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RationaleIn droplet‐assisted ionization (DAI), intact molecular ions are generated from molecules in aerosol droplets by passing the droplets through a temperature‐controlled capillary inlet. Ion formation is explored through the effects of analyte mass flow, droplet solvent composition, and capillary temperature on ion signal intensity. MethodsA Waters SYNAPT G2‐S is adapted for DAI by reconfiguring the inlet with a temperature‐controlled capillary. Droplets are generated by atomization of a solution containing analyte and then sampled through the inlet. If desired, solvent can be removed from the droplets prior to analysis by sending the aerosol through a series of diffusion dryers. Size distributions of the dried aerosols allow the mass flow of analyte into the inlet to be determined. ResultsAnalyte signal intensities are orders of magnitude higher from droplets containing a protic solvent (water) than an aprotic solvent (acetonitrile). The highest signal intensities for DAI are obtained with inlet temperatures above 500°C, though the optimum temperature is analyte dependent. At elevated temperatures, droplets are thought to undergo rapid solvent evaporation and bursting to produce ions. The lowest signal intensities are generally obtained in the 100–350°C range, where slow solvent evaporation is thought to inhibit ion formation. As the temperature decreases from 100°C down to 25°C, the signal intensity increases significantly. When 3‐nitrobenzonitrile, a common matrix for solid‐state matrix‐assisted ionization (MAI), is added to droplets consisting of 50/50 v/v water and acetonitrile, the matrix enhances ion formation to produce a signal intensity comparable to DAI in 100% water. ConclusionsThe results are consistent with other inlet ionization techniques, suggesting that similar ion formation mechanisms are operative. Optimized ion yields (the combined effects of ionization probability and ion transmission) for DAI are currently in the 10−5to 10−6range, which is sufficient for many aerosol applications.
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Spray Mechanism of Contained-Electrospray Ionization
Analytical characteristics of contained electrospray ionization (ESI) are summarized in terms of its potential to modify the analyte solution during the stages of droplet formation to provide opportunities to generate native versus denatured biomolecular gas-phase ions, without the need for bulk-phase analyte modifications. The real-time modification of the charged microdroplets occurs in a cavity that is included in the outlet of the contained-ESI ion source. Close examination of the inside of the cavity using a high-speed camera revealed the formation of discrete droplets as well as thin liquid films in the droplets wake. When operated at 20 psi N2 pressure, the droplets were observed to move at an average speed of 8 mm/s providing ∼1 s mixing time in a 10 mm cavity length. Evidence is provided for the presence of highly reactive charged droplets based on myoglobin charge state distribution, apo-myoglobin contents, and ion mobility drift time profiles under different spray conditions. Mechanistic insights for the capture of vapor-phase reagents and droplet dynamics as influenced by different operational modes are also described.
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- PAR ID:
- 10161496
- Date Published:
- Journal Name:
- Journal of the American Society for Mass Spectrometry
- ISSN:
- 1044-0305
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1021/jasms.0c00044
