MALDI Mass Spectrometry on High-Density Droplet Arrays: Matrix Deposition, Selective Removal, and Recrystallization.

High-density droplet arrays are emerging as a powerful tool for high-throughput bioanalytical applications. These arrays are formed of thousands of nanoliter droplets, which can be analyzed by various optical and spectroscopic methods as well as label-free matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS). However, special precautions are required for the massive number of small droplets, particularly in the deposition of matrix compounds. Here, we introduce a new workflow for the analytical preparation of an array comprising 6048 droplets, which significantly improves the intensity of the MALDI-MS signals. We deposited matrix compounds in a custom-made sublimation chamber followed by a recrystallization step to achieve significant signal intensity increases for three model proteins with low, medium, and large masses, respectively. Furthermore, selective removal of the matrix before recrystallization enhanced the spatial resolution and increased the signal intensity by an average of 57%. This method can be easily standardized and upscaled for the preparation of an even larger number of droplets per array for MS analysis.