Improvement of the new NO detection method using laser-induced two-photon ionization with a TOF mass spectrometer

We have developed a new method to detect low concentrations of tropospheric NO, using laser induced two-photon ionization (Lee, S.-H.; Hirokawa, J.; Y oshizumi, Y.; Akimoto, H. Rev. Sci. Instrum. 1997, 68,2891). This method us es a frequency-doubled pulsed-dye laser operating near 226 nm to photoioniz e MO by resonance enhanced two-photon ionization via its A(2)X(2) (0,0) ban d. This work reports our recent results regarding the improvement of the ov erall instrumental performance obtained by applying a time-of-flight (TOF) mass spectrometer and further discusses the water vapor influence in this m ethod. NO ion signals were discriminated efficiently from the other ions ge nerated by impurities in the ionization cell by using a TOF mass spectromet er. The obtained sensitivity of this method was 10 pptv (S/N = 2), at a las er power of 44 muJ and an integration time of 1 min. Also, the background s ignal was decreased from 5 to 0.25 Hz as compared to the previous study, in dicating that a nearly signal-limited condition was obtained, which enables NO detection free from background noise. In our laser ionization instrumen t, the uncertainty in NO ion intensity caused by the fluctuation of laser p ower was deviated less than 3% from a long-term average. Moreover, by compa rison with the ozone-chemiluminescence method, water vapor influence has be en investigated by introducing humid sampling gases into the molecular beam . As the relative humidity in the sampling gases varied from 0 to 85% at ro om temperature, no significant change in the NO detection efficiency was ob served. This demonstrates an important advantage over the ozone-chemilumine scence method, although further field intercomparisons are required to cert ify the applicability of this new NO ionization method into the atmospheric study.