Chemistry Reference
In-Depth Information
quartz-glass carriers, and concentrated nitric acid was added to the samples.
In this way, the organic matrix was decomposed. Scandium and rubidium
were used as internal standards and up to eight elements were determined.
Lower values for Na, Mg, and Zn and higher values for calcium were found in
the ALS group compared to the control group. In parallel to these measure-
ments, serum samples from both groups were taken and investigated with a
raised concentration of bromine.
Amniotic fluid is a clear slightly yellowish liquid in the amniotic sac of
pregnant females protecting the fetus. In a first approach, the cellular content
and suspended particles were separated by centrifugation [103]. In the second
approach, samples were treated with nitric acid and subsequently digested by
oxygen
plasma. After that,
the
metal
composition
could
be
determined
quantitatively [105].
5.3.3TissueSamples
Organ tissues can be analyzed after ashing and/or digestion. It may, however,
be preferable to simply cut a tissue sample in thin sections as known from
histology, and directly place these sections on TXRF carriers [96-98]. Besides
simplicity, this method offers the advantage of preventing contamination and
losses caused by chemical preparation.
5.3.3.1Freeze-CuttingofOrgansbyaMicrotome
A small piece of tissue with a dimension of 4-8 mm, a volume of less than
0.5 cm
3
, and a mass below 500 mg is frozen at a temperature of
25
°
C.
The frozen sample is cut by a microtome (Reichert-Jung, Nußloch, Germany)
in thin sections, each about 5-15
μ
m thick. Every section is slid onto the scalpel
with the aid of a device called a stretcher. After about 30 s, the section can be
placed in the center of a glass carrier by slightly touching it with the surface of a
cleaned carrier. Quartz-glass carriers must not be siliconized because hydro-
phobic carriers lead to tears and droplets. Furthermore, the carriers must have
a moderate temperature. If a carrier is too cold the section may be repelled, and
if it is too warm it may be attracted too fast and may develop waves. Figure 5.9
shows a microtome and a thin section of mussel tissue on a quartz-glass
carrier [98]. After positioning, the carrier is warmed up to room temperature.
The section dries within a few minutes and adheres evenly to the surface. Then,
a first spectrum can be recorded in order to choose a suitable internal standard
element. Ga, Se, or Y are usually absent in the sample so that they can serve as
the internal standard. For that purpose, the sections are spiked with a droplet of
this standard, for example, 10
μ
l of a gallium-standard solution with a 1
μ
g/ml
concentration. The droplet is soaked up by the section and the section is
thoroughly dried by evaporation on a hot plate or in a small clean oven (about
30 min at 50
°
-80
°
C depending on the kind of tissue). At the same time, the
section shrinks to a thickness of 5-10
μ
m. After that, a rest time of about 1 h is
15 to
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