Positive and negative analyte ion yield in matrix-assisted laser desorption/ionization

The ratios of positive to negative analyte ion yields for matrix-assisted laser desorption ionization were studied for fibrinopeptide A, angiotensin I and bradykinin in combination with six matrices (CHCA, DHB, 4-NA, ATT, ANP, 5-AQ). The selection of these particular compounds was based on their acid/base properties. The measurements were carried out on two different time-of-flight instruments, one of which was equipped with a charge collection detector. The findings are: (i) for a desorption from a non-metallic substrate the total positive/negative ion yield ratio is ≈1. (ii) The analyte positive/negative ion yield is strongly dependent on many factors, and generally scatters a lot. More acidic matrices were found to produce a positive/negative analyte ion yield ratio >1, whereas for basic matrices, the ratio was ≤1. (iii) For selected matrices, the positive/negative analyte ion yield ratio follows the acid/base properties of the analyte. (iv) Clear indications for a significant contribution of clusters as carriers of the total charge in MALDI were obtained from the data.