Positive and negative chemical ionization mass spectrometry using a Townsend discharge ion source

Abstract. The efficiencies of charge exchange reaction in dopant-assisted atmospheric pressure chemical ionization (DA-APCI) and dopant-assisted atmospheric pressure photoionization (DA-APPI) mass spectrometry (MS) were compared by flow injection analysis. Fourteen individual compounds and a commercial mixture of 16 polycyclic aromatic hydrocarbons were chosen as model analytes to cover a wide range of polarities, gas-phase ionization energies, and proton affinities. Chlorobenzene was used as the dopant, and methanol/water (80/20) as the solvent. In both techniques, analytes formed the same ions (radical cations, protonated molecules, and/or fragments). However, in DA-APCI, the relative efficiency of charge exchange versus proton transfer was lower than in DA-APPI. This is suggested to be because in DA-APCI both dopant and solvent clusters can be ionized, and the formed reagent ions can react with the analytes via competing charge exchange and proton transfer reactions. In DA-APPI, on the other hand, the main reagents are dopant-derived radical cations, which favor ionization of analytes via charge exchange. The efficiency of charge exchange in both DA-APPI and DA-APCI was shown to depend heavily on the solvent flow rate, with best efficiency seen at lowest flow rates studied (0.05 and 0.1 mL/min). Both DA-APCI and DA-APPI showed the radical cation of chlorobenzene at 0.05-0.1 mL/min flow rate, but at increasing flow rate, the abundance of chlorobenzene M +. decreased and reagent ion populations deriving from different gas-phase chemistry were recorded. The formation of these reagent ions explains the decreasing ionization efficiency and the differences in charge exchange between the techniques.

Section: Introductionmentioning

confidence: 99%

Charge Exchange Reaction in Dopant-Assisted Atmospheric Pressure Chemical Ionization and Atmospheric Pressure Photoionization

Vaikkinen

Kauppila

Kostiainen

2016

“…Available coated filaments increase operational lifetime two to three times [26], a modest improvement upon isobutane CI results. The filament may be entirely replaced with a Townsend discharge at the expense of higher signal-to-noise [27,28].…”

Section: Introductionmentioning

confidence: 99%

Isobutane Made Practical as a Reagent Gas for Chemical Ionization Mass Spectrometry

Newsome

Steinkamp

Giordano

2016

Abstract. As a reagent gas for positive-and negative-mode chemical ionization mass spectrometry (CI-MS), isobutane (i-C 4 H 10 ) produces superior analyte signal abundance to methane. Isobutane has never been widely adopted for CI-MS because it fouls the ion source more rapidly and produces positive CI spectra that are more strongly dependent on reagent gas pressure compared with methane. Isobutane was diluted to various concentrations in argon for use as a reagent gas with an unmodified commercial gas chromatograph-mass spectrometer. Analyte spectra were directly compared using methane, isobutane, and isobutane/argon mixtures. A mixture of 10% i-C 4 H 10 in argon produced twice the positive-mode analyte signal of methane, equal to pure isobutane, and reduced spectral dependence on reagent gas pressure. Electron capture negative chemical ionization using 1% i-C 4 H 10 in argon tripled analyte signal compared with methane and was reproducible, unlike pure isobutane. The operative lifetime of the ion source using isobutane/ argon mixtures was extended exponentially compared with pure isobutane, producing stable and reproducible CI signal throughout. By diluting the reagent gas in an inert buffer gas, isobutane CI-MS experiments were made as practical to use as methane CI-MS experiments but with superior analytical performance.

“…One of the gaseous anions found in the plasma of discharge ion sources under negative-ion-generating conditions is the superoxide radical-anion, O 2 -• [5][6][7][8][9]. Sometimes, O 2 -• has been generated under pulsed positive-and negative-ion chemical ionization conditions, using a mixture of hydrogen and oxygen as the reagent gas [10].…”

Section: Introductionmentioning

confidence: 99%

“…The attachment of this superoxide radical-anion to several gaseous molecules has been documented. For example, the formation of adducts with O 2 -• has been noted for alcohols [6,11], ketones [12], phenols [13], tetrachlorodioxins [14], polyethers [15], phthalates [16,17], and some tricothecenes [7]. Recently, Cody and Dane reported the formation of O 2 -• adducts of linear aliphatic hydrocarbons, alcohols, and esters under direct analysis in real time (DART) conditions [18].…”

Section: Introductionmentioning

confidence: 99%

Competitive Deprotonation and Superoxide [O₂^-•] Radical-Anion Adduct Formation Reactions of Carboxamides under Negative-Ion Atmospheric-Pressure Helium-Plasma Ionization (HePI) Conditions

Hassan

Pinto

Weisbecker

et al. 2015

Abstract. Carboxamides bearing an N-H functionality are known to undergo deprotonation under negative-ion-generating mass spectrometric conditions. Herein, we report that N-H bearing carboxamides with acidities lower than that of the hydroperoxyl radical (HO-O ], respectively. For somewhat acidic carboxamides, the association between the two entities is weak. Thus, upon mildest collisional activation, the adduct dissociates to eject the superoxide anion. Superoxide-adduct formation results are useful for structure determination purposes because carboxamides devoid of a N-H functionality undergo neither deprotonation nor adduct formation under HePI conditions.