The residence time distribution (RTD) of fluid elements in combustion chambers is a key feature of the flow field, and its knowledge is therefore important for the design and improvement of combustion systems. The hostile yet sensitive environment of burners, in particular with high particle loads of solid-fuel firing, impede direct access to this quantity. Two strategies to obtain the RTD based on hydrogen chloride injection with subsequent detection by tunable diode laser absorption spectroscopy are directly compared and applied to gas and solid-fuel combustion under two different combustion modes. Through the direct comparison of this work, the experimentally more challenging and thus rarely utilized pulse injection was found to be superior to the commonly used step injection.