For many years, gas chromatographers have considered the normal variability of manual injections to be in the range of 5% RSD for an individual, with 10% RSD common between individuals. It is entirely possible however to reduce this variability to less than 1% RSD for one person and easily to less than 2% RSD for two or more individuals. The “secret” is in reducing the individual sources of variability by a systematic injection technique.
The difficulty of setting the plunger repeatedly to the same volume is the greatest source of error. Plunger throw (the distance the plunger moves during the injection) is often short, hence a small change in plunger position represents a large volume difference. Parallax (viewing the plunger other than 90º from the bore axis) is also a large source of error. Ideally use a repeating adaptor to overcome all parallax reading error. If your technique does not allow for a repeating adaptor, choose a syringe with graduation lines that extend to the rear side of the syringe to enhance readability.
Use a straight wall, well deactivated liner, with quartz wool packing positioned to wipe the needle (ie. FocusLiner®) where possible. This liner configuration give both good vaporization and precision on most gas chromatographs. Cup liners usually give poorer reproducibility than straight liners. Also a hint, don’t overtighten the septum cap and make sure the system is leak free.