The CIMS is designed to measure sulphuric acid. It was build by Dr. Greg Huey's group at Georgia Tech. The working principle is a selective ionisation of sulphuric acid molecules via a reaction with nitrate ions. The nitrate ions are generated by an Am241 source and mixed with the sample flow, where sulphuric acid transfers a proton to the nitrate. The products of this proton exchange are a single charged sulphate ion and a neutral nitric acid molecule. Ionised species enter the actual mass spectrometer through a pinhole into the Collision Dissociation Chamber (CDC) which destroys clusters of ions with neutral molecules by collision of accelerated ions with molecules in a low pressure environment – for example clusters with one sulphate ion and nitric acid molecules attached to it. After passing through the CDC the ion cores are separated by their mass with a quadrupole mass analyser and detected.
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