Setting the splitless injection purge valve time in gas chromatography

In splitless injection GC we set the purge valve time to sweep the inlet liner of excess solvent so that it doesn't "tail out" through the whole chromatogram.  But what time do we set?  A good "rule of thumb" is to sweep the inlet liner 1.5 to 2 times with carrier gas prior to turning on the purge valve so that all sample analytes are completely (as possible) transferred from inlet liner to GC column.  GC inlet flow can be estimated using a flow calculator and liner volume can be calculated using the inside diameter, pi, and liner length.  After that, it's a simple matter to calculate the 1.5 to 2x sweep time. 

You can see the effect of splitless purge valve time on  peak response for a volatility range of PAHs (naphthalene being relatively volatile, benzo[ghi]perylene being relatively involatile), including the EFSA PAH4, in the chart below.  A properly set purge valve time becomes even more important when analyzing dirty "real world" samples.  More on that later...