Transcript L16

Atomic Absorption
Spectroscopy
Lecture 16
1
Interferences in Atomic
Absorption Spectroscopy
There are two major classes of interferences
which can be identified in atomic absorption
spectroscopy. The first class is related to
spectral properties of components other than
atomized analyte and is referred to as
spectral interferences. The other class of
interferences is related to the chemical
processes occurring in flames and
electrothermal atomizers and their effects on
signal. These are referred to as chemical
interferences and are usually more important
than spectral interferences.
2
Spectral Interferences
1. Spectral line Interference
Usually, interferences due to overlapping lines
is rare since atomic lines are very narrow.
However, even in cases of line interference, it
can be simply overcome by choosing to
perform the analysis using another line that
has no interference with other lines.
Therefore, line interference is seldom a
problem in atomic spectroscopy.
3
2. Scattering
Particulates from combustion products
and sample materials scatter radiation
that will result in positive analytical
error. The error from scattering can be
corrected for by making a blank
measurement. Scattering phenomenon
is most important when concentrated
solutions containing elements that
form refractory oxides (like Ti, Zr, and
W) are present in sample matrix.
4
Metal oxide particles with diameters
larger than the incident wavelength will
make scattering a real problem. In
addition, samples containing organic
materials or organic solvents can form
carbonaceous (especially in cases of
incomplete combustion) particles that
scatter radiation.
5
3. Broad Band Absorption
In cases where molecular species from
combustion products or sample matrix are
formed in flames or electrothermal
atomizers, a broad band spectrum will result
which will limit the sensitivity of the
technique. It should be indicated here that
spectral interferences by matrix products
are not widely encountered in flame
methods. Even if matrix effects are present
in flames, they can be largely overcome by
adjusting various experimental conditions
like fuel/oxidant ratio or temperature.
6
Another method for overcoming matrix
interferences is to use a much higher
concentration of interferent than that
initially present in sample material, in
both sample and standards (this
material is called a radiation buffer).
The contribution from sample matrix
will thus be insignificant.
Spectral interferences due to matrix are
severe in electrothermal methods and
must thus be corrected for.
7
Background Correction Methods
a.
The Two Line Correction Method
In this method, a reference line from the source
(from an impurity in cathode or any emission
line) is selected where this line should have
the following properties:
1.
Very close to analyte line
2.
Not absorbed by analyte
If such a line exists, since the reference line is
not absorbed by the analyte, its intensity
should remain constant throughout analysis.
8
9
However, if its intensity decreases, this
will be an indication of absorbance or
scattering by matrix species. The
decrease in signal of the reference line
is used to correct for the analyte line
intensity (by subtraction of the
absorbance of the reference from that
of the analyte). This method is very
simple but unfortunately it is not
always possible to locate a suitable
reference line.
10
b.
The Continuum Source
Method
This background correction method is the
most common method although, for reasons
to be discussed shortly, it has major
drawbacks and fails a lot. In this technique,
radiation from a deuterium lamp and a HCL
lamp alternately pass through the graphite
tube analyzer. It is essential to keep the slit
width of the monochromator sufficiently wide
in order to pass a wide bandwidth of the
deuterium lamp radiation.
11
In this case, the absorbance by analyte
atoms is negligible and absorbance can
be attributed to molecular species in
matrix. The absorbance of the beam
from the deuterium lamp is then
subtracted from the analyte beam
(HCL) and thus a background
correction is obtained.
12
13
14
Problems Associated with Background
Correction Using D2 Lamp
1.
The very hot medium inside the graphite
tube is inhomogeneous and thus signal is
dependent on the exact path a beam would
follow inside the tube. Therefore, exact
alignment of the D2 and HCL lamps should be
made.
2.
The radiant power of the D2 lamp in the
visible is insignificant which precludes the
use of the technique for analysis of analytes
in the visible region.
3.
Addition of an extra lamp and chopper
will decrease the signal to noise ratio.
15
c.
Background Correction Based on
Zeeman Effect
Zeeman has observed that when gaseous atoms (but
not molecules) are placed in a strong magnetic field
(~ 1 tesla), splitting of electronic energy levels takes
place. The simplest splitting of one energy level
results in three energy levels, one at a higher energy,
another at a lower energy (two s satellite lines) and
the third remains at the same energy as the level in
absence of the magnetic field (central p line).
Furthermore, the p line has twice the absorbance of
a s line and absorbs polarized light parallel to
direction of the magnetic field while the two s lines
absorb light perpendicular to magnetic field.
16
17
Light from a HCL lamp will pass through
a rotating polarizer that passes
polarized light parallel to external
magnetic field at one cycle and passes
light perpendicular to field in the other
cycle. The idea of background
correction using this method is to allow
light to traverse the sample in the
graphite furnace atomizer and record
the signal for both polarizer cycles
using the wavelength at the p line.
18
19
First cycle: light parallel to field; the p
line of the analyte absorbs in addition to
absorbance by matrix (molecular matrix
absorb both polarized light parallel or
perpendicular to field)
Signal a = Ap + AMatrix
b.
Second cycle: light perpendicular to field;
the p line of analyte will not absorb light
perpendicular to field and s lines will also
not affect absorbance at the p line
wavelength. Only matrix will absorb.
Signal b = AMatrix
a.
20
The overall signal is the difference of the
two signals = Ap
Therefore, excellent background
correction is achieved using the
Zeeman effect. This background
correction method results in good
correction and is usually one of the
best methods available.
21