Matrix‐assisted ionization (MAI) is a relatively new ionization technique for analysis by mass spectrometry (MS). The technique is simple and has been shown to be less influenced by matrix effects than e.g. electrospray ionization (ESI). These features are of interest in the targeted analysis of proteins from biological samples.
Targeted protein determination by MAI‐MS was evaluated using a triple quadrupole mass analyzer equipped with a stripped nanoESI source in selected reaction monitoring (SRM) mode. The proteins were analyzed using the bottom‐up approach with stable isotopic labeled peptides as internal standards (IS). The MAI matrix was 3‐nitrobenzonitrile dissolved in acetonitrile. Aqueous sample and matrix solution were mixed in a 1:3 volume ratio. One microlitre of the dried matrix/analyte sample was introduced into the inlet of the mass spectrometer where ionization commences.
SRM settings established for ESI‐SRM‐MS of the peptides here investigated were applicable in MAI‐SRM‐MS for all evaluated peptides except one that is poorly soluble in water. Addition of IS provided efficient correction at most levels (relative standard deviation (RSD) ≤28% (except lowest digest level), r2 ≥ 0.995). This was also true for the more complex biological matrices, diluted urine (1:1; RSD = 20% a synthetic peptide, NLLGLIEAK) and diluted digested serum (1:100; RSD = 7% digested cytochrome C). Biological matrix influenced the signal intensity unless sufficiently diluted.
The results demonstrate that MAI‐SRM‐MS has promising potential in targeted protein determination by the bottom‐up approach because of its simplicity, ease of use, and speed. However, more data is needed to confirm the results prior to application in a clinical setting.