DEALING WITH INTERFERENCE IN ICP-OES

by Thomas Bower, Quality Control Chemist

ICP-OES can often provide accurate results in determining the concentration of metals in a solution under the right conditions. However, often, the conditions are not ideal and the analyst must address two possible sources of error: background interference and spectral overlap (Gaines 2004). This paper will explain how background interference and spectral overlap can influence instrument results and provide suggestions on how to address these issues if they present themselves.

Background interference can pose as a problem when the background radiation is from a combination of sources that are difficult to control by the operator (Gaines 2004).  These sources include various elements in the sample such as calcium which is known to contribute to some of the background interference when present in high concentrations (Gaines 2004). The noise from the radiation disrupts an accurate analysis because the instrument is unable to differentiate it from other analytes under study. Background interference can be accommodated in a number of ways. In some cases, adjusting instrumental parameters can lower background radiation (Gaines 2004). “It can be argued that matrix-matched standards and samples will eliminate the need for background correction where the analyst only has to measure peak intensity”(Gaines 2004). “However, the problems with matrix matching are obvious and may offset any advantage gained when you don’t make them”(Gaines 2004). In some cases, the instrument will allow the analyst to correct for the background radiation by using algorithm or a correction mode built into the software.  Ultimately, the best strategy to handle background radiation is using the “avoidance” approach (Gaines 2004). This method simply involves choosing an alternate line that is not influenced by the background radiation whenever available.

“A spectral line overlap occurs when spectral lines of two (or more) elements contained in the sample are so close to one another that they cannot be resolved by the spectrometer”(Thomsen et. al. 2006). This occurrence can influence the results because the signal in the region of interest has more than one point of origin (Thomsen et. al. 2006). As line overlap varies for the same spectral line, instrument resolution will determine how much variation occurs (Thomsen et. al. 2006). To address the problem of spectral overlap, the analyst can use either interelemental correction or, once again, use the avoidance approach.

The avoidance approach should be used first in most cases since it is the simplest way to bypass the problem. However, when this method is not possible, interelemental correction should be used to correct the problems with spectral overlap. Interelemental correction involves using the instrument software to determine the interference and mathematically correct the instrument readings. It should be used in compliance with the recommendations in the instrument’s operating manual where a defined procedure may be outlined with the instrument’s software (Gaines 2004).

In conclusion, background interference and spectral overlap are potentially significant sources of error that must be considered when analyzing metals in solution. In both cases, the best choice available is to select an alternative line that is unaffected by the background radiation or spectral overlap if available. By taking the necessary measures to either avoid or correct these problems, the ICP chemist will be better prepared for testing metals in solution in order to achieve accurate results.   

 

References
Gaines, P. & Brolin, B. (Ed.). (2004). ICP operations: 8. spectral interferences: types, avoidance, and correction. Inorganic Ventures. Retrieved September 23, 2008, from
http://www.ivstandards.com/tech/icp-ops/part08.asp.
  
Thomsen, V., Schatzlein, D., & Mercuro, D. (2006, July). Interelement corrections in
spectrochemistry. Spectroscopy, 32.

 

For more information please contact Thomas Bower at t.bower@apgqa.com.