2.3. Speciation analysis
In the 1960s, determination of summation
parameters such as total heavy metals in water by
sulfide precipitation and gravimetry, was replaced
by the determination of the individual elements,
as it was recognized that the various heavy metals
differ significantly in their toxicity. One of the
driving forces at that time was the introduction of
AAS which made these determinations easy. In
the meantime it is well established that the total
content of an element does not give sufficient
information, as several elements may be essential
for humans or animals, and may be toxic at the
same time, depending not only on their concentration,
but even more importantly on their oxidation
state or the chemical form in which they are
present. A typical example is chromium, which is
essential in its trivalent form, but carcinogenic in
its hexavalent compounds. Another example is
arsenic, the inorganic forms of which are well
known to be highly toxic, whereas organic com-
pounds, such as arsenobetain, which are found in
high concentration in some seafood, are non-toxic,
and may even be essential.
A limiting factor for the introduction of speciation
analysis into routine use is undoubtedly that
most of the research work in that field is currently
done using high-pressure liquid chromatography
. HPLC as a separation technique, and
ICP-MSfor detection. This combination, although very
powerful, is far too expensive for routine use,
considering that a fast multi-element system is
typically waiting 10]20 min for a few species of a
single element arriving at the detector. There is
no doubt that this kind of research has to be
carried out in order to gain all the knowledge
necessary for speciation analysis. However, for
routine purposes, a good percentage of speciation
analysis can be done without prior separation of
w x the individual compounds by chromatography 28 .
A simple procedure has, for example, been proposed
to distinguish between ‘toxic’, i.e. inorganic,
mono- and di-methylated arsenic, and non-toxic
arsenobetaine using hydride generation HG
w x
AAS 29 . FI on-line column preconcentration
and separation is typically selective for one oxidation
state of an element only, and may hence be
used for the separation of redox species, as was
demonstrated for the differential determination
. w x of Cr VI and total chromium in water 30 .
Another example of speciation analysis by on-line
separation and preconcentration is shown in Fig.
3 for an activated alumina microcolumn, which at
. pH 2 selectively preconcentrates Cr VI , whereas
. w x Cr III is retained selectively at pH 7 31 . There
is no question that this field of routine speciation
analysis is far from being completely explored.
In addition,
AAS with a quartz tube atomizer is
an extremely attractive detector for gas chro-
. matography
gc , as already proposed back in
w x 1976 by Van Loon and Radziuk 32 . This door
was, however, closed by instrument manufacturers
with the introduction of digital electronics in
the 1980s that no longer permitted the recording
of a series of peaks over an extended period of
time, i.e. the duration of a chromatogram. It
should not be difficult to open this door again,
and
AAS could then become a sensitive, highly
specific, and not too expensive detector for
gcw x 33 , and after post-column derivatisation, even
for HPLC 34]36 . This aspect of
AAS as an
attractive detector for chromatography in speciation
analysis will be brought up one more time in
connection with diode laser
AAS see Section
. 3.2 .