Using labelled internal standards to improve needle trap micro-extraction technique prior to gas chromatography/mass spectrometry

Background When working with humid gaseous samples, the amount of water vapor collected in a needle trap along with VOCs may vary from sample to sample and decrease during the storage. This has a major impact on analyte desorption efficiency and recovery. We propose the addition of a labeled internal standards (ISs) to virtually cancel the effect due to humidity variability on the analytical performance of NTME combined with GC-MS. Methods Triple-bed (Divinylbenzene/Carbopack X/Carboxen 1000) and single-bed (Tenax GR) needles were tested with standard gaseous mixtures prepared at different relative humidity (RH) levels (85, 50 and 10%). The standard mixtures contained 25 analytes representative of breath and ambient air constituents, including hydrocarbons, ketones, aldehydes, aromatics, and sulfurs, in the concentration range 0.1-700 ppbv. Results The tested needles showed different behaviours, as recovery was independent of humidity for single-beds, whereas a low recovery (10-20%) was observed when triple-beds trapped very volatile compounds (e.g. pentane and ethanol) at humidity level as low as 10% RH. Triple-beds showed an almost quantitative recovery (>90%) of all the analytes at 50 and 85% RH. The addition of 6D-acetone and 8D-toluene to the sorbent material before gas sampling and the normalization of raw data almost nullified this effect, thereby lowering the variations of analyte recovery at different RH levels down to 20%. After normalization, the inter- and intra-day method precision were halved to 5% and 10% in the case of singlebeds, respectively, and to 15% and 20% with three-beds. Conclusions The addition of ISs to the sorbent in the needle trap helps to keep under control data reproducibility and improves data reliability of NTME-GC-MS for the determination of VOCs in breath and ambient air samples.