Method development and validation for air sampling and quantification of methylisocyanate

Status:

completed 10/2021

Aims:

Internal investigations performed by a power plant operator detected methyl isocyanate (MIC) during the heating of new insulation materials in co-generation plants. This substance is listed in TRGS 900 (occupational exposure limits) with an occupational exposure limit of 0.024 mg/m³. MIC is toxic in contact with skin and when swallowed. Inhalation of MIC can be life threatening. Furthermore, MIC is a sensitising agent and induces allergic reactions affecting the skin and respiratory tract. Two occupational diseases are recognised as being linked to the use of isocyanates at places of work. These are occupational disease no. 1315 (diseases of the respiratory tract caused by isocyanates) and occupational disease no. 5101 (skin diseases).

On behalf of BG ETEM, whose member companies include operators of co-generation plants, tests were performed with various insulation materials in A 2.5. To do this, the materials were heated to different temperatures in a test setup that was designed for this purpose. During this process, released MIC is collected on a sample holder, which is based on an existing sampler for diisocyanates. The individual insulation materials were inhomogeneous, however, a large amount of MIC was detected when some materials were heated. Since the inhomogeneous nature of the insulation materials resulted in large fluctuations in the measured values, the results are not transferable to the real-life conditions in power plants. In addition, it was not possible to collect MIC quantitatively in the pilot project. This also raised questions about the suitability of the sampling method used.

Because the pilot project was unable to draw conclusions regarding the actual exposure of employees in power plants to MIC, this resulted in the need to develop a suitable measurement method for air sampling of MIC.

After successful validation, the method was to be implemented in the MGU.

Activities/Methods:

The method to be developed for the quantification of MIC in the air at a workplace was to be based on the method for quantification of isocyanates that are already contained in the MGU. The reason for this was to keep the possibility of simultaneous quantification of total reactive isocyanate groups (TRIG), polyisocyanates, and monomeric diisocyanates.

The previously used sample holders were modified by adding an additional fibreglass filter to each filter capsule, which was also impregnated with derivatising reagent, and the sampling process was reduced to a maximum of 1 hour, in order to minimise breakthrough during sampling. High-performance liquid chromatography coupled with mass spectrometry (HPLC-MS) was used for quantification. This analysis technique, which is much more sensitive than routine analysis of isocyanates, requires careful monitoring of blank values in reagents and materials used in the measurement method. This is the only way to ensure that a sufficiently low limit of quantification is achieved, which opens up the possibility of shorter sampling durations.

The measurement method had to by fully validated using sampling tests in accordance with DIN EN 1076. The loading of the sample carrier was performed using a gas collecting tube with air supplied via a test gas line. The influence of the MIC concentration on the measurement result at different humidities was tested. Because the measurement method will be used at high temperatures in practice, the influence of the temperature on the measurement result was also to be tested. This was carried out in a climatic chain using combinations of different MIC concentrations and temperatures. For these tests, the sample holders had to be doped. The use of gas collecting tubes/a test gas line is not practically feasible in this case, because none of the test gas lines available in the IFA are located in areas with adequate temperature control possibilities.

Depending on the result of the validation, a review of the measurement method under real-life conditions is to be pursued. This must be discussed with the German Social Accident Insurance Institutions that have responsibility for the operation of plants in which insulation material is used to insulate boilers, for example.

The results of the validation must then be documented in an understandable way and the measurement method must be published in the MGU.

Results:

Determining methyl isocyanate (MIC) using a method based on one of the existing MGU measurement methods for isocyanates proved ineffective in the sampling process.

Even when using a modified sample holder composed of three nested, impregnated filters in a GSP system, the measurement results achieved in sampling tests showed significant fluctuations. This could be down to the high reactivity of MIC, which reacts with atmospheric moisture, meaning that the substance does not fully reach the derivatising agent on the filters. As we know from other volatile isocyanates, the level of volatility of the MIC can be a cause of poor repeatability results.

Under the current conditions, quantitative determination on the filter was not possible.

The analytical determination of MIC using LC-MS is very sensitive, however, without reproducible sampling, determination of the risk posed by MIC in the air at the workplace is not possible.

A fundamental problem when it comes to the analytical determination of MIC is the limited commercial availability of this substance. At one time, it was possible to acquire MIC from a Russian supplier. However, this supplier no longer offers MIC. For quantitative determinations, the calibration of the analyser system using MIC standard material with a significant level of purity as a comparative substance is an important pre-requisite that is not guaranteed in the longer term. Therefore, a suitable measurement method for use in the MGU could not be developed. The IFA recommends that anyone working at workplaceswith risk of exposure to MIC wears suitable PPE. The specialist department of the IFA (PPE to protect against chemical and biological hazards) offers advice on this topic.

The A 2.5 is of the opinion that IFA project 2088 can be ended.

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