Observation and measurement of anomalous responses in a differential mobility analyzer caused by ultrafine fibrous carbon aerosols

We observed anomalous instrument responses above certain voltages when characterizing aggregates of airborne carbon nanotubes or nanofibers using a differential mobility analyzer (DMA). These were associated with sudden increases in measured number concentrations at high voltages, fluctuations in DMA voltage and audible high-frequency sounds from the DMA column. Onset of the anomalies depended on the material forming the aerosol and DMA sampling flow rate. The low density of the nanotubes relative to the nanofibers is thought to be the main reason the anomalous responses occur more easily and strongly with the nanotubes. Two possible mechanisms are suggested to explain the observations. The results indicate that measurement of nanometer-diameter conducting fibrous material by electrical mobility analysis may present a unique challenge.