The amount of Pep-NH + in the solid sample should be close to 100% because the pKa of the matrix such as CHCA is about 1.17 as described earlier, and that of Pep-NH + can be as high as 14. Again, these numbers are for the cases with water as the medium and thus not accurate for solid CHCA sample. However, it should be reasonable to assume that a high percentage of peptide analytes exists in the form of Pep-NH + in the sample in the context that the MALDI MS ion yields of peptides are in the order of 10 -4 . In the gas phase, the tendency for neutralization via ion pair formation in equations (5-6) should be similar as discussed earlier. The tendency for neutralization via proton transfer as shown in equations (5-6) can be estimated using the corresponding equilibrium constants, and they are 291 kJ/mol or 10 15 and 447 kJ/mol or 10 , respectively when a pKa of 903 kJ/mol of guanidinium is used for Pep-NH + , pKa of 1194 kJ/mol of sulfuric acid is used for HX, pKa of 1350 kJ/mol of carboxylic acid is used for M, and the plume temperature is assumed to be 1,000 K. At 2,200 K, the numbers become 347 kJ/mol or 10 8 for the case of equation ( ) and 509 kJ/mol or 10 12 for the case of equation ( ). In both cases, the equilibria strongly favor the neutral species.