The substance fraction of the volatilized component in the total desolvated component. The gaseous state includes free atoms, molecules and radicals. This quantity is measured in a defined part of the flame, usually the observation space. The fraction volatilized varies inversely with the size of the desolvated particles. Since χ v varies markedly with the height in the flame, its observed value represents an average.
Sources:
Orange Book, 2nd ed., p. 168 (https://media.iupac.org/publications/analytical_compendium/)
PAC, 1986, 58, 1737. 'Quantities and units in clinical chemistry: Nebulizer and flame properties in flame emission and absorption spectrometry (Recommendations 1986)' on page 1741 (https://doi.org/10.1351/pac198658121737)