When doing headspace injection, what can I do to avoid peak splitting of some analytes? A direct injection on same system is giving the same split peaks.

Headspace injections may initially have poor peak shapes if there is mixing or diffusion occurring in the system. This typically occurs in the inlet. Use a 2.0 mm ID (or smaller) liner to keep the volume for dilution and mixing smaller, which will lead to sharper peak shape.  

Additionally, most solvents when using headspace boil at relatively low temperatures (100°C or lower). If this is the case, then the inlet only needs to be hot enough to keep the analytes and solvent from condensing. If the inlet temperature is higher, the temperature actually starts to hurt because it causes the injection gas to over-expand which can lead to backflash. 

The following suggestions can help peak splitting with headspace injection:

1. Use a 2.0 mm ID (or smaller) liner
2. Lower the inlet temperature to 50-75°C higher than the headspace vial temperature
3. Lower the initial oven program temp if much higher than 50°C. 
4. Depending on the solvent, there may be some large amount of solvent that is getting onto the system. If that is the case, a guard column may help.
5. Pay attention to the injection technique. If using an autosampler, please remove any delays in the injection. If there is a hesitation in the injection, you will get two peaks no matter what else is optimized.