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By Anonymous on Tuesday, June 22, 2004 - 11:57 am:
Does anybody know why ion trap is not good for quantitation?
Thanks so much!
jin
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By Anonymous on Tuesday, June 22, 2004 - 01:21 pm:
What do you want to quantify ?
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By MG on Wednesday, June 23, 2004 - 07:36 am:
You can use a trap for quantitation, but in my experience the linearity won't be as good, and you might have to use a quadratic fit.
As to why, I think it has to do with space-charging effects in the trap. The automatic gain control (goes by different names due to trademark issues) ought to compensate, but for some reason it doesn't do a perfect job. Hopefully someone else can give a better explanation.
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By HW Mueller on Wednesday, June 23, 2004 - 07:46 am:
How about the isotope dilution technique, would that compensate any fluctuations?
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By Anonymous on Wednesday, June 23, 2004 - 11:33 am:
The trap can be used to acheive good quantitation results, but it cannot acheive the limits of detection/quantitation or as good %CVs (i.e., precision values) as a triple quadrupole MS operating in selective reaction monitoring (SRM) mode. The primary reason is duty cycle.
Simply put, duty cycle is the amount of time the MS measures the ions being generated from the ion source. On a triple quad in SRM mode, the duty cycle is nearly 100%. However, on the ion trap, you must store, fragment, then eject the ions from the trap, which lowers the duty cycle. For the ion trap, this equates to a lesser number of scans across a chromatographic peak, which leads to higher %CVs. The reduced sensitivity of the ion trap in MS/MS mode versus the triple quad in SRM mode is also attributed to the lower duty cycle of the ion trap.
Concerning the linearity of the ion trap, it is generally as good as the triple quad (because of the automated gain control), that is until you reach the Limit of Quantitation on the ion trap.
Kevin
Does anybody know why ion trap is not good for quantitation?
Thanks so much!
jin
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By Anonymous on Tuesday, June 22, 2004 - 01:21 pm:
What do you want to quantify ?
-------------------------------------------------------------------------------------------------------
By MG on Wednesday, June 23, 2004 - 07:36 am:
You can use a trap for quantitation, but in my experience the linearity won't be as good, and you might have to use a quadratic fit.
As to why, I think it has to do with space-charging effects in the trap. The automatic gain control (goes by different names due to trademark issues) ought to compensate, but for some reason it doesn't do a perfect job. Hopefully someone else can give a better explanation.
-------------------------------------------------------------------------------------------------------
By HW Mueller on Wednesday, June 23, 2004 - 07:46 am:
How about the isotope dilution technique, would that compensate any fluctuations?
-------------------------------------------------------------------------------------------------------
By Anonymous on Wednesday, June 23, 2004 - 11:33 am:
The trap can be used to acheive good quantitation results, but it cannot acheive the limits of detection/quantitation or as good %CVs (i.e., precision values) as a triple quadrupole MS operating in selective reaction monitoring (SRM) mode. The primary reason is duty cycle.
Simply put, duty cycle is the amount of time the MS measures the ions being generated from the ion source. On a triple quad in SRM mode, the duty cycle is nearly 100%. However, on the ion trap, you must store, fragment, then eject the ions from the trap, which lowers the duty cycle. For the ion trap, this equates to a lesser number of scans across a chromatographic peak, which leads to higher %CVs. The reduced sensitivity of the ion trap in MS/MS mode versus the triple quad in SRM mode is also attributed to the lower duty cycle of the ion trap.
Concerning the linearity of the ion trap, it is generally as good as the triple quad (because of the automated gain control), that is until you reach the Limit of Quantitation on the ion trap.
Kevin
