TANDEM DIFFERENTIAL MOBILITY ANALYZER.

An analytic expression for the response of a TDMA system is presented and its validity discussed. A least-squares minimization technique is used with this analytic result to extract inter-DMA size changes from experimental data. A FORTRAN computer code that demonstrates the method is included as an appendix. Using this method and available instrumentation, diameter changes can be measured with a precision of about 0. 3%. For aerosols in the 0. 01-0. 2 mu m diameter range where the technique works best, this corresponds to a precision in measuring diameter of 0. 03-0. 6 nm, or the characteristic thickness of a molecular monolayer. Guidelines for laboratory implementation of the TDMA are suggested and specific examples discussed. A more accurate numerical solution is used to estimate the systematic error introduced by the approximate, analytic expression.