INVESTIGATION OF THE ABILITY OF MDHS METHOD-25 TO DETERMINE URETHANE-BOUND ISOCYANATE GROUPS

Method 25 for the Determination of Hazardous Substances (MDHS 25) of t he Health and Safety Executive of the United Kingdom attempts to ident ify and quantify all isocyanate species in an air sample. Isocyanate s pecies are derivatized with 1-(2-methonyphenyl)piperazine (MOPP) and a nalyzed by high-performance liquid chromatography (HPLC) with tandem u ltraviolet/electrochemical (UV/EC) detection. The method identifies pe aks as being isocyanate-derived if the EC/UV detector response ratio i s between 0.75 and 1.5 times that of the derivatized monomer. This inv estigation sought to determine if the method correctly identifies and accurately quantifies intermediates created during polyurethane format ion that possess free isocyanate groups. Model compounds derived from 2,4-toluene diisocyanate (2,4-TDI) and ethylene glycol were prepared. These urethane species contained two (''dimer'') and three (''trimer'' ) TDI units and terminal MOPP-derivatized isocyanate groups. Like mono meric 2,4-TDI/MOPP urea, each contained two derivatized isocyanate gro ups per molecule, This investigation found that neither the UV nor the EC response is proportional to the number of isocyanate groups presen t in the model compounds, Therefore, ii is concluded that MDHS 25 is n either capable of correctly identifying TDI-urethane intermediates pos sessing MOPP-derivatized isocyanate groups nor is it capable of accura tely quantifying these isocyanate groups, The proposed solution to thi s problem is the utilization of a derivatizing reagent that yields der ivatized isocyanate species whose detector responses come more exclusi vely from the derivatized isocyanate moiety and, therefore, are more p roportional to the number of derivatized isocyanate groups.