DIFFERENT FIELDS OF APPLICATIONS - Total-Reflection X-Ray Fluorescence Analysis and Related Methods - page 293

Figure5.1. Typical TXRF spectrum of a rainwater sample. Gallium was added as internal standard
with a concentration of 60 ng/ml. All values indicated in ng/ml. Figure from Ref. [3], reproduced
with permission. Copyright1996, John Wiley and Sons.
steps have to be taken, as already shown in Figure 4.16. A volume of about
100 ml is first provided, and an aliquot of 1 or 2 ml is acidified with nitric acid
(pH 2). A single-element standard, for example, Se, Co, or Ga, is added on the
μ g/ml level with a nitric acid base. The standard is homogeneously mixed and
aliquots of 10-100 μ l are pipetted onto cleaned hydrophobic sample carriers.
The droplets are dried on a hot plate or under infrared light, and the residue of
0.1-10 μ g is analyzed within a counting time of 100-1000 s. Quantification is
carried out according to Equation 4.12.
Figure 5.1 shows a typical spectrum of rainwater. Detection limits go down
to some ng/ml. Table 5.1 shows the quantitative results of TXRF applied to the
reference material NIST 1643c“Water”[1,2]. 1 No further preparation steps
were necessary, besides the addition of yttrium as an internal standard and the
evaporation of 50 μ l on a clean carrier. Mo and W excitation was used with a
counting time of 1000 s. Repeated determinations show a precision of 1-6%
(squared mean, 4%). The relative deviations from the certified values had an
accuracy of 1-12% (squared mean, 6%). For most elements, measured and
reference values agree quite well—only for Cr, Sr, and Mo a relative deviation
of about 8% is significant.
1 NIST stands for the National Institute of Standards and Technology (Gaithersburg, MD).