Determination of ammonium in aqueous samples using new headspace dynamic in-syringe liquid-phase microextraction with in situ derivitazation coupled with liquid chromatography–fluorescence detection Original Research Article

A new simultaneous derivatization and extraction method for the preconcentration of ammonia using new one-step headspace dynamic in-syringe liquid-phase microextraction with in situ derivatization was developed for the trace determination of ammonium in aqueous samples by liquid chromatography with fluorescence detection (LC–FLD). The acceptor phase (as derivatization reagent) containing o-phthaldehyde and sodium sulfite was held within a syringe barrel and immersed in the headspace of sample container. The gaseous ammonia from the alkalized aqueous sample formed a stable isoindole derivative with the acceptor phase inside the syringe barrel through the reciprocated movements of plunger. After derivatization-cum-extraction, the acceptor phase was directly injected into LC–FLD for analysis. Parameters affecting the ammonia evolution and the extraction/derivatization efficiency such as sample matrix, pH, temperature, sampling time, and the composition of derivatization reagent, reaction temperature, and frequency of reciprocated plunger, were studied thoroughly. Results indicated that the maximum extraction efficiency was obtained by using 100 μL derivatization reagent in a 1-mL gastight syringe under 8 reciprocated movements of plunger per min to extract ammonia evolved from a 20 mL alkalized aqueous solution at 70 °C (preheated 4 min) with 380 rpm stirring for 8 min. The detection was linear in the concentration range of 0.625–10 μM with the correlation coefficient of 0.9967 and detection limit of 0.33 μM (5.6 ng mL−1) based on S N−1 = 3. The method was applied successfully to determine ammonium in real water samples without any prior cleanup of the samples, and has been proved to be a simple, sensitive, efficient and cost-effective procedure for trace ammonium determination in aqueous samples.