How to Challenge an Analytical Method's Selectivity

Eurachem emphasizes that defining the measurand is critical when a substance can appear in different forms. Quantifying a particular form is not the same question as quantifying a group of related forms. Before evaluating interference, the report must make clear what its result includes.

A hypothetical example is an assay recognizing both the complete peptide and a fragment. It may be suitable for a total signal defined that way, but insufficient if the conclusion is intended to report only intact material. Eurachem guide.

ICH Q2(R2) allows absence of interference to be demonstrated using appropriate materials or comparison with an independent procedure. For a separation, discrimination from nearby components is especially relevant; showing that the detector responds when only the analyte is present is insufficient.

When reading a report, identify which alternatives were evaluated and why they were plausible. A blank rules out some background signals but does not necessarily represent a structural variant absent from that blank. The conclusion should retain this distinction. ICH Q2(R2).

A study of neuropeptide Y analogs examined variants with configurational changes. An initial chromatographic separation distinguished many, but others required changing the separation system or using capillary electrophoresis. The difficulty was not the same for every variant.

The example shows how to formulate a demanding test: challenge the method with materials that could be confused. It does not turn the article's conditions into a general recipe or show that one technique is always superior to another. Original research.

ICH Q2(R2) recommends combining procedures when one alone does not discriminate sufficiently. A second measurement is useful according to the information it adds: repeating the same limitation under a different instrument name does not resolve ambiguity.

When evaluating the comparison, ask which identity or interference each procedure can distinguish and whether both results actually describe the same material.

Eurachem proposes investigating foreseeable interferents and reviewing scope when a method's application changes. A useful conclusion therefore specifies the substances and matrices examined, along with outstanding limitations.

Instead of declaring absolute selectivity, a report can explain that no interference from evaluated components was observed within the studied range. That wording helps determine whether a new sample is represented by the validation.