Transcript Atomic Absorption Spectroscopy

Atomic Absorption
Spectroscopy
Lecture 17
1
Chemical Interferences
These are interferences resulting from
chemical processes occurring in flames and
electrothermal atomizers and affect the
absorption signal. To quantitatively assess
the effects of the different chemical
processes occurring in flames, one should
regard the burnt gases as behaving like a
solvent. This is necessary since our
knowledge of gaseous state reaction
equilibria is rather limited. Chemical
interferences include three major processes:
2
1. Formation of Compounds of
Low Volatility
Anionic species forming compounds of low
volatility are the most important. The
formation of low volatility species will result
in a negative error or at least will decrease
the sensitivity. For example, the absorption
signal of calcium will be decreased as higher
concentrations of sulfate or phosphate are
introduced. Cations forming combined
products with the analyte will also decrease
the signal obtained for the analyte. For
example aluminum forms a heat stable
compound with magnesium.
3
Elimination of Low Volatility
Compounds
1. Addition of a releasing agent: cations that
can replace the analyte (preferentially react
with the anion) are called releasing agents.
In this case the analyte is released from the
compound of low volatility and replaced by
the releaseing agent. Lanthanum or
strontium are good releasing agents in the
determination of calcium in presence of
phosphate or sulfate. Also, lanthanum or
strontium are good releasing agents in the
determination of magnesium in presence of
aluminum since both can replace
magnesium.
4
2. Addition of a protective agent: organic
ligands that form stable volatile
species with analytes are called
protective agents. An example is
EDTA and 8-hydroxyquinoline which
will form complexes with calcium
even in presence of sulfate and
phosphate or aluminum.
3. Use of higher temperature is the
simplest procedure to try if it is
possible
5
2. Dissociation Equilibria
Dissociation reactions occur in flames
where the outcome of the process is
desired to produce the atoms of
analyte. For example, metal oxides
and hydroxides will dissociate in
flames to produce the atoms as in the
equations
MO = M + O
M(OH)2 = M + 2 OH
6
Remember that we are not working in solution
to dissociate the compounds into ionic
species. In fact, not much is known about
equilibrium reactions in flames. It should
also be remembered that alkaline earth
oxides and hydroxides are relatively stable
and will definitely show characteristic broad
band spectra (more intense than line
spectra), except at very high temperatures.
The opposite behavior is observed fro alkali
metals oxides and hydroxides which are
instable even at lower flame temperatures
and thus produce line spectra.
7
An equilibrium can be established for the
dissociation of compounds containing atoms
other than oxygen, like NaCl where:
NaCl = Na + Cl
Now, if the signal from a solution of NaCl was
studied in presence of variable amounts of Cl
(from HCl, as an example), the signal will be
observed to decrease as the concentration of
Cl is increased; a behavior predicted by the
Le Chatelier principle in solutions.
8
The same phenomenon is observed
when a metal oxide is analyzed using a
fuel rich flame or a lean flame. Signal
will be increased in fuel rich flames
since the dissociation of metal oxides
is easier due to less oxygen while the
opposite takes place in lean flames
(oxygen rich).
9
A good example on dissociation equilibria can
be presented for the analysis of vanadium in
presence of aluminum and titanium, fuel rich
flames result in higher absorbance signal for
vanadium since the little oxygen present in
flames will be mainly captured by Al and Ti,
thus more V atoms are available. However, in
lean flames, excess oxygen is present and
thus vanadium will form the oxide and
addition of extra Ti and Al will not affect the
signal.
10
3. Ionization Equilibria
Ionization in fuel/air flames is very limited due
to relatively low temperatures. However, in
fuel/nitrous oxide or fuel/oxygen mixtures,
ionization is significant. Therefore, at
higher temperatures an important portion
of atoms can be converted to ions:
M = M+ + e
K = [M+][e]/[M]
11
Ionization in flames may explain the decrease
in absorption signal for alkali metals at very
high temperatures where as the temperature
is increased signal will increase till an extent
at some temperature where it starts to
decrease as temperature is further
increased; a consequence of ionization.
Therefore, usually lower flame temperatures
are used for determination of alkali metals. A
material that is added to samples in order to
produce large number of electrons is
referred to as an ionization suppressor, the
addition of which results in higher
sensitivities.
12
13
Practical Details in AAS
Sample Preparation
The most unfortunate requirement of AAS may
be the need for introduction of samples in
the solution form. This necessitates the
dissolution of the sample where in many
cases the procedure is lengthy and requires
very good experience. Care should be
particularly taken in order not to lose any
portion of the analyte and to make sure that
the reagents, acids, etc. used in the
dissolution and pretreatment of the sample
are free from analyte impurities.
14
I suggest that you follow exact
procedures for preparation of specific
samples for analysis by AAS. In some
cases where the sample can be
introduced directly to an electrothermal
atomizer without pretreatment (like
serum samples), definitely,
electrothermal atomizers will have an
obvious advantage over flame methods
which require nebulization.
15
Organic Solvents
1. Increased nebulization rate due to lower
surface tension of organic solvents which
produces smaller droplets as well as faster
evaporation of solvents in flames will result
in better sensitivities.
2. Immiscible organic solvents containing
organic ligands are used to extract metal
ions of interest and thus concentrate them
in a small volume (thus increasing
sensitivity) and excluding possible
interferences due to matrix components.
16
Calibration Curves
The absorbance of a solution is directly
proportional to its concentration but due to
the large number of variables in AAS, usually
this direct relationship may slightly deviate
from linearity. The standard procedure to do
is to construct a relation between the
absorbance and concentration for a series of
solutions of different concentrations. The
thus constructed graph is called a calibration
curve.
17
The unknown analyte absorbance is
found and the concentration is
calculated or located on the curve.
Neither interpolation nor extrapolation
is permitted to the calibration curve. A
sample can be diluted or the calibration
curve may be extended but always the
analyte absorbance should be within
the standard absorbance range
recorded. Usually, the concentration
axis has the ppm or ppb units.
18
19
Standard Addition method
Chemical and spectral interferences can be
partially or wholly overcome by the use of a
special technique of calibration called the
method of standard addition. In addition, the
use of this method provides better
correlations between standards and sample
results due to constant nebulization rates.
The method involves addition of the same
sample volume to a set of tubes or
containers.
20
Variable volumes of a standard are added
to the tube set followed by completion
to a specific volume. Now, all tubes
contain the same amount of sample but
different concentrations of analyte. A
plot is then made for the volume of
standard and absorbance. This plot will
have an intercept (b) with the y axis and
a slope equals m.
21
22
23
The concentration of the analyte can be
determined by the relation:
Cx = bCs/mVx
Where, Cx and Vx are concentration and
volume of analyte and Cs is the
concentration of standard.
One can only use two points to get the
analyte concentration using the
relation:
Cx = AxCsVs/(At –Ax)Vx
24