The 3M™ Organic Vapor Monitor 3520 with Backup Section uses the proven scientific principle of diffusion, which is the gradual mixing of molecules of two or more substances. Primary and back-up sorbent pads are included for sampling demanding environments. Especially suited for monitoring compounds like acrylonitrile, butadiene and methylene chloride for which activated carbon shows limited capacity. It has two stages for increased sampling capacity. Increased sampling may be necessary when sampling volatile compounds (compounds with low boiling points), high or unknown concentrations, complex mixtures or when sampling in high humidity conditions. It has an 18-month shelf life.