## 1 Introduction
Very few halogenated compounds have so far been found in hydrogen samples. Only dichloromethane, tetrachloroethylene and tetrachlorohexafluorobutane have been reported so far. The most commonly identified sulphur compounds in hydrogen samples were hydrogen sulphide (H\({}_{2}\)S), carbonyl sulphide (COS) and carbon disulphide (CS\({}_{2}\)) whereas tert-butyl mercaptan (T-BM) and tetrahydrothiophene (THT) have been found in one sample. Most of these compounds are organic. Different containers can be used for collecting gas samples. These include gas cylinders (of different materials, with or without internal passivation), sampling bags (of different materials), canisters [9], adsorbent tubes and impingers. There are two principal ways to sample: sampling with or without enrichment. Sampling without enrichment can be performed in cylinders, canisters or bags. For sampling with enrichment, the compounds to be determined are either adsorbed onto a solid collection phase (using adsorbent tubes), or absorbed in a solution (using impingers). Due to the stringent thresholds to reach for hydrogen purity testing, methods allowing enrichment can be a good alternative. Thermal desorption often coupled with gas chromatography is a common separation method that can be used for speciation used to determine the content of organic impurities. The analytical method has the advantage that it indirectly lowers the detection limits by a considerable factor. By means of pumped sampling, according to ISO 16017:2000 [10], organic impurities are trapped onto the sorbent, whereas the hydrogen matrix passes through without being retained. The analytes' boiling point is the key factor when deciding which sorbent type to use. The amount preconcentrated onto the sorbent can be calculated by measuring the gas flow rate and sampling time during sampling on the sorbent tube. However, no existing sorbent tubes are sufficiently universal as to trap all possible impurities, and therefore a selection process is required. In this study, the selection process was based on a literature review of the state-of-the-art regarding the suitability of different sorbents for the targeted compounds. The suitability of sorbents for a given compound or group of compounds is often assessed by measuring the recovery yield and the storage stability for a given compound. The recovery yield is defined as the ratio of the measured and the spiked content and expressed as a percentage. In order to assess the fit for purpose of the analytical method for hydrogen purity testing, ISO 21087:2017 [11] states that, \"the bias shall be small enough to have a relative combined standard uncertainty below 10% of the concentration\". If precision and trueness are considered to equally contribute to the uncertainty, a bias of up to around 7% will be acceptable. In the case of a total family, the overall bias includes the contribution of many compounds. To evaluate the different sorbents, the following criteria were then adopted: recoveries above 90% were considered as very good; recoveries between 85 to 90% were considered as good; recoveries from 50 to 84% were considered as low and finally, for recoveries below 50%, the sorbent material was considered as not suitable for the targeted compound. The storage stability is defined as the change in content for a given compound as determined at the end of the storage time compared to that at the start of the stability test. In this paper, the results from short-term stability studies in different sorbents are presented for hydrocarbons (including oxygenated hydrocarbons), halogenated hydrocarbons and sulphur compounds. Storage periods of two weeks were evaluated since this is the period typically required to complete the collection, transport and analysis of hydrogen samples.