Repeatability problems

[Virolarufula]

Hi,

I'm devolping a method to detect simultaneous six pesticides (metaldehyde, metazachlor, propyzamide, pendimethalin, carbetamide and quinmerac) by LC-ESI-MS, using MRM (2 transitions).
Equipment:
Alliance 2695 (LC)
Quattro premier XE (MS)
Solvents:
UpH2O + 0.1% AcOH
Methanol + 0.1% AcOH

The elution seems pretty ok!

I'm injecting x10 from the same vial 50uL of 10ug/L pesticides mix and the precision is about 70-75%, for all pesticides.
Example:
Injection 1 - Metaldehyde area = 68766.26
Injection 2 - Metaldehyde area = 31420.48
Injection 3 - Metaldehyde area = 63153.2
Injection 4 - Metaldehyde area = 43335.82

"Narrow the problem"
LC
I already change the syringe for a new one, performed the adjusting seal, needle wash, primed the solvents lines, changes the solvents......but the repeatability "saga" continues....

MS
Clean cone, check the baseline noise (Ok, done that with Waters support!), but the repeatability "saga" continues....


Any ideas?

[tom jupille]

Since you have more than one peak, pretend that one of them is an internal standard. Calculate the ratio of the areas for two of the peaks.
If the reproducibility of the ratio is much better than the reproducibility of the area, then the chances are good that you are looking at a problem that affects both peaks in the same manner (sample prep, injection volume, possibly MS response).
On the other hand if the area is more reproducible than the ratio, that suggests that the problem affected different peaks in different ways (so possibly things like baseline noise, integrator settings, peak shape issues, ionization efficiency).

[Virolarufula]

Hi Tom,

Thanks a lot! The reproducibility of the ratios is much better.
Great help!

Kind regards,
Andre