Chromatography Forum

Dear all,
I was working with an GC to measure fatty acids. Recently, many people are talking about a triple quadrupol and how cool it is. But I didn't get the point what is the advantage when compared to an GC-MS. What is an quadropol? Could anybody give me easy answers?

Thanks a lot!!!!

I know this page already. Its not so helpful .... I need a more applied perspective. Maybe some examples

The short answer: MS/MS is indeed cool. But cool is not necesarily better.

GC/MS separates compunds with the GC part and obtains information on individual ions formed from the compunds in the MS. And it this gives adequate information to do the job, it is adequate.

Sometimes compounds coelute and have both fragment to give ions at the same masses in the mass spectrum. If two compunds give large ions of the same mass, there is a good chance that the large ions are of different structures and perhaps formulae. If that large fragment for each of the two molecules is fragmented, it is highly likely the fragmentation pattern for each of those large precursor ions will differ. And by obtaining the MS/MS signal, one can obtain unique signals for each of the two original compunds.

A quadrapole is a flight guide for ions and can be used as a mass filter to obtain a mass spectrum. in a tripple quadrapole, the first quadrapole is used to separate the ions generated by fragmenting a molecule. The second is used as an ion guide and the ions selected by the first ion guide are borken into smaller fragments. The third quadropole is used to separate the ions generated in the second quadrople.

And, MS/MS techniques are indeed helpful. But, they are still subject to interferences. So, other methods are sometimes required.

... I'd add that a triple quadrupole is going to be a lot more expensive than a straightforward GC-MS (which is a single-quadrupole instrument relying on source-fragmentation to make its beautiful spectra), and that is already more expensive that a straightforward FID system. For many applications, there is absolutely no need to use the most illustrious instrument available; in fact there may be no benefit in doing so.

There's a real danger that somewhere, someone will use a stupidly over-specified piece of equipment to do a job because they happen to have it. They want to publish the results, so they think up some marginal (or downright spurious) reason why their equipment should confer benefits over the cheaper systems people have used for years. Once it's in print, everyone else is obliged to follow, or look cheap-and-sub-standard; not being "cutting edge" can be bad news for your scientific reputation, even if the cutting edge doesn't contribute anything new...

If you need a triple quad, buy one. If a simple GC-MS will do (and you don't envisage novel applications that need future-proofing*), get a simple GC-MS. And if you don't need MS at all, and an FID is doing fine, consider sticking with it.

(a related issue is people developing LC methods for analytes that work better in GC, and vice versa, because they happen to have one or other instrument/expertise; that's fine, but I hate it when they claim advantages that don't exist! How I wish we could write "We analysed this by LC because the GC was broken at the critical time in the project"!)

(* note: beware of future-proofing that actually gets you an instrument that does a worse job on the tasks that you know you will have to do, in order to make it able to do hypothetical tasks it may never have to do...)

[/mode: general complaint]

One other advantage to the MS/MS approach is that it can eliminate most background noise compared to normal MS. If you use the first quad to filter to a specific mass(eliminating all other masses)then break that mass into a spectrum(MS/MS technique) then you have eliminated a large percentage of background noise that you no longer have to sort through to find your target analyte, which can give lower detection limits(sometimes, not always).

The downside to MS/MS is you must know what your target analytes are, and set up the instrument to only look for those analytes, you lose most of your ability to see unknown analytes in your sample, which can be both good or bad depending on your application.