GC X GC

[npaz]

Hi. Ive heard of GC X GC, and two things come to mind. One is that I need a modulator and Two I need a GC TOF that is capable of fast scan rates. Why need a modulator when you can use a column union? Ive used two different columns before at one time without such a modulator. (no problem) And finally is it possible to use a single quadrapole for this analysis?

[jh1]

Sounds like you're talking about the LECO Pegasus GCXGC TOF. From what I recall of the system the advantage with the setup is that you can run the 2 columns at different temperature ramps, something you can't do with 2 columns connected by a simple union. A GC/GC system could be hooked up to a quad, I believe LECO uses a TOF because it allows for high resolution with very short run times.

[Suresh Seethapathy]

Hello jh1,

When you use a column union to connect two columns and run the GC, there are chances for the compounds separated in the first column to join up again as they go through the second column. The key thing with GCxGC is that you have to preserve the separation as much as possible that occured in the first column.

What the modulator does is, it traps whatever is coming from the first column for a brief period of time, say 4 seconds. Then, it allows the trapped portion to go through the second column while simultaneously trapping the compounds eluting out of the first column (for 4 secs). The result is a a bunch of 1 D chromatograms of 4 secs each. Fignal result - high peak capacity and orthogonality (separation by two different mechanisms).

There are various types of modulators and some like the one which i use (single stage cryogenic modulator) can result in peaks with width of about 30ms!! That is the reason why you need a very high scan rates which can be provided by ToF but not Quadrupole (atleast not yet). remember that you need atleast 10 points across the peak for proper quantification.

You will never be able to separate compounds in gasoline or diesel with the way you are doing. You can try googling GCxGC chromatograms and you can see how complex they can be and how powerful they are.

Hope that helps
Suresh.

[Ron]

You can do GCxGC with a newer quadrupole mass spectrometer, most newer mass specs can collect data at 30 to 50 mass spectra per second.

The LECO TOF is a low mass accuracy, low resolution instrument just like a quad. The LECO does have faster data acquisition rates, but there are not the mass resolution and accuracy necessary to do molecular formula calculations.

[Don_Hilton]

I use GCxGC-TOFMS and typically acquire at rates as fast as 150/second to allow for spectral deconvolution in complex samles. And, I can easily see low picograms of compounds, on column, at this acquisition rate - with sufficient spectral quality to identify the compunds. And I am acquiring over a mass range from about 25 Da to 800 Da. (The mass range acquired does not affect the spectral quality, as the ions are sorted in mass rather than filtered in the analyzer portion of the spectrometer.)

What you need in the way of instrumentation depends on the problem you need to solve. For some samples I have done, I would use nothing less than a cryofocusing modulator (for sharp peaks in the second dimension) and a TOFMS to allow for deconvolution of chromatograms that are still complex in the second dimension. For other projects, modulator such as the Deans switch modulator and a quadropole would be entirely fine - profided that there was adequate software to handle the GCxGC-MS data.

If you are picking an instrument, have the prosepective vendors run samples and see what they can show you in your sample. And, you pick the sample. The results from somethign selected for marketing purposes and your sample may differ.

[AICMM]

npaz,

To take Don's comment a little further, do you really need GC X GC? Awful lot of money if the problem is not complicated separation. If only a few critical pairs need resolution, much better to go with a heartcut (simpler, cheaper, easier to get running) as the two dimensional GC of choice. And, yes, at that point, Quad-MS is very useful.

Depends on the problem.

Best regards.

[HW Mueller]

Now, for someone who hasn´t done GC for many years and has, therefore, not been diligent regarding reading the literature, this chain is very confusing. It used to be that heart cutting was done via Deans switching, either with a cold trap (on the main column) or without. The word module was not used, do some of these merely delay some of the eluent of the first column before releasing it to the second column??

[Ron]

HW, one of the hot topics in GC today is comprehensive GCxGC, where all the analytes go through both columns. It is like doing a heart-cutting experiment every 2 to 8 seconds. The switching device for this is usually called a modulator, either thermal or pneumatic. There is a nice example of a comprehenive 2-D application in the Mass Spectrometry supplement to the latest issue of LC-GC.

[Suresh Seethapathy]

Ron,

I am not convinced yet that you can do an ideal GCxGC with quadrupole. With the aquisition rate of 30-50, you will probably be able to quantify only those peaks with a minumum width of 150 ms to 200 ms(approx). Further, with that aquisition rate, you would probably be scanning only about 200 Da. Please correct me if my mathematics seems wrong. If you are quantifying volatiles with cryogenic modulators, i don't think you can use quadrupole. There may be applications which result in peak widths greater than say 150-200 ms and that which does not require a long scan range for which quadrupole may work.

Other complications with quadrupole would be spectral skewing. With the LECO ToF you can scan full range (Approximately 10 to 850 Da) at high frequency that is absoultely necessary for spectral deconvolution. Although you will need only 10 points across a peak for quantitation, you will need upto 20 data points for deconvolution in case of coelutions..

Cheers,
Suresh.

[Ron]

I would agree that it is not possible at this time to do an ideal GCxGC experiment on a quad, just as it is not possible to do an ideal experiment on a TOF or any other type of mass spectrometer. The suitability of a system for an experiment depends on the goals of that experiment. If the goal is to collect the absolute maximum number of scans at the widest mass range to separate the maximum possible number of compounds then a TOF is required. If you are looking at identifying a specific set of analytes or class of compounds in a complex matrix you probably don't want that much information.

If a set of compounds is being analyzed that covers a limited mass range then scanning a narrow mass range is not a disadvantage, it actually eliminates mass data that is not relevent to the analysis and reduces the need for deconvolution. Spectral skewing is related to scan rate, at a higher scan rate the skewing is reduced. 10,000 amu/sec or above is common today, and there is much less skewing than at 1,000 amu/sec.

I'm not claiming that quads are perfect for all GCxGC applications, but they are a realistic alternative for some applications, and as scanning rates increase more applications will be feasible on quads.

[Suresh Seethapathy]

Ron, I fully agree with you that quad's could be a realistic alternative for some applications, if not all. It will be very interesting to see how far the quads can be improved for this application. Considering that the ToF costs atleast 5 or 6 times that of a quad (to my knowledge), it will certainly be a big advantage if the quads can do better.

[Don_Hilton]

Get the quote before you rule out the TOF. I am not sure that they are quite that much more than a quad. It's been a while since I've looked at prices...

[DanCooper]

I'm not sure about the LECO, but the Almsco BenchTOF is certainly not that expensive. I think it's of the order of 3x the price.