Figure 3. Chemoselective probe design for facilitating MS-based structural analysis and quantification.

The probe design includes the carbonyl-selective aminooxy for QDA [1] or thiol-selective iodoacetamide groups for QDE [4], plus two common structural motifs of a dodecyl chain for extractability into organic phases and a quaternary ion for ready ionization in MS. Each probe can also be prepared as a pair with R equal to CH₃ and ¹³CD₃ (*QDA or *QDE). The resulting adducts of metabolites with a 1:1 mixture of QDA and *QDA can be readily discerned by an m/z difference of 4.02188 by UHR-FTMS, as illustrated by a “bar-code”-like profile of the adduct pairs in the spectrum on the top right (adapted from [1]).