Abstract In order to establish a simple and reproducible sampling device and method, which is crucial for a wide application of breath analysis, the original membrane extraction with sorbent interface (MESI) system was improved by coupling with a palm-size CO 2 sensor. Variations in analyte concentrations due to mass losses and different breathing patterns were normalized by simultaneously measuring the partial pressure of CO 2 and the concentrations of target analytes in the breath sampled. Analyte concentrations can then be expressed normalized to CO 2 as in the alveolar air. The MESI system was applied to study light hydrocarbons such as methane and ethane, which are difficult to analyse by other methods. A systematic study of breakthrough, which relates to the sorbent capacity and is characteristic of the analytical efficiency, was performed through the effects of analyte concentration, trap temperature, sample humidity and extraction time on breakthrough. Continuous on-line monitoring of breath methane and ethane was carried out under the optimum operation conditions based on the breakthrough study.