The amount of Pep-CO2 -in the solid sample can be estimated using the pKa of carboxylic acid group in amino acids. Assuming that the pKa of Pep-CO2 -is 2.2 and the concentration of [M+H] + in the sample with CHCA being the matrix is 0.26 M as discussed earlier, the concentration of Pep-CO2 -would be 7.3 × 10 -5 M when 10 pmol of peptide with one carboxylic acid group is dispersed in 25 nmol of CHCA and 133 mL/mole is taken as the molar volume of CHCA. It is noted that the concentration of Pep-CO2 -may be far away from 7.3 × 10 -5 M because the sample is a solid and it contains other materials besides the peptide and the matrix.