New method for measuring low NO concentrations using laser induced two photon ionization

Abstract

A new method based on laser induced two photon ionization was developed for detection of low levels of NO concentration in the troposphere. A frequency-doubled pulsed dye laser operating at near 226 nm was used to photoionize NO by a (1ϩ1) resonance enhanced two photon ionization via its A 2 ⌺←X 2 ⌸(0,0) band. NO ions are detected by an electron multiplier. Air gas samples are injected through a pulsed nozzle into a vacuum chamber. Cooling of rovibrational levels during the expansion process allows better selectivity for NO detection from other nitrocompounds. Measurement conditions were optimized as a function of X/D for the nozzle ͑X is the distance from the nozzle orifice to the ionization area, and D is the diameter of the nozzle orifice͒, laser power, and electron multiplier voltage. For the first time, selectivities for NO measurement, which were obtained from ion intensity ratios of NO and other nitrocompounds such as NO 2 at the same concentrations, are discussed. A selectivity of 45 for NO vs NO 2 was obtained using a 44 J pulse energy and an X/D of 120 for the nozzle. The current detection limit determined for NO was 16 pptv with an integration time of 1 min and a pulse energy of 44 J.

Development of medium pressure laser ionization, MPLI. Description of the MPLI ion source

“…This is an order of magnitude lower than the value reported by Lee et al [11,22] in which an ionization volume of 5000 mm 3 and integration time of 1 min was used.…”

Unexpected Signals in Gas Chromatograph/Multiphoton Ionization/Time-of-Flight Mass Spectrometer Designed for the Detection of Negatively Charged Species

“…These spectra were nearly identical to the reported one; the signals can be assigned to the transition from the ground state to the first excited state (X 2 Π1/2, X 2 Π3/2 ← A 2 Σ + ). 11,12 This result strongly suggests that signals (1) and (2) in Fig. 2 do not arise from contaminants but are produced as a result of multiphoton ionization of the analyte (nitric oxide).…”

“…2(B). On the other hand, signals (4) and (12) arise from electrons that are formed at the surface of the repeller by collisions of molecular ions that were induced by multiphoton ionization of the analyte. Signals (2) and (3) and (8) - (11) originate from electrons induced by collisions with the fragment ions.…”

High sensitive detection of nitric oxide using laser induced photoacoustic spectroscopy at 213 nm

“…Trace gas detection NO using quartz fork coupled with an acoustic organ pipe type microresonator has been investigated [12,13]. In other studies, photoionization techniques have been applied for the measurement of NO using resonance enhanced multiphoton ionization (REMPI) approaches at 226 and 384 nm [14]. However, the sensitivity achieved in all these studies is not as high as those achieved in this work at 213 nm using selfdeveloped PA cells having special geometry.…”