Finally, some increased sample loading capacity for PAHs! Part 4: 30m x 0.32mm x 0.50µm Rxi-5ms29 Oct 2014
This may be the last post in my series on sample loading capacity for PAHs on Rxi-5ms GC columns.
I finally got an obvious increase in sample loading capacity for PAHs by going to a 30m x 0.32mm x 0.50µm Rxi-5ms: bigger bore x thicker film. I say “obvious” because now I can see it by the peak shapes in the chromatograms (essentially answering Jaap’s question – “What do chromatograms tell us?”). I think in the interest of not being long-winded, I’ll let the chromatograms tell the story and summarize the series below. As previously, the EZGC Method Translator was used to provide the GC conditions when going from the 30m x 0.32mm x 0.25µm Rxi-5ms to the 30m x 0.32mm x 0.25µm Rxi-5ms.
- PAHs, especially larger ones like benzo[b]fluoranthene (5 rings) up to benzo[ghi]perylene (6 rings), and higher, easily overload 0.25mm x 0.25µm and even 0.25mm x 0.50µm 5% phenyl-type columns like the Rxi-5ms (and Rxi-5Sil MS) and corrupt overall peak capacity, separation efficiency, qualitative identification (particularly due to coelutions of isobaric species), and quantitative accuracy.
- If extra sample loading capacity (above ~10 ng each on column?) for PAHs is very important, a 30m x 0.32mm x 0.50µm column offers a balance between sample loading capacity and efficiency of separation, at the expense of increased run time versus 0.25mm columns. (Note that the efficient flow rate for the 0.32 x 0.50µm column is going to be around 1.8 ml/min helium, so a mass spectrometer must be capable of pumping this flow.)
- Sample loading capacity estimates in ng on column may be lower than is typically stated and needs additional investigation with other compounds.