Hydrogen Plasma-Based Medium Pressure Chemical Ionization Source for GC-TOFMS

Abstract

The construction, critical evaluation, and performance assessment of a medium-pressure (2–13 mbar), high-temperature chemical ionization (CI) source for application in GC-MS is described. The ion source is coupled to a commercial time-of-flight (TOF) mass analyzer. Reagent ions are generated in a two staged process. The first stage uses a filament free, helical resonator plasma (HRP) driven ion source for H3 + generation. Reagent gases, for example, nitrogen, isobutane, and methane are added in a second stage to the H3 + stream, which leads to the formation of final protonation reagents. The GC effluent is added subsequently to the reagent ion gas stream. Designed for the hyphenation with gas chromatography, this GC-CI-TOFMS combination produces GC limited Gaussian peak shapes even for high boiling point compounds. Limits of detection for the compounds investigated are determined as 0.4–1.2 pg on column with nitrogen, 0.6–12.6 pg with isobutane, and 2 pg to >25 pg with methane as reagent gas, respectively. An EPA 8270 LCS mix containing 78 main EPA pollutants is used to evaluate the selectivity of the different reagent ions. Using nitrogen as reagent gas, 74 of 78 compounds are detected. In comparison, 41 of 78 compounds and 62 of 78 compounds are detected with isobutane or methane as CI reagent gas, respectively.

Parallel Operation of Electron Ionization and Chemical Ionization for GC–MS Using a Single TOF Mass Analyzer

“…Details on the custom CI source in combination with a GC-TOF were presented previously. 29 Both sources operate continuously. A custom ion optical device consecutively extracts the ion populations from each source and pulses them into the TOF analyzer.…”

“…The EI source was operated with 70 eV electron energy and an emission current of 1 mA. Details on the custom CI source in combination with a GC-TOF were presented previously …”

“…A fatty acid methyl ester (FAME) C 4 –C 24 saturated carbon mixture and an EPA 8270 LCS mix were diluted with hexane to 1 and 2.5 ng/μL, respectively. The CI source was operated as described in ref . For the perfume analysis, 0.2 mL headspace (HS) of a brand perfume was sampled using a manual HS sampler (Ellutia, Ely, UK).…”

Gas chromatography coupled to time‐of‐flight mass spectrometry using parallel electron and chemical ionization with permeation tube facilitated reagent ion control for material emission analysis

“…In Ref. [19], the selectivity was altered by adding different reagent gases such as nitrogen, methane, and isobutane directly into the second stage of the CI source. In this study, however, a main flow of nitrogen enriched with dopants such as water or ammonia provided by a reagent gas supply unit is used for reagent variation as shown in Figure 1.…”

“…The plasma-based CI source used in combination with the EI source was presented earlier. 19 Low hydrogen flow rates obtained from a small and laboratory-safe on-demand generator are used for ion generation. Earlier it was shown that the primarily generated H 3 + population can be specifically converted to a variety of other reagent ions, e.g., [N 2 H] + /[N 4 H] + , [H 3 O] + , or [C 4 H 9 ] + , which enables the adjustment of the gas phase acidity and consequently balance between selectivity and the degree of fragmentation.…”