Electrospray ionization mass spectrometry supported by mobility classification (ESI-DMA-MS) is a strong technique for a comprehensive analysis of organic and inorganic clusters and small nanoparticles. In-depth understanding and optimization of the electrospray process are key for unlocking new fields of application and for extension of the accessible range of data. We show that the combination of mobility-classified mass spectrometry (DMA-MS) with an electrospray operated in a well-defined cone-jet mode is capable of providing a full picture of the outcome of the electrospray process including the evaporation residues of the parent droplets. Based on ESI-DMA-MS measurements of lysozyme, we find that clusters up to almost 1 MDa (around 60 lysozyme molecules per cluster) formed as evaporation residues of the parent droplets can be detected. The escape probability of lysozyme molecules from the parent droplets is polarity-dependent. A quantitative analysis of clusters is possible at analyte concentrations where the likeliness of finding more than a single molecule in the generated droplet is low. At higher concentrations, one can, however, quantify the analyte concentration from the detected size of the droplet evaporation residues. The approach is widely applicable for organic and inorganic nanostructures. The results allow clear statements on the conditions under which mixtures of larger molecules, clusters, and nanoparticles are accessible to quantitative analysis by mass spectrometry.
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