ICP-OES Measurement

Matrix Effects

Matrix effects are arguably the most subtle danger to the ICP-OES analyst. Slight differences in the matrix can cause a considerable systematic error having a definite bias. Internal standardization has been the most common approach toward correcting this problem and this approach does work well in many cases. The choice of the internal standard is critical.

Please consider the following questions before using the internal standard approach for matrix correction:

1. Is the internal standard (IS) element compatible with your matrix?  (Avoid using rare earths in fluoride matrices).
2. Are there any possible spectral interferences upon the IS line?
3. Is the concentration of the IS sufficient to give a good signal to noise ratio?
4. Can your sample possibly contain the IS element as a natural component?
5. Is the IS clean? Are the trace impurities reported on the certificate of analysis?
6. Is your method of addition of the IS very precise? Is the same amount added precisely to all standards, blanks, and samples?
7. Do you always use the same lot of IS for the standards and samples? (Using the same lot is very important).
8. If your plasma temperature were to go up or down is the IS likely to follow the same pattern of intensity change as the analyte? This is where many IS problems occur (i.e. - an IS with the same plasma / temperature behavior as the analyte is difficult [at best] to find for each analyte while avoiding other issues listed above).

The technique of standard additions is a tedious but more reliable approach for matrix correction. For unknown matrices it may well be the fastest approach. When using standard additions on unknown matrices, it is possible to have severe spectral and background correction problems. It is cautioned here that at least two spectral lines should be used and the spectral region carefully scanned and studied. Attempt to make the additions approximately 2x and 3x, where x is the analyte concentration.

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