Measurement and Results - Concentration of Internal Standard
Since you are using EPA 200.8 to analyze environmental water, we recommend that you follow the internal standard specification set forth in the method. (Y, Sc, In, Tb, and Bi). The IS solution we sell for 200.8 (2008ISS) contains 20 ppm of Y, Sc, In, Tb, and Bi, and is designed to be diluted to the recommended working levels set forth in Rev 4.4 (200 ppb) and Rev 5.4 (20-200 ppb). However, the method does say that the concentration of each element can depend on the sensitivity of the instrument. If the suggested concentration of IS elements is not sufficient to produce a good signal, then optimization would be required. To optimize the internal standard, you would increase the concentration of the IS element until you reach a concentration that produces a sufficient signal. However, we recommend following the method as written unless there is technical justification from your laboratory to deviate from the prescribed method. Please keep in mind that changes in internal standard concentration may need to be vetted with additional method validation activities.
As a general rule, the IS element(s) should not be found in your samples/standards and should be compatible with your matrix. Multielement internal standards are common for MS applications. For MS applications, you should cover the mass range of the analytes of interest and the internal standard element(s) should have a similar mass.