EFFICIENCY AND EVALUATION - Total-Reflection X-Ray Fluorescence Analysis and Related Methods - page 401

numbers, with different concentrations down to extreme traces, and in quite
different matrices, have to be determined. Consequently, methods are required
that are capable of a multielement detection, have a high detection power, and
are applicable to a multitude of sample materials. Moreover, the methods
should give accurate results and work economically. With regard to all these
features, TXRF plays an important role in atomic spectroscopy [84].
Total reflection XRF is a powerful method of microanalysis and is also
suitable for trace element detection. Masses of only a few μ g of a solid or
volumes of few μ L of a liquid sample are sufficient for analysis. Specimens have
to be deposited in the center of a flat and ultraclean carrier, for example, on a
quartz-glass carrier or on a cheap Plexiglas support. The time necessary for
recording a spectrum is of the order of minutes.
All elements can be detected simultaneously with the exception of light
elements, that is, TXRF is a real multielement method. Detection limits are
usually on the picogram level, but for low- Z and transition elements the
detection limits decrease by one or two orders of magnitude. Concentrations
can be determined down to 1 ng/g for solid samples or 1 pg/ml for liquid
samples. For a simple quantification, one prerequisite has to be met absolutely;
samples have to be deposited with a restricted covering and thickness. Small
samples with a covering of μ g/cm 2 and a thickness of μ m, do not show annoying
matrix effects and do not destroy the coherence of the incoming and totally
reflected beam. In this case, quantification is made possible by a simple
addition of a single internal standard element.
Competing methods of TXRF for laboratory analyses are ET-AAS (electro-
thermal atomic absorption spectrometry, also called GF-AAS or graphite
furnace AAS), ICP-OES, ICP-MS, and INAA. The group of classical methods
including gravimetry, titrimetry, voltammetry, and chromatography cannot
compete in any way with one of the universal and effective methods of modern
spectrometry like TXRF. Most of the nonspectrometric methods are only
suitable for single-element detection and have only mediocre detection limits—
at μ g or ng levels. However, the simplicity of their equipment and the accuracy
of their results assure them firm places in the analytical laboratory.
Atomic absorption spectrometry with flames (FAAS) was the most common
method of atomic spectroscopy several years ago. However, only the technique
using graphite furnaces (ET-AAS) is suitable for trace analysis. It is a micro-
analytical method like TXRF and shows low detection limits (at picogram
levels) even for light elements. On the other side, it is mostly applied as a single-
element method so that it is time-consuming especially for the successive
determination of many elements. Furthermore, the standard-addition method
used for calibration is rather laborious. A simultaneous multielement detection
has been envisaged for laser AAS. At present, only an oligoelement technique
has been developed, using four hollow-cathode lamps and an Echelle

spectrometer.
A very common excitation source for optical emission spectrometry is ICP-
OES. It is a multielement method for macrosamples, applicable to solutions of