lc ms contamination 353.12 and 705.23

[rachelg]

I am trying to identify secreted proteins. I need to eliminate and identify a contaminant from the culture media that has a major peak at 353.12 and a smaller one at 705.23. It elutes like peptides from a c18 column and does not bind to scx stage tips. I thought it was triton n-101 as depending on the number of repeat units this surfactant has peaks at 353.2686 and 705.4784, but due to the sensitivity of the esi mass spec a difference of 0.1 means that they are in fact different. Back to the drawing board...

i have bought all new culture media stocks and tested them to no avail, still contaminated. I have also ruled out the culture process, eppendorf tubes, pipette tips, culture flasks, mineral oil, milliQ water, syringe filters. The contamination peaks are larger when BSA is present in the media than when it is absent and there is no contamination when just water is tested.

Do surfactants always exhibit the 44 m/z peaks?

Thanks

[Alp]

Are you using negative or positive ion mode?

By ESI you mean electrospray ionization?

I find it interesting... I supposed you were doing positive ion mode electro spray. I gave 705.23 the title "dimer +H" and I subtracted 1.0078 (H).
Then divided by 2.
Then added 1.0078 (H) to give monomer+H and got a mass of 353.1189, which is pretty close to the 353.12 you report.

If you modify the whatever lens voltages that are used for declustering water molecule adducts on your system, does the 705.23 ion decrease in signal with an increase in the signal at 353?

Alp

[rachelg]

positive ion mode and yes it is electrospray, sorry for the shorthand i have been writing it so much . I saw the relationship between the peaks too, but am not sure where to go with it apart from searching databases of contaminants. The 353.12 peak is the bigger peak (abundance 100%) compared to the 705.23 peak (~30%), there are also some much smaller ones that correlate to polysiloxanes and are not large enough to be interfering. I do not know about the lens voltage as the mass spec facility people set up the program for me to use. I have been told that part of this problem is that i am looking for very low abundance proteins compared to the contamination. If my proteins were in higher abundance this would not be such an issue. I will ask though about the lens voltage, Thanks

I am going to test the bsa in milliq vs milliq to see if we get the same contamination peaks as previously the media with bsa exhibited more contamination than the media without bsa (although it was still present).

rachel

[Kostas Petritis]

Is the 353 a single or double charged ion?

[RochesterProteomics]

Are you using any Triton X-100? This detergent expresses at both 353.27 and 705.47. I know that these don't match up identically with your masses, but based on your instrument, it is possible that you have a mass error of .2 daltons.