Improving Mass Measurements in MALDI TOFMS by Measuring Initial Ion Position and The Investigation of Sample Preparation Parameters in the Cationization of Synthetic Polymers
Authors:   , Piotrowski Michelle Louise , Owens Kevin Glenn 
Published: April 2022

Mass spectra collected with a Bruker Autoflex III MALDI TOFMS were found to have poor intensity precision and mass accuracy. Both the within- and between-sample reproducibility of peak areas were significantly higher than results seen in previous work for samples deposited using electrospray deposition. A correlation between TOF peak area and sample location was observed. It was found that the misalignment of the instrument sample stage, multiple insertions and mountings of the target plate, and the different shapes of the sample plate were the four sources of variation in height along instrument flight axis. These changes in height produced reproducible changes in the measured ion TOF. In addition, a small but reproducible change in the position of the laser spot 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 sample plate can be calculated, exactly defining changes in the ion flight pathlength 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 TOF values collected from different locations on the sample plate using the laser images, with the relative standard deviation (RSD) of TOF 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 absolute misalignment of the sample stage. A LabView based virtual instrument was used to simulate flight times of ions at various heights along the z-axis, which were compared with the measured flight times taken from the instrument. MALDI is a soft ionization technique often used to analyze synthetic polymer analytes and often a cationization agent is needed. For the analysis of a polymer analyte both amount and type of the matrix and cationization agent (termed "salt") should be considered. The choice of matrix must be compatible with the polymer; the matrix, polymer and cationization agent used must all be soluble in the solvent chosen. The cationization agent can improve the ion signal intensity with either the salt present in the sample or MALDI matrix as an impurity (as in the case of Na⁺ or K⁺) or added during the sample preparation process. Polymers, such as PEG have shown different affinities towards salt used and their selectivity can change as the matrix changes. Previous work in our group investigated the effect of salt-to-analyte (S/A) ratio for cationization for the polymer PMMA, looking at the choice of matrix and concentration of the salt. In some cases the addition of salt can decrease the signal intensity of the polymer analyte. S/A ratio plots are used in this work to investigate the effect of cationization agent concentration on the analysis of PEG with different matrices. The cationization agent used in this work is Cs+, and it was found that higher amounts of matrix were needed to improve the signal of the analyte. Several matrix clusters were observed in the low mass region of the mass spectra that could indicate a competition between the matrix and analyte for the cationization agent.