Transfer capillaries are the preferred means to transport ions, generated by electrospray ionization, from ambient conditions to vacuum. During the transfer of ions through the narrow, long tubes into vacuum, substantial losses are typical. However, recently it was demonstrated that these losses can be avoided altogether. To understand the experimental observation and provide a general model for the ion transport, here, we investigate the ion transport through capillaries by numerical simulation of interacting ions. The simulation encompasses all relevant factors, such as space charge, diffusion, gas flow, and heating. Special attention is paid to the influence of the gas flow on the transmission and especially the change imposed by heating. The gas flow is modeled by a one-dimensional gas dynamics description. A large number of ions are treated as point particles in this gas flow. This allows to investigate the influence of the capillary heating on the gas flow and by this on the ion transport. The results are compared with experimental findings. Graphical Abstract ᅟ.

A detailed study of the transfer of ions in transfer capillaries of electrospray ion sources is presented. The laminar flow field for various capillary sizes and wall temperatures is calculated. It forms the base of ion transfer simulation of a large number of ions with space charge. This allows to study the thermodynamical conditions of the ions during transfer, which are found to vary strongly with the capillary dimensions. The dependence of mass flow and ion current on the size of the capillary is presented. Simple scaling relations are derived and tested. The method also allows to predict a transfer bias between different ion species depending on the difference in ion mobility and the composition of transferred ions.