Photodecomposition of o-phthaldialdehyde-derivatized amino acids by the photodiode array detector during their high-performance liquid chromatographic analysis

During the high-performance liquid chromatographic (HPLC) analysis of o-phthaldialdehyde (OPA)-derivatized amino acids such as arginine and homoarginine, we observed that the response of the fluorescence detection (FLD) system is decreased when the photodiode array detection (DAD) system (located before the FLD system) was simultaneously switched on. The decrease in the FLD response, i.e., the difference in the FLD peak area (ΔA) obtained with DAD modes off (Aoff) and on (Aon) was dependent upon the flow rate, but the relative FLD response decrease (ΔA/Aoff) was practically independent of the amount of analyte injected. For example, decreasing the flow rate from 1 to 0.5 mL/min resulted in the relative decrease of FLD response from ∼5% to 11%. It was concluded that DAD caused a photoinduced partial decomposition of the OPA-derivatized amino acids flowing through the cell. The photoinduced dissociation of OPA derivatives was independently supported by using off-line photodiode array spectrometric measurements with long and short irradiation pulses. Based on the experimental results, for description of the variation of FLD responses with the flow rate upon the irradiation by DAD a simple mathematical model is proposed and reported.