Inversion of tandem differential mobility analyser (TDMA) measurements

The tandem differential mobility analyser (HTDMA) technique is used to detect size changes of submicron particles after a treatment such as exposure to high relative humidity (RH). Measured diameter growth factor distributions must be inverted, because they are only a skewed and smoothed integral transform of the actual growth factor probability density function (GF-PDF). We introduce a new approach, TDMAinv, representing the inverted GF-PDF as a piecewise linear function. Simulated measurements are used to prove the concept. Measurements of an aerosol with a bimodal GF-PDF show that TDMAinv provides equivalent information to TDMAfit, the most widely used inversion algorithm. The major advantage of TDMAinv is that convergence of the inversion is robust and independent of the initial guess. This makes TDMAinv a reliable tool to analyse large TDMA data sets. A methodology is also demonstrated for analysis of TDMA data in cases where the dominant fraction of selected particles is doubly or triply charged.