Internal Standardization and Isotope Dilution

Internal Standardization

The calibration curve technique is the most popular calibration technique. If the sample matrices are known and consistent then matrix matching the calibration standards to the samples is an excellent option. Even when matrix matching is an option, many analysts still use an internal standard. It is suggested that the analyst consider the following questions before using an internal standard:

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).

As discussed in the last part of this series, the matrix can influence the plasma as well as the nebulizer. Internal standardization is very effective in correcting for nebulizer related effects and may be effective for correcting plasma related effects. It is obviously important that the matrix effect influence both the internal standard to the same extent as the analyte. This should be the case for nebulizer related effects but it may not be so for plasma related effects where the matrix influence is related to the excitation potential of the emission line (as discussed in Part 10). It may be difficult to find an internal standard that has a similar excitation potential as the analyte in measurements where several analytes are involved. The analyst is advised to confirm that the matrix influences the internal standard and analyte signal intensities proportionately.

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