Atomic Absorption Spectroscopy Lecture 16

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.

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.

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.

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.

 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.

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.

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.

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.

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.

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.

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.

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.

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.

a.       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

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.