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. Chemical ionization mass spectrometry (CIMS) using iodide as a reagent ion has been widely used to classify organic compounds in the atmosphere by theirelemental formula. Unfortunately, calibration of these instruments ischallenging due to a lack of commercially available standards for manycompounds, which has led to the development of methods for estimating CIMSsensitivity. By coupling a thermal desorption aerosol gas chromatograph (TAG) simultaneously to a flame ionization detector (FID) and an iodide CIMS, we usethe individual particle-phase analytes, quantified by the FID, to examine thesensitivity of the CIMS and its variability between isomers of the sameelemental formula. Iodide CIMS sensitivities of isomers within a formula arefound to generally vary by 1 order of magnitude with a maximum deviation of 2 orders of magnitude. Furthermore, we compare directly measured sensitivityto a method of estimating sensitivity based on declustering voltage (i.e.,“voltage scanning”). This approach is found to carry high uncertainties forindividual analytes (0.5 to 1 order of magnitude) but represents a central tendency that can be used to estimate the sum of analytes with reasonableerror (∼30 % differences between predicted and measuredmoles). Finally, gas chromatography (GC) retention time, which is associated with vapor pressure and chemical functionality of an analyte, is found to qualitatively correlatewith iodide CIMS sensitivity, but the relationship is not close enough to bequantitatively useful and could be explored further in the future as apotential calibration approach.

Quantification of isomer-resolved iodide chemical ionization mass spectrometry sensitivity and uncertainty using a voltage-scanning approach