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‘The signal-to-noise ratio is used to define the system sensitivity. The limit of quantitation (corresponding to a signal-to-noise ratio of 10) is equal to or less than the reporting threshold.’
This requirement concerns all LC or GC analytical procedures stated in the TESTS section of a monograph and which include a reporting threshold (or disregard limit in older monographs); it does not concern those included under the ASSAY section.
In monographs, a reporting threshold is only stated in tests:
- where the total of impurities is limited,
- when normalisation is applied for the quantitation of only one impurity, or
- when there is a potential sensitivity issue.
Therefore, this requirement is not to be applied to assays or tests that do not include a defined reporting threshold/disregard limit.
The term ‘tests’ covers tests for both organic and inorganic impurities.
It should be emphasised that the previous version of the general chapter mentioned ‘in a related substances test, the limit of quantification (corresponding to a signal-to-noise ratio of 10) is equal to or less than the disregard limit.’
Past experience showed that this statement was sometimes misunderstood, as it was wrongly interpreted to apply only to tests under the heading ‘Related substances’, although it was meant to apply to any related substances (i.e. impurity) test in monographs. The term ‘in a related substances test’ was therefore omitted in the course of the international harmonisation work.