Isocyanates - A Sampling Primer
Isocyanate sampling can be rather complicated and intimidating with all of the available methods and media options. For successful method selection, today’s hygienist must educate themselves on the type of process they will be sampling prior to selecting a method. The common isocyanate species are methylene diphenyl diisocyanate (MDI), hexamethylene diisocyanate (HDI), toluene diisocyanate (TDI), isophorone diisocyanate (IPDI), and methylene bis-(4-cyclohexyl isocyanate) (HMDI). After determining the isocyanates to be tested, evaluate the atmosphere where collection will occur. Are the isocyanates in a vapor or aerosol form or both? Selection of either a dry sampler (filter) or wet sampler (impinger) will depend onvapor or aerosol phase for isocyanates. Filters have typically been best for collection of vapors and aerosol particulates smaller than 2 µm while impingers are best for aerosol collection.
Below is a guide to assist in proper sampling and method selection.
Monomers are single molecules that are capable of reacting with either themselves or different molecules to form polymers. All isocyanate species encountered in the field have a monomer component to them. Even whensampling for the oligomeric isocyanate species, some component of the monomer will be present in the sampling environment. All monomeric species are capable of being present in both a vapor and aerosol form.
Oligomers, also known as polymers, consist of fewer than five monomer molecules. Many of the common isocyanatespecies are capable of forming oligomers. Common oligomers encountered in sampling are MDI, HDI, and IPDI. TDI is the only isocyanate species that is not commonly found as an oligomer. All oligomeric species will be present in an aerosol form.
OSHA National Emphasis Program (NEP)
In June, 2013, OSHA announced a new National Emphasis Program (NEP) for occupational exposure to isocyanates that will focus on workplaces in the general, construction, and maritime industries in which isocyanate compounds are used. Over a three-year period, OSHA will focus outreach efforts and inspections on isocyanate hazards in an effort to reduce occupational illnesses, injuries, and deaths, and to raise awareness of the health effects associated with exposure to isocyanates, which can cause occupational asthma; irritation of the skin, eyes, nose, and throat; and cancer. Workers in jobs such as painting, blowing foam insulation, and the manufacture and thermal degradation of polyurethane products may be exposed to these hazardous chemicals.
Within this program OSHA issued an official statement that it is now their recommendation that all isocyanatesregardless of physical state (vapor or aerosol) shall be field desorbed.The rationale behind this decision is backed by the article by Karloy, American Industrial Hygiene Journal 59:645-647 1998, which showed under reporting of MDI when samples were not field desorbed.There is a consensus in the industry that the majority of the isocyanate sampling, especially when aerosols reach a certain size (> 2 µm) the sample collected will underestimate the airborne concentration.
Basically, OSHA is now recognizing that using a dry sampler underestimates isocyanate exposures. This means that field description will be required for dry samplers or another sampling technique, such as impingers or the Supelco Asset sampler, must be used. Please note that under this program even the back filter of the Iso checksystem would need field description since OSH is not discerning between aerosols and vapors.
Bureau Veritas Laboratories have recommended field desorption as a standard practice for MDI and any polymeric species when collected on a dry sampler. If you want to be compliant with the OSHA NEP you will either need to request field desorbing solution or collect your samples on the Asset sampler.
Common Isocyanate Sampling and Analytical Methods:
OSHA methods 42, 47, and PV2092 utilize the same sampling media consisting of a glass fiber filter treated with 1-(2-pyridyl)piperazine (1-2PP). Methods 42 and 47 have been validated for the monomeric forms of TDI, HDI, and MDI only. Oligomeric isocyanates have not been validated. These two methods are best for sampling of the monomer species that are in vapor form. If aerosols or MDI are to be collected using these methods, field desorption is required. Sampling occurs open faced and analysis is performed by high performance liquid chromatography (HPLC) equipped with ultra violet or fluorescence detection. Reporting limits for the isocyanate species range between 0.1 to 0.5 µg per sample. The advantage of these methods is the use of a dry sampler with an industry accepted analytical method. The disadvantage is field desorbing when dealing with aerosols and MDI.
NIOSH 5521 utilizes an impinger filled with a solution of 1-(2-methoxyphenyl)-piperazine in Toluene. This method is a modification of Method MDHS 25 published by the Health & Safety Executive of Great Britain. This sampling system is capable of collecting both vapor and aerosol. Analysis is performed by HPLC equipped with electrochemical and ultraviolet detection. As with the OSHA methods, reporting limits for the isocyanate species range between 0.1 to 0.5 µg per sample for the monomers and 1 to 2 µg per sample for the oligomers. Advantages of this method are the ability to collect for the vapor and aerosol in one sampler without the need for field desorption and the ability to collect a full shift sample. Disadvantages include the use of impingers and ashort holding time (7 days) for the impinger reagent.
MDHS 25/3 utilizes both an impinger and glass fiber filt. Both the impinger and filter utilize the same derivitizing agent, 1-(2-methoxyphenyl)-piperazine. This sampling system is capable of collecting both vapor and aerosol and has been validated for all the common monomeric forms and the oligomeric forms of HDI and MDI. Sampling occurs using an impinger/treated glass fiber filter sampling train and analysis is performed by HPLC equipped with ultraviolet detection. Reporting limits for the isocyanate species range between 0.1 to 0.5 µg per sample for the monomers and 1 to 2 µg per sample for the oligomers. Advantages of this method are the ability to collect multiple isocyanate species along with both vapor and aerosols simultaneously and no need for field desorption of the filte. Disadvantages include the use of impinger sampling.
ISO-Check is a dry sampler utilizing a dual filter system.The front filter is an untreated PTFE which collects the aerosol forms of the isocyanates. The back filter is a glass fiber filter impregnated with 9-(N-methylaminomethyl anthracene (MAMA) for the vapor phase of isocyanates. The ISO-check system has been validated for the monomeric and oligomeric forms of the common isocyanates: HDI, MDI, IPDI, HMDI, 2,4-TDI, and 2,6-TDI. The method requires that the front filter be field desorbed since it is untreated and leaves the isocyanate collected on the filter with the ability to react. Analysis is performed by HPLC equipped with ultraviolet detection. Reporting limits for the isocyanate species range between 0.2 to 0.5 µg per sample for the monomers and 1 to 2 µg per sample for the oligomers. The advantage of this sampler is the ability to collect both the monomeric and oligomeric forms of the common isocyanate species together. Disadvantages of the sampler are the short, 15 minute sample duration and the need for field desorption. This method is new technology and has not been officially accepted by either NIOSH or OSHA.
Newer Isocyanate Methods:
NIOSH 5525 utilizes both an impinger and glass fiber filter. Both the impinger and filter utilize the same derivitizing agent of 1-(9-anthracenylmethyl)piperazine (MAP) and is capable of collecting both vapor and aerosol. The method has been partially validated for all the common monomeric forms and the oligomeric forms of HDI and MDI. Analysis is performed by HPLC equipped with both ultraviolet and fluorescence detectors The value of this method is the lower reporting limits that can be achieved, 50 ng for both the monomer and oligomer species, due to the strong fluorescence response of the MA derivatives. Another benefit is the ability to calculate a total isocyanate for the collected atmosphere when all the possible isocyanate species are not known. This is due to the MAP isocyanate derivatives all exhibiting the same equivalent response in the ultraviolet analysis making it possible for the laboratory to calculate all confirmed isocyanate species. This is accomplished by comparing both the fluorescence and ultraviolet chromatograms and calculating the total using the response of one isocyanate species. The disadvantages are using impingers for collection and the lack of commercial availability of the MAP derivitizing agent.
Supelco Asset Sampler is a dry sampler with the capability to collecting both vapor and aerosol forms of monomeric and oligomeric forms of common isocyanates. This is possible due to the design of the sampler. The sampler is composed of two components: 1) a filter seated at the bottom of the sampler and 2) a denuderthat runs the length of the sampler. Both components are treated with a dibutylamine derivitizing agent. The denuder is impregnated with a higher concentration of DBA to continuously refresh the filter with reagent during sampling. This process reduces the risk of analyte breakthrough and prevents the loss of isocyanate particles. The analytical methodology of the sampler follows ISO method 17734 with analysis by HPLC equipped with a mass spectrometer (MS) or tandem mass spec (MS/MS) detection. Reporting limits of the isocyanate species are in the nanogram range. An advantage of the sampling method is that it is a dry sampler capable of collecting both the monomeric and oligomeric forms simultaneously without the need for field desorption. This method is new technology and has not been officially accepted by either NIOSH or OSHA.