Nitrophenyl boronic acids as derivatizing agents in chromatography
The suitability of nitrophenyl boronates as derivatives for GC and HPLC analysis has been studied.
Samples of ortho, meta and para nitrophenyl boronic acids were prepared and purified, the purity of these products was examined by GC. The derivatization performance of these acids was studied employing a number of bifunctional model compounds such as: diols, aminoalcohols, hydroxy acids etc.
The derivatives formed were studied to determine their stability, speed of formation, quantitative analysis and overall chromatographic behavior. The derivatization reactions were found to be very fast and essentially quantitative, reaching equilibrium in just a few minutes at room temperature. The boronates formed are of different stabilities depending on their structure, the single most important stability factor is the boronate ring size, this is followed by the alkyl substitution present and the position of the nitro group on the aromatic ring.
Excellent quantitative performance was obtained with flame ionization and electron capture detectors. Detection limits in the femtomole range were achieved with the EC detector. Solvent background and adsorption problems were found at the picomole level in some cases.
The applicability of nitrophenyl boronates to HPLC analysis is seriously limited by the column activity due to residual silanol groups, this tends to decompose all but the most stable boronates. However, excellent results were obtained in the case of 2,4-pentandiol boronates, reaching detection limits in the picomole level with a variable wavelength ultraviolet detector.
Nitrophenyl boronic acids can also derivatize ortho disubstituted aromatic molecules containing hydroxyl, amino and carboxylic acid groups. Molecules which contain the reactive groups on the side chain attached to an aromatic ring can present a steric obstruction when reacting with the ortho nitrophenyl boronic acid. This same problem was found in the case of 1,3-propandithiol which reacts only with the meta and para nitro phenyl boronic acids.