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Abstract Nano-second, capillary discharges (nCDs) are unique plasma sources in their ability to sustain high specific energy deposition ω dep approaching 10 eV/molecule in molecular gases. This high energy loading on short timescales produces both high plasma densities and high densities of molecular exited states. These high densities of electrons and excited states interact with each other during the early afterglow through electron collision quenching and associative ionization. In this paper we discuss results from a two-dimensional computational investigation of a nCD sustained in air at a pressure of 28.5 mbar and with a voltage amplitude 20 kV. Discharges were investigated for two circuit configurations—a floating low voltage electrode and with the low voltage electrode connected to ground through a ballast resistor. The first configuration produced a single ionization wave from the high to low voltage electrode. The second produced converging ionization waves beginning at both electrodes. With a decrease of the tube radius, the velocity of the ionization fronts decreased while the shape of the ionization wave changed from the electron density being distributed smoothly in the radial direction, to being hollow shaped where there is a higher electron density near the tube wall. For sufficiently small tubes, the near-wall maxima merge to have the higher density on the axis of the capillary tube. In the early afterglow, the temporal and radial behavior of the N 2 (C 3 Π u ) density is a sensitive function of ω dep due to electron collision quenching. These trends indicate that starting from ω dep ⩾ 0.3 eV/molecule, it is necessary to take into account interactions of electrons with electronically excited species during the discharge and early afterglow.
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Performance of a new coaxial ion–molecule reaction region for low-pressure chemical ionization mass spectrometry with reduced instrument wall interactions
Abstract. Chemical ionization mass spectrometry (CIMS) techniques have becomeprominent methods for sampling trace gases of relatively low volatility.Such gases are often referred to as being “sticky”, i.e., havingmeasurement artifacts due to interactions between analyte molecules andinstrument walls, given their tendency to interact with wall surfaces viaabsorption or adsorption processes. These surface interactions can impactthe precision, accuracy, and detection limits of the measurements. Weintroduce a low-pressure ion–molecule reaction (IMR) region primarily builtfor performing iodide-adduct ionization, though other adduct ionizationschemes could be employed. The design goals were to improve upon previouslow-pressure IMR versions by reducing impacts of wall interactions at lowpressure while maintaining sufficient ion–molecule reaction times. Chambermeasurements demonstrate that the IMR delay times (i.e., magnitude of wallinteractions) for a range of organic molecules spanning 5 orders ofmagnitude in volatility are 3 to 10 times lower in the new IMR compared toprevious versions. Despite these improvements, wall interactions are stillpresent and need to be understood. To that end, we also introduce aconceptual framework for considering instrument wall interactions and ameasurement protocol to accurately capture the time dependence of analyteconcentrations. This protocol uses short-duration, high-frequencymeasurements of the total background (i.e., fast zeros) during ambientmeasurements as well as during calibration factor determinations. Thisframework and associated terminology applies to any instrument andionization technique that samples compounds susceptible to wallinteractions.
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- Award ID(s):
- 1652688
- PAR ID:
- 10168494
- Date Published:
- Journal Name:
- Atmospheric Measurement Techniques
- Volume:
- 12
- Issue:
- 11
- ISSN:
- 1867-8548
- Page Range / eLocation ID:
- 5829 to 5844
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.5194/amt-12-5829-2019
