Matrix Effects in Desorption Electrospray Ionization across Mineral and Rock Substrates

Desorption electrospray ionization mass spectrometry imaging (DESI-MSI) is increasingly used for in situ chemical imaging of geological and planetary materials. However, quantitative comparisons across mineral and rock substrates are often complicated by matrix effects. For geological samples, these effects can arise not only from ion suppression caused by indigenous compounds but also from substrate-dependent variations in signal response imposed by the physical and chemical properties of mineral and rock surfaces. Here, we focus on the latter and systematically evaluate DESI matrix effects across 25 representative mineral and rock substrates by continuously infusing a six-component internal-standard mixture into the spray solvent and quantifying ion responses. The signals of different internal standards generally changed in the same direction across the substrate series. Signal intensities decrease with increasing surface roughness, indicating that physical sampling and transfer efficiency exert first-order control. At comparable roughness, different mineral classes still exhibit systematic response differences, consistent with additional crystal-chemical modulation through substrate-derived ionic backgrounds and thin-film interfacial chemistry. Substrates also systematically shift ion-form distributions among protonated and deprotonated species, as well as Na/K/Cl/formate adducts. Together, these findings demonstrate that substrate properties systematically shape DESI signal behavior, affecting both ion yield and ion-form partitioning, and provide a practical basis for mitigating matrix effects in geological and planetary DESI imaging, for example, through normalization using spray-added internal standards to reduce substrate-driven variability in analyte ion intensities.