Where A stands for analyte, and M stands for a matrix that contains an acidic group such as carboxylic acid group. Met + and X -are as defined earlier. In the solid sample, the ratio of isolated AH + , [M-H] -, Met + , X -and A⁞Met + surrounded and stabilized by neutral matrix molecules over neutral species can be low but can be considered significant when compared with the ion yield detectable by MS, which can be as low as 10 -8 . Upon laser irradiation, in the gas phase, AH + and [M-H] -, and AH + and X -will become more associated with each other to form ion pairs, which will have a strong tendency to form the neutral molecules via proton transfer (equations 12-13). The ions and ionic species Met + , X -and A⁞Met + will have a strong tendency to form the neutral A⁞Met + ⁞X -, which is more stable than isolated A, Met + , and X -, or as isolated A⁞Met + and X -(equation 14). Polyethylene glycol (PEG), oligosaccharides, and polystyrene belong to this group of analytes. Polystyrene lacks electron pair for proton and main group metal cations, but can coordinate with transition metals using its π and π* orbitals.