The substance fraction of component entering the flame in the amount of component consumed. The efficiency of nebulization is related to the amount of component and not to the amount of solvent. It cannot be calculated directly from the ratio: volume rate of sample drained from the spray chamber divided by volume rate of fluid consumed. Corrections usually have to be made to take account of differences in component concentrations in the drained and consumed solutions, respectively, because of evaporation of some solvent from mist droplets deposited on the walls.
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 1740 (https://doi.org/10.1351/pac198658121737)