Time-of-flight detection coupled to a flowing afterglow: Improvements and characterization

• Large gain in signal-to-noise switching from QMS to reflectron ToF mass spectrometer.
• Minor mass discrimination for cations > ∼10 m/z, less mass discrimination for anions.
• Very light ions discriminated against; likely a surface coating effect.
• Dynamic range in signal sensitivity has been increased by one order of magnitude.

An orthogonally accelerated time-of-flight mass spectrometer has been added to a flowing afterglow–Langmuir probe apparatus, coupled by a rectilinear ion guide. The TOF has improved the signal-to-noise (S/N) ratio by about an order of magnitude over a quadrupole mass filter/electron multiplier detection typical for flow tube instruments of this type and used previously. The improved S/N reduces our data acquisition time commensurately. Additionally, mass discrimination is low enough that it is minimal for ions heavier than ∼10 amu. The lack of mass discrimination not only reduces experiment time but also leads to greater accuracy in the measurement of reaction rate coefficients. Finally, simultaneous detection of both heavy and light ions is improved. H+ can now be detected and WF5+ isotopes can be separated without reducing sensitivity. We have found that discrimination against low-mass ions can occur when certain electron-attaching reactants are used (e.g. SF6), and possibly due to radicals formed after certain other reactants attach electrons. Evidence points to adsorption of these species on surfaces of the ion guide immediately behind the flowing afterglow sampling orifice, which separates the flow tube and TOF region. The adsorption follows a Langmuir isotherm profile and is greatly reduced by heating the gas in the flow tube.

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