Your GC Column Matters!
In liquid chromatography, changing column to improve the selectivity of your separation is a no brainer. In fact, it’s a standard method development approach testing different stationary phases to find the chromatographic optimum.
On the other hand, in gas chromatography there is a tendency to underestimate the contribution of the column type to the quality of the final chromatography. Furthermore, when working with GC-MS systems, this inclination is possibly increased by the general reluctance to vent the MS to allow column changing.
If this is keeping you from testing different stationary phases during your method optimisation, Agilent Capillary Flow Technology might be an interesting solution. For instance, the QuickSwap Capillary Flow module allows vent-free columns, removal of columns and back-flush mode to increase your column life time.
Now, going back to our columns, below is an example on how your GC column choice can impact your final results.
Direct transmethylation of fatty acids was performed on dried oregano and the same sample was injected on two different GC columns (Agilent HP-5MS 30m x 0.25mm x 0.25 mm and SGE BPX-70 25m x 0.22 mm x 0.25 µm) using the same GC conditions (e.g. injection volume, inlet temperature, temperature ramp).
Figure 1 and 2 show the extracted ion chromatograms (m/z 74) for fatty acids methyl esters (FAMEs) and thymol (m/z 135), respectively, obtained on the two selected columns.
BPX-70 is a polar phase 70% Cyanopropyl Polysilphenylene-siloxane custom designed for separation of Fatty Acid Methyl Esters (FAMEs). As shown in Figure 1, the BPX-70 column gives a nice and even distribution of the FAMEs across the chromatographic space when compared to the more commonly used HP-5MS (non-polar, 5%-Phenyl-methylpolysiloxane), despite the lower number of theoretical plates (25 m versus 30m).
Figure 1: Extracted Ion Chromatogram (m/z 74) for fatty acids methyl esters using a HP-5MS column (top) and a BPX-70 (bottom).
The Thymol (2-isopropyl-5-methylphenol) peak not only shows dissimilar selectivity on the two columns, but we can also observe differences in peak shape and relative ratio of the interfering unresolved peak on the right.
Figure 2: Extracted Ion Chromatogram (m/z 135) for Thymol using a HP-5MS column (top) and a BPX-70 (bottom).
They do say it, “The difference is in the details”…
If you would like to discuss this further, please do not hesitate to contact me on +44 (0)1223 279210 or email: enquiries@anatune.co.uk.