For the neutral matrix molecule CHCA, the experimental ion yield, which was defined slightly different from that of permanent salts, and is the quotient of total number of ions detected by MS over the total number of molecules including ions and neutral molecules in the sample, is in the order of 10 -7 -10 -8 . These are significantly lower than the yields of permanent salts such as BTPP-Cl. The lower values are probably caused by the lower concentration of the ion pair [M-H] -⁞[M+H] + (see individual ion pair analysis with equation 1 in supporting information) in the plume than ion pairs of BTPP-Cl. For BTPP-Cl, 100% of the species can be assumed to exist as ion pairs in the plume. For CHCA, the majority of the molecules in the plume would be neutrals. At 1,000 K, the ratio of ion pair over the matrix is estimated to be 10 -13 . The estimated ion yield would be 4.6 × 10 -18 (4.6 × 10 -5 × 10 -13 ), which is significantly lower than the experimental data.