A Method for Defining the Position of Ion Formation in a MALDI TOFMS by Analysis of the Laser Image on the Sample Surface.

A method is developed to determine the position of ion formation along the flight axis of a MALDI TOFMS instrument using the image of the laser on the sample surface. Previous work (JASMS 2018, 29, 422-434) showed that misalignment of the sample stage in a Bruker Autoflex III MALDI TOFMS as well as multiple insertions/mountings of the target plate and differences in target plate shape itself produced reproducible changes in the measured ion time-of-flight which could be attributed to changes in the position of ion formation along the instrument flight axis. Here, a small but reproducible change in the position of the laser in the sample-viewing camera image was observed, with the movement depending on both the sample position and target plate used. Using the change in coordinates of the laser position in the camera image and the known angle of incidence of the laser on the sample surface, the initial z-axis position of the ion at different locations on the plate can be calculated, exactly defining changes in the ion flight path length and the distance between the sample plate and first extraction plate/grid with sample position on the target plate. A correction method is developed to correct the time-of-flight values collected from different locations on the sample plate using the laser images, with the relative standard deviation (RSD) being reduced from 23 ppm to below 6 ppm. The laser images, along with the measured target plate heights, are also used to calculate the misalignment of the sample stage. Graphical Abstract.