A method of performing an optical measurement of an analyte in a processed biological sample using a cartridge is provided. The cartridge is operable for being spun around a rotational axis. The method comprises: placing the biological sample into a sample inlet; controlling the rotational rate of the cartridge to process a biological sample into the processed biological sample using a fluidic structure; controlling the rotational rate of the cartridge to allow the processed biological sample to flow from the measurement structure inlet to an absorbent structure via a chromatographic membrane, and performing an optical measurement of a detection zone on the chromatographic membrane with an optical instrument. An inlet air baffle reduces evaporation of the processed biological sample from the chromatographic membrane during rotation of the cartridge.