Gas phase titrations are titrations done in the gas phase, specifically as methods for determining reactive species by reaction with an excess of some other gas, acting as the titrant.  In one common gas phase titration, gaseous ozone is titrated with nitrogen oxide according to the reaction
:O + NO → O + NO.
After the reaction is complete, the remaining titrant and product are quantified (e.g., by Fourier transform spectroscopy) (FT-IR); this is used to determine the amount of analyte in the original sample.
Gas phase titration has several advantages over simple spectrophotometry.  First, the measurement does not depend on path length, because the same path length is used for the measurement of both the excess titrant and the product.  Second, the measurement does not depend on a linear change in absorbance as a function of analyte concentration as defined by the Beer–Lambert law.  Third, it is useful for samples containing species which interfere at wavelengths typically used for the analyte.