Effects of Ambient CO2 on Monitoring of the International Space Station Atmosphere with the Air Quality Monitor

Since 2009, gas chromatography-differential mobility spectrometry (GC/DMS) has been used on-board the International Space Station (ISS) to monitor the atmosphere for volatile organic compounds. The technology was originally tested as part of a Station Detailed Test Objective (SDTO) and then transitioned to operational hardware. The operational version of this hardware, the Air Quality Monitor (AQM), currently monitors 22 compounds, though the target list is flexible and can be adjusted depending on changes in materials or the spacecraft environmental control systems. After separation on the GC column, target compounds are ionized via charge transfer from a Reactant Ion (RI). In the positive mode, H+(H2O)n is the RI while O2-(H2O)n acts as the RI in the negative mode. In the early stages of the SDTO, it was discovered that the position of the RI Peak (RIP) in the negative mode was shifting with time on orbit and the instrument was losing sensitivity to certain compounds. This shift of the RIP appeared to be correlated with increasing concentrations of CO2 in the recirculation system of the instrument. The operational version of the AQM uses larger, replaceable sieve packs to clean the recirculated carrier gas. It was hypothesized that incorporation of the large sieve packs would minimize the effect of CO2 on the position of the RIP. Unfortunately, this phenomenon has also been observed on the first two sets of AQMs that were operated on the ISS. In this paper, we will discuss the mechanisms behind the shifting RIP as well as the effects on the ionization of selected target compounds. Additionally, we will discuss potential approaches to mitigate the impact of the RIP shift and extend the current 6-month life of sieve packs on-orbit.