Isotop ratio

[agilent2]

Hi everyone

Why in isotop ratio determination (for example 15N/14N) magnetic sector mass spectrometer should be used while almost all other mass spectrometers (ion trap, qudrapole) are able to separate isotops with 1 amu difference? Could I use GC/iontrap-MS or GC/TOF-MS for isotop ratio determination of gases instead of magnetic sector mass spectrometer?

[CE Instruments]

A GC Ion trap is poor at gases and normally will not trap masses below 50 (?) A GC TOF is not cheap especially if accurate mass. Not many people use magnetic sectors for isotope work nowadays ! Are you looking at gases ? Then a quadrupole system should be fine. If you are looking at complex molecules , you need a GC combustion IRMS for best sensitivity
e.g. Delta V
Does this help ?

[agilent2]

Thanks
My main question is why TOF or quadrupole analyzers are not used commonly for isotope ratio determination when separation of 1 amu is adequate.
As you mentioned, IRMS is commonly used which equipped with magnetic sector analyzer. I can't understand the necessity of using magnetic sector.

I want to determine 15N/N14 ratio in a high grade N2 gas by GC/TOF-MS but I heard that magnetic sector (or IRMS) is needed for precise determination.

[Loekie]

Usually the differences in the ratios that need to be measured are very small. differences of less than 1% are not uncommon. The quantitative capability and stability of the signal on a tof, trap, or quad is not good enough to differentiate and quantify to these accuracies. the errors on the replicate injections, even when using the ratios, are often higher than the differences that are required to be measured.

Loekie

[agilent2]

Dear Loekie,
Thanks for your logical response.

Is using faraday cup dectore insted of elecrron multplier tube related to low ration in isotops abundance?

Thanks