GCMS Vs GCMS - RT Variation

Chromatography Forum: GC Archives: GCMS Vs GCMS - RT Variation
Top of pagePrevious messageNext messageBottom of pageLink to this message  By Anonymous on Thursday, April 8, 2004 - 03:49 am:

We got ~ 5 min. difference in retention time when analysing a sample using GC-FID and GC-MS with almost no change in any other parameter.
This may be due to change of carrier cas from N2 to He and due to vacuum in MS.
Can anybody suggest how to match the RT's of both detectors.We tried with flow but of no use.


Top of pagePrevious messageNext messageBottom of pageLink to this message  By Anonymous on Thursday, April 8, 2004 - 07:28 am:

One easy way is to simply reduce the temperature of the GC oven in the MS until it matches the RT of your FID system or do the reverse, increase the temperature in the GC-FID system.


Top of pagePrevious messageNext messageBottom of pageLink to this message  By Jason Ellis on Thursday, April 8, 2004 - 07:45 am:

Changing oven temperature may change elution order or resolution of the peaks. I recommend using the "Method Translator" program which is available free on the Agilent website. You can find it at this page:

http://www.chem.agilent.com/cag/servsup/usersoft/main.html#mxlator

The program will allow you to calculate a new lower headpressure to use on your MSD system to achieve equivalent retention.

Best regards,
Jason Ellis
GC Column Tech Support
Agilent Technologies


Top of pagePrevious messageNext messageBottom of pageLink to this message  By Anonymous on Thursday, April 8, 2004 - 02:06 pm:

Anonymous,
There will certainly be a shift when going from nitrogen to helium as a carrier, the helium will give a shorter retention than nitrogen for the same flow rate. The vacuum from the MS will also cause a slight shift in the r.t. but not much. I think most of your shift is carrier gas difference.

Regards,
Mark


Top of pagePrevious messageNext messageBottom of pageLink to this message  By Leon on Thursday, April 15, 2004 - 10:20 am:

It is not as complicated as it might appear from some previous comments.

Of course there might be a shift if it is not done right. But there does not have to be a shift, and there will be no shift if it is done right. And, in this simple case, it can be easily done without Agilent's GC Method Translation software (although the software will certainly do the job, and I recommend to have and to use it).

If one does not want to bother with the Method Translation software in this simple case, and to better understand whatís going on, one can do the following.

Step 1. Measure or calculate the hold-up time in the original nitrogen/FID method.

Step 2. Make sure that the hold-up time in the new helium/MS method is the same as the one found in Step 1. Set up this hold-up time by calculating the new column pressure or by trial and error. In the latter case, keep in mind that the hold-up time in GC/MS is inversely proportional to absolute pressure on a column. If, for example, the measurement shows that the hold-up time is 17 % greater than it needs to be, increase the absolute pressure (that is, atmospheric pressure + gauge pressure) by 17 %.

Thatís it. If it is done right, the retention times of ALL peaks in both methods will be the same (within minor errors typically not exceeding statistical errors). And there is no magic here. It is based on a simple and solid theory and, as confirmed it tons of experiments, works exactly as predicted. All this is well documented in literature (some of which is available from Agilent site).

Also, Jason is right. Most likely, changing oven temperature would not work. Theory and experiments say that it does not have to work.


Top of pagePrevious messageNext messageBottom of pageLink to this message  By Ron on Friday, April 16, 2004 - 07:25 am:

Leon is correct, what is is doing is adjusting the linear velocity to match in the two systems. The separation equations are written in terms of linear velocity, not in terms of flow. With the same flow in a GC-FID system and a GC-MS system the linear velocity is higher in the mass spec system due the the larger pressure drop in the column going to vacuum. I have matched retention times in mass spec and FID systems by setting linear velocity instead of flow and it works quite nicely.


Top of pagePrevious messageNext messageBottom of pageLink to this message  By St MOREAU on Tuesday, July 27, 2004 - 02:14 am:

Ron is right. On a GC2010 FID SHIMADZU and
a QP2010 MS SHIMAZDU, you will get exactly
the same RT since they are working in an
innovative way to keep the linear velocity
constant. So, try to calculte your pressure
program to keep the average linear velocity
cosntant in both cases.


Top of pagePrevious messageNext messageBottom of pageLink to this message  By Leon on Thursday, August 5, 2004 - 11:53 am:

Dear St Moreau,
I am embarrassed to confess that I don't known what does it mean to "keep the linear velocity constant" in a temperature-programmed analysis. What are the benefits of this feature? How much this mode of operation is better than, say, constant pressure mode or constant flow mode? Could you please be specific? Is it, at least in one way, potentially 1% better, 10% better, 100% better, more? Are there disadvantages?
Thank you


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