Investigations into Ambient Ionization Mechanisms: Electrospray Ionization and Direct Analysis in Real Time Mass Spectrometry

Mass spectrometry (MS) is an important tool for chemical analyses. Despite the MS requirement for generation of analyte gas-phase ions, many ion source designs afford little-to-no fragmentation, allowing characterization of intact molecules. However, this does not assure that detected ions are representative of the analytes’ natural state. Ionization mechanisms are generally complex and rarely fully understood. Fundamental research into these mechanisms provides greater insight into the relationship between solution chemistry and mass spectra. Work herein addresses aspects of two ambient ionization mechanisms: electrospray ionization (ESI) and Direct Analysis in Real Time (DART).

Ions produced by ESI are dispersed into a fine aerosol to encourage droplet evaporation, ultimately resulting in bare gaseous ions. Evaporation will induce cooling of emitted droplets over time. In this research, ratiometric fluorescence thermometry was used to probe droplet temperature evolution, and to assess whether it is adequate to impact probed equilibria. Under typical ESI conditions, droplet temperatures were observed to decrease ~30 K axially within ~0-5 mm from the emitter, before rewarming ~3 K over ~5 mm. These profiles were fit using diffusion- and surface-controlled evaporation models. Both fit well, (R ≥ 0.994), but the latter required unrealistic droplet radii for a good fit. In lateral profiles near the emitter tip, temperatures are lower in the periphery than on-axis (by ≤ 10 K), consistent with expected enrichment of the spray periphery with smaller droplets. At longer axial distances, lateral profiles were relatively flat. At lower flow rates, droplet temperature was observed to fall more rapidly, possibly attributable to changes in droplet size and/or velocity with flow rate.

DART studies of selected compounds in a range of solvents were performed to assess gas-phase ion chemical effects on the relationship between detected ion abundances and bulk solution composition. When the DART gas stream contacts a sample solution, desorption/ionization of the matrix can inhibit analyte ionization, suppressing analyte signal. The effect depends on the components’ relative proton affinity and ionization energy. This effect was determined to be present with quantities ≥ 10 nL liquid or 10 μg [microgram] solid and at analyte-to-matrix ratios less than 1:100.