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Use of dielectric barrier discharge (DBD) as an ion source for sensitivechemical analysis is uncommon because barrier discharges generate excess noisedue to spatial and temporal instability. This design uses contacted, crossedglass-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 chemicalfragmentation. Positive-ion-mode proton transfer, chemical fragmentation from amicro-discharge, and NO+ adducts combine to allow broad chemical sensitivity.We analyze noise properties of the ion source and report chemical responsivityfor a wide range of volatile organic compounds. Source noise spectral densityis compared for three systems: the contacted coated wires source, a gappeddielectric barrier discharge source, and a 5 mCi Ni-63 radioactive source. Thecrossed-wires source shows noise properties approaching those of thewhite-noise Ni-63 source, while gapped discharge exhibits 1/f noise fromarea-discharge random path and intensity variations. For chemical sensitivitytesting, dilute samples are delivered by vapor flow injection or by gaschromatography, and then detected by differential ion mobility spectrometry(DMS / FAIMS) and, in a few cases by mass spectrometry. The compounds tested inpositive ion mode include ketones and alcohols, simple aromatics (benzene,toluene, xylenes, ethyl and propyl benzene), chlorinated and nitrated solventsand aliphatics (hexanes and n-octane), with sensitivities from ppt to ppblevels. The wires source for trace vapor detection is stable under a range ofenvironmental conditions from very low (ppb) humidity levels upward, has widerchemical coverage than the radioactive nickel source with nearly equivalentnoise properties, and is adjustable to higher intensity.

A gapless micro-dielectric-barrier-discharge ion source for analytical applications