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High Noise Level in MRM Analyses
The background noise in MRM analyses can either be electronic or chemical. To
distinguish between chemical noise and electronic noise an acquisition should be
performed with and without ions being transmitted to the detector. Ions are best
prevented from reaching the detector by setting the ion energy 1 (MS1) and ion energy
2 (MS2) fully negative. If there is a significant decrease in the background noise with
the ion energies set negative then the major contribution to the overall noise is
chemical noise. Any residual noise is electronic noise.
If the dominant source of noise is chemical, a reduction in electronic noise does not
yield significant improvements in overall signal to noise ratio.
Chemical Noise
The most common source of noise is chemical noise.
•
If the auto injector, probe or connecting tubing have been exposed to a high
concentration of the compound to be analysed then this may be giving a high
background due to “carry over”. This can occur if concentrations of a few ng/µl
are used for tuning prior to attempting sub pg/µl detection levels. If the injector
is contaminated the signal level normally changes when injections of mobile
phase are made.
Repetitive injections of 10% formic acid and / or isopropanol may help reduce
the noise. If the probe or connecting tubing are contaminated then infusing 10%
formic acid and / or isopropanol with a syringe pump may help.
•
Check that the LC system is not adding contaminants into the mobile phase.
Using a syringe pump, infuse a syringe of mobile phase taken from the solvent
reservoir. Compare this with the MRM background when the LC system is
delivering the solvent.
•
Try a different MRM transition.
This may reduce the noise level if the compound(s) contributing to the chemical
noise do not yield the same set of daughter ions as the compound being
analysed.
•
Check the purity of solvents and additives.
Try a different type of solvent or the same type of solvent from a different
manufacturer. Ensure all solvents and additives are HPLC grade. Check the
cleanliness of any glassware used.
Maintenance and Fault Finding
Page 207
Quattro Ultima
User's Guide
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