Introduction - Determination of Anions in Natural and Treated Waters - page 30

for a variety of sample types. The fundamental advantage of TXRF is its capability to
detect elements in the picogram range in comparison to the nanogram levels typically
achieved by traditional energy-dispersive X-ray fluorescence spectrometry.
The problem in detecting atoms in the nanogram per litre or submicrogram per litre
level is basically one of being able to obtain a signal which can be clearly distinguished
from the background. The detection limit being given typically as the signal which is
equivalent to three times the standard deviation of the background counts for a given unit
of time. In energy-dispersive X-ray fluorescence spectrometry the background is
essentially caused by interactions of radiation with matter resulting from an intense flux
of elastic and Compton-scattered photons. The background, especially in the low-energy
region (0-20keV), is due in the main to Compton scattering of high-energy
Bremsstrahlung photons from the detector crystal itself. In addition, impurities on the
specimen support will contribute to the background. The Auger effect does not contribute
to an increased background, as the emitted electrons, of different but low energy, are
absorbed either in the beryllium foil of the detector entrance windows or in the air path of
the spectrometer.
A reduction in the spectral background can be effectively achieved by X-ray total
reflection at the surface of a smooth reflector material such as quartz. X-ray total
reflection occurs when an X-ray beam impinges on a surface at less than the critical angle
of total reflection. If a collimated X-ray beam impinges onto the surface of a plane
smooth and polished reflector at an angle less than the critical angle, then total reflection
occurs. In this case the angle of incidence is equal to the angle of reflection and the
intensities of the incident and totally reflected beams should be equal.
The principles of TXRF were first reported by Yaneda and Horiuchi [14] and further
developed by Aiginger and Wodbrauschek [15]. In TXRF the exciting primary X-ray
beam impinges upon the specimen prepared as a thin film on an optically flat support at
angles of incidence in the region of 2-5min of arc below the critical angle. In practice the
primary radiation does not (effectively) enter the surface of the support but skims the
surface, irradiating any sample placed on the support surface. The scattered radiation
from the sample support is virtually eliminated, thereby drastically reducing the
background noise. A further advantage of the TXRF system, resulting from the new
geometry used, is that the solid-state energy-dispersive detector can be accommodated
very close to the sample (0.3mm), which allows a large solid angle of fluorescent Xray
collection, thus enhancing a signal sensitivity and enabling the analysis to be carried out
in air at atmospheric pressure.
The sample support or reflector is a 3cm diameter wafer made of synthetic quartz or
perspex. The water sample can be placed directly onto the surface. The simplest way to
prepare liquid samples is to pipette volumes between 1 and 50µL directly onto a quartz
reflector and allow them to dry. For aqueous solutions the reflector can be made
hydrophobic (eg by silicon treatment) in order to hold the sample in the centre of the
plate. Suitable elements for calibration can be achieved by a simple standard addition
technique.
Since Yaneda and Horiuchi [13] first reported the use of TXRF various versions have
been developed [15-18]. Recently an X-ray generator with a fine focus tube and multiple
reflection optics has been developed by Seifert & Co and coupled with an energy-