Chromatography Forum

I have run some samples for extractables from tubings using a LC/MS (QTOF).

One of the peaks was confirmed to be methyl paraben: http://en.wikipedia.org/wiki/Methylparaben. I just wonder why I have such excellent response of this molecule in negative mode? I cannot see where it is possible to pull off a proton.

Obviously it works fine, but it would be nice to know. The mobile phase was just water and acetonitrile in this case.

phenolic hydroxyls are quite acidic.

Phenol has a pKa of 9.95 (Wikipedia). It should be neutral at pH 7, but I can imagine that normal rules does not apply inside the source (300°C and thousands of volts).

Mattias, anything at a pH = 7 in H2O is neutral regarding the H2O. Do you mean it is ~ nonionized at pH = 7 in water? Now, the gas phase (in the MS source) reactions have little to do with wet chemistry.

There are many suprising ionization schemes in MS, but I actually would have expected that phenols ionize well in negative mode, especially without pH control. I also got still quite excellent ionizations for basic analytes with alkaline mobile phases in positive mode.

Thanks for your replies,

It seems rather unpredictable how well different molecules ionise at different polarities. I guess the gas phase chemistry inside the source is quite complicated.

I have another example that is puzzling me, it was posted here about a month ago. I then had good response of dibutylphthalate in positive mode. That molecule has no basic functions what-so-ever. Or did I kick off an electron from the aromatic ring, making the molecule positively charged?

I am no expert on this, but:

The way I see it, you have to consider what is going on at the end of the electrospray needle. There is a voltage creating a field where ions move. The idea of electrospray is that ions are dragged out of the tip of the needle in droplets of solvent with the ions towards the outside of the droplet and all repelling one another. Presumably the original counterions are dragged back towards the conductive needle itself. I.e. the needle is acting like an electrode in electrochemistry. As the ions are dragged away, the neutral, unionised molecules remaining will tend to ionise to restore the equilibrium.

So the relevant feature of a molecule isn't so much its actual extent of formation of ions at the pH of the solvent, but its ability to form ions. If it can, it will in electrospray, even if it normally wouldn't to any great extent.

Dibutylphthalate ionises nicely as do all the phthalate esters. I'd guess their oxygens can pick up a hydrogen by donating an electron pair, and I suspect (chemists correct me) that the charge can be delocalised over the benzene ring which would make it more stable. But this is only half the story; in electrospray, things that can exist easily on the outsides of droplets seem to do very well too, and anything with alkane chains has that hydrophobic side. Detergent-like molecules do well.

One can try to be too clever about this. My (very limited) experience is that a molecule with absolutely no polar bonds will not show up, ever, and a molecule with no way to behave acidically at all will probably not run well in negative mode, but almost everything with any polar bonds (i.e. containing oxygen or nitrogen) gives some sort of signal in positive mode if you try hard enough. But it does depend on solvent, instrument, and probably the phase of the moon.

That is interesting, coming from magnetic sector MS I am always wondering about people who are surprised that a certain molecule ionizes. With the voltages invoved on our apparati it even never occurred to us that there could be any molecule or atom that can not be ionized.

As far as basicity/acidity of molecules in the condensed (liquid) phase is concerned I recommend reading about the work of Nobel price winner George Olah.

Sorry, I just assumed electrospray, simply because it's so universal on LC-MS systems nowadays.

Things like anthracene have never given a peep of a signal for me in electrospray. I leave that sort of stuff to my GC friends who can ionise anything...

I am running electrospray...

Regarding what will ionise and not, I guess I should not think too much about it. Didn't Newton say: why think - just do the experiment.