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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union
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Discussion papers
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 01 Jul 2019

Research article | 01 Jul 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Measurement Techniques (AMT).

Performance of a new co-axial ion-molecule reaction region for low-pressure chemical ionization mass spectrometry with reduced instrument wall interactions

Brett B. Palm1, Xiaoxi Liu2, Jose L. Jimenez2, and Joel A. Thornton1 Brett B. Palm et al.
  • 1Department of Atmospheric Sciences, University of Washington, Seattle, WA, USA
  • 2Department of Chemistry and Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO, USA

Abstract. Chemical ionization mass spectrometry (CIMS) techniques have become prominent methods for sampling trace gases of relatively low volatility. Such gases are often referred to as being sticky, i.e. having measurement artifacts due to interactions between analyte molecules and instrument walls, given their tendency to interact with wall surfaces via absorption or adsorption processes. These surface interactions can impact the precision, accuracy, and detection limits of the measurements. We introduce a low-pressure ion-molecule reaction (IMR) region primarily built for performing iodide-adduct ionization, though other adduct ionization schemes could be employed. The design goals were to improve upon previous low-pressure IMR versions by reducing impacts of wall interactions at low pressure while maintaining sufficient ion-molecule reaction times. Chamber measurements demonstrate that the IMR delay times (i.e., magnitude of wall interactions) for a range of organic molecules spanning five orders of magnitude in volatility are 3 to 10 times lower in the new IMR compared to previous versions. Despite these improvements, wall interactions are still present and need to be understood. To that end, we also introduce a conceptual framework for considering instrument wall interactions and a measurement protocol to accurately capture the time-dependence of analyte concentrations. This protocol uses short-duration, high-frequency measurements of the total background (i.e., fast zeros) during ambient measurements as well as during calibration factor determinations. This framework and associated terminology applies to any instrument and ionization technique that samples compounds susceptible to wall interactions.

Brett B. Palm et al.
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Brett B. Palm et al.
Brett B. Palm et al.
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Short summary
We introduce a co-axial, low-pressure ion-molecule reaction (IMR) region for iodide-adduct chemical ionization mass spectrometry, designed to decrease the effects of IMR wall interactions with organic/inorganic gases. This IMR has 3–10 times shorter delay times than previous IMRs. We introduce a conceptual framework for understanding and subtracting the background signal due to analyte molecules interacting with IMR walls. This framework can be applied to other tubing and instrument systems.
We introduce a co-axial, low-pressure ion-molecule reaction (IMR) region for iodide-adduct...