DIFFERENT FIELDS OF APPLICATIONS - Total-Reflection X-Ray Fluorescence Analysis and Related Methods - page 309

Total reflection XRF was first applied to the detection of trace elements in
tea by Xie etal . [70]. Infusions of 39 different kinds of tea were performed after
the Chinese standard procedure for judging the flavor of tea. According to this
instruction, 1 g of tea leaves was steeped in 50 ml of boiling water. In order to
prevent contamination, distilled water was used instead of tap water. After
5 min, each infusion was filtered, cooled, acidified, and spiked with gallium. For
analyses, aliquots of 5 μ l of an infusion were pipetted onto cleaned quartz-glass
carriers and dried by IR radiation. TXRF was carried out with Mo excitation
and a Si(Li) detector in a live time of 200 s.
This technique was first evaluated by the analysis of a certified tea sample
(GWB 08505: tea, Research Center for Eco-Environmental Sciences, Acade-
mia Sinica, Beijing, China). The detection limits were of the order of 0.1-1 μ g/g.
The reliability for the determination of 14 elements was determined by 12
replicate measurements. The precision was shown to be 4% for most elements,
the accuracy for six elements was about 1.5%, for another seven elements it was
about 9%. Apart from light elements, the concentration values for the other
elements were lying within the confidence interval of the reference values.
After this quality check, the 39 different infusions were analyzed and the
influence of origin, type, and quality of the tea samples was studied. A
particular result was found for the concentration of selenium in green tea of
the Enshi district in Hubei Province, which is one of the two Se-rich regions of
China. Values go up to 7.5 μ g/g, being much higher than the mean selenium
concentration for green Chinese tea, which is only 0.1 μ g/g.
It was considered that the Keshan disease leading to serious heart failure is
caused by a Se deficiency and can be prevented by the consumption of this kind
of tea. The trace element selenium is a constituent of particular proteins and
enzymes, which play an important role in metabolic processes. The deficiency
disease was first noted and treated in 1966 in the Keshan county (Heilongjiang
Province in the northeast of China). In 1975, a study on children in Sichuan
Province (central part of China) showed that the symptoms of disease are
reduced by supplementation of selenium. We usually receive this trace element
in our diet; the daily requirement of selenium is about 50 μ g. Foodstuff, such as
kidney, liver, fish, seafood, and Brazil nuts with concentrations of some 10 μ g/g,
can provide high amounts of this essential trace element. This demand can also
be met by drinking 1 l of the particular green tea per day.
Total reflection XRF was applied to coffee samples by Haswell and Walms-
ley in 1991 [71]. Coffee with a mass of 0.3 g was treated with 1 ml of a vanadium-
standard solution, and diluted to 25 ml with ultrapure water. Aliquots of 10 μ l
were applied to a hydrophobic quartz-glass reflector and dried. The prepara-
tion took place in a clean bench. Afterward, up to 11 elements were determined
by TXRF (Mo excitation, 500 s).
A most simple visualization of the data was made by star plots, which are a
valuable tool for simple and rapid screening and classification of items [72].
This technique was applied to different coffee samples of different manufac-
turers as demonstrated in Figure 5.6. The concentration of 10 elements was