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Measurement and Results - Concentration of Internal Standard

 
I have a question about the concentration of internal standard. We are using EPA 200.8 to analyze environmental water. I heard that "the concentration of Sc should be 100ppb and Y, In, and Tb is 20-30ppb.". Because we are using multi-element internal standard in ICP-MS, the concentrations of Sc, Y, In, and Tb are the same and I can't make like 100ppb Sc nad 20-30ppb others as what I heard. (1) Can you let me know why the concentration of IS should be different? (2) Please let me know why the concentration of Sc should be higher than others. Also, I heard that I have to use individual internal standard to make internal standard like below. "Sc can start at 50ppb: add 25ul to 500ml bottle or 50ul to 1L bottle. Spike in additional amounts until counts are 2.5-3.5M (guess total of 100ppb) All the other IS (Y, In, and Tb) start at 10ppb: add 5ul to 500ml bottle or 10ul to same 1L bottle. Spike in additional amounts until counts are around 500k (guessing 20-30ppb).". "This is for the optimization of internal standard." (3) Can you explain what is the optimization of internal standard, if you know? (4) Who can use the multi-element internal standard? Thanks and I am looking forward to hearing from you.
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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.

Posted: 07/24/20 12:18:45

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