Determination of ethane, pentane and isoprene in exhaled air using a multi-bed adsorbent and end-cut gas-solid chromatography

A method for the determination of exhaled ethane, pentane and isoprene was developed and validated. The method was based on pre-concentration of the analytes on a multi-bed solid adsorbent tube containing Tenax TA, Carboxen 569 and Carboxen 1000, thermal desorption and gas chromatography (GC) with flame ionisation detection (FID). A pre-column in an end-cut GC system was used to avoid problems with water and strongly retained substances. The detection limits were 5, 2 and 6 pmol per sample for ethane, pentane and isoprene, respectively, using a sample volume of 500 ml. The linearity was good for all analytes with correlation coefficients exceeding 0.999. The repeatability for exhaled air samples was 7, 10 and 12% for ethane, pentane and isoprene, respectively. Analysis of a certified reference material of ethane and pentane did not differ significantly from the certified values. Ethane and pentane levels were stable up to six days of storage in sample tubes. Isoprene levels were not stable during storage in the sample tubes used here, but using Carbopack X instead of Carboxen 569, levels were stable up to two days. The levels of exhaled ethane, pentane and isoprene in healthy subjects (n = 4) were 8.1 +/- 5.8 pmol l(-1), 11 +/- 5.8 pmol l(-1) and 2.4 +/- 0.90 nmol l(-1), respectively. The method could, with minor modifications, be used to determine other low-molecular hydrocarbons in exhaled air as well.