By Anonymous on Thursday, July 18, 2002 - 02:13 pm:
Hi all,
I am getting saw baseline on my LC/MS (m/z 100-500) run with blank. My mobile phase is MeCN and water with no buffer. My PDA does not have saw baseline (200 - 500nm). The MS peaks in the baseline are mainly m/z 185.4, and 229.5. I do not know what are these peaks come from. Does any one know what are these peaks? I am using a C18 column.
Thank you in advance.
By dan on Thursday, July 18, 2002 - 06:13 pm:
Is the saw at the same frequency as the pump heads? If the answer is yes a good idea would be to isolate the noise. I would start by removing the column and replacing it with a union. If that does not do the trick I would passivate the system including the autosampler. This is described in a recent message topic. Only a mass spec jock would care what 185.4 and 229.5 are from. The more important thing is to make it go away.
By Anonymous on Thursday, July 18, 2002 - 08:28 pm:
Actinium -> Rhenium or polyethylene oxide. Goes away when you fix the system.
By Anonymous on Friday, July 19, 2002 - 06:45 am:
Thanks for helping. I took the columns off and did a blank injection. The noise did not go away. How do you passivate the system? I guess I have never done that before.
By Anonymous on Friday, July 19, 2002 - 05:23 pm:
I would not go pumping nitric acid through the system to passivate it until I knew it was due to a dirty system. What happens if you stop the flow or incease the flow the the frequency of the saw also change. What happens if you pump plain water?
By dan on Friday, July 19, 2002 - 06:14 pm:
I don't disagree with Anon 523. However passivation is routine instrument mantainence. In my experience 90% of all issues are due to dirty instruments. If the frequency is independent of flow rate then the issue may be electronic. Tell us the results of the flow rate study and we will respond. However the following link to Waters web site will give you info on the passivation technique:
http://www.waters.com/waters_website/pdfs/lah376.pdf
Also you could go to your instrument manufacturers website and search for "passivation". Good luck and let us know how it works out.
By Anonymous on Monday, July 22, 2002 - 01:57 pm:
I cleaned MS on Friday and today the baseline is much better. I actually passivated my mass spec with formic acid.
The baseline still have some saw mostly during the gredient portion of the run.
Here are the tests I did today:
1) isocredient - better than gredient
2) double the flow rate -- better but appeared to be independent of flowrate
3) tried a different solvent line - no difference
4) new bottle of MeCN - not much difference
5) changed solvents filters - not much difference
Is there anything else I can try? I am trying methanol today to see if that will improve the baseline.
By dan on Monday, July 22, 2002 - 03:20 pm:
I DON'T UNDERSTAND 2 BUT I STILL THINK THAT YOU NEED MORE CLEAN UP. IF YOU WASH UNDER YOUR ARMS AND FEET YOU SMELL BETTER BUT YOU NEED A SHOWER TO BE CLEAN. USE A FULL PROTOCOL THAT WILL REMOVE ORGANICS THEN PASSIVATE WITH 6N HNO3. YOU WILL BE AMAZED AT HOW MUCH BETTER YOU WILL BE. AGAIN LET US KNOW THE OUTCOME.
By Anonymous on Monday, July 22, 2002 - 03:53 pm:
I have no clue why passivation should do anything. What is the nature of the plastic tubing that you are using? Same questions for frits etc, anything that is prone to bleed.
By dan on Wednesday, July 24, 2002 - 06:15 pm:
Anon722
Passivation deactives the steel and provides an inactive oxide layer on all steel surfaces. Also it is typical to find occluded organic stuff in small crevices in the system. A thorough cleaning and passivation will make the system like new. Ghost peaks and noise are usually due to the elution of something. This something is often something on the system. Modern columns tend not to bleed so much anymore and frits don't have an organic layer to bleed. To the original writer how does the system look now?
By Anonymous on Thursday, July 25, 2002 - 01:10 pm:
Dan and all the anonymous,
Thanks for helping. I tried MeOH and it did not make much difference. Then today I changed water from HPLC water to in-house water. Changed back to acetonitrile. The baselines seem better after a few injections. The peak 229 and 185 were smaller compare to before.I do not know if it is because of the methanol washed off something or the water.
The background looks better now after I did all these(cleaned mass spec, changed acetonitrile,ran methanol through,changed water). I think it is acceptable now (the baseline is still jagged, but the magnitude is much smaller now)
I often do full scan and look for trace unknowns or contaminents (compare good and bad lots of chemicals). I need a good baseline in my analysis.
Thanks again.