Sample Introduction:

The most general sample introduction techniques are as follows:

• Electrokinetic injection
• Pressure injection

With electrokinetic injection, one end of the capillary and its electrodes are erased from their buffer compartment and placed in a little cup holding the sample. A potential is then applied for a period of time causing the sample to enter the capillary through a combination of ionic migration and electroosmotic flow. The capillary end and electrode are then placed back within the regular buffer solution for the duration of the separation. This injection method discriminates through injecting larger amounts of more mobile ions associative to the slower moving ions.

With pressure injection, a sample introduction end of the capillary is also placed momentarily within a small cup holding the sample and a pressure difference is then used to drive the sample solution within the capillary. The pressure difference could come from applying a vacuum at the detector end through pressurizing the sample or through elevating the sample end. Pressure injection does not discriminate because of ion mobility but cannot be used in gel field capillary. For both the injection process, the volume injected is controlled through the duration of the injection. Injections of 5-50 nL are general, and the injection of volumes below 100 pL has been reported. For a buffer along with density and viscosity near the values for water, a height differential of 5 cm for 10 s injects about 6 nL along with a 75 µm inside diameter capillary.

Microinjection tips constructed from capillaries pulled down to extremely small diameters permit sampling from picolitres environments like as single cells or substructures inside single cells. This method has been employed to study amino acids and neurotransmitters from single cells.