Negative peaks

[wheeltops]

I have a problem which is new to me and I can't work it out. I'm running PFOA on a wax column in MeOH. The run is starting at 150C and the PFOA peak comes off at ~1.4 min. Detection is by MS using SIM 100 131 & 181m/z.

The problem is that there is a big dip in the baseline following the PFOA peak. Why is this? What is the cause of negative peaks with MS detection?

Cheers folks.

[Don_Hilton]

I suspect that if you run the chromatogram in full scan mode, you will discover a sizable peak present where the baseline dips.

[wheeltops]

I'm a little confused by this. If I'm running in SIM and only looking at three ions why do the other ions present in the eluent cause a drop in the baseline?

Yes, there is a big peak following the PFOA (it's Methanol) but if there is a baseline level of my chosen ions before the peak why does it not return to this level after the peak? Is the relative abundance of the SIM ions dependant on the flood of Methanol ions (even though I haven't selected them for detection). I can see that there may be a rise in the baseline due to the presence of SIM ions in solvent fragmentation but the drop doesn't make any sense to me.

[Yuva]

If there is a very big, most the electronic from the filament will be used to ionize the molecule from the compound from big peak. So less electrons will be available for 100 131 & 181m. Hence you may see a negative peak or shift in the baseline to negative side.

[wheeltops]

Ah, of course. It all seems so simple now. Thanks for explaining it to me.Now I'm curious to know if I could cancel out the baseline drop by increasing the emission current?

[AICMM]

wheeltops,

If you have enough room, try turning the filament off for the methanol peak. Otherwise, you might change to a heavier solvent like ethanol or DMSO.

Best regards.

[Don_Hilton]

If you attempt to overcome ion suppression with an increase in filament current, you will affect the baseline, but you will also change the sensitivity of the instrument -- and worse yet, increasing the ionization with a significant quantity of material in the ion source will result in increased sputtering of the filament material - and shorter filament life. (Reason to turn off the filament, as mentioned above.) (spelling corrected WITHOUT help from cat – and reposted.)

[BZK]

The negative peak, as confirmed from other people, is probabily due to an eluted compound in high concentration. The effect increase drammaticaly if you use ion trap as MSD due to autogain caracteristic.