Figure 3.

Variant screening. (A) When a patient sample with a missing signal for the ADET peptide was acquired in discovery DDA, a fragment spectrum was found that could be annotated as TDETQALPQR (A376T). (B) The same sample was then reacquired in MRM, this time targeting the mutated peptide by precursor mass and by two b-ions that contain the mutation; a clear signal could be picked up. Adding these to the assay now allows detection of the mutation in all patients in the batch. (C) By checking the GISAID database,³³ the frequency of this mutation in Belgium showed that this variant was circulating around the time the samples were taken. Not long after, the Delta variant, which contains the (D377Y) mutation, completely replaced the other variants. The figure is composed of two consecutive screenshots. (D) Therefore, a similar approach was applied to specifically identify a biomarker peptide for the Delta VoC and the resulting high resolution MSMS spectrum is shown. (E) This D377Y mutation was still immuno-enriched by the SISCAPA antibody reagent (somewhat less efficiently), and again the target can easily be added to the MRM, albeit at a slightly shifted retention time.