contamination problem in HPLC/MS system

[pyao]

Hi, we are testing levogluocosan by HPLC/MS system, however, the whole system is easily contaminated by levogluocsan and hardly to clean. Some details are bellow:

HPLC: Agilent technologies 1200 Series
- Quarternary Pump _15% LC-MS level MeOH
_85% pure water(TOC 2.9ppb)
- Isocratic Pump_75% 5mM NH4COOH solution
- column_Synergy Hydro(C18) 50mm*2.1mm, 4um, Phenomenex
MS: Finnigan LCQ Advantage MAX
-positive mode
Target m/s:180

I have tested 5ng/ml, 10ng/ml levogluocosan with this system, and now the m/s 180 is very high when I inject only water and MeOH. As a result, there is no significant peak when I test 5ng/ml levogluocsan again, since the background is too high.

Do you have some advices to solve this contaminated problem?
Thank you!

[zokitano]

In which solvent are your samples/standards dissolved?
Did you get this high background only with standard solutions, samples or both?

[pyao]

Thank you for your replay.
I dissolved my samples in pure water.
The background is high in both sitiation, testing sample and only solvent.

[zokitano]

Ok, let's make some things clear here (since rereading of your original post I cannot clearly understand):
Do you have high background due to the presence of other ions different from m/z 180 or you have only high residual mass peak at m/z 180 (due to probably some memory effect), for both samples and standards?

This is very important to know in order to suggest some possible solution for your problem.

Regards

[pyao]

I think it is only high at m/z 180, for solvent. Memory effect?

[zokitano]

Levoglucosan has MW of 162.14. So do you monitor the protonated molecule [M+H]+, which should be at m/z 163 (unit mass resolution)?
What actualy are you monitoring at m/z 180?
You're probably using ESI, right?

edit: memory effect could be explained as presence of some ion(s) in the total ion chromatogram which remained in the system from the previous analysis/use and is not characteristic for the present analysis.

And probably the peak at m/z 180 is your analyte adduct with ammonium ion? (162 + 18 = 180)

[pyao]

Yes, its m/z is 163, but this MW is hard to detect, so we add ammonium format, then we monitor [M+NH4]+,so m/z 180 is the target.
Yes, we use ESI.

[zokitano]

Try to clean the ion source and then observe if you'll get the same memory effect. Also monitor the spray from the capillary if it is regular and well defined.

Try to change some of the ion source settings (gas heaters, desolvation temperature, curtain plate gas - I am not familiar with your system) in order to get complete desolvation of the spray and prevent fast contamination.

Good luck!

[pyao]

Ok, I will try.
Thank you!

[lmh]

are you sure it's the right thing, and not some other mass of 180? One reason I ask is that 5ng/mL is really quite dilute for the instrument you are using. I doubt you are injecting more than 1pmole, and I find it hard to believe that residual contamination from such a small injection could be a problem.