What is spectral interference, when does it occur and how can it be minimised?

Spectral interference, or spectral overlap, is a term used by scientists who are interested in looking at the emission wavelengths of elements to classify data from a source of excited ions which contain a mixture of elements.

It is defined as an absorbing wavelength of an element, not being determined but present in the sample which falls within the measuring line of the analyte of interest.

The absorbance of the element will be measured together with the analyte of interest, increasing the detected emission wavelength.

This bloating effect of the detected wavelength would then cause an increase in signal, telling the instrument that more analyte of interest is present in the sample than there actually is.

When can spectral interference occur?

An element emits a unique spectrum of light when excited by an energy source.

This spectrum of light can be subcategorised into strong emission lines (meaning that these wavelengths are being emitted frequently) and weak emission lines (wavelengths are emitted infrequently).

When we look at the emission spectra of Calcium in comparison to Potassium and even Sodium, we can see that in their emission spectra, the spectras overall are unique.

However, if you were to list every individual wavelength, you would notice correlation between a few radiative emission lines. This is known as spectral interference.

This can be classified as a type of matrix interference, as it is the mixture of different elemental species being present in the solution.

However, it is not the only type of matrix interference that is possible. See phosphate and sulphate interferences of calcium for an example of non-radiative matrix interferences.

Spectral Overlap

How can the spectral overlap effect be stopped or minimised?

A physical solution to this problem is by way of sample and calibration standard preparation.

This involves introducing the element that is causing the signal increase into all of your standards as a blank, so that it is subtracted from the sample’s signal via the calibration curve.

Alternatively, the BWB-Tech range of flame photometers include functionality that allows for this spectral overlap to be calibrated out of the curve and therefore reduces, or completely removes, the impact on the sample analysis.