Comparison of Odorant Losses at the ppb-Level from Sampling Bags of Nalophan™ and Tedlar™ and from Adsorption Tubes

摘要

The issue of volatiles and odorant losses has already been addressed by different authors. The motivation came from the fields of breath analysis (Mochalski et al. 2013; Mochalski et al. 2009), atmospheric chemistry (Sulyok et al. 2001; Kim et al. 2012) and odour measurement (van Harreveld 2003; Hansen et al. 2011; Parker et al. 2010; Trabue et al. 2006). The aim in these fields is to collect samples for subsequent laboratory measurements. Frequently manufacturers of sample bags claim excellent stability of the stored substances. A closer look on the measurement procedure often shows an unrealistic background: The measurements were made with high concentrations of substances at an unrealistic level. The losses are different at a trace level, e.g. at the ppb-level near the odour thresholds of odorants, because the contribution of wall adsorption in the polymeric bags is comparatively greater. The published contributions show some general trends but often suffer from deficits of the measurement procedure. The initial losses due to adsorption at the walls of the bags contribute to a great extent to the losses. Therefore a measurement starting with the freshly filled bags as the reference level for losses does not reflect the real situation of bag sampling. Other obvious problems are the stability of the sample gas generation at trace level concentrations. The measurements for this study were made with a high sensitive trace gas measurement system. It uses online thermal desorption and a gas chromatography - time-of-flight mass spectrometry system. A calibration gas generator on basis of continuous evaporation and dilution has been used for the preparation of the sample gases in the ppb-range. Odorants from different chemical classes and of different molecular weight have been included in the study. Special care has been taken to avoid any losses due to leakage or cold spots in the measurement chain. Extensive measurements with different bag materials have been made, including the most common bags from Nalophan and from Tedlar. For comparison the same measurements have been made with adsorption tubes as a sampling method.