Shoot-and-Dilute GC – Used Motor Oil and PAHs – Rxi-5ms GC Column Performance

In a previous post, I introduced the Shoot-and-Dilute GC concept to keep GC systems up longer.   A split injection with a Restek Premium Precision split inlet liner with wool pays huge dividends in good sample transfer of active and polar and involatile analytes while simultaneously reducing the impact of “dirt” on the expensive GC column.

In that experiment, I made 200 1µL split injections (10:1) of used motor oil at 5000 ng/µL interspersed with PAH standards to determine if “dirt” in the inlet over time could impact the transfer of the less volatile PAHs (e.g. indeno[1,2,3-cd]pyrene, dibenz[ah]anthracene, benzo[ghi]perylene).  As reported in that earlier post, there was no significant degradation of peak response for the PAHs even though 1000 µg of used motor oil had been injected in the GC inlet liner.  What I didn’t show was the impact on the GC column.  Read on…

Example used motor oil chromatograms at 5000 and 50,000 ng/µL are overlaid with a PAH standard at 2 ng/µL in the first figure below to give the ChromaBLOGraphy reader an idea of compound response.  It’s important to note that even though much of the used motor oil gas chromatographs, there are also many involatile hydrocarbons, and potentially, metals, that don’t chromatograph, or don’t even make it out of the GC inlet liner.  The next figure of non-baseline corrected chromatograms illustrates not only how PAH response is about the same after all the used motor oil injections (Shoot-and-Dilute is working as advertised!), but also shows the ruggedness for the 15m x 0.25mm x 0.25µm Rxi-5ms GC column.  Column bleed at 330°C, which in this case is probably less volatile components of used motor oil baking out, has barely increased during the life of this experiment.  Based on the number of analyses up to this point and the oven temperature program, the column has spent over 22 hours at 330°C.

Astute readers likely noticed that I mentioned 50,000 ng/µL used motor oil injections, but didn’t show any data yet for that work.  Stay tuned to Shoot-and-Dilute TV!