Chemistry Reference
In-Depth Information
that the amount will always be minimal, as these techniques work with much smaller samples amounts than
conventional digestion/combustion approaches, and still get equal or better sensitivity owing to the fact that
samples are not diluted.
Of the toxic elements typically controlled in polymers, probably the most difficult to determine is Hg,
owing to the extreme volatility of its species, which are prone to be lost before the measurement takes place.
This element is also not simple for GFAAS monitoring, but the arrival of the HR CS GFAAS technology has
improved considerably the situation, as it brings a more powerful and accurate way to correct for background,
as demonstrated in [77]. With this technique, it becomes possible to directly determine Hg in polymers in a
straightforward way, even when most matrix components are almost simultaneously vaporized together
with the analyte, because it is feasible to achieve a signal that is directly comparable with that obtained for
aqueous standards.
The benefits of HR CS GFAAS have also been shown clearly for analysis of oils and biodiesel samples.
While these are not solid samples, they are very complex to bring into solution and in fact, several works have
explored the advantages deriving from treating them as if they were solid samples (weighing them and
depositing them into the graphite furnace using a solid sampling accessory) instead of as liquids, owing to
their high viscosity [79-81]. Analysis of these samples is very tedious and requires again the use of acids or
of organic solvents to prepare emulsions. Instead, direct analysis using a solid sampling strategy requires just
10-minute homogenization in an ultrasonic bath and micro amounts of a surfactant (Triton X-100) and Pd
solution as chemical modifier [79, 80]. Moreover, the use of HR CS AAS makes it possible to determine more
than one element simultaneously, providing their atomic lines are very close, as discussed in Section 3.3. This
feature is shown in the papers published by Dittert
et al.
, in which Cr and Fe [79] and Co and V [80],
respectively, are determined in simultaneous mode.
Another important advantage of the HR CS GFAAS technique discussed before is the possibility of
monitoring molecular lines. These typically correspond to the hyperfine rotational structure of molecular
electronic transitions that, owing to the high resolution of the instrument (1 to 2 pm, depending on the
wavelength), can be fully appreciated. Most of these 'lines' show the typical width of an atomic line
(approximately 10 pm). This feature opens possibilities to determine elements that were not accessible before
to AAS and these can be determined directly in solid samples. For instance, Ferreira
et al.
investigated the
monitoring of CS lines for S determination in solid biological samples [82] with satisfactory results. As
discussed before, even for elements for which atomic lines are available (e.g., P), monitoring of molecular
bands (PO) may be useful to develop milder atomization procedures that result in the consumption of less
energy [15].
Other types of samples that are hard to dissolve are those of environmental interest, such as soils and
sludges. Again, dissolution of these samples requires the use of concentrated and dangerous reagents, such as
HF. The use of solid sampling is an appealing alternative [22, 83]. Moreover, it has to be considered whether
true quantitative results are always needed. In many situations, important decisions are made on the basis of
whether an analyte is above or below a determined value (e.g., a legal limit), rather than on detailed, specific
chemical information. Therefore, one of the current trends in Green Analytical Chemistry is to develop
screening systems, defined as those systems which can provide a clear binary (Yes/No) response with the
maximum simplicity and the minimum possible delay [84, 85]. These methods can be used in a two-stage
system to filter out a large number of samples, such that only in those situations in which the response is
considered as not totally reliable, would a confirmatory answer be required by means of an alternative
technique. In this regard, it has been demonstrated that many of the solid sampling approaches discussed in
this chapter, which perhaps may suffer to provide very precise results owing to the lack of homogeneity of
these environmental materials, are instead suitable to be used as screening methods.
In Reference [22], Resano
et al.
proposed a method for Hg screening in soil samples to be used for
agricultural purposes. This is a clear example of a situation in which only a binary response (Yes/No) is
