Parameterising matrix-assisted laser desorption/ionisation (MALDI): strategy for matrix—analyte selection and effect of radical co-additives on analyte peak intensities

Dihydroxybenzoic acid (DHB) analogues substituted at the 5-position can act as UV matrices at a wavelength of 337 nm, even when their absorption maxima are shifted past this particular wavelength.Modification of a matrix with a chiral ligand γ-(3-carboxy-4-hydroxy-anilide) (GCA) allowed it to differentiate between chiral isomers of tryptophan and also gave different intensities for glucose isomers, including structural dimers of glucose (cellobiose and maltose).An analogue that had one free hydroxyl group at the 2-OH position and a modification at the 5-position (MY10) gave protonated substance P (SP, analyte) peaks, similar in intensity to the not derivatised parent 2,5-dihydroxybenzoic acid indicating that the 5-OH position is not an important structural component.Another analogue that resembled a ‘dendrimeric’ structure of DHB (M552), also acted as a matrix, although its absorption maxima was at 552 nm suggesting the possibility of it being used at other wavelengths in addition to 337 nm.The DHB radical was complexed to a nitrone ‘spin-trap’. On complexion, the peptide (SP) peak intensity decreased. Addition of either radical initiators, such as 2,2-azobis(iso-butyronitrile) AIBN and tert-butylperoxide, or other radicals such as 2,2,6,6-tetramethyl-1-piperidinyloxy free radical (TEMPO) gave rise to higher analyte peak intensities for [SP+Na]⁺.It is thought that the DHB neutral radical is an intermediary in the protonation of the analyte. The photo-fragments of DHB, specifically the m/z 137 species, may also take part in proton transfer since possible mass analogues (hydroquinone, (deoxy)benzoin) can lead to analyte enhancement. Stabilization of or an increase in the matrix radical can also lead to analyte signal enhancement.