A method of performing an optical measurement of an analyte in a processed biological sample using a cartridge, wherein the cartridge is operable for being spun around a rotational axis, wherein the cartridge comprises: - a support structure comprising a front face; - a fluidic structure for processing a biological sample into the processed biological sample, wherein the fluidic structure comprises a sample inlet for receiving the biological sample; and - a measurement structure recessed from the front face, wherein the measurement structure comprises a chromatographic membrane, wherein the measurement structure comprises a measurement structure inlet connected to the fluidic structure to receive the processed biological sample, wherein the measurement structure comprises an absorbent structure, wherein the chromatographic membrane extends from the measurement structure inlet to the absorbent structure, wherein the chromatographic membrane comprises a detection zone, wherein the measurement structure comprises an inlet air baffle (128, 128') connected to the front face, wherein there is a gap between the air inlet baffle and the measurement structure inlet, wherein the gap is over the chromatographic membrane, wherein the inlet baffle is configured for regulating a flow of air over the measurement structure during rotation of the cartridge about the rotational axis; wherein the method comprises: - placing the biological sample into the sample inlet; - controlling the rotational rate of the cartridge to process the biological sample into the processed biological sample using the fluidic structure; - controlling the rotational rate of the cartridge to allow the processed biological sample to flow from the measurement structure inlet to the absorbent structure via the chromatographic membrane, wherein the inlet air baffle reduces the evaporation of the processed biological sample during rotation of the cartridge; and - performing the optical measurement of the detection zone with an optical instrument. - a support structure comprising a front face; - a fluidic structure for processing a biological sample into the processed biological sample, wherein the fluidic structure comprises a sample inlet for receiving the biological sample; and - a measurement structure recessed from the front face, wherein the measurement structure comprises a chromatographic membrane, wherein the measurement structure comprises a measurement structure inlet connected to the fluidic structure to receive the processed biological sample, wherein the measurement structure comprises an absorbent structure, wherein the chromatographic membrane extends from the measurement structure inlet to the absorbent structure, wherein the chromatographic membrane comprises a detection zone, wherein the measurement structure comprises an inlet air baffle (128, 128') connected to the front face, wherein there is a gap between the air inlet baffle and the measurement structure inlet, wherein the gap is over the chromatographic membrane, wherein the inlet baffle is configured for regulating a flow of air over the measurement structure during rotation of the cartridge about the rotational axis; - placing the biological sample into the sample inlet; - controlling the rotational rate of the cartridge to process the biological sample into the processed biological sample using the fluidic structure; - controlling the rotational rate of the cartridge to allow the processed biological sample to flow from the measurement structure inlet to the absorbent structure via the chromatographic membrane, wherein the inlet air baffle reduces the evaporation of the processed biological sample during rotation of the cartridge; and - performing the optical measurement of the detection zone with an optical instrument.