Combination of headspace single-drop microextraction with smartphone-based on-drop colorimetric detection for determination of ammonia and total nitrogen in environmental and food samples

[Display omitted] •A simple and green methodology was proposed for ammonia and total nitrogen assay.•HS-SDME was coupled with smartphone-based on-drop colorimetric detection.•On-drop sensing provided the extraction and in-situ detection of ammonia in one step.•Micro Kjeldahl digestion was used to convert total nitrogen of samples to ammonium.•Analysis of diverse real samples showed the good practicability of the method. In the present study, a simple, environmentally friendly and inexpensive analytical method was proposed based on the combination of headspace single-drop microextraction with smartphone-based on-drop colorimetric detection for determination of ammonia (free ammonia and ammonium ion) in environmental and food samples. This method was also coupled with micro Kjeldahl digestion for determining total nitrogen content (or protein) in foods. Ammonia was extracted into an aqueous microdrop containing o-phthalaldehyde (OPA)-Na2SO3 reagent, and simultaneously converted to a colored derivative (1-sulfonatoisoindole). The colored microdrop was directly imaged by smartphone camera as a detector, and the captured image was processed with a color recognition app for ammonia quantification. The influence of experimental variables on the sensitivity of this method was studied. The linear range of the method was achieved from 0.25 to 10 mg L−1 for ammonium with the detection limit of 0.07 mg L−1. The intraday and interday relative standard deviations were between 2.5 and 5.5 % at the concentration levels of 0.5 and 5 mg L−1. This method was satisfactorily applied to determining ammonia and total nitrogen in environmental and food samples with ammonia and total nitrogen recoveries in the range of 90–109 %. In comparison with the Berthelot and classical Kjeldahl methods, there was no considerable difference in accuracy between these methods and the present method.