Does my LC/MS ALS have problem?

[thohry]

Hi all,


We are analyzing Clenbuterol by LC/MS/MS, the peak is fine, but the reproducibility is bad (for the lowest concentration of 0.1ppb). I injected 4 times in sequence and the areas are as follow: 212, 155, 213, 300

Are there anything wrong with my analysis or that is just normal?

Thanks for any help.

[yangz00g]

What's S/N at this concentration?

[thohry]

What's S/N at this concentration?

Thanks for the reply. The S/N values are not bad, around 15 or so.

[yangz00g]

If you are using a Thermo instrument, the true S/N is way below what the software gives you. If you use others, try use 6sigma or valley to valley to determine the S/N.

I suspect this concentration is below your method's true LOQ.

[lmh]

The question is whether the autosampler is at fault. The lowest calibration point is poor, but from this I assume other points are good.

If your lowest calibration point is made by injecting a minute volume from a vial that you also use for other (higher) calibration points, then some of the variability may be that you are pushing the limits of the autosampler.

If your lowest calibration point is a sensible injection from a much diluted standard, then the autosampler cannot be at fault, since it is doing a good job with similar sized injections of higher concentrations.

It is normal for reproducibility to be low at low concentrations; that is what defines your limit of quantification. What constitutes "low" depends on your instrument and method.

In my personal view, S/N isn't always a good measure, depending on instrument. In our ion trap, for example, noise vanishes virtually to zero in SRM, which means S/N can be phenomenal, but the signal can still become so small that it is very unreliable and variable.

[thohry]

Thanks lmh for the reply. You are right when suspecting other points. In fact they are not so reproducible. I will post some results later. In constructing the calibration, if I run only one time (each point), the R2 is not good. But if I run triplicate runs and take the average instead, the R2 is very good: 0.998. That's why I suspect the ALS. The instrument is 1200/6410 and only 8 months old.

[bisnettrj2]

You may want to run the Pressure Test in the Diagnostic View to see if you have a leak in the injection valve - if you have poor reproducibility at all levels of your calibration, you might have a leak there.

Edit - you may also have a leak in the metering valve, so there could be several places that you're losing your reproducibility.

[yangz00g]

In my personal view, S/N isn't always a good measure, depending on instrument. In our ion trap, for example, noise vanishes virtually to zero in SRM, which means S/N can be phenomenal, but the signal can still become so small that it is very unreliable and variable.

With all due respect, you mean in your ion trap, as long as you use SRM, you s/n is always indefiniite?!!!!! Sorry, not in my world. IMO, s/n is very good indicator for measurment precision.

It's very clear, based on Theroy's description, autosample is not the culprit, because only the 0.1ppb point is bad. For many LC-MS/MS, 0.1ppb for Clenbutero is close to LOD.

A simple Google will tell

[bisnettrj2]

Ahem...

You are right when suspecting other points. In fact they are not so reproducible.