Production of test gases for low-volatility substances on a dynamic test gas stream facility

Status:

completed 09/2008

Aims:

The indoor measurement of volatile organic compounds (VOCs) is a task frequently conducted within the BGMG hazardous substances measurement system of the accident insurance institutions for the purpose of risk assessment. In the context of the quality assurance of measurement methods and for the conducting of round-robin tests, test gases for VOCs must be generated on the large dynamic test gas stream. The VOCs extend in this context from substances of higher volatility, with boiling points of around 60 °C, to substances of lower volatility, with boiling points of up to 250 °C. Test gases for high-volatility substances have already been produced successfully. However, the existing vaporizers on the test gas stream did not permit the production of test gases containing VOCs with low-volatility components (with boiling points > 150 °C).
Owing to this requirement, other techniques were to be tested for the manufacture of test gases containing VOCs with low-volatility components, in order to enable test gases also to be produced for these components in the future.

Activities/Methods:

The production of test gases by means of capillary diffusion was trialled. This method also enables low-volatility substances with boiling points of around 250 °C to be vaporized. For many of the components concerned, the coefficients of diffusion required for calculation of the concentration are unknown, and had to be determined experimentally. The coefficients of diffusion can be used to calculate the vaporization rate of the individual components and to produce a reproducible test gas which can be used in the context of round-robin tests and the development of methods.
In the first step, the diffusion coefficients were to be determined for limonene and for 2-(2-butoxyethoxy)-ethanol, in order to identify the suitability of these substances for the production of test gases. Were positive results to be obtained, the method was to be applied to further substances.

Results:

During the project, the vaporization rates of the following hazardous substances were determined: octamethylcyclotetrasiloxane,
R(+)limonene, pentadecane, (2-2-butoxyethoxy)-ethanol, 2-(2-butoxyethoxy)-ethyl acetate, 2-butoxyethyl acetate, 1,2,3-trimethylbenzene.
The vaporization rates were determined for a range of capillary diameters and in a temperature range from approximately 80 °C below the boiling point to 20 °C below the boiling point of the substance concerned. Suitable selection of the temperature and of the appropriate capillary diameter enables the desired concentration range to be attained in the test gas.
The values obtained permit estimation for the purpose of selection of suitable vaporization conditions for further substances where the physical data (boiling point, coefficient of diffusion) are known. It will therefore be possible to extend the selection of substances without difficulty in the future. With immediate effect, compounds are available with a boiling temperature of >150 °C for use in round-robin tests of VOCs.
In addition to the round-robin tests, the technique can be employed for validation of methods and for quality assurance of test methods for VOCs in the context of the BGMG hazardous substances measurement system of the accident insurance institutions.

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