MS/MS signal & LOQ

Discussions about GC-MS, LC-MS, LC-FTIR, and other "coupled" analytical techniques.

8 posts Page 1 of 1
Hi,
The fact is that:
…. the signal of the MS/MS detector not should be greater than 5*e7
…. For my method, I have signals > 5*e7 for the highest calibrators
…. For my method, there is some carryover and also a lack of sensitivity for the LOQ I would like to use (but the precision of this LOQ is good, <20%)
So, if I’m not wrong, I think that I should reduce the signal <5*e7, maybe diluting my sample.
But then, my LOQ will increase, too!!!
How to manage, to have a lower signal improving the LOQ at the same time?

Thank you very much!!!
Are you trying to reduce the effect of the carryover by reducing the response?

With the detail given, It sounds like you need to get rid of the carryover to provide reliability at the low end of the range. Or is this a problem folowing a sample at the high end of the range? And, you need to have a way to handle samples that give response at the high end of the range, perhaps by diluting and reinjecting them.

Do you know where the carryover is occuring so that you can get rid of it?
I would say make a different calibration curve starting at the current LOQ with same sensitivity and make smaller steps in the calibration concentrations with the highest calibrator being a lower concentration that falls within acceptable signal limits.

This will give a smaller dynamic calibration range and possibly eliminate carryover from the high calibrator. If you happen to have a sample with a result above the range of your calibration, then dilute that sample to bring it within range, you do not need to worry about a higher LOQ for that sample since you already know the result will be well above the LOQ even at a dilution.
The past is there to guide us into the future, not to dwell in.
Thqnks for your comments,

Now, I think that is clear that I have to change the calibration range I built up.
But not decreasing the highest point, so that increasing the lowest point. The reason is that I need the highest one for my plasma samples and in fact, the lowest one I'll need for the analysis of the drug in other matrix, and in that case I think that I should remake the method by using another calibration range (lower highest calibration point).

So.... I think that I can't have one method for the two analysis!!!

BTW, what could you say me about the "saturation of the MS/MS detector"? What does it mean exactly? Which are the consequences?

Thank you very much!!!
Saturation of a detector means that ions are arriving at the detector too fast for the detector to count them all. This results in non-linear response. You don't really want to be working in this region. (Depending, you can do good work in the low end of the range where the detector is saturated - but you are running on the edge.) Dilute your samples or run with a split injection rather than operate at this range.
There is very good advice in the responses here.

Sometimes you need 2 methods for 2 different concentration ranges.

If your detector is close to saturating you are not helping yourself. I have been told (is it true?) that constantly saturating the detector can actually shorten it's life.

As previously stated...
If you want your LOQ then lowering the concentration range of your curve makes good sense. If a sample is too concentrated to fit the new range you will have to do dilutions.

If you want to keep your high end without saturating the detector, you will have to dilute your standards and raise your LOQ.
Thanks a lot for all your adices !!

It seems I don’t have choice; definetily I should have two different methods for two different objectives ;-( as I need the upper calibration point, but if I dilute my samples I will increase the LOQ required for the second one.

Thank you very much for all your replies!!!
But sometimes it is really hard to dilute more the sample….
If I doing a precipitation method, then the most dilution factor will be by applying it at the final dilution with the mobile phase, isn’t it?
So, if now I’m mixing 100µl sample + 200µl of mobile phase, and I obtain a signal around 6*e8 for the highest calibration…
If I mix 50µl sample+200µl of mobile phase, do you think that the signal will decrease enough?
Otherwise, I should dilute more, and then I’m not sure about the shape of the peak obtained will be really good….
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