Evaluation of total isocyanate-in-air method using 1-(2-methoxyphenyl)piperazine and HPLC.

The United Kingdom Health and Safety Commission has set a 'common control limit' for workplace exposure to all isocyanates. This limit replaces the previous United Kingdom threshold limit values, which were set for just four species of monomeric isocyanate compounds. This new control limit is set at 20 micrograms of isocyanate group per cubic meter of air expressed as an eight-hour weighted average, and 70 micrograms of isocyanate group per cubic meter of air as a 10-minute weighted average. These new control limits make it necessary that analytical methods should be capable of measuring total isocyanate concentration, including monomers and prepolymers. A previously available method for determining isocyanate monomers using 1-(2-methoxyphenyl)piperazine with high performance liquid chromatography (HPLC) has been modified to measure both monomer and prepolymer. A dual detection system employing electrochemical and ultra-violet detectors is used to identify isocyanate-derived HPLC peaks which are then quantified by reference to a monomer standard, thus avoiding the necessity of isolating prepolymer standards. The total isocyanate-in-air concentration is then calculated from the total area of the isocyanate-derived HPLC peaks. The precision of the method is better than 10% over the range 35 to 140 micrograms(NCO)/m3 for a 10 min sample.