Applicability of canisters for sampling high volatile alkanes

Status:

completed 08/2022

Aims:

The sampling of very volatile low molecular weight substances has often proved difficult in practice. Conventional adsorption materials had only limited capacity for these substances, so that breakthrough occurred when samples were collected on adsorbent tubes. When collecting inert gas sampling bags, so-called Tedlar© bags, wall reactions can occur despite deactivation of the surfaces, and loss-free storage of gas samples is only possible to a limited extent. One alternative option is to use surface-treated stainless steel canisters as sampling vessels. The canisters were evacuated prior to collection, and sampling was carried out using a pressure gradient and a critical nozzle for adjusting the volume flow. Depending on the selection of the critical orifice, this allowed time-integrated sampling at the workplace or measuring station for periods ranging from a few minutes to 24 hours.

The aim was to develop a procedure for the use of these stainless steel canisters. Initially, a method was to be developed for highly volatile alkanes (pentanes and hexanes), which can be adapted to other analytes as the project progresses. Once the boundary conditions for the use of stainless steel canisters were established, methods for sampling highly reactive gases such as ethylene oxide were to be developed, as these posed particular challenges when measuring hazardous substances, due to their high levels of volatility and reactivity. At present, there are no suitable measurement methods for ethylene oxide in the risk assessment applied in the measurement system for exposure assessment (MGU) of the German Social Accident Insurance Institutions.

Activities/Methods:

The first step was to generate relatively small quantities (up to 25 litres) of static test gases in the Tedlar© bags and then to transfer them to the stainless steel vessels. This made it possible to determine whether the resulting gas mixtures could be collected in the stainless steel canisters without modification. The canister validation tests were carried out on the IFA’s dynamic test gas line.

At the same time, a gas chromatographic (GC) method was validated for these alkanes, so that the gas samples could then be analysed directly. This method allowed the chromatographic separation of the eight isomeric substances (n-pentane, isopentane, n-hexane, 2,2-dimethylbutane, 2,3-dimethylbutane, 2-methylpentane, 3-methylpentane, cyclopentane and cyclohexane). Also, an online procedure was developed using selected ion flow tube mass spectrometry (SIFT-MS) for the continuous monitoring of the test gases.

A method to collect and analyse ethylene oxide was developed and validated based on the knowledge gained from the development of methods for alkanes through canister sampling. Thus, the alkanes served as test substances in the initial suitability tests carried out on canister sampling. In test measurements at workplaces using canisters as sampling devices, the aim was to prove and test the practical suitability of canisters.

Results:

A sampling method was developed using stainless steel canisters and a gas chromatographic analysis. Measuring procedures were developed and validated for the determination of aliphatic C5 and C6 hydrocarbons, as well as for ethylene oxide. The methods were validated on a dynamic test gas line, creating a selected ion flow mass spectrometry method that was used for the real-time monitoring of the generated test gases. The method to determine isomeric short-chain aliphatic alkanes (pentanes and hexanes) covers a work area of 1-10 mg/m³ and has an expanded measurement uncertainty of approx. 15%. The measurement method for ethylene oxide has a measurement range of 10 to 2,000 ppb and an expanded measurement uncertainty of U = 15%. The methods therefore fully satisfy the requirements for measurement methods for carcinogenic agents.

The usability of the developed method for ethylene oxide was tested in initial measurements at workplaces with exposure. The results show the full usability of the measurement method. An expansion of the measurement programme in the MGU should permit a comprehensive statement on ethylene oxide exposures in different sections (e.g. ethylene oxide production or application in the sterilisation of medical devices) with different types of references to ethylene oxide. The method will be introduced as a new standard procedure in the MGU in early 2023.

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