Chemistry Reference
In-Depth Information
produces a few so-called main components by correlating all the variables with
each other and building special linear combinations between them. The
individual coefficients can be interpreted as weighing factors. Altogether,
10-20 variables per sample may be reduced to two to four clusters or main
components. A meaning of components has to be found, which sometimes may
be difficult and the results may be ambiguous and doubtful.
As already described earlier, tissue samples can be freeze-cut and analyzed
directly by TXRF [97,98]. The method is simple and fast and provides the
simultaneous detection of about 10-15 elements within a few minutes. Car-
valho and coworkers applied this technique for breast, colon, rectum, stomach,
lung, uterus, and prostate tissues of several patients during surgery [74,110].
Healthy and cancerous tumor tissue was taken from the same individual during
surgery. After a representative excision of about 0.1 cm
3
tissue each, the
samples were kept separately in clean formalin at room temperature for several
weeks until freeze-cutting.
3
Many sections with a diameter of about 4-6mm
and a thickness of about 5-10
μ
m were cut by a freezing microtome. Contiguous
but nonconsecutive sections were taken, that is, only each sixth section was
chosen, and about five sections of each tissue were analyzed by TXRF.
Each section was placed on a clean quartz-glass carrier and dried at room
temperature. After recording a first spectrum, the best internal standard was
chosen between Ga, Y, and Se and added to the section. After drying at 50
°
C
for roughly 15 min, the final X-ray spectrum was recorded within 180 s. A Mo
tube adjusted to 50 kV and 38 mA was used for excitation, and a Si(Li) detector
was applied for spectral registration. After a rest time of 1 h, the mass of each
section was determined by a microbalance as mentioned in Section 5.3.3.1 with
typical masses of about 100
μ
g.
A few but significant results may be given here. The concentration values
determined by TXRF are roughly between 0.1
μ
g/g and 3000
μ
g/g. It was shown
that the concentration of several elements and their behavior is tissue depen-
dent. Phosphorus and potassium show increased concentrations in most can-
cerous tissues, copper and zinc are increased for cancerous breast, lung, and
prostate tissue, selenium is decreased in cancerous colon tissue but increased
for cancerous breast and lung tissue, bromine is strongly increased for cancer-
ous breast tissue, whereas it is decreased for all other cancerous samples.
Copper and zinc show increased levels of concentration not only for the tissue
but also for the serum of cancer patients. Consequently, the Cu concentration
and moreover the Zn/Cu ratio with a better precision and a higher significance
can be a useful screening tool for a preliminary and fast cancer diagnosis as
already pointed out by Marco-Parra
etal
. [111]. Three groups of elements with
3
Formalin is an aqueous solution of formaldehyde (CH
2
O) used for the germination and
immobilization of tissue samples. For TXRF investigations it should be extremely clean. Never-
theless, there is a risk of introducing and/or washing out certain elements. Such a possible effect
should be controlled by TXRF of the formalin bath before and after use.
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