Chemistry Reference
In-Depth Information
microextraction and quartz tube-atomic absorption
spectrometry has been used for the selective and sen-
sitive determination of methylmercury in seafood
(Fragueiro
et al.
, 2004).
It can be anticipated that these simple, rapid, and
inexpensive procedures, based on AAS detection,
will be readily accepted once routine laboratories get
involved in speciation analysis.
used as a single element detector. It is easy to couple
online with LC, because it can accept a continuous
fl ow of eluent. The disadvantages are the overall inef-
fi ciency of the nebulizer and the plasma's sensitivity
to organic solvents. The poor tolerance of the plasma
source to common mobile phases, such as ion-pair
reagents, limits the applicability of the technique. The
fact that many ion exchange chromatography elutions
are not isocratic (i.e., the elution is effected under
variable, usually increasing, ionic strength) requires
special protocols to circumvent the problem of vary-
ing analyte response during the elution (Zhang and
Zhang, 2003).
10.2.2 Atomic Fluorescence Spectrometry
When species can be converted into hydrides, such
as is routinely the case for Hg, Se, As, and Sb, atomic
fl uorescence spectrometry (AFS) becomes a very eco-
nomical elemental detection technique. This method
is based on measuring the intensity of the specifi c
resonance fl uorescence of the atom (Kirkbright and
Sargent, 1974). It is, however, necessary to keep in
mind that the conversion of the different species of an
element into hydrides is not happening to the same
extent and at the same rate. This has been documented
(e.g., in the case of As). The conversion of methylated
arsenic species into methylated hydrides gives a differ-
ent response than the conversion of inorganic arsenite
or arsenate to AsH
3
(Zhang
et al.
, 1996).
10.2.4 Mass Spectrometry
10.2.4.1 Inductively Coupled Plasma-Mass
Spectrometry (ICP-MS)
ICP-MS is a remarkably powerful technique for
(ultra) trace element determinations. It is characterized
by extremely low limits of detection and a wide lin-
ear dynamic range, multielement capability, and high
sample throughput (Vanhaecke and Köllensperger,
2003). This method is based on measuring m/z ratios.
The very low detection limits are due to the very high
degree of atomization in the argon plasma at approxi-
mately 7000 K (Dean, 2004). This extreme temperature
makes it far superior to the graphite furnace as an
atomization source for atomic absorption spectrometry.
When a quadrupole mass spectrometer is used, there
are problems caused by spectral interference, because
the resolution is limited to
10.2.3 Atomic Emission Spectrometry
Inductively coupled plasma atomic emission spec-
trometry (ICP-AES) has become the most common
emission spectrometric technique. It is sometimes
referred to as ICP-optical emission spectrometry (OES).
The argon-based plasma is compatible with aqueous
aerosols and offers high energy for drying, dissocia-
tion, atomization, and ionization of the analytes. The
temperature reached by an argon ICP is 5500-6500 K,
high enough to destroy to a great extent the molecular
bonds and to ionize many elements. The high degree
of excitation results in a high atomization yield and
thus high sensitivity. The standard confi guration of
an ICP includes a pneumatic nebulizer for the forma-
tion of the aerosols and a spray chamber acts as a fi lter
selecting droplets with a maximum cutoff diameter.
The light emitted by the atoms on their return from the
excited to a lower energetic state is resolved into a line
spectrum by either a polychromator or a monochroma-
tor, depending on the equipment. The wavelength is
specifi c for the atom and the intensity for its concentra-
tion. The incidence of possible interferences caused by
matrix constituents is very large and requires a careful
study.
More information on ICP-AES can be found in spe-
cialized handbooks (Boumans, 1987; Dean, 2005)
ICP-AES is in principle multielemental, although in
the case of elemental speciation, it will most often be
m/m = 1. For instance,
when measuring
52
Cr (most abundant Cr isotope
.
=
83.8%), mass 52 will experience interference from the
isobars of
40
Ar
12
C
+
,
35
Cl
16
OH
+
,
36
S
16
O
+
, occurring in
samples having a high C, Cl, S, . . . content.
50
Cr and
54
Cr suffer from interferences from high background
counts because of
36
Ar
14
N
+
and
38
Ar
16
O
+
, respectively.
Today two major tools exist to reduce the spectral
interferences so as to become negligible. The dynamic
reaction cell allows chemical reactions in a collision cell
so that the interfering isobars are neutralized or trans-
form the analyte into another more heavy polyatomic.
Another very reliable, but very expensive tool to ban
isobaric interferences is the high-resolution ICP-MS
with
θ
m/m = 1/4000 till 1/10000 (Houk, 2003).
Besides spectral interferences, there may be numer-
ous nonspectral interferences, which are matrix-
induced signal suppression or enhancement. An
additional hurdle is signal instability and/or drift. In
case of total element determination, these, together
with the nonspectral interferences, can often be cor-
rected for by use of a carefully selected internal refer-
ence. To all blank, sample, and standard solutions an
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