Introduction - Determination of Anions in Natural and Treated Waters - page 29

characteristic X-rays from the elements in the specimen. The most commonly used
source of such X-rays (in the 2-100keV range) are radioisotopes and X-ray tubes. An
EDXRF spectrometer such as the XR300 uses a compact, low power (10-100W typical)
X-ray tube capable of delivery of X-ray photons with a maximum energy of 30 or 50keV.
Why is the technique referred to as 'energy-dispersive' XRF?
The classical XRF spectrometer which has been commercially available since the 1950s
uses crystal structures to separate (resolve) the X-rays emanating from the fluorescence
process in the irradiated specimen. These crystals diffract the characteristic X-rays from
the elements in the specimen, allowing them to be separated and measured. The
characteristic fluorescent X-rays are said to have been separated from each other by the
process of 'wavelength dispersion' (WDXRF). Each element emits characteristic lines
which can be separated by WDXRF before being individually counted. For each line and
diffracting crystal, we can set a detector at a particular angle (from the Bragg equation)
and collect X-rays, which are primarily from the selected element.
The EDXRF system uses the Si(Li) (lithium-drifted silicon) detector to simultaneously
collect all X-ray energies emitted from the specimen. Each detected X-ray photon gives
rise to a signal from the detector. The magnitude of this signal is proportional to the
energy of the detected X-ray and when amplified and digitised can be passed to a multi-
channel analyser which displays a histogram of number of X-rays (intensity) against
energy. The incident photons, therefore, have been electronically separated (dispersed)
according to their energy. The energy of each of the X-rays from all the elements is
readily accessible from published tables.
Due to the simple spectra and the extensive element range (sodium upwards) which
can be covered using the Si(Li) detector and a 50kV X-ray tube, EDXRF spectrometry is
perhaps unparalleled for its quantitative element analysis power.
Qualitative analysis is greatly simplified by the presence of few peaks which occur in
predictable positions and by the use of tabulated element/ line markers which are
routinely available from the computer-based analyser.
To date, the most successful method of combined background correction and peak
deconvolution is to use the method of digital filtering and least squares (FLS) fitting of
reference peaks to the unknown spectrum [13]. This method is robust, simple to automate
and is applicable to any sample type.
The combination of the digital filtering and least squares peak deconvolution method
and empirical correction procedures has application throughout elemental analysis. This
approach is suitable for specimens of all physical types and is used in a wide selection of
industrial applications.
1.1.8.4 Total reflection X-ray fluorescence spectrometry
The major disadvantage of conventional energy dispersive X-ray fluorescence
spectrometry has been poor elemental sensitivity, a consequence of high background
noise levels resulting mainly from instrumental geometries and sample matrix effects.
Total reflection X-ray fluorescence (TXRF) is a relatively new multi-element technique
with the potential to be an impressive analytical tool for trace-elemental determinations