Measurement and Results - Selenium specifications for EPA Method 200.8
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We’re not aware of the exact reason as to why the method stipulates that Se should be 5x higher than the other analyte concentrations, but we do have a hypothesis. The method section referenced regards calibration standards and the relevant statement in our opinion is “The element concentrations in the standards should be sufficiently high to produce good measurement precision and to accurately define the slope of the response curve.”. Selenium has a high ionization energy and therefore has relatively poor signal response at a given concentration, so we suspect that the method is trying to ensure that the signal response is high enough to meet the requirements for a good calibration curve. Another 200.8 element with a similar high ionization energy behavior is As, but it is monoisotopic so intensities are proportionally higher than the numerous Se isotopes you have to choose from. This is a technically reasonable answer, particularly for ICP-MS instruments prevalent when the method was first promulgated, and that lack the sensitivity of newer models. p.s., thank you very much for including the method reference! While we have that revision in our technical library, it was very helpful to have the passage front-and-center, and we hope the ideas discussed prove equally useful.
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