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Spectral Interference: Types, Avoidance and Correction 8
Types of Spectral Interference: ICP-OES
The types of spectral interferences most commonly encountered for ICP-OES are discussed in the Spectral Interferences
section of Part 15: ICP-OES Measurement of our Reliable Measurements series. You may wish to review this information
before continuing.
As noted in part 7 of this guide, the collection of spectra at different concentrations on all elements and lines available will
save a lot of time in the line selection process.
Avoidance: ICP-OES
Several modern ICP instruments have the capability of avoiding the spectral interference by going to another line. Many
instruments can make measurements simultaneously on several lines for 70+ elements in the same time it used to take to
make a measurement on a single line/element combination. If you have the opportunity, I would strongly encourage the
avoidance approach over attempting to make correction on a direct spectral overlap or wing overlap interference. Background
corrections are another manner and can be routinely dealt with.
Correction: ICP-OES
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.
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 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
Figure 8.1:
Spectrum of 6% Ca solution vs. nitric acid blank
Figure 8.2:
Flat background correction