Spectral Interference: Types, Avoidance and Correction

Correction: ICP-OES

Examples are provided below for background interference and spectral overlap.

Background Interference

Background radiation is a potential source of error that requires correction. The source of the background radiation is from a combination of sources that cannot be easily controlled by the operator. Figure 8.1 shows the spectra for a highly concentrated Ca sample as compared to a nitric acid blank.

Figure 8.1:
Spectrum of 6% Ca solution vs. nitric acid blank



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The background radiation intensity for the nitric acid blank is ~ 110,000 counts at 300 nm whereas the background radiation for the Ca containing solution is ~ 170,000 counts at the same wavelength. Although background radiation can be lowered somewhat by adjusting instrumental parameters, it cannot be eliminated and corrections are typically necessary. It can be seen that the highly concentrated Ca matrix contributes some to the background radiation but there are greater contributions from other sources independent of the sample matrix.

It can be argued that matrix-matched standards and samples will eliminate the need for background correction where the analyst only has to measure the peak intensity. It would follow that the precision of the measurement would be better (lower) and for some instruments the measurement time will be shorter. However, the problems with matrix matching are obvious and may offset any advantage gained when you don’t make them.

The correction for background radiation is typically made by first selecting background points or regions and then a correction mode or algorithm. The ‘algorithm’ or ‘correction mode’ depends upon the curvature of the background, as is illustrated below.

Figure 8.2:
Flat background correction



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Figure 8.2 shows a flat background where correction was made on both sides of the line. In this case the instrument allows for the selection of background regions thereby improving the accuracy of the estimated background radiation. If the instrument only allows for selection of background points then intensities are taken at set wavelengths, averaged and subtracted from the peak intensity. For flat backgrounds the distance of each point from the peak intensity is not important provided there is no interference from other lines in that vicinity. Figure 8.2 demonstrates that care was taken to avoid The Re line on the long wavelength side of the Zn 213.856 nm line and that a straight line that accurately determines the background intensity in the peak area is obtained.

Figure 8.3 shows a sloping but linear background. If the instrument only allows for selection of background points then intensities are taken at set wavelengths, averaged and subtracted from the peak intensity. Here, background points must be taken equal distance from the peak center in order to make an accurate correction. Again, a linear fit was used.

Figure 8.3:
Sloping background correction



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Curved backgrounds are encountered when the analytical line is near a high intensity line, as is the case shown in Figure 8.4 below. In this case an algorithm estimating a curve (parabola) was used. For some instruments, depending upon design and software, this type of correction can be very difficult. This is a case where the 589.592 nm Na line would allow for the easier linear correction without loss in sensitivity.

Figure 8.4:
Curved background correction



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