local fraction volatilized, χ v, β v

in flame emission and absorption spectrometry
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.
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
Orange Book, p. 168