Dear All,
We are about to upgrade our Leco BT GC-MS to 2D GC. I would like to hear people's experiences of using cryo modulators, and whether they would recommend them over a flow modulator. I understand that a nitrogen cooled cryomodulator is able to detect much smaller volatiles, and without the loss of sensitivity of a lot of flow modulators. What are people's personal experience of working with them? Especially liquid nitrogen cooled versions? Do they work well, do they break down alot, would you recommend them? Have you had any health and safety issues with using them? Or practical problems?
Specifically, we are interested in hearing individuals' experience of which modulator they would recommend for breath analysis, and why.
Flow modulation using a modulator capable of reverse flow, with an MS that can take a reasonable flow rate. If choosing flow modulation, chose a flow modulator which doesn't lose sensitivity either. This means, for doing fully comp GCxGC, you should chose a modulator that is capable of transferring all eluent from primary column to secondary column. You can also use dual FID/MS so no flow goes to waste.
Use of liquid N2 is fine, plenty of good breath studies use this, it will modulate most compounds but for breath you might have inefficient modulation of acetone or struggle to see the very volatile compounds. So depends if you're trying to see everything or specific targets. If you can't get a flow modulator as above then liquid N2, cryomodulation will be better then a crappy flow modulator. Thermal modulators have sharper modulations (and perhaps more to consider if doing petrochemicals for example) but the latest flow modulators are good at matching performance and for breath you are analysing volatiles and very volatile compounds so would go with a decent flow modulator.
From a practical perspective also consider the number of samples and study length, and what this means in terms of either regularly topping up liquid N2 (thermal), or replacing helium cylinder slightly more frequently (flow).
Thanks Michael. I had understood that the use of liquid nitrogen and a thermo modulator was to improve the detection of compounds such as acetone and is the best option for breath analysis. Do I understand correctly that you find that a good flow modulator is superior in this regard? Do you have any particular devices that you would recommend?
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