Development of a miniaturized system for the sampling of vapour/droplet mixtures

Status:

completed 11/2014

Aims:

Substances are generally present at workplaces either in gas/vapour form, or in a condensed phase in the form of droplets or particles. Some substances however cannot be clearly assigned to one or other of these categories. These are substances that exhibit a low but non-negligible vapour pressure at room temperature. They may be liquids or sublimating solids. When inhalative exposure to such substances is being measured, it must always be considered whether an aerosol can be formed by the work process.

For substances with a vapour pressure of between 0.001 Pa and 100 Pa at room temperature, a sampling method should therefore generally be used in which vapour and aerosol are collected simultaneously by a single sampling system. Liquids with a boiling point of between 180 °C and 350 °C generally fall within this category. A sampling system for the inhalable fraction should be selected for the aerosol.

The mass transfer between the vapour and the condensed phase is a dynamic process subject to constant external influences, including the temperature and air flow. The distribution of the mass between the vapour and particle phases is not constant and cannot therefore be measured. Consequently, the sum of the vapour and aerosol must always be determined and assessed. The procedure is described comprehensively in the EN 13936 standard, "Workplace exposure – Procedures for measuring a chemical agent present as a mixture of airborne particles and vapour – Requirements and test methods".

For this combined sampling of inhalable aerosol and vapour phases, a new sampling system was to be developed for use at relatively low flow rates of 0.066 to 0.5 l/min. This enables filters to be used in combination with standard adsorption tubes, without the methods used for the vapour phase having to be modified.

Activities/Methods:

A flow rate of 0.333 l/min was initially selected, this being the intake rate most commonly used in workplace measurements conducted by the statutory accident insurance institutions for the sampling of organic substances. By the use of readily interchangeable intake cones however, the new GGP Mini was designed for flexibility, permitting the use of other intake rates in the range from 0.066 to 0.5 l/min.

The new sampling head had to be examined for its suitability for the collection of organic substances. For this purpose, laboratory tests were first to be conducted in which a range of relevant substances were applied directly to the filter. Tests were also performed at a range of atmospheric humidities, as were tests of the storability. Suitable tests for verification of the laboratory results were to be performed on the flow-tube reactor at the Helmholtz Zentrum, the German Research Center for Environmental Health in Munich. Tests were also necessary in the IFA's dust tunnel in order to determine the separation performance of the sampling head.

Following successful testing and establishment of the new sampling system, all methods for hazardous substances with critical physical properties were to be switched over to the new system.

Results:

In the course of the project, a new system for the sampling of vapour/particle mixtures was developed and validated. For this purpose, a sampling head was first developed for the inhalable aerosols for use at low flow rates. The sampling head is manufactured from high-grade steel. It has a total diameter of 22 mm and a length of 40 mm. Besides the sampling head proper, it features a plastic sleeve with a length of 170 mm for mounting of the collection tube used.

The droplets are deposited on a 13 mm filter. A short length of hose serves as the connection to the adsorption tube on which the vapours are adsorbed.

Eleven substances were selected for the laboratory tests. All laboratory tests were performed at a flow rate of 0.333 l/min. The results show that the sum of the vapour and droplets always yields a good recovery level. The scatter of the summated values is substantially lower than that of the discrete values for the filter and the activated carbon tube, which deviated in some cases widely. This is particularly evident with substances that can be detected in significant quantities on both the filter and the activated carbon tube. This particularly holds true for diethylene glycol monobutyl ether, diethylene glycol, triethylene glycol monomethyl ether and n-hexadecane.

Changes in concentration, the influence of atmospheric humidity, and the temperature dependency were studied in further tests. All tests showed that for the critical substances, the ambient conditions had a major influence upon the discrete values for the filter and the adsorption tube, whereas the sum of the two remained constant.

Studies of the storability revealed no losses over a four-week period of storage.

The laboratory results were validated by a series of tests conducted on the flow-tube reactor at the Helmholtz-Zentrum in Munich. A droplet aerosol of defined size and composition was introduced into a primary gas stream. The comparative tests with the liquid aerosol show excellent correlation with the results of the laboratory tests.

The studies of the dust collection properties of the sampling head show that compared to the established GSP sampling system, the quantity collected by the new Mini GSP system is too low. At a flow rate of 0.333 l/min and an intake velocity of 1.25 m/s, the collection of inhalable dusts is 50% that of the GSP system. Increasing the intake velocity improves collection to over 90%, i.e. the aperture of the sampling head must be modified slightly (reduced in size) for an intake rate of 0.333 l/min.

Both the laboratory tests and the series of tests conducted in the flow-tube reactor show the absolute necessity of determining the vapour and particle phases together in a single sampling system, since only the sum of the two yields reproducible results. The tests confirm that the sampling system developed is suitable for reliable collection of the sum of the vapour and particle phase. The standard methods for the critical substances such as diethylene glycol, triethylene glycol monobutyl ether and n-hexadecane must be converted to the new system in the MGU hazard measurement system of the accident insurance institutions.

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