INFLUENCE OF PH ON THE INTENSITY AND STABILITY OF THE FLUORESCENCE OFP-AMINOBENZOIC ACID IN AQUEOUS-SOLUTIONS

It is known that the fluorescence of p-aminobenzoic acid (PABA) is pH- dependent. However, recent observations showed that the fluorescence o f PABA dissolved in water and in buffered solutions at physiological p H (7.4) decreases with time. A study was therefore performed to invest igate these changes and find solutions for the problem. The first part of this study was to investigate the interdependence of the fluoresce nce intensity and the pH of a PABA solution in various acid-base mixtu res. It turned out that this dependence is sigmoid, i.e. the fluoresce nce intensity is very low below pH 4, and it maximizes at pH 6 and bey ond. In the second part, the fluorescence intensity was studied as a f unction of time. PABA was dissolved at the concentrations of 10, 50, 1 00 ng/ml and 1 mu g/ml in various standard solutions: 0.1 N HCl, phosp hate buffered saline (PBS) (pH 7.4), TRIS buffer (pH 7.4), berate buff er (pH 9.0) and 1 N NH4OH. PABA did not show any fluorescence at all i n 0.1 N HCl, while showing stable fluorescence up to 7 days in berate buffer (pH 9.0) and in 1 N NH4OH. In water, PBS and TRIS buffer, the f luorescence of PABA decreased with time, with a higher decay rate in m ore diluted solutions. After 4 days, the fluorescence intensity of 10 and 50 ng/ml PABA in these solutions was practically zero. When concen trated NaOH was added to those solutions at time zero, the fluorescenc e intensity could be maintained. Apparently, at any pH below 9.0, a tr ansformation of the fluorescent form of PABA to the unionized form tak es place. Hence, pH should be taken into careful consideration when an alysing PABA in aqueous solutions using fluorescence spectrometry.