Interference Subtraction

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Page ID: 76356

Alexander Scheeline & Thomas M. Spudich
Analytical Sciences Digital Library

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What changes if the intensity observed at a given wavelength is due not only to the line of interest but also to emission from a line of another element whose concentration is independent of the concentration of the primary line being observed at a given wavelength? Once again, one must subtract those photons due to something other than the sought-for substance. However, one has to be careful of the order of operations. How do we account for the continuum underlying both the sought-for line and the interfering line? How do we know how much of the interfering substance is present? Where does the needed information come from?

At each wavelength, intensity is the sum of I(sought-for element), I(continuum background), and I(interfering elements). While methods exist to deal with elements of comparable concentration, the correction is most easily explained if the interfering line is a weak line of a high concentration element interfering with a strong line of a low concentration element. That means that, at some other wavelength, there's a strong line of the interferent that does not itself suffer from interference. We thus can independently measure the concentration of the interferent. In turn, the weak, interfering line has an intensity that is some known fraction of the intensity of the strong line used to assay the interfering element. Thus, I(interfering element) = k(λ) C(interferent). If one has sufficient data, the dependence of k on the indirect influence of the sample on the emission source can also be included.

We now find

\[I_{atom} = I(λ) - I_{background} - k(λ) C(interferent).\]

A hidden assumption is that I_background is not influenced by the interferent. What if the interfering line does appear in the wavelength region where one is trying to measure background? In many ways, this is tricky. The instrument has no way to distinguish whether intensity is due to continuum or line emission unless one scans across a line. If one simply says

\[I_k,atom = I(λ) - I_background_adjacent, \]

there is the possibility of oversubtraction or undersubtraction of continuum or interferent emission. No single wavelength pair observation can clarify the problem. We defer to the section on Chemometrics how to deal with the messy condition where many elements contribute to intensity at many observed wavelengths.