Fast GC – Another Approach

Jaap de Zeeuw and I are often discussing fast GC, including low-pressure (or vacuum-outlet) GC and smaller inside diameter columns (such as 0.10 or 0.15mm).  I dug out some work I’d done in 2005 with a 0.18mm x 0.18µm Rxi-5ms column, but I cut the column down to 4m to REALLY go fast for some drinking water safety work I was doing with pesticides (Azinphos methyl, Chlorpyrifos, Diazinon, Dichlorvos, Dimethoate, Ethoprop, Malathion, Methyl parathion, Monocrotophos, Parathion, Terbufos, and other organophosphorus pesticides, and Aldrin, gamma-BHC (Lindane), Chlordane, DDT, Endosulfan, Endrin, Heptachlor, Methoxychlor, and other organochlorine pesticides.

Check out the sub 2 min chromatogram for pesticides ranging from fast eluting Dichlorvos to the relatively late eluting Coumaphos.  I was able to achieve this by:

Using a 20:1 split injection with an oven temperature start of 90°C and a sensitive (low pg) TOFMS as a detector to make up for the split injection.

Employing a flow of 1 mL/min He, which gave a linear velocity of ~105 cm/sec, very high, but lack of chromatographic separation was recovered with mass spectrometry and spectral deconvolution.

Programming the GC oven at 80°C/min to 330°C, accomplished by using an oven temperature insert and 240V “fast GC” option on the Agilent 6890 GC.

Benefits of this approach include being able to start at a higher oven temperature by employing a split injection, which increases turnaround time.  And, the ultra fast oven program rate keeps peaks very narrow, some less than 1 sec in base peak width, which increases detectability (a narrower peak is a taller peak).   The LECO Pegasus TOFMS at 40 spectra/sec supports peak definition and spectral deconvolution with such narrow peak widths.

A split injection, while leading to less detectability than a splitless injection, is often better matched with the capacity of the 0.18mm column.

I challenge readers (and Jaap!) to beat my pesticide analysis time record...

P.S. Another benefit of the 4m length approach is that I got 5 columns out of the 20m I purchased.

Pesticide standard analyzed in less than 2 min on a 4m x 0.18mm x 0.18µm Rxi-5ms GC column. Not all pesticides are labeled due to space limitations.

 

Select pesticides showing good detectability and reference spectra match factors due to fast GC-TOFMS (narrow peaks and spectral deconvolution).

Methyl parathion in a drinking water extract is located with TOFMS automatic peak find and spectral deconvolution even when almost coeluting with matrix co-extractives.