Identification of molecules and elucidation of their chemical structure are ubiquitous problems in chemistry. Mass spectrometry (MS) has frequently been applied for these purposes due to its sensitivity and versatility, as it can be applied to single compounds or complex mixtures. Ideally, MS patterns are chemical fingerprints that can be used to detect known compounds or characterize new molecules. However, the process is sensitive to the nature of parent medium (matrix effects) and instrument conditions, making unambiguous identification often difficult. Toward improved MS detection and analysis, we are developing atomistic computational approaches to understand and predict how the original environment and MS conditions regulate the ionization fate of analytes.<br/> <br/>In order for detection to occur, analytes must be charged and must transfer from the condensed phase to the gas phase, in a process called ionization. Our current efforts focuses on soft ionization processes, such as electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI), for detection of moderately polar and nonpolar analytes. Although these methods tend to retain the analyte’s structure, rather than fragmenting it as in hard ionization approaches, chemical transformations can still occur in the microsolvated phase between condensed and gas phases, and may even be accelerated relative to the condensed phase. To understand these processes, we use computational methods (density functional theory, molecular dynamics and enhanced sampling approaches) to probe ionization propensity of analytes, as well as microsolvated cluster energetics and structures. We will discuss how computational methods provide precious insights on how the nature of parent condensed phase influences the fate of an analyte in the gas phase (the so-called matrix effects). Using amino acids in water clusters as a testbed, we show how microsolvated environments can change the ionization propensity as well as the tautomeric form of the analyte, which dictates which species enter the mass spectrometer.

Using computation to probe microsolvated phases during soft ionization processes in mass spectrometry

ACS Fall 2021

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