The time needed for the component to pass through the observation space (in s). It depends on the geometry of the observation space, the flame temperature and the solute nebulized.
Source:
PAC, 1986, 58, 1737. 'Quantities and units in clinical chemistry: Nebulizer and flame properties in flame emission and absorption spectrometry (Recommendations 1986)' on page 1742 (https://doi.org/10.1351/pac198658121737)