The Ultimate Separation Machine
Posted on July 15th, 2014
One useful principle that can be applied to GC-MS method development is to get the mass spectrometer to do as much of the work as possible and rely upon the chromatographic column as little as possible. The reasoning behind this rule is that it takes time to generate theoretical plates, whereas mass spectrometers generate their separations virtually instantaneously.
The application of this principle leads to the creation of fast and rather elegant analytical methods.
I hate hanging around waiting for results.
You could argue that this principle is what drives the increasing adoption of high-end mass spectrometers in GC-MS.
If so, where does GCxGC-MS fit into the picture?
Some samples (hydrocarbons for example) have complex, underlying structures that GCxGC can reveal visually (beautifully in fact), in a way that mass spectrometry can’t. Most samples, however, lack any real structure (landfill leachate anyone?). For these samples, GCxGC produces a chromatogram that looks like a dog’s breakfast.
Ah! I hear you say, but doesn’t GCxGC have greater peak capacity than one dimensional GC?
Yes it does – but not that much more!
Coupling two columns in series via a modulator involves a lot of compromises in column lengths, column i.d., carrier gas flow rates and modulation frequency that inevitably leaves each column a long way away from its optimum performance.
In truth, the ultimate in GC separation is delivered by 2 dimensional GC (2D GC), where co-eluting compounds can be cut from one column (of any dimension, operating under optimum conditions), via a cryo-trap, to a second dissimilar column (of any dimensions, operating under optimum conditions).
In this case, your peak capacity is huge – equal to the peak capacity of the first column, multiplied by the peak capacity of the second column.
If you are interested to know more, here is a great article on the power of 2D GC.
For my money, the ultimate separation machine is a high resolution accurate mass GC/Q-TOF with selectable 1D 2D heart-cutting capabilities.
With this system, you get the best of all Worlds; even quite tricky separations can be dealt with, using the tremendous separating power of the high resolution TOFMS preceded by a simple 1 dimensional GC separation. Where you need more horse-power you can invoke tandem MS.
On the other hand if some of the compounds that you need to separate are isobaric (have identical empirical formulae) then high resolution MS can’t help. For these compounds, heart-cutting co-eluting, components from one column (of any length, operated under optimum temperature and flow conditions), to a dissimilar column (also of any length, with a dissimilar phase, operated under optimum flow and temperature conditions) will generate plenty of theoretical plates, when and where you need them – many more than GCxGC ever can.
The photograph at the top of the blog post shows just such a system, built by Nobuo Ochiai and his team at GESRTEL KK in Tokyo. The Research Institute for Chromatography in Belgium (Pat Sandra’s lab) also have one.
In May at the Riva symposium, both groups presented some truly impressive separations using this approach.
If you would like to discuss this further, please do not hesitate to contact me on +44 (0)1223 279210, or email firstname.lastname@example.org.