Silicone sampling tubes can cause drastic artifacts in measurements with aerosol instrumentation based on unipolar diffusion charging

Aerosol instrumentation based on unipolar diffusion charging has become popular in the recent years. These instruments can be made very small, making them suitable as personal monitors. In many applications, including personal monitoring, the use of flexible sampling tubes is required. We found that degassing from these sampling tubes can alter the gas composition of the aerosol, which changes the ion properties in the unipolar charger. As a result, the particle concentrations, measured with a unipolar diffusion charger are biased. The strongest effect was found with new, conductive silicone tubes, because of the degassing of siloxanes. Results obtained with one miniature diffusion size classifier unit were by a factor of approximately two too low. Partector and Nanoparticle Surface Area Monitor showed in principle the same behavior, but to a lower degree. Other tube materials were found to have even the opposite effect, i.e., the measured concentrations increased, when measuring through a tube. The largest observed increase was, however, only approximately 14%. Tygon (TM) tubes were found to be the best compromise considering particle losses and effect on the diffusion charger.