Chromatography Forum

Hello everybody,

Can anyone guide me about the response factor of ECD, FID and TOF-MS detector used with GC X GC? Which one has got better?


Thanks

For compounds that are sensitive on all 3 instruments:

FID<TOF-MS<ECD

In GCxGC, detector sensitivity is only part of the question. Sampling rate is as well. If you are using cryomodulated GCxGC, your second dimension peak widths are 0.1 sec or less. (depending on vendor)

FID and TOFMS can sample quickly and obtain adequate data density across a peak. ECD is a bit trickier as it has a slower sampling rate - and depending on manufacturer, it may not sample quickly enough to obtain a good representation of the chromatographic peaks.

For compunds such as hydrocarbons, you can expect to get more sensitivity out of an FID. For compunds containing multiple chlorine or bromine atoms, the ECD would be expected to be more sensitive. But, if the sampling rate is too low, you will not be able to reliably integrate the peaks.

For compunds that form stable ions easily, you may get more sensitivitiy out of the MS. But the strong advantage with the MS is the abilty to use specific ion traces to pick out individual compunds where the total signal (which would be similar to the FID trace) is a jumble.

All three techniques work well, but each is best for specific applications. If you want to share more details, perhaps we can give you a more focused answer.

Hello,

My company wants to characterize volatile compounds responsible for off odors. We've reached the conclusion that GCxGC systems can give a more complete result to our questions comparetivle to 1D GC-MS. However i came across this problem: we have two close research groups with two distinct detector types.

One group as a Leco Pegasus 4D GCxGC ToFMS
Other group has a GCxGC-FID.

Which one do you think could better solve our questions?

Thanks

For off-odour work you need selectivity as well as sensitivity. The most powerful possible detector for off-flavours is the human nose. You need to be set up for GC-sniffing in order to locate which peaks on the chromatogram correspond with the off-odour. Then you can focus on identifying the compound in that peak. With natural materials you can expect lots of peak overlaps, and so you need two-dimensional separations, but classical heart cutting suits your application better then GCxGC because the very narrow peaks from GCxGC have never been shown to be sniffable, and if you are looking for relaible identifications accurate masses are a huge help, and accurate mass MSs do not scan fast enough for GCxGC.

Peter

So would you suggest such a technique as multidemensional GC-O?

The power in a GCxGC technique is that it will show you many peaks that are associated with the off-odor samples. The unfortunate thing is that none of the peaks that you would identify may turn out to have significant olfactory response. I have watched a a couple of colleagues who did flavor/fragrance discovery by running samples on a GC system with a sniff port and a strip chart recorder (this was some years ago) and one would sniff and the other would write the comments on the strip chart as peaks emerged from the chromatograph. There were a few comments early in the run, among the large peaks. But as the baseline became flat, the descriptions of fragrances became more frequent.

GC with a sniff port is the most direct to lead you to the compund of interest. Multidimentional depends on your sample.

Hi,

The thing here is: we have samples resultant from natural products with high chemical variety. In fact i understand that it would be very complicated to correlate an off-odor with a specific chemical compound without sniffing it simultaneously. So you believe that the best system would by heart-cut MDGC? Do you have any suggestions of who should i contact to do this? Thank you very much for your help. You've been very helpful, specially because chomatography it's not my thing

You have summarised it perfectly. There is off the shelf hardware available - Google is your friend.

Peter

zupahfly,

I don't think you necessarily need multi-dimensional at all here. I think the point Peter was making (not wanting to speak for Peter) is that you should look at an olfactory detector in parallel to a simple chromatograph (single dimension FID could be completely sufficient.) Brechbueler (sp?) and SGE both make olfactory accessories by the way.

Best regards,

AICMM

zupahfly,

I don't think you necessarily need multi-dimensional at all here. I think the point Peter was making (not wanting to speak for Peter) is that you should look at an olfactory detector in parallel to a simple chromatograph (single dimension FID could be completely sufficient.) Brechbueler (sp?) and SGE both make olfactory accessories by the way.

Best regards,

AICMM

Hi,

I'm not sure if a simple approach will have the enough resolution. I'm talking about a natural product that i need to identify off-odours. I believe that a multidimensional might be more accurate, but i'm not sure.

You can do off-odour identifications with a single dimension separation, using sniffing to locate which peak elutes at the same time as the off-odour. Beware though that the peak that you see on the chromatogram may be hiding a smaller one with a potent odour - and then you need a 2D system to pull those two peaks apart. Whether you get co-elutions is fundamentally a matter of luck. In general 2D-GC will be a more straightforward approach than trying to find a selective sample clean-up, although extractions at different pHs (checking all the time where the off odour is going), simple TLC or column chromatography fractionations can also be very useful.

You can use GC-FID with a sniff split to locate the off-odours, but you have to have MS (at the very least) to identify them.

If your business is off-odours I think that you need to be set up for GC with heart cutting, sniffing on both dimensions and MS. How you do this in terms of instruments will, as ever, be guided by what you already have and what you can afford to spend.

Peter

Thanks guys,

You've being very helpful. Just one more thing. The group owning a GCXGC-ToF-MS says they can identify the responsible compunds for maloudor by comparing samples classified by our panelists nose as good against samples we classify as having off-flavours. Do you think this will work properly?

Thanks

Using GCxGC-TOFMS to compare good with bad will reliably find off-odours if, and only if, the off-odour compound is the only difference between the samples. What are the chances of that ?

Peter

Hi,

Thats what i thought, but the guy garantees that he cam make a lot of samples and statistically find the compounds... But i think that will be much more expensive and not that accurate right?

I can upload later a chromatogram of a "good" and a "bad" samples so you can see (now i'm at work and upload images to sites is forbidden)

Thank you,