Two gas-phase quantification methods for Chemical Ionization Mass Spectrometry using Iodide reagent ions (Iodide-CIMS) were developed in this work: the integration method and the flow rate method. Both methods utilize a Hamilton syringe pump to relate the detected signal to the gas-phase concentration, also known as a calibration coefficient, injected into a VOCUS 2R API-LToF mass spectrometer configured with an AIM source and equipped with VUV light to generate iodide reagent ions. These quantification methods can be combined with any chemical ionization mass spectrometer utilizing a large variety of target analytes. The integration method was used to determine a calibration coefficient, which was applied to partitioning experiments in order to determine the Henry’s Law coefficient for 1,2-ISOPOOH. Henry’s Law describes the ratio of the presence of a compound in the aqueous-phase and the gas-phase when partitioning is in equilibrium. Henry’s Law applies only to dilute solutions. The 1,2-ISOPOOH Henry’s Law coefficient was determined to be 2.61E+05 M/atm +/- 1.81E+04 M/atm. The reported value is bracketed by available literature values, which range from 1.00E+05 M/atm to 1.30E+06 M/atm. The flow rate method was used during initial calibration development to determine preliminary calibration coefficients for a suite of atmospherically relevant acids. This method will continue to be optimized for future laboratory and aircraft calibrations.