A system for determining a concentration of acetone in a fluid, said system comprising: a sensor configured to generate a differential response to a constant concentration of acetone, wherein the sensor comprises a hydroxylammonium salt, and whereby exposure of the fluid to the hydroxylammonium salt in the presence of a liquid produces a change in a variable, said variable being dependent upon the pH of the liquid; and an acquisition device configured to determine the concentration of the acetone from the differential response, wherein the acquisition device comprises: a detector for measuring the change in the variable; and a processor configured to calculate the concentration of the acetone in the fluid from the change in the variable, optionally wherein the fluid is breath; wherein the differential response is spatially differential, whereby the sensor is configured to respond to the acetone at a plurality of detection positions on the sensor, said detection positions having different sensitivities to the acetone; and wherein: the sensor comprises a spatially differential local concentration or local amount of the hydroxylammonium salt at different points of the sensor so as to provide spatially differential sensitivity to the sensor; or the sensor comprises a coating on a detection surface, and the coating has spatially differential permeability to the analyte so as to provide spatially differential sensitivity to the sensor. a sensor configured to generate a differential response to a constant concentration of acetone, wherein the sensor comprises a hydroxylammonium salt, and whereby exposure of the fluid to the hydroxylammonium salt in the presence of a liquid produces a change in a variable, said variable being dependent upon the pH of the liquid; and an acquisition device configured to determine the concentration of the acetone from the differential response, wherein the acquisition device comprises: a detector for measuring the change in the variable; and a processor configured to calculate the concentration of the acetone in the fluid from the change in the variable, optionally wherein the fluid is breath; wherein the differential response is spatially differential, whereby the sensor is configured to respond to the acetone at a plurality of detection positions on the sensor, said detection positions having different sensitivities to the acetone; and wherein: the sensor comprises a spatially differential local concentration or local amount of the hydroxylammonium salt at different points of the sensor so as to provide spatially differential sensitivity to the sensor; or the sensor comprises a coating on a detection surface, and the coating has spatially differential permeability to the analyte so as to provide spatially differential sensitivity to the sensor. a detector for measuring the change in the variable; and a processor configured to calculate the concentration of the acetone in the fluid from the change in the variable, optionally wherein the fluid is breath; wherein the differential response is spatially differential, whereby the sensor is configured to respond to the acetone at a plurality of detection positions on the sensor, said detection positions having different sensitivities to the acetone; and wherein: the sensor comprises a spatially differential local concentration or local amount of the hydroxylammonium salt at different points of the sensor so as to provide spatially differential sensitivity to the sensor; or the sensor comprises a coating on a detection surface, and the coating has spatially differential permeability to the analyte so as to provide spatially differential sensitivity to the sensor.