US Air Force Research Laboratory Chemical Ionization Mass Spectrometer (CIMS)

Measurement Description: Chemical ionization mass spectrometry has proven to be a versatile and extremely sensitive tool for monitoring trace neutral gases in the atmosphere. In the CIMS technique, trace neutral gases are converted to unique ions by reaction with a suitable precursor ion. The ions can then be detected without interference from background atmospheric species since there is virtually no ion background in the atmosphere. In principle the chemical ionization technique is similar to a laboratory fast flow experiment for kinetics measurement but with the objective to sensitively determine absolute concentrations of neutral species using already determined kinetics. The sensitivity of the method comes from the fact that ion-molecule reactions are extremely fast, often occurring on the order of 10^-9 molec cm^-3 s^-1. The selectivity of the technique is dependent upon choosing an appropriate ion precursor. The CIMS instrument is configured to measure HNO₃ concentrations for SOLVE, with the bulk of the time devoted to HNO₃ measurements. Improvements made since the SONEX mission are expected to result in a sensitivity of 10 pptv and a 20% accuracy for a 1 second integration period.

A schematic of the AFRL CIMS instrument is shown in the figure. This instrument is similar to that employed in the 1997 SOXEX program with modifications to enhance sensitivity and accuracy. Air enters an inlet outside of the boundary layer of the airplane. The air is carried undiluted down a 3.5 cm (i.d.) tube by a high speed fan. The flow speed in the tube is fast, ~10 m/sec in order to minimize reactions of the trace species with the flow tube walls. The air exhausts from the airplane through a second hole in the airplane skin. The Reynolds number in the flow tube is approximately 14,000 and the flow is in the turbulent regime. Reactant ions are introduced just upstream of the mass spectrometer inlet. The ions are formed from a corona discharge ion source from an oxygen-CO₂ mixture to produce CO₃^-. The trace neutrals react with the ions across the flow tube. A sample of the gas is extracted by a small orifice forming the entrance to the mass spectrometer. The bulk of the gas flow is exhausted back out of the aircraft. The sampled gas expands into the vacuum system forming a supersonic molecular beam. The ions in the beam are mass analyzed by a quadrupole mass spectrometer and detected by a discrete dynode electron multiplier. The instrument measures 100 mass peaks per second in the peak stepping mode and the 100 peaks can be for the same mass for maximum time resolution or divided into several peaks for more complete information. Every few seconds a complete mass scan is performed to monitor the instrument performance.

The system is calibrated by injecting square wave flows of HNO₃ from permeation tubes into the flow tube. Combining the flow rate of the calibration gas, the increase in the ion signal, and the air flow rate as determined by a pitot tube yields the instrument sensitivity. The flow tube walls are coated with Teflon and heated to 50°C to inhibit HNO₃ sticking to the walls.