The most common example is correcting for matrix differences between the calibration standards and samples, and the key criteria for using an internal standard are: 1) it must not be present in the sample, 2) it must be free from interferences, 3) it’s added in equal concentrations to all analytical solutions (blanks, calibration standards, and samples).

It is suggested that the analyst consider the following questions before using an internal standard:

• Is the internal standard (IS) element compatible with your matrix? (Avoid using rare earths in fluoride matrices.)
• Are there any possible spectral interferences upon the IS line?
• Is the concentration of the IS sufficient to give a good signal to noise ratio?
• Can your sample possibly contain the IS element as a natural component?
• Is the IS clean? Are the trace impurities reported on the certificate of analysis?
• Is your method of addition of the IS very precise? Is the same amount added precisely to all standards, blanks, and samples?
• Do you always use the same lot of IS for the standards and samples? (Using the same lot is very important.)
• 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).