Chemistry Reference
In-Depth Information
The basic principle underlying AAS is that atoms
formed in the ground state in a gas will absorb radia-
tion at characteristic ground-state energy levels. A light
beam with the spectral composition of the ground state
lines of the element will be specifi cally absorbed by
the ground-state atoms of that element. For example,
ground-state cadmium atoms thus absorb radiation
at a characteristic wavelength of 228.8 nm, one of the
ground-state cadmium lines, and lead atoms absorb at
283.3 nm. The light source is a high-intensity, ground-
state line emitting hollow cathode lamp of the specifi c
element. The decrease in intensity of the light beam
compared with a blank is proportional to the amount
of the element in the ground state in the plasma.
There are two main methods for atomization of a
sample in AAS: the fl ame (F-AAS) and the graphite
furnace method (GF-AAS). The fl ame method uses
different gas mixtures for creating a high temperature
atomization fl ame (e.g., air-acetylene). Different types
of fl ames are required for different metals. The liquid
sample is aspirated directly into the fl ame. Because of
the relatively low atomization of the fl ame (approxi-
mately 500 °C), the method may not be sensitive
enough to measure most elements in biological materi-
als. In the graphite furnace, temperatures of approxi-
mately 2000 °C prevail, increasing by several orders of
magnitude the number of atoms and hence the detec-
tion sensitivity. Typically, the detection limit for Cd by
F-AAS is 1
The diffi culty, if not the impossibility, of making
fl ame atomic absorption (F-AAS) and graphite fur-
nace atomic absorption spectrometry (GF-AAS) an
online method for measuring the elements in liquid
chromatography fractions make them less popular for
elemental speciation purposes. They have, however,
earned their merits in the fi eld. GF-AAS has been used
for the offl ine measurement of elements in the elution
fractions of LC, although insuffi cient detection limits
proved to be a serious drawback in the case of many
clinical applications in which the concentrations of the
elemental species in the biological fl uids and tissues
are very low (Zhang and Zhang, 2003).
When species can be converted to hydrides, as is
routinely done for Hg, Se, As, and Sb, hydride genera-
tion atomic absorption spectrometry is a very interest-
ing and cheap online detection technique. It has been
applied for the speciation of arsenic in persons with
abnormally high arsenic concentrations in serum, such
as dialysis patients (Zhang
et al.
, 1996; 1998). An online
method was developed for the speciation of arsenic
species in human serum, including monomethylar-
sonic acid (MMA), dimethylarsinic acid (DMA), arse-
nobetaine (AsB), and arsenocholine (AsC). The method
is based on cation-exchange liquid chromatography
(LC) separation, UV-photooxidation for sample diges-
tion, and continuous hydride generation (HG) atomic
absorption spectrometry (AAS) for the measurement
of arsenic in the LC eluent. By developing the tech-
nique of argon segmented fl ow in the postcolumn
eluent, a substantial improvement in chromatographic
resolution for the separation of these four arsenic spe-
cies was obtained. The LC separation, photooxida-
tion, hydride generation, and AAS measurement can
be completed online within 10 minutes. The detection
limits for MMA, DMA, AsB, and AsC in serum were
1.0, 1.3, 1.5, and 1.4
g/L
−1
and only 8 ng/L
1
with GF-AAS. A
small aliquot of the sample is injected into the oven.
A matrix modifi er may be added to assist the charring
and atomization processes.
Nonspecifi c absorption may occur because of the
presence of matrix atoms and molecules in the fl ame
or the furnace. Salts such as sodium chloride and phos-
phates are especially apt to cause interference. This may
be avoided by background correction, for which a deu-
terium lamp is generally used. A more sophisticated
method uses the Zeeman effect that splits the lines with
a magnetic fi eld. In case the background correction fails,
or when a concentration step is needed, it is necessary
to mineralize the sample and to extract the element. A
common chelating agent is APDC (ammonium pyrro-
lidine dithiocarbamate), which is extracted into MIBK
(methylisobutyl ketone). This procedure has been used
for several elements (e.g., cadmium, lead, and nickel).
For mercury with its high volatility at room temper-
ature, the best detection method is cold vapor atomic
absorption spectrometry.
Elements such as As, Se, Sb, and Hg that can easily
be transformed into hydrides can be very advanta-
geously measured by hydride generation (HG)-AAS.
This method allows elimination of background inter-
ferences to a very large extent.
µ
g/L
1
of arsenic, respectively. The
concentration of the four species was determined in six
serum samples of patients with chronic renal insuffi -
ciency. Only AsB and DMA were signifi cantly detected
by this method. The main part of arsenic in human
serum is AsB. No MMA, AsC, or inorganic arsenic was
detected.
Atomic absorption spectrometry with a quartz
tube atomizer is a very sensitive, specifi c, rugged, and
comparatively inexpensive detector for GC. Gas chro-
matography coupled with atomic absorption spectrom-
etry has been described as a sensitive instrumentation
for mercury speciation (Emteborg
et al.
, 1996). Online
solid-phase extraction, coupled to graphite furnace
AAS, has also been explored.
Cold vapor atomic absorption spectrometry
(CV-AAS) is the most widely used technique for
measuring Hg species. Direct coupling of solid-phase
µ
