These training notes provide a brief introduction to x-ray fluorescence (XRF) analysis of soils. XRF has been used to characterize a broad range of materials for over twenty years. Recent advances in digital electronics and semi-conductor technology has yielded very portable XRF analyzers for field analysis of many sample types including soils. These notes will cover the following subjects:
- Introduction to XRF, basic theory of operation
- EPA Method 6200
- Field use of XRF analyzers for soil
o In-situ testing
o Prepared sample (or ex-situ) testing
- Basic quality assurance and sample preparation strategies
During the training session, most of the time will be spent performing measurements on prepared and unprepared soil samples with XRF instruments provided.
1. Introduction to XRF
Basic Atomic Structure:
A model of an atom is shown in Fig. 1. In this model, the atom consists of a nucleus occupied by protons and neutrons. Surrounding this nucleus are negatively charged particles called electrons. In this, the Bohr model of the atom, it is assumed that the electrons orbit around the nucleus of the atom in fixed orbits, much like the planets orbit the sun. While this atomic model is not phyiscally accurate, it is perfectly satisfactory to explain most of the principles encountered in x-ray fluorescence analysis. For an uncharged atom, the number of electrons equals the number of protons. For each element, the electrons are orbiting the nucleus at different energy levels. These 'orbits' or 'shells' each contain a specific number of electrons. The shells closest to the nucleus get filled first and the shells get filled from the inner-most to the outer-most shell. Shells are named with the inner-most being the K-shell, then L-shell, etc., alphabetically named. The K-shell electrons can be thought of as having the lowest level of stored energy. The further out the electron shells are, the higher the energy level they have stored (the L-shell electrons have more stored energy than the K-shell electrons, the M shell electrons have more stored than the L shell, etc.).