Quantitative transfer of polar analytes on a solid surface to a liquid matrix in MALDI profiling

In profiling of a specimen by matrix‐assisted laser desorption ionization (MALDI) using a solid matrix, the solvent of the matrix solution extracts an analyte(s). A quantitative profiling cannot be achieved if the solvent evaporates before the complete extraction of the analyte. The extraction can become more quantitative when a liquid matrix dissolved in a solvent is used, which remains a liquid even after the evaporation of the solvent. To check this, radii of an analyte circle (r_A), a matrix solution drop (r_D) and a liquid matrix (r_M) remaining after the solvent evaporation were controlled. Three types of samples were prepared, case A (r_A, r_D < r_M), case B (r_D ≤ r_M < r_A) and case C (r_M < r_D < r_A). In case A, the analyte amount in the matrix layer determined by MALDI was the same as the prepared amount inside the analyte circle. In case B, the analyte amount was the same as the amount inside the matrix circle. Only the analytes in contact with the liquid matrix layer, not more and not less, are transferred to the matrix layer. In case C, the analyte amount was greater than the amount inside the matrix circle, presumably because some of the analyte outside the matrix circle was dissolved by the solvent of the matrix solution. Copyright © 2016 John Wiley & Sons, Ltd.