Detection of weak acid anions.

[Jam]

Dear all,

I've been looking for an information concerning the detection of weak acid anions, especially borate. I know that there is a column f. ex. manufactured by Hamilton, that can easily separate fluoride, borate, etc. The detection mode is conductivity, but I'm more interested using indirect-UV. Regarding the information from Hamilton, fluoride can be detected by indirect-UV (like nitrite, phosphate, chlorate, and bromide among others), but I haven't found anything to Borate by indirect-UV. Why? What am I missing here? Is there a reason for not detecting Borate by indirect-UV?

Best regards,
JAM

[Chris Pohl]

Jam,

The problem with detecting borate in indirect UV is rather simple: no suitable reagent is in use for indirect UV with a suitable basicity to maintain ionization of borate. On paper, NaOH might work for this since hydoxide absorbs in the low UV but I suspect the extinction coefficient may not be sufficient for reasonable detection limits. On the other hand, it’s possible that some more weakly basic aromatic compounds might work such as phenols or aromatic sulfonamides, since these compounds are only ionized at higher pH where borate is also ionized (borate pKa is 9.2). I’m not sure why these haven’t been reported in the literature. One possible option is to increase the acidity of borate through the formation of a polyol adduct. For example, addition of mannitol to the eluent will significantly increase the pKa to around 6 which may be enough to achieve detection with standard indirect UV eluent systems. Of course, as you undoubtedly know if you've visited the Hamilton web site, borate can certainly be detected using indirect conductivity detection but detection sensitivity for the other ions you mentioned will be compromised under such operating conditions (not to mention the fact that hydroxide isn't a very good eluent for use with the PRP-X100 and can only be used for weakly retained anionic species unless one adds a "scavenger ion" such as thiocyanate which further compromises detection sensitivity). Using direct conductivity detection with a suppressor device, one can readily detect borate even though it is weakly ionized by using the trick I mentioned above (i.e. adding mannitol to the mobile phase).

[Bill Tindall]

borate is a good high pH buffer because it is UV transparent. It makes a UV absorbing complex with some sugars. Maybe you could post column add some sugar and get detection.