Assessing an imaging ammonium sensor using time correlated pixel-by-pixel calibration

In this study, a ratiometric imaging ammonium sensor (optode) and a novel calibration technique for imaging optodes were developed, tested and thoroughly evaluated. The overall sensing scheme is based on ammonium coextraction together with a solvent sensitive dye in a hydrogel–ether emulsion. Quantification of ammonium during 10 days of experiments was made through an image ratio (excitation:emission/excitation:emission; 572 nm:592 nm/520 nm:572 nm ) coupled to the calibration technique i.e. time correlated pixel-by-pixel calibration (TCPC). Sensor performance was validated according to International Union of Pure and Applied Chemistry (IUPAC) recommendations for within-laboratory requirements of reproducibility, and compared with the more frequently used approach of image pixel calibration. Using the TCPC protocol; a limit of detection (LOD) of 1.2 ± 0.2 × 10 −6 M was calculated. The preliminary estimate of precision at 200 × 10 −6 M, using relative standard deviation (R.S.D.) as a measure, was 5.2% over the 10-day period. The within-batch repeatability was 2.3% (R.S.D.) at 200 × 10 −6 M. A linear fit of the data compared the drift over time for the various calibration techniques. Without time correlation, the ratiometric pixel-by-pixel calibrated signal drifted significantly over time. The drift was more or less completely removed using the TCPC protocol. The image resolution, experimentally determined at the distance applied (3 × 10 −1 m) by the 10–90% edge-response approach, was 210 × 10 −6 m. Imaging concentrations and diffusive transport of ammonium in soil during dissolution of a fertilizer stick was given as an example of a versatile sensor application.