Experimental determination of the transfer function of a differential mobility analyzer (DMA) in the nanometer size range

This paper describes the evaluation of the transfer function for the TSI-short differential mobility analyzer (DMA) in the nanometer particle size range. The TSI-short DMA is identical with the TSI-long DMA (Model 3071) but has shortened inner and outer electrodes and the insulator material Teflon was replaced by black Delrin. The DMA transfer function was determined by operating two identical DMAs in series. The DMA was investigated at a sheath to aerosol flow rate ratio of between 10 and 1. Usually for particles above 50nm the parameters of the assumed triangular transfer function, i.e. height and half-width, approach the ideal values, which depend only on the flow ratio. For particle size below 50 nm the height decreases from 0.9 at 50nm to 0.18 at 5 nm. The half-width increases from the ideal value 0.1 to about 0.17. The observed changes of the transfer function are due to diffusional effects resulting in losses mainly in the inlet and outlet and also broadening of the aerosol stream inside the DMA. The determined transfer functions were also compared with transfer functions calculated using a theoretical model. The agreement proved to be satisfactory. The half-widths and the transmission efficiencies of the conventional TSI-DMA (Model 3071), TSI-short with Teflon and also black Delrin insulator as a function of particle size are compared to demonstrate the improvements in performance.