ESI Negative mode for acidic analytes

[kellylen1412]

Hi currently im using Shimadzu LCMS 2010A MS with ESI probe.
I need to identify phenolic compounds in my liquid products ie phenol, catecho, guiacol, o-cresol and so on. My liquid product is acidic about ph 3.8
My question is what kind of mobile phase is advisable? i was made to understand that negative mode will give me better results. i have tried using water/acetic acid 0.1 and acetonitrile 50/50 but seems not working. any suggestion?
Should the mobile phase be basic in nature or acidic in nature due to the fact my liquid product is acidic. Thank you for the comments and advice.

[Gaetan Glauser]

Hi,

I have never analysed these small phenols but I can still give you my feeling and how I would proceed:

1. Negative mode seems definitely more appropriate than positive mode.
2. An acidic mobile phase may reduce ionisation. But the pKa of phenol is about 10, you will hardly find a mobile phase that makes phenols entirely ionised. However, gas phase acid-base chemistry can be surprising. I would first have a try without any buffer, using flow injection analysis on pure standards.
3. If you have an APCI probe, it would be worth to give it a try, ionisation may be better.
4. These compounds absorb in UV. If you have a UV detector before the MS, you can verify if you get at least a signal.
5. My first impression is that these small phenols would be better detected by GC-MS after derivatization. Maybe someone can confirm or invalidate.

[orcicdejan]

I don't work with simple phenolics, but rather plant phenolics (flavonoids, phenolic acids, lignans etc). For my LC with ESI-MS, the combination of 0.05 % HCOOH and MeOH works just fine. Negative mode will give you *way* better sensitivity. Now, for the mobile phase, you have 2 opposite effects - with the acidic mobile phase, you will decrease your sensitivity (because of the protonation), but you will improve the separation (because of the same protonation). Thus, it is advisable to use not-too-acidic mobile phase.

[Gaetan Glauser]

Indeed plant phenolics such as flavonoids or quinic acids work well in negative mode using acidic additives. But very small phenolics may behave quite differently, that's what need to be assessed using standard compounds.