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- Posts: 6
- Joined: Fri Dec 22, 2017 11:31 am
I would like to ask for advise on a persistent carry over issues that I am currently experiencing.
We are working on multiresidue pesticides analysis using Querchers method on a Sciex 4500QTrap unit with Sciex Exion LC.
I have set an external and internal rinsing of the autosampler after every injection. I have tried numerous wash solvent which according to some journals and papers are effective as wash solvents for pesticide analysis.
The solvents that I have tried are: Methanol only, Methanol:Water (1:1), Acetonitrile only, Acetonitrile with 0.1% formic acid, Methanol:Acetone (80:20), the magic solution of ACN:IPA:Methanol:H20 (1:1:1:1).
Injection of blanks give high intensity of a few pesticides (especially TPP as we are using TPP as our 'internal std').
I tried to isolate the carry over/ghost peak issue and the brief is as below:
i) Only use the MS and do an infusion of all the solvents used to check if its solvent contamination issue or MS issue
ii) No high signal was observed - all suspected carry over 'peak' MRM transition was less than 5k cps intensity - so I think it's not an MS or buffer/solvent issue
iii) Next, I have purge and flush the system (both pump A and B) without column with the following solvent at high flow rate (2mL/min) for app 15min each
-first with water
-then with methanol
-next with magic solution of ACN:IPA:Methanol:H20
-then with acetonitrile
iii) After this, I tried to inject a high load (max 50uL injection) without column of the blank solvents - no ghost peaks were observed
iv) After numerous high load 50uL injection (to clean the sample loop as well) with methanol, IPA, water, acetonitrile - all with no peaks observed
At this point, I assumed that the carry over or contamination does not come from the MS or the LC.
v) I connected the column, did a flushing/conditioned on the column (basically, 30min 98% A, then 60min 98% B and 10min 95% A (my run initial gradient)
A - Water with 0.1% formic acid and 5mm amm formate
B - Methanol with 0.1% formic acid and 5mm amm formate
vi) After doing the above, I test run the blank injections and ghost/carry over peaks were observed (same RT of std run on those 'detected analytes')
vii) Suspecting it to be column issue, I changed the column to a brand new column and to my disappointment, the ghost peaks were observed (at an even higher intensity)
I have not passivate my LC yet.
Should I do that as my last resort, or is there any other advice from the kind experts here? Did I miss out any troubleshooting steps?
Hope there is someone kind to guide and give some advise.
Thanks a lot in advance !
Sincerely,
Ken