poor repeatability on LC-MS Sciex 3200 QTRAP

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

7 posts Page 1 of 1
Hello,
I am writing to you for advice, because I have a problem with LC-MS/MS.

My equipment:
HPLC - Agilent 1200 series, column Agilent Zorbax XDB C18 150x4.6 mm, flow 0.5 mL/min, injection volume is 10 uL,
eluent A - water, eluent B - methanol (both with 5mM ammonium acetate).
Mass spectrometer - AB Sciex 3200 QTRAP with ESI source.

I am analyzing pesticide residues and for some time I have very poor repeatability.
I inject 6 times pesticide mix standard in pure solvent (concentration 100 ng/ml) from the same vial.
For some analytes the RSD% is 1-5% but for others RSD > 20%.
The ion source, orifice plate are cleaned.

Some time ago an ion source temperature error message appeared. The message said that the set temperature of 600’C is not reached. The service technician from the spectrometer stated that the heaters in the ion source are in poor condition and would be replaced (upgrade to the new type od heaters) Unfortunately after upgrade, sometimes an ion source temperature error message still appears. I lowered the temperature of the source to 500'C and the error no longer appears but still repeatability of pesticide standards are very poor.
Is it possible that the ion source works in an unstable way and hence the lack of repeatability of peak area fields? Even though at 500'C the message about ion source temperature error does not appear anymore?
It is necessary to optimize again all of the MRM pairs, because we upgrade the heaters?
Or it is necessary to calibrate mass spectrometer by PPGs?
Maybe ion source works unstable, because still is something wrong with temperature?

Best regards
Marcin from Poland
Hello,
Maybe some od you have any suggestions about my problem with LC-MS?

Best regards
Marcin
Did any of you even have problems with repeatability on LC-MS? Is this to be a problem with the ion source?
Marcin,

What is the nature of your pesticide? Some pesticides will bind to silanol groups on the surface of the vial thus reducing concentration over time. You can add a drop of strong acid to prevent this or use plastic vials (if they don't cause problems)
Vlad Orlovsky
HELIX Chromatography
My opinions might be bias, but I have about 1000 examples to support them. Check our website for new science and applications
www.helixchrom.com
Vlad Orlovsky wrote:
Marcin,

What is the nature of your pesticide? Some pesticides will bind to silanol groups on the surface of the vial thus reducing concentration over time. You can add a drop of strong acid to prevent this or use plastic vials (if they don't cause problems)


Hello Vlad,
we have a whole package of different pesticides and so far there was no problem. Now after using 1% acetic acid in acetonitrile as a solvent, the problem still exists.
If it used to work, and now it doesn't, and genuinely the only thing that's changed is the reduction in temperature from 600 to 500, it might be that ionisation efficiency is reduced because things aren't drying adequately. If you're running a gradient method, the efficiency of drying will vary with the solvent mix, so some pesticides may be better than others. Ionisation efficiency is difficult to predict in LC-MS anyway, and very dependent on the properties of the target analyte.
It might be worth trying a run at reduced flow rate, or using your existing flow-rate but splitting the flow before the MS so that less actually goes into the MS. This will reduce the drying burden on the ion source so the drying efficiency will be improved once again. (splitting shouldn't greatly reduce sensitivity; electrospray approximately measures concentration, not amount).
With the Sciex source, the temperature is normally increased when there is more water in the mobile phase, or high boiling solvent so that it will evaporate fast enough. I can normally achieve good results with 500c with 100% water with lower flow rates.

With the change in temperature you may want to do the optimization again with a flow that is the same as your analytical run using the three way connector that should come with the instrument, so you can infuse with syringe into a full flow from the LC pump and adjust the gas flows and probe position to match the new temperature.

The other problem with the pesticides is their solubility in water, some are not very soluble and will give unstable results while the very soluble pesticide will work better.

If you have to keep the temperature at or below 500c you may also want to use a smaller internal diameter column for the low flow rate, since 0.5ml/min is at or below optimum flow in your 4.6mm ID column. Have you tried using a 3.0mm ID column and a flow rate of about 0.3 ml/min? This might give better evaporation of the mobile phase than the 0.5ml/min flow rate at 500C. I have moved mostly to 2.1mm ID columns for LCMSMS work so I can use lower flows for better response and sharper peaks.
The past is there to guide us into the future, not to dwell in.
7 posts Page 1 of 1

Who is online

In total there is 1 user online :: 0 registered, 0 hidden and 1 guest (based on users active over the past 5 minutes)
Most users ever online was 1117 on Mon Jan 31, 2022 2:50 pm

Users browsing this forum: No registered users and 1 guest

Latest Blog Posts from Separation Science

Separation Science offers free learning from the experts covering methods, applications, webinars, eSeminars, videos, tutorials for users of liquid chromatography, gas chromatography, mass spectrometry, sample preparation and related analytical techniques.

Subscribe to our eNewsletter with daily, weekly or monthly updates: Food & Beverage, Environmental, (Bio)Pharmaceutical, Bioclinical, Liquid Chromatography, Gas Chromatography and Mass Spectrometry.

Liquid Chromatography

Gas Chromatography

Mass Spectrometry