I am doing amino acids analysis using GC-MS. The MS that I was using before was a quadrupole and things always OK. My new GC-MS is a ion trap, I am using the same derivatization method and the same GC conditions but I cannot detect any peaks on my standard samples. Does any one have any suggestion why amino acids detection is not working with the ion trap detector?
I cannot believe that an ion trap is generally unable to do amino acid detection, but there are some differences between ion trap und quadrupole MS.
One possible explanation is that ion traps are sensitive to coeluting peaks. Most traps use AGC (automatic gain control), which means the ion storage times and the resulting LOD are influenced by coeluting substances.
Do You use an ion trap with internal or external ionization?
By the way, a very good general introduction to ion traps is
http://www.chem.uky.edu/research/lynn/pdfs/traps.pdf
Do you see any peaks at all or are just the peaks of interest missing?
With your previous gc/ms where you working at detection limits or did you have a big signal?
A trap has a more complex ion trajectory and works in an accumulate/readout sequence so you'll have a lower efficiency.
Do you use a linear or orbital ion-trap instrument for the analysis. I never had a problem with sensitivity using a linear ion trap, but have encountered some problems using orbital traps. You should be able to see some peaks though. The fact that you don't see any peaks is strange and I am not sure whether this is a straight up sensitivity problem or whether some of your parameters are off set.
Hello Zelia,
about sensitivity quadrupole versus ion trap, is very similar, but linearity is the better in quadrupole than ion trap. But normally from ion trap u receive highest response for heavy ion (molecular ion for example) than quadrupole ms. But in your case is most problem in derivative of AA is artefact's. If u use some procedure for purification/concentration of sample AA there is no problem with sensitivity on both MS.
Usually boiling point of derivative AA is from 160 - is depend witch derivative you prepare - mtbsfa or bstfa - this is very imoprtant) Check the witch temperature have your MS - source and transfer line (quadrupole can be set from 230 C even higer), but ion trap have lower temp's. This Can be insufficient. (i observe this in derivative of some monoterpenes glicosides). Check scan properties (33-500amu) etc.
Please, let Us know how it goes....
Hi,
You have got very good answers from different researchers and I agree with some observations. In my experience, ion traps are less sensitive than modern quadrupoles, particularly in comparison to some of the newest ones working in SIM mode. I think that for some compounds I have seen nearly a factor 20 of difference in favor of a modern quadrupole. The figures become particularly bad in the analysis of very polar and adsorptive compounds, most surely due to the limitations of the temperature of the transfer line, as was mentioned by Mariusz. Check this aspect and if such is the problem do not connect directly the column into the trap, but through a uncoated well deactivated tube (be careful with the connector).
As noted above sensitivity is comparable in trap vs qusdruploe detectors. I assume you are using a single quad detector. In this case there should be no real difference in detecing the generated ions. A problem may arise if you are overloading the trap with co-eluting impurities, but if you have optimised the trap conditions by infusing a single amino acid derivative, there should not be a problem in defining a small enough m/z range for your acids to mimic SIM detection in quadrupole analysis.
You may also make use of the MSn ability of the ion trap to try and exclude any interfering co-elutents. This may increase the S/N ratio provided the co-elutents do not supress the ionidsation of your acids completely. Trials with single amino acid derivatives should allow you to optimise experimental parameters to produce the highest ion current, then include varying amounts of possible impurities (from the synthesis?) to see how strongly they supress the signal.
If supression is severe then you must interpose either on-line chromastography or prior sample preparation SPE techniques to elute the products separately from impurities. Possible techniques can be sourced from ion Chromatography texts in terms of stationary and mobile phases - these can RP or HILIC stationary phases and buffered aqueous/ACN mobile phases.
best of luck
Hello Tanks for your help. I am able to detect my amino acids however the sensitivity is much lower than using the quadrupole. I believe that the temperature of the transfer line may be the main problem. Up until now I have only injected mixtures of two standards amino acids, the internal standard (norvaline) and another one, so I do not think that co-elution may be a problem now. However I may have that problem with my samples. I know the mass spectrum of all the amino acids in which I am interested, the dominant mass is always between 100 and 250. Leo according to your suggestion I could decrease the m/z range, do you think that there is no problem in define it from 100 to 250. Thanks for helping me
100 to 250 should be ok. I think you have problem With The optimalization of The ionisation conditions
I agree to Dag Ekeberg. You should have a look on the ionisation conditions. And it is important which type of instrument you use. A GC/ion trap with internal or external ionization are completely different.
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