Chemistry Reference
In-Depth Information
alcoholic content (above 20%), that is, of commercial or homemade spirits [78].
Vanadium was used as internal standard and Fe, Cu, and Zn could be
determined as trace metals. The concentration of copper was above the allowed
limit for similar beverages.
5.2.2VegetableandEssentialOils
The simplest preparation consists in a dilution of oil samples and has been
checked for soya oil, peppermint oil, lime oil, and cumin oil by Reus and
Prange [2,79]. An aliquot of 1 ml is diluted with toluene (1 : 1), and an internal
standard, for example, organo-copper or -vanadium in toluene or an oil-based
standard (E. Merck, Darmstadt, Germany), is added to a concentration of
10
μ
g/ml. Then, 20
μ
l of this solution is pipetted onto a quartz-glass carrier,
which is heated to 100
°
C on a hot plate for 5 min to remove the volatile parts of
the matrix. The dry residue is analyzed by TXRF within 1000 s. Detection limits
are in the range of 3-20 ng/ml.
In order to improve the detection power, a cold plasma ashing is recom-
mended [2] in which 1 ml of an oil sample containing the internal standard is
bottled in a small quartz beaker and ashed in a low-temperature oxygen plasma
(1 h at 300 W and 500 Pa O
2
pressure). The residue is dissolved with 500
μ
lof
half-concentrated nitric acid, and 50
μ
l of this solution is applied to TXRF. As
compared to the direct method described in the previous paragraph, this
method lowers detection limits by about one order of magnitude.
However, ashing requires approximately 2 h, and volatile elements like
halogens or Hg, Se, Ti, and S are lost. For these elements, pressure digestion
and subsequent plasma ashing are recommended but require about 10 h. For
several elements, the detection limits of the aforementioned direct method are
fortunately sufficient, in particular with respect to legal regulations for toxic
elements.
5.2.3PlantMaterialsandExtracts
Plant materials that have been investigated by TXRF are algae, tea plants,
corn, hay, lichen, moss, pine, parsley, dill, poplar, beans, rye, and wheat.
Analyzed samples stem from blades, bulbs, fine roots, leaves, needles, roots,
rosettes, seedlings, seeds, spikes, stalks, tubers, and xylem saps. The usual
preparatory technique for plant materials is nitric acid digestion of freeze-dried
and pulverized components. If volatile elements have to be determined, a
pressure digestion is preferable. Otherwise, an open digestion or even a cold
plasma ashing can be carried out. Plant extracts like vegetable oils were
analyzed after a simple dilution [67]; aqueous extracts like tea were analyzed
directly [70].
The individual components of plants are thoroughly cleaned, shredded,
freeze-dried, and finally pulverized in a cleaned porcelain mortar with a
pestle [80,81]. Then, 100 mg of the powdered and possibly sieved plant material
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