Postcolumn void volume.

Post column dead volumes are the most critical part of a nanoflow system. In most set-ups, dispersion occurs in an improper connection or a not well designed ESI emitter. Peak broadening increases rapidly in low flow, small ID columns (see below). Example. Volumes of peaks leaving the column in a width of 3 sec (FWHM) are typically  3, 12 and 25 nL on a 25, 50 and 75 umID column, resp. When connected to an emitter with a dead volume of e.g 60 nL, the 3 sec wide bands broaden by 20, 6 and 2.6 times, and are observed in width's of  60, 18 and 8 s, resp. (see for more details peak broadening)

Peak broadening on 50 umID column (flow rate 120 nL/min) when connected to an ESI emitters with a void volume of 90 nL (top), 20 nL (middle) and 5 nL (bottom), resp..