A sensor system for monitoring the simple gases hydrogen, carbon monoxide, hydrogen sulfide, ammonia and ethanol in exhaled breath.

A sensor array system was constructed incorporating electrochemical sensors for hydrogen, carbon monoxide, hydrogen sulfide and ethanol, a ceramic sensor for total volatiles and a dye-based optical ammonia sensor. The system was calibrated using standard gases balanced with dry air. Limit of detection and % relative standard deviation values (n = 10) for the sensors in the array are hydrogen (0.1 ppm, 2.6%), carbon monoxide (0.4 ppm, 2.1%), ethanol (0.5 ppm, 1.5%), hydrogen sulfide (0.1 ppm, 1.5%) and ammonia (0.6 ppm, 10.7%). Humidity effects were assessed by calibrating with humidified standard gases (hydrogen, carbon monoxide) or spiked breath samples in Tedlar bags (hydrogen sulfide, ethanol and ammonia). The calibration data were used to establish a cross-sensitivity matrix. The concentration of breath volatiles was found to be dependent on exhalation rate and exhalation volume. A test protocol based on these data required volunteers to exhale 1 litre of breath at a rate between 7.5 and 17.5 l min(-1). Sensor responses were measured for 40 s then purged at 7 l min(-1) (150 s). A longitudinal study was undertaken of ten asymptomatic volunteers over a five-day period. Volunteers ate an ad hoc diet, but fasted prior to giving the first breath sample and then gave samples every hour for 8 h. Breath hydrogen levels for volunteers showed large variations within a day and also from day to day. Fasting levels ranged between 0.3 and 34.1 ppm (mean 9.1 ppm). The carbon monoxide levels for non-smokers were between 0.6 and 4.9 ppm (mean 2.1 ppm), whilst for smokers they were between 8.3 and 18.7 ppm (mean 12.8 ppm). The measured levels of other gases on breath were as follows: hydrogen sulfide (0-1.3 ppm, mean 0.33 ppm), ethanol (0-3.9 ppm, mean 0.62 ppm) and ammonia (0-1.3 ppm mean 0.42 ppm). The system was capable of direct quantitative measurements of low concentrations of a range of volatiles on exhaled breath. The measured values for compounds on the breath of asymptomatic volunteers were in broad agreement with quoted literature ranges. The system will now be assessed in a clinical setting.