title: Humidity Effects on Fragmentation in Plasma-Based Ambient Ionization Sources.
abstract: Post-plasma ambient desorption/ionization (ADI) sources are fundamentally dependent on surrounding water vapor to produce protonated analyte ions. There are two reports of humidity effects on ADI spectra. However, it is unclear whether humidity will affect all ADI sources and analytes, and by what mechanism humidity affects spectra. Flowing atmospheric pressure afterglow (FAPA) ionization and direct analysis in real time (DART) mass spectra of various surface-deposited and gas-phase analytes were acquired at ambient temperature and pressure across a range of observed humidity values. A controlled humidity enclosure around the ion source and mass spectrometer inlet was used to create programmed humidity and temperatures. The relative abundance and fragmentation of molecular adduct ions for several compounds consistently varied with changing ambient humidity and also were controlled with the humidity enclosure. For several compounds, increasing humidity decreased protonated molecule and other molecular adduct ion fragmentation in both FAPA and DART spectra. For others, humidity increased fragment ion ratios. The effects of humidity on molecular adduct ion fragmentation were caused by changes in the relative abundances of different reagent protonated water clusters and, thus, a change in the average difference in proton affinity between an analyte and the population of water clusters. Control of humidity in ambient post-plasma ion sources is needed to create spectral stability and reproducibility.
authors.affiliation: Nova Research, Inc., 1900 Elkin St. Suite 230, Alexandria, VA, 22308, USA. graham.newsome.ctr@nrl.navy.mil.
authors.forename: G Asher
authors.identifier: http://orcid.org/0000-0003-1683-2197
authors.initials: GA
authors.lastname: Newsome
authors.affiliation: US-FDA Center for Food Safety and Applied Nutrition, Office of Regulatory Science - Division of Analytical Chemistry, 5100 Paint Branch Parkway, College Park, MD, 20740, USA.
authors.forename: Luke K
authors.initials: LK
authors.lastname: Ackerman
authors.affiliation: US Naval Research Laboratory, Naval Technical Center for Safety and Survivability, Code 6180, 4555 Overlook Ave. SW, Washington, DC, 20375, USA.
authors.forename: Kevin J
authors.initials: KJ
authors.lastname: Johnson
mesh_terms: D000818:Animals; D001923:Brain Chemistry; D006813:Humidity; D013058:Mass Spectrometry / Q000379:methods*; D051379:Mice; D057054:Molecular Imaging / Q000379:methods*; D058626:Plasma Gases
chemical_list: D058626:Plasma Gases
keywords: Ambient ionization; DART; FAPA; Fragmentation; Humidity; Mechanism; Water cluster