XRF Laboratory

Elemental analysis using the XRF technique

Introduction

The XRF spectroscopy is widely used for the qualitative and quantitative elemental analysis of environmental, geological, biological, industrial and other samples. Compared to other competetitive techniques, such as Atomic Absorption Spectroscopy (AAS), Inductively Coupled Plasma Spectroscopy (ICPS) and Neutron Activation Analysis (NAA), XRF has the advantage of being non-destructive, multi-elemental, fast and cost-effective. Furthermore, it provides a fairly uniform detection limit across a large portion of the Periodic Table and is applicable to a wide range of concentrations, from a 100% to few parts per million (ppm). Its main disadvantage is that analyses are generally restricted to elements heavier than fluorine.

The XRF method

Basic principle

The X-ray fluorescence principle is depicted in Figure 1. An inner shell electron is excited by an incident photon in the X-ray region. During the de-excitation process, an electron is moving from a higher energy level to fill the vacancy. The energy difference between the two shells appears as an X-ray, emitted by the atom. The X-ray spectrum acquired during the above process reveals a number of characteristic peaks. The energy of the peaks leads to the identification of the elements present in the sample (qualitative analysis), while the peak intensity provides the relevant or absolute elemental concentration (semi-quantitative or quantitative analysis).

A typical XRF spectroscopy arrangement (Figure 1) includes a source of primary radiation (usually a radioisotope or an X-ray tube) and an equipment for detecting the secondary X-rays.

XRF sources

The irradiation of a sample is usually performed by radioisotope sources or X-ray tubes [1-5]. The energy of the primary radiation should be higher than, but close to the binding energy of the K- and L-shell electrons of the excited atom.  

The most widespread radioisotope sources include Fe-55, Co-57, Cd-109 and Am-241. These sources emit X-rays of definite energy, therefore being capable of efficiently exciting a limited number of atoms (table 1).  As a result, to analyze a broad range of elements, a combination of radioisotope sources is necessary.

Alternatively, X-ray tubes may be used to excite the sample with characteristic or continuum X-rays. Depending on the elements to be analyzed, the anode material of the tube is selected (table 2). 

X-ray detectors

Solid state detectors (table 3) have prevailed for the secondary X-rays measurement. In particular, Si(Li) and HPGe detectors, operating under liquid nitrogen temperature, are most commonly being used due to their high resolution.    

In recent years, the development in semiconductors technology has furnished a number of small sized, thermoelectrically cooled (Peltier effect) detectors, such as HgI₂, Si-PIN, Si-DRIFT and CdZnTe. Although their resolution is inferior to that of Si(Li) and HPGe crystals, their convenient size and cooling requirements have rendered them popular in portable XRF equipment.

Calibration

Quantitative XRF analyses require calibration of the measuring arrangement, which may be performed by two major approaches: empirical and fundamental parameters (FP) calibration. 

The empirical calibration is based on the analysis of standards with known elemental compositions.  To produce a reliable calibration model, the standards must be representative of the matrix and target element concentration ranges of the sample to be analyzed. Maintaining the same sample morphology (particle size distribution, heterogeneity and surface condition) and source/sample geometry for both standard and sample measurements is essential in empirical calibrations. 

Alternatively, “standardless” FP techniques may be used, which rely on built-in mathematical algorithms that describe the physics of the detector’s response to pure elements. In this case, the typical composition of the sample must be known, while the calibration model may be verified and optimized by one single standard sample.

Detection limits

Two types of detection limits should be considered in XRF analysis: a) instrument detection limits, which represent the threshold concentration of a given element that a particular instrument can resolve and b) method detection limits, related to sample preparation and analysis time. Depending on the element to be analyzed and the sample matrix, typically achieved detection limits vary between 10 and 100 ppm.    

Sample preparation

Procedures for sample preparation vary considerably in the cases of in situ or intrusive measurements. Solid sample must be polished to assure surface homogeneity, while powders are usually pressed into pellets. In all cases, x-ray transparent supporting media should be used (polyethylene, Kapton, Mylar etc.).

XRF applications

Bibliography

[1] Kalnicky D.J., Singhvi R. (2001). Field portable XRF analysis for environmental samples. Journal of Hazardous Materials 83: 93-122.

[2] Jenkins R. (1999). X-ray Fluorescence Spectrometry. Wiley-Interscience, New York.

[3] Lachance G.R., Claisse F. (1994). Quantitative X-ray fluorescence analysis: theory and application. John WILEY & Sons, New York.

[4] Cesareo R., Gigante G.E., Castellano A. (1999). Thermoelectrically cooled semiconductor detectors for non-destructive analysis of works of art by means of energy dispersive X-ray fluorescence. NIM A 428: 171-181.

[5] Iwanczyk J.S., Patt B.E., Wang Y.J., Khusainov A.Kh. (1996). Comparison of HgI₂, CdTe and Si (p-i-n) X-ray detectors. NIM A 380: 186-192.

[6] Loupilov A., Sokolov A., Gostilo V. (2001). X-ray Peltier cooled detectors for X-ray fluorescence analysis. Radiation Physics and Chemistry 61: 463-464.

[7] Omote J., Kohno H., Toda K. (1995). X-ray fluorescence analysis utilizing the fundamental parameter method for the determination of the elemental composition in plant samples. Analytica Chimica Acta 307: 117-126.