Chromatography Forum

I am quantifying some anionic surfactants by LC-ESI MS, and I am getting really poor linearity for some compounds. The calibration standards were prepared as a mix (so I wasn't pipetting each one individually), and are autoinjected onto the LC as a mix. Two have good linearity (>0.99 r^2), so I don't think it is bad pipetting technique. The others are quite terrible (<0.95 r^2). Around the middle of the calibration line, it levels off then begins increasing again (almost like a cubic curve). One in particular: increases, levels off around the middle of the calibration line, increases again for two more points, then decreases for the last point! My LC method is very long (>1 hour), so perhaps my compounds are aggregating while sitting on-board? Has anybody seen anything like this?

Sorry that this response came so late but quite possibly it sounds like there is ion pairing that is occurring. Are your samples sitting in glass vials? It quite common to have Na+ and K+ leeching into solution. This results in neutral Na[Anion] and K[Anion] species which are not detected in the mass spectrometer.

If you're using plastic, vials, maybe samples are adhering to or adsorbing onto the surface? In that case you may have to reconfigure your analysis method to be shorter (unfortunately).