Controlled Generation of Peracetic Acid Atmospheres for the Evaluation of Chemical Samplers

The goal of the experimental design and research presented herein was to produce peracetic acid (PAA) atmospheres suitable for evaluating PAA sampling methods in a controlled environment. To achieve this, generated test atmospheres must: enable simultaneous exposure of multiple samplers to equivalent atmospheres; maintain a continuous stable atmosphere for a duration to exceed experimental time frames; and produce a range of analyte concentrations, air velocities, temperatures and humidities. The American Conference of Governmental Industrial Hygienists (AGCIH) Threshold Limit Value (TLV) for PAA is 0.4 ppm. The goal was to generate PAA atmospheres with concentrations from one tenth of the TLV up to 100 times the TLV.\n\nPAA is a reactive chemical, which presents some inherent challenges to generation of standard reference materials and atmospheres. PAA is a strong oxidant. PAA solutions are labile, thus it is difficult to keep reference standards or atmospheres. In solution, PAA and water are in equilibrium with acetic acid (AA) and hydrogen peroxide (HP). The equilibrium shifts as solutions of PAA evolve oxygen gas through the degradation of HP. Dynamic PAA atmospheres which are flowing and continuously refreshed are the preferred method for exposing samplers to equivalent atmospheres. Generation of static atmospheres by evaporating PAA solution into a closed container cannot be maintained for very long before needing to be refreshed. Dynamically generated atmospheres are created by a continuously renewed flow of air as a carrier and a source of PAA vapors, which are necessary for evaluating sensor performance over time frames relevant to occupational safety.\n\nGeneration of PAA vapors can be accomplished from assisted vaporization, including applying heat and/or aerosolization with a nebulizer to accelerate evaporation. Glass reacts with PAA to hydroxylate the silica. used a glass syringe to deliver PAA solution to a nebulizer; however, gas bubbles formed in the syringe leading to uncontrollable ejection of the PAA solution from the syringe. To mitigate this, used acid washed syringes. They generated atmospheres of up to 2 ppm with a reported 95% recovery of theoretical delivered PAA. Dilute solutions of PAA can help to mitigate gas evolution but require more energy to evaporate and introduce additional humidity.\n\nFor these experiments, PAA vapors were swept directly from the headspace above a PAA solution into our generation system. The flow rate of the air sweeping the headspace was varied to achieve the desired concentration after dilution into the carrier gas stream. When evaporating diluted PAA solutions using a nebulizer, the minimum humidity range is limited due to the water content in the dilute PAA solution. Thus, a wider humidity range on the low end is available when sweeping the headspace. The ability to maintain the concentration of PAA in the headspace above the solution is limited by the kinetics at the PAA solution-air interface. A practical solution was achieved using a PAA delivery system where air was passed through the headspace above a PAA solution in a vial. An impinger with a N,N-diethyl-p-phenylenediamine (DPD) analysis was used as the reference measurement for the PAA concentrations in the generated atmospheres.