  Use of dielectric barrier discharge (DBD) as an ion source for sensitive
chemical analysis is uncommon because barrier discharges generate excess noise
due to spatial and temporal instability. This design uses contacted, crossed
glass-coated micro-wires to focus the field into a gradually vanishing gap,
suppressing spatial and temporal variability, reducing pressure, temperature,
and humidity effects, stabilizing discharge initiation and limiting chemical
fragmentation. Positive-ion-mode proton transfer, chemical fragmentation from a
micro-discharge, and NO+ adducts combine to allow broad chemical sensitivity.
We analyze noise properties of the ion source and report chemical responsivity
for a wide range of volatile organic compounds. Source noise spectral density
is compared for three systems: the contacted coated wires source, a gapped
dielectric barrier discharge source, and a 5 mCi Ni-63 radioactive source. The
crossed-wires source shows noise properties approaching those of the
white-noise Ni-63 source, while gapped discharge exhibits 1/f noise from
area-discharge random path and intensity variations. For chemical sensitivity
testing, dilute samples are delivered by vapor flow injection or by gas
chromatography, and then detected by differential ion mobility spectrometry
(DMS / FAIMS) and, in a few cases by mass spectrometry. The compounds tested in
positive ion mode include ketones and alcohols, simple aromatics (benzene,
toluene, xylenes, ethyl and propyl benzene), chlorinated and nitrated solvents
and aliphatics (hexanes and n-octane), with sensitivities from ppt to ppb
levels. The wires source for trace vapor detection is stable under a range of
environmental conditions from very low (ppb) humidity levels upward, has wider
chemical coverage than the radioactive nickel source with nearly equivalent
noise properties, and is adjustable to higher intensity.
