3
$\begingroup$

I want to perform chemical stability studies for different pH Values. To achieve pH Values from 1 to 6 I can prepare solutions with trifluoric acid and formic acid and ammonium formate and acetate. I know that the very high and low values are outside the optimal buffer range, but it still works because the sample concentration is small compared to an high excess of testing buffer. Bufferranges for volatile reagents - sepscience.com

For the range of 7 to 12 I can achieve these values by mixing ammonium bicarbonate and ammonium hydroxide in specific quantities. Still, I am wondering if there is a way to also test the high pH range 12-14 with a system that is LCMS compatible. Normally these pH ranges are accessed by using sodium hydroxide. But this is not compatible with LCMS, because hard ions are harmfull for mass detectors. I know that normally silica columns for rp18 are used in the pH 2-8 range. The solutions are therefore obviously diluted with LCMS mobile phase that contains formic acid before they are injected in the systems to achieve an acceptable pH value and sample concentration. Still, sodium formate would not be volatile. I know, that it is possible to divert the first minutes of an hplc run into the waste before they enter the mass detector. Still, if your product is being deteriorated into very polar substances (which I suspect for my case), you would not observe these species. Is there a possibility to access these pH values with a volatile reagent that is not harmfull for LCMS detectors? Alternatively I am thinking of doing NMR testing, still here you would be limited to heteroatoms to avoid water contamination in the spectra and you can not time the measurements as flexible as LCMS runs.

Improve this question
$\endgroup$
2
  • $\begingroup$ The decomposition of a compound possibly could be monitored by UV-Vis (at pH this high perhaps better to substitute glass by an other window material), or ATR-FT-IR spectroscopy (Mettler Toledo for instance commercializes this as ReactIR), too. $\endgroup$
    – Buttonwood
    Commented Nov 5, 2023 at 15:10
  • $\begingroup$ Most of my compounds are UV inactive. That complicates things a lot. Especially purification appears to be very challenging in most cases because you can not rely on UV bus detectors. A specific compound is even UV inactive, very polar and inert for staining reagents. Here your only change is doing mass coupled purification, screen every fraction or a purification system that is either equiped with an ELSD, conductivity or refractive sensor. (The latter two I don't have seen in a main stream purification setup yet). $\endgroup$
    – raptorlane
    Commented Nov 5, 2023 at 18:36

1 Answer 1

Reset to default
4
$\begingroup$

First off, as a comment implies you need the right column. Silica-based columns are not suitable for high $\pu{pH}$, even $\pu{pH }8$ can cause problems according to a now-deleted comment in the thread. AChem recommends a pure graphitic carbon alternative.

Assuming you have the right column, you may want to consider guanidine, an organic strong base made of carbon, hydrogen and nitrogen and forming a multiatomic cation upon protonation. With a $\pu{pK}_b$ of $0.4$ this will be $>90$% ionized below $\pu{pH } 12.6$, but becomes more of a true buffer above that level.

Improve this answer
$\endgroup$
9
  • 1
    $\begingroup$ @AChem does this comment belong to the answer or the question? $\endgroup$
    – Oscar Lanzi
    Commented Nov 4, 2023 at 16:50
  • $\begingroup$ The comment belonged to both the Q and A. Let me clarify. Injecting a highly basic solution will eventually create a void at inlet of the OP's HPLC column. Not a good idea to employ highly basic sample solutions or even basic mobile phase. Only Hypercarb column can handle basic pH which is made of pure graphitic carbon. $\endgroup$
    – ACR
    Commented Nov 5, 2023 at 3:24
  • 1
    $\begingroup$ @AChem I prefaced my answer with your recommendation. $\endgroup$
    – Oscar Lanzi
    Commented Nov 5, 2023 at 12:29
  • $\begingroup$ @OscarLanzi I am aware of the requirements of RP-Phase columns. I thinks I stated out, that the extreme pH - values of the testing buffer is reduced before injection. I am more interested in finding a solution for utilizing this extreme pH regime in a way that is not harmfull for the detector, even after neutralisation and will not overlap with the retention time frame of polar substances (I know, that is a lot all at once and this might not even possible). $\endgroup$
    – raptorlane
    Commented Nov 5, 2023 at 18:46
  • $\begingroup$ @OscarLanzi I was not able to find a supplier for free guanidin. Only HCl salt. Have you used this base in this fashion already? $\endgroup$
    – raptorlane
    Commented Nov 12, 2023 at 18:02

Not the answer you're looking for? Browse other questions tagged or ask your own question.