For quantitative work, an auto-injector, set to its maximum injection speed, is recommended.   In a slow injection, such as a manual injection, they syringe needle itself provides a surface for heating and evaporating the sample.  Evaporation happens inside the needle!  As described above, as solvent evaporates, a vapor layer forms between the remaining solvent and the needle inside surface, insulating the remaining solvent, but causing some less volatile analytes to be trapped on the inside surface of the needle.  If injection is fast enough, the liquid can be fully ejected from the needle before this evaporation occurs. 

 If performing a manual injection, consistency is key; practice and develop your own rhythm.  Over the years, many syringe handling techniques have been described for manual injection.  These include: filled needle, hot needle, cold needle and solvent flushing.  None of these methods completely eliminates discrimination.  In a filled needle injection, the needle is filled with sample liquid.  This is the technique performed by auto-injectors: pull the plunger up to the desired volume and inject.  In a cold needle injection, the sample is drawn all the way into the barrel of the syringe, the needle is inserted into the inlet and the plunger is depressed immediately.  The needle is assumed not to have heated so rapidly, and therefore is “cool.”  A hot needle injection employs the same procedures as the cold needle injection, except that there is a short (a few seconds) delay after the needle is inserted into the hot inlet to allow it to heat-up.  In a solvent flush injection, a plug of solvent is drawn into the syringe prior to drawing the sample materials into the syringe.  This plug is said to force the sample quickly through the needle, while also flushing out any solute material that has been left behind. 

The main key for avoiding or limiting discrimination is that the plunger depression and ejection of sample from the needle be performed as quickly as possible, to avoid evaporation of the solvent from the hot needle surface and the consequent retention of high-molecular weight solutes in the needle.