I'm trying to find a reason for an experimental observation, I noticed that when the absorbance of $\ce{Ca}$ is measured with FAS(flame atomic spectroscopy) that it decreases when metals such as $\ce{Al}$ and $\ce{K}$ is present(the ions). I used a $\ce{Ca}$ hallow cathode lamp.

Any idea as to why?

  • $\begingroup$ My guess is that the transition used for measuring calcium is very close to transitions in aluminum and potassium. You would need to talk to an AA operator. $\endgroup$
    – LDC3
    Commented Sep 16, 2014 at 1:48

2 Answers 2


Well whatever metal you add will interfere in the absorbance of Ca, because the metal will absorb some of the light too, that's why that of the Ca decreases and thats why the analyte is almost every time mixed only with water which does not absorb the light.( and ofcourse you can add a reagent to form the complex which helps inorganic compounds to absorb the light )


Very late to the party. In flame atomic absorption spectroscopy (Flame AAS or FAAS), apparent absorbances can differ from true absorbances because of a number of inter-related factors, such as the presence of easily ionizable elements (EIEs) or refractory elements. The latter form light scattering oxides, resulting in higher apparent absorbance. With EIEs present, the formation of ions of the desired analyte species is suppressed, so ionic absorbance is reduced.

In general, it is quite complicated to figure out exactly what causes enhancement or suppression of the absorbance of a given species in FAAS and it is even harder in inductively coupled plasma - atomic emission spectroscopy (ICP-AES). A useful paper in this regard has been published by Zaranyika et al. 1.

Note that other elements are generally not a direct spectral interference problem in terms of absorbing the very narrow hollow cathode lamp (HCL) light output. For calcium absorbance, the following figure (of mine) shows the Ca I spectral profile and the HCL emission profile.

Ca I and HCL profiles

And in FAAS and ICP-AES, the goal is to not have any multi-particle species: just atoms and ions, in their various excited or ground states.


1 M.F. Zaranyika, A.T. Chirenje, C. Mahamadi, "Interference Effects of Easily Ionizable Elements in ICP-AES and Flame AAS: Characterization in Terms of the Collisional Radiative Recombination Activation Energy", Spectroscopy Letters, 45 (2012) 1-12.

  • $\begingroup$ This is the correct answer, not the accepted one. Win some, lose some. $\endgroup$
    – Ed V
    Commented Jul 18, 2023 at 0:56

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