Cannula transfer or cannulation is a subset of air-free techniques used with a Schlenk line, in transferring liquid or solution samples between reaction vessels via cannulae, avoiding atmospheric contamination. While the syringes are not the same as cannulae, the techniques remain relevant. There are two methods of transfer: vacuum, and pressure. Both utilize differences in pressures between two vessels to push the fluid through. Often, the main difficulty encountered is slow transfer due to the high viscosity of the fluid.