Combined liquid phase microextraction and fiber-optics-based cuvetteless micro-spectrophotometry for sensitive determination of ammonia in water and food samples by the indophenol reaction.

The Berthelot reaction for ammonia is revisited with the aim of miniaturization and addressing interferences as encountered with food and water samples. Headspace single drop microextraction of ammonia in phosphoric acid served to attain selectivity in complex matrices, and liquid-liquid microextraction of red or blue indophenol species into 1-octanol-isooctane (60:40, v/v) resulted into high sensitivity. Fiber-optics-based cuvetteless micro-spectrophotometry has been used for colorimetric determination on microliter volumes of extract. The linear dynamic range, limit of detection and enrichment factor have been found to be 0.2-3 mg kg, 0.14 mg kg and 38, respectively, measuring red species for milk, cheese and beer (4.9-5.5% error; 4.8-6.3% RSD; n = 5); and 5-400 µg L, 0.4 µg L and 137, respectively, measuring blue species for water samples (3.3-5.7% error; 3.6-6.8% RSD; n = 5). A plausible reaction scheme has been proposed for nitroprusside catalysis in indophenol reaction.