Optimization of Surface-Assisted Laser Desorption/Ionization Mass Spectrometry

Abstract

In recent years, there has been a growing interest in the study of small molecules (e.g. metabolites, lipids, drugs) as they play important roles in various research areas, including biomedicine, biotechnology, environmental science and drug discovery. Mass spectrometry (MS) has become a predominant technology to analyze these low molecular weight compounds. However, the analysis of small molecules by the established matrix-assisted laser desorption/ionization (MALDI) MS technique can be difficult because of the interference generated by the matrix in the low m/z range. Therefore, new innovating ionization techniques that do not require an organic matrix have been developed. Among these alternatives is surface-assisted laser desorption/ionization (SALDI) MS, which relies on nanostructured surfaces to promote the desorption/ionization process. Yet, the implementation of this novel ionization technique introduces new challenges, which include the understanding of the fundamental mechanisms that govern the SALDI process, and the optimization of the experimental factors. In particular, the design and optimization of the assisting nanosubstrates are not straightforward, because of the wide range of available nanomaterials (in terms of chemical nature and morphology), characterized by different properties, which impact the performance of the SALDI MS experiments. Thus, it is essential to find a model to compare the capabilities of the SALDI nanosubstrates, in order to optimize their characteristics based on the same reference. In this context, thermometer ions have been used as reference compounds to test novel instrumentation, new methodologies, or to tune instruments. In this article, we review the study of thermometer ions in SALDI MS, which have enabled to rationalize the effect of the experimental factors (i.e. the nanosubstrate morphology and chemical nature, and the instrumental settings) on the performance of SALDI MS experiments. These studies allow the rational design of optimized nanosubstrates for advanced applications, such as SALDI MS imaging, which will be discussed at the end of this article.