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Developing repeatable measurements for reliable analysis of molecules at surfaces using desorption electrospray ionization.

Desorption electrospray ionization (DESI) is a powerful ambient ionization technique that can provide high-sensitivity mass spectrometry information directly from surfaces at ambient pressure. Although a growing amount of research has been devoted to exploring different applications, there are few studies investigating the basic parameters and underpinning metrology. An understanding of these is crucial to develop DESI as the robust and reliable technique required for significant uptake by industry. In this work, we begin with a systematic study of the parameters affecting the repeatability, sensitivity, and rate of consumption of material with DESI. To do this we have developed a model sample consisting of a thin uniform film of controlled thickness of Rhodamine B on glass. This model sample allowed assessment of optimal sensitivity and spot shape under different conditions. In addition, it allowed us to study the surface in more detail to understand why and how each parameter affects these. Using the model sample to optimize the instrument parameters for DESI led to an absolute intensity repeatability of better than 15%, achieved over a period of 1 day. This model sample provides valuable insight into the electrospray-sample interaction and the desorption mechanism. Confocal microscopy of areas analyzed by DESI allow droplet distribution, material utilization, and spot size to be determined. Studying surface erosion also gives the erosion rate of material, analogous to the sputtering yield in secondary ion mass spectrometry. The results of the study provide a clear description that explains the differences observed with changing electrospray parameters allowing optimization of the technique, for both spatial resolution and sensitivity.