Articles | Volume 9, issue 4
Research article
06 Apr 2016
Research article |  | 06 Apr 2016

Constraining the sensitivity of iodide adduct chemical ionization mass spectrometry to multifunctional organic molecules using the collision limit and thermodynamic stability of iodide ion adducts

Felipe D. Lopez-Hilfiker, Siddarth Iyer, Claudia Mohr, Ben H. Lee, Emma L. D'Ambro, Theo Kurtén, and Joel A. Thornton

Abstract. The sensitivity of a chemical ionization mass spectrometer (ions formed per number density of analytes) is fundamentally limited by the collision frequency between reagent ions and analytes, known as the collision limit, the ion–molecule reaction time, and the transmission efficiency of product ions to the detector. We use the response of a time-of-flight chemical ionization mass spectrometer (ToF-CIMS) to N2O5, known to react with iodide at the collision limit, to constrain the combined effects of ion–molecule reaction time, which is strongly influenced by mixing and ion losses in the ion–molecule reaction drift tube. A mass spectrometric voltage scanning procedure elucidates the relative binding energies of the ion adducts, which influence the transmission efficiency of molecular ions through the electric fields within the vacuum chamber. Together, this information provides a critical constraint on the sensitivity of a ToF-CIMS towards a wide suite of routinely detected multifunctional organic molecules for which no calibration standards exist. We describe the scanning procedure and collision limit determination, and we show results from the application of these constraints to the measurement of organic aerosol composition at two different field locations.

Short summary
We present the maximum sensitivity of a TOF-CIMS using the collision limit and iodide adducts. We also present an ion adduct declustering scanning procedure which determines the effective binding energies of the detected ion adducts and therefore their approximate sensitivity. The combination of declustering scanning and the collision limit provides an approximate calibration for many compounds in the mass spectrum which would otherwise be impossible to obtain by traditional methods.