CIMS

Chemical Ionization Mass Spectrometer

The principle of continuous long term measurements of ultra-low H₂SO₄ concentrations with the chemical ionization mass spectrometer (CIMS) is based on the reaction between H₂SO₄ and NO₃^- primary ions and the detection of HSO₄^- ions by a mass spectrometer. This reaction occurs in an ion drift tube operated under atmospheric pressure where the sulfuric acid in the sample gas is mixed with the NO₃‾ primary ions.

H₂SO₄ + NO₃‾ → HSO₄‾ + HNO₃

A calibration system has recently been developed for the H2SO4-CIMS. This system is based on the production of H₂SO₄ through the reaction of known concentrations of SO₂ molecules and OH radicals. OH is formed by the photodissociation of water vapor inside a quartz tube, which is illuminated by a bandpass-filtered 185 nm light from a mercury lamp. In order to produce known amounts of water vapor, a nitrogen flow is pushed through deionised water in a temperature-controlled bubbler. The components of the calibration unit (lamp, quartz tube, optical system, photodiode) are installed in a metal box, which is purged with a constant N₂ flow rate in order to provide a stable photon flux. The concentration of H₂SO₄ is derived from the gas concentrations the flow rates, the geometry and the UV light intensity.  

Contact: Linda Rondo, Andreas Kürten, Joachim Curtius

last changed 13. April 2011  E-Mail: Webmasterkissmann@iau.uni-frankfurt.de