Thanks Michael, I appreciate your thoughts.  It's not plastic....I sampled directly out of the brand new 4L Optima bottle. Previous run contamination is not likely either as none of my samples have large amounts of these peaks.

Hi Steven, they are probably alkybenzensulphonates, common detergent components.

255 and 283 are palmitic and stearic acids. 311, 325 and 339 are C11, C12 and C13 alkylbenzene sulfonates.

I'm having a problem with these same signals, did you identify what causes and or how to remove this contaminating signal from the analysis?

Hi Larissa, the answers regarding linear alkylbenzene sulfonates (LAS) are correct. I should have mentioned 297 in my original post....it is always present whenever 311, 325, 339 are observed. I did daughter scans of 297, 311, 325, and 339 and they all fragment to 183. I am now convinced this sequence of masses are in fact LAS compounds. Regarding 255 and 283, although these masses do happen to match palmitic and stearic, I am less certain that that is what they are.

Thanks for the feedback. I am suspecting detergent in the solvent bottles. I did a big wash, but the contaminating signal remains in the ICT. I am doing a sequence of washes of the column in order to visualize the reduction in the intensity of these signals. What strategy are you using in your laboratory?

If you have a question about MassLink, is it possible to change the way of presenting data on the vertical axis? Or is only in %?

How did you wash? Exactly what solvents did you use? What brand solvents do you use?

Also, I don't know of a way to change vertical axis. But, since you see the TIC (which is in "counts") in the upper right corner of your chromatogram, you can always know the absolute intensity of any peak. Alternatively, you can go to "Edit"..."Copy Chromatogram List". This copies the list to the "clipboard" as an x,y pair where y is raw intensity of each timepoint in the chromatogram. You can then paste that list into Excel and plot your own chromatogram.

Hello, sorry my delay.

I have been washing with water 01% formic acid and acetonitrile, using cleaning methods from 10 until 100% acetonitrile.

Thank you for the MassLink explication.

About the 297, 311, 325, 339, the alkyl benzene sulfonates (LAS), we see them in very high concentration after each maintenance of our instrument, so i assume they must be related to cleaning products or parts that Waters is using for their maintenances, but does anyone know how to get rid of them :)?

I am in trouble with same problem. After maintenance and changing some parts, I see big LAS peaks. Did you find the solution?

Steven J Symes Jongmin Ahn I have the same problem in a palmitic acid method in which a peak appears at the same retention time with a high area. so if anyone has found a solution please tell m, thank you in advance.