Fig. 4.
Dependence of ion signal intensities on analyte/matrix ratios. (A and B) Plot of relative intensity of trifluoroacetate (A) and stearate (B) anions versus MAILD matrices (DMAN, solid lines; DMA, dashed lines), concentrations plotted for clarity as log2 [matrix]. The amount of analytes was kept constant at 250 pmol, and the matrix concentrations were increased to have the following matrix-to-analyte molar ratios: 0.02:1. 0.05:1. 0.1:1. 0.2:1. 0.5:1. 1:1. 2:1. 5:1. 10:1 and 100:1, for which MAILD-TOF/MS measurements were made; 100% = 40,000 counts (A); 100% = 35,000 counts (B). (C) A plot of pKa versus limit-of-detection (LOD) for 4 different acids, namely TFA, TCA, PA, and SA. The solid line represents the LOD studies with DMAN as matrix (TFA 300 fmol; TCA 750 fmol; PA 7.8 pmol; SA 15.6 pmol). The dashed line represents the LOD studies with N,N-dimethylaniline as the matrix (TFA 1 pmol; TCA 2.5 pmol; PA 10 pmol; SA 20 pmol). Inset shows the same curve for just 2 acids, TFA and TCA to highlight the difference in the LOD obtained for the 2 acids. Error bars represent SEM (n = 5, 20 scans with 20 laser shots per scan).