statistical Process Control) control charts are shown to be a very useful tool for determining optimum restandardization strategies. Optimal strategies (least scatter in calculated concentrations with fewest standards) which emerged from this study included not restandardizing at all except when indicated by an SPC chart of a reference standard intensity run concurrently with each unknown, and a moving range of 3, with standards run every 5 unknowns, coupled with SPC monitoring of the standards used for this restandardization.

Sporadic displacements of substantial magnitude were observed during routine analysis in the concentration intercepts for some trace elements, together with a similar change in the sensitivity (slope) for magnesium when determined using a synthetic multilayer. These effects were traced to the electronic circuit providing correction for pulse amplitude shift with intensity.

Corrections for scattering on XRF intensity have commonly used integral approaches and isotropic approximations, which give a limited knowledge on this subject. An exact theoretical expression for XRF enhancement due to coherent and incoherent scattering is presented in this paper. It has been obtained as a solution of the Boltzmann transport equation for photons. This equation describes the diffusion of photons in an infinite homogeneous target. Both the excitation and the emission beams have been assumed to be well collimated. The solution accounts for the mixed, scattering and photoelectric interactions. Complete kernels including atomic form factors have been used for both types of scattering. An appropriate kernel has been written for the photoelectric interaction. The correction is computed for several pure elements and is compared with isotropic approximations.

XRF intensity computations, for polychromatic x-ray sources and multicomponent samples, need a high number of low speed floating-point operations, making them a hard bone for any computer. The evaluations of the Intensity expression require the computation of complex attenuation coefficients. They involve also a spread quantity of atomic data for a high number of elements. And the users need to insert additional information on the experiment, like the excitation-detection geometry or the x-ray tube spectrum, interactively. Personal computers are an optimum target for this kind of application mainly due to the low cost of the computation time and the possibility to display high-resolution graphics. Moreover the increasing power of microprocessors makes it possible to attack hard computations like this on a PC.

A theoretical expression for the fourth-order XRF-intensity has been found using the transport theory. An exact solution of the Boltzmann equation for the photon flux in a semi-infinite homogeneous medium has been applied iteratively to find the intensity, XRF emission has been isolated from the scattering processes by considering only the photoelectric effect in the interaction kernel. The incident and emitted beams have been assumed as well-collimated.

Fourth-order intensity computations for a multicomponent material are reported as an example. Results show that the fourth-order intensity remains below a tenth of the tertiary and therefore may be neglected in most cases.

A method to measure low level iodine concentrations by means of a Total Reflectance X-Ray Spectrometer (TXRF) was presented at the 1986 and 1988 Denver X-Ray Conferences (Ref. 1 and 2.). The optimum set-up of the spectrometer for this purpose was obtained by using a Cu-anode tube. The collimated beam was two times totally reflected: first on the mirror and then on the synsil sample substrate. By these means the undesired high energy bremsstrahlung was almost eliminated. This operational mode is very convenient for iodine measurement by L-lines. The detection limit of iodine was as low as 100 pg.

For simultaneous measurement of several elements (I2<Z<83) it was necessary to change the set-up of the spectrometer.

The need for process control has led to the development of improved instrumentation for measuring chemical contents of process streams. In many instances, on-line X-ray Fluorescence (XRF) analysis represents a straightforward method to accomplish these kinds of analyses. The sample preparation is often easy compared to many other methods and the XRF analyzer can be calibrated with process stream or artificial standards. Results are obtained in a time frame suitable for closed-loop control strategies. The Courier 10 is a new, compact on-line XRF analyzer which has been developed for chemical and petrochemical process applications such as sulfur and/or lead in gasoline; calcium in polymers; and chrome, iron ,nickel in metal treatment baths.

An x-ray fluorescence macroprobe was built for intermediate-scale compositional mapping to bridge the gap in spatial resolution between bulk x-ray fluorescence and electron beam methods. The macroprobe was optimized for quantitative whole rock mapping on a millimeter scale to evaluate changes in bulk composition of fine-grained mineral aggregates.

In many applications of energy-dispersive XRF analysis, quantitative information concerning the chemical composition of the samples is not required. Rather, one is interested in whether a given sample is similar to some reference material or whether the chemical composition is changing from one sample to the next. We have investigated the use of pattern-recognition techniques in such applications. It will be demonstrated with experimental data that the pattern-recognition approach is extremely simple and fast. It uses only a single parameter, the normalized correlation coefficient, and can be applied directly to raw data. The efficacy of the method is illustrated with Si(Li) spectra of geological and pigment samples, and proportional counter spectra of geological samples. The pattern-recognition method should be ideally suited for field XRF applications, and the algorithm can be easily implemented on a personal computer.

X-ray windows are used in sources and detectors to separate the neighborhood of the x-ray generation or detection from the use environment. While each use has its own requirements, there are some principles that should be used in designing an optimal x-ray window. Because x rays are absorbed to some extent by all materials, minimizing absorption is one criterion in preparing windows. Also, for most uses there is a pressure difference across the window so that ensuring pinhole-free structure and sufficient mechanical strength to support the differential is another criterion for window design. Traditionally, absorption, is minimized by fabricating the window with lower atomic number (low Z) elements such as Be, B or C. However, the wavelength (energy) region of interest becomes a very important complicating factor.

By utilizing the total reflecting properties of x-rays inside glass capillary tubes, intensive fine- or microbeams of x-rays can be attained. By using a conventional x-ray tube in combination with capillary technique, microbeams with a diameter of a few μm and with an intensity sufficient for trace elements, analyses have been achieved. By moving the sample, lineand areascan can be performed and trace element information can be directly superimposed on a micrograph from a built-in optical microscope.

The background of the capillary technique is reviewed and an EDXRF microbeam spectrometer for trace elements is described. Examples of applications for the microbeam spectrometer are given.

As the Environmental Protection Agency adds ever more sites to the National Priorities List for Superfund clean-up, there is a tremendous need for a fast method of screening for contaminants. In the case of inorganics, we can apply field-portable X-ray fluorescence technology.

With commercially available field-portable X-ray fluorescence equipment using in-situ measurements we can quickly screen a site (hundreds of measurements), input the data to a portable computer, process it, and print a colored concentration isopleth map of the contaminant of interest, all in realtime.

The in-situ measurement approach pioneered by Lockheed is the key to rapid screening capability. Problems arising from particle size distribution, soil heterogeneity, and bulk density are minimized by employing site-specific standards in the construction of our calibration curves. Inherent in these site-specific standards are all the matrix problems which occur in the routine samples.

The strength of field-portable X-ray fluorescence technology is the accuracy of analysis above the quantitation limits. The difficulty in measuring low concentrations near the detection limit is the weak point of field-portable units.

NOTICE: Although this research was funded in part by the U.S. EPA through Contract 68-03-3249 to Lockheed Engineering & Sciences Company, it has not undergone Agency review and does not necessarily reflect Agency policy.

Recent years were marked by a rapid increase of interest in the ase of x-ray fluorescence for on-site chemical characterization of contaminated soils and waste materials.

The known advantages of the XRF technique, such as capability of simultaneous, multielement analysis, speed of measurement (10 to 100 seconds), little or no sample preparation, wide concentration range, “portability,“ and ease of integration into environmental monitoring station(s), make it specifically suited for this application.

Major difficulties still inhibiting wider use of the XRF for hazardous waste screening stem from the lack of suitable calibration samples and proper protocols, as well as from the low awareness of the industry about the technique.

Effective assessment and remediation of hazardous waste sites dictates that analytical methodologies be developed which assist in the evaluation of site contamination and simultaneously make efficient use of sampling time and resources (1). Optimally, a technique would provide on-site personnel with immediate and accurate information concerning the identity and concentration of inorganic soil contaminants (2).

“HAZARDOUS WASTE SITE” - What picture do those words bring to mind? Most people visualize a remote area behind a tall chain-linked fence with signs prohibiting entrance and warning of the dangers within. For these sites already identified as hazardous waste sites, access is limited, and all who enter must wear appropriate protective clothing when working on the site. However, a yet to be identified “hazardous waste site” may be as close by as the back lot of a manufacturing facility in your town. Or, it may be a vacant lot in your neighborhood or even your own property, land on which children play and vegetable gardens grow. Thus, those involved in locating hazardous contamination and determining the extent of that contamination are not only with the U.S. Environmental Protection Agency and its contractors, but are also with state and local departments of health as well.

Freshwater mussels (Mollusca: Unionidae) are filter feeders that are relatively immobile, widely distributed and are known to concentrate trace metals in their shells (1,2,3). These characteristics make them good candidates for monitoring metal pollution in lakes and rivers. Another characteristic of mussels that make them particularly attractive as pollution monitors is the fact the shell is deposited in distinctive, annual growth layers. The concentrations of metals in these shell layers may provide a history ol the metals present in the environment where the mussel was collected.