There has been much focus wIThin the industry of late on ways of improving the way in which HPLC method development is carried out – which has LED to a more widespread adoption of modelling and optimization software combined with dedicated method development platforms from the major instrument vendors.
I don’t see the same degree of rigour or innovation applied to gas phase separations. In fact, until recently, it was many months since I last had a discussion on optimizing elution variables for gas chromatography – and here of course, we primarily mean optimizing the temperature program. A colleague recently remarked that there aren’t too many folks around the world developing new GC methods these days, other than the instrument vendors. I’m not sure if this is borne out in the statistics – but anecdotally it ‘feels’ right.
However, for those of us still developing GC methods, please find below a collection of guiding principles on GC temperature program development, which have proved invaluable in improving the effectiveness of our own GC method development.
There are two ways to affect the selectivity (α) of a GC separation. Changing the chemical nature of the stationary phase and altering the temperature – either the ISOthermal temperature or the ramp rate (oC/min) in a temperature programmed separation. Here, I’m going to deal with developing a temperature program and highlight the thinking which goes behind establishing the major parameters with the GC temperature program.
OK – so our friendly colleague has left a vial of colourless, odourless liquid on our desk with the helpful note ‘compositional analysis please’. How do we go about screening the sample for information on how to analyze the volatile components by GC (or more probably GC-MS)?
If the sample is aqueous, dilute x10 with methanol and make a split injection under the conditions shown below. If the sample is organic in nature, dilute x10 with ethyl acetate and do the same thing.
Column: 5% Phenyl dimethylpolysiloxane, 30 m x 0.25 mm x 0.25 μm
Injection: Split (4 mm i.d. liner, straight, deactivated, no packing)
Injection Volume: 1 μl
Split ratio: 100:1 (adjust according to the sample component concentration seen in the first injection)
Carrier: Helium at 35 cm/sec. or Hydrogen at 45 cm/sec.
Initial Oven Temperature: 40 oC
Oven Program Rate: 10 oC/min.
Final Temperature: 330 oC (or Gradient temperature maximum for the column you are using)
Final Time: 10 min
Detector: FID at 300 oC (ensure all supplied gases are optimized) MS using total ion acquisition mode 50–550 amu
One should make the usual caveats that not all components are detected by FID and there is a possibility that volatile components may have masses above 550 amu, therefore where analysis is critical these possibilities should be further investigated.
