A microfluidic device includes a sample inlet for a fluid sample, a degassing chamber having a gas-permeable membrane and defining first and second portions separated by the gas permeable membrane, and a detection chamber. The sample inlet and the detection chamber are fluidly coupled to the first and second portions, respectively of the degassing chamber. The detection chamber is operably coupled to a light source and a detector. Assessing a concentration of chlorine gas in an aqueous sample includes providing an aqueous sample to a microfluidic device, separating gas from the aqueous sample in the microfluidic device, providing the gas to a detector, assessing the absorbance of the gas sample at a known absorption wavelength of chlorine, and based on the assessed absorbance of the gas sample at the known absorption wavelength of chlorine, assessing a concentration of chlorine gas in the aqueous sample.