Even when the average molecular weights of the PEG components of the inks are very close to each other (e.g., panels B and C in Figure ), a distinction between them can be made based on the composition of the end groups. Indeed, close examination of masses of PEG ions in LDI mass spectra of all four inks (insets in Figure ) reveals very significant differences. The mass of each PEG ion is a sum of the mass of the chain, end groups, and a cation (usually either sodium or potassium):