EXPERIMENTAL-DETERMINATION OF THE TRANSFER-FUNCTION OF A DIFFERENTIALMOBILITY ANALYZER (DMA) IN THE NANOMETER-SIZE RANGE

This paper describes the evaluation of the transfer function for the T SI-short differential mobility analyzer (DMA) in the nanometer particl e size range. The TSI-short DMA is identical with the TSI-long DMA (Mo del 3071) but has shortened inner and outer electrodes and the insulat or material Teflon was replaced by black Delrin. The DMA transfer func tion 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 50 nm the parameters of the assume d triangular transfer function, i.e. height and half-width, approach t he ideal values, which depend only on the how ratio. For particle size below 50 nm the height decreases from 0.9 at 50 nm to 0.18 at 5 nm. T he half-width increases from the ideal value 0.1 to about 0.17. The ob served changes of the transfer function are due to diffusional effects resulting in losses mainly in the inlet and outlet and also broadenin g of the aerosol stream inside the DMA. The determined transfer functi ons were also compared with transfer functions calculated using a theo retical model. The agreement proved to be satisfactory. The half-width s and the transmission efficiencies of the conventional TSI-DMA (Model 3071), TSI-short with Teflon and also black Delrin insulator as a fun ction of particle size are compared to demonstrate the improvements in performance.