An approach based on the calculation of the probabilities of the random displacements of particles around their regular trajectories enables us to derive analytical expressions for the normalized apparent mobility spectrum. A particular derivation is carried out for the case of a DMA with one collecting electrode, with inclined electric field, and variable electric field strength. Calculations show that an inclined electric field actually results in a sharper apparent spectrum, i.e., in a higher resolution than a transverse electric field does. A way to improve resolution by calculations is outlined.