Determination of dissolved zinc in seawater using micro-Sequential Injection lab-on-valve with fluorescence detection

This paper introduces the preliminary design and optimization of a micro-Sequential Injection lab-on-valve system (μSI-LOV) with fluorescence detection for the direct determination of trace Zn(2+) in an unacidified seawater matrix. The method capitalizes on the sensitivity and selectivity of FluoZin-3, which was originally designed to measure zinc in living cells. The optimum reaction conditions, sources of blank signal and physical parameters of the μSIA-LOV are evaluated with the requirements of trace metal analysis in mind, namely high sensitivity and low background signals. A detailed investigation of the effect of sample and reagent sequencing on sensitivity is presented for the first time using μSIA-LOV. We find that the order of sequencing greatly influences peak shape and analytical sensitivity with the highest and smoothest peaks obtained when a large volume of sample (75 μL) is aspirated last in the sequence prior to flow reversal and detection. The optimized reaction conditions and reagent/sample sequencing protocol yield a detection limit of 0.3 nM Zn(2+), high precision (RSD < 2.5%), a linear quantification range up to 40 nM and an analytical cycle of ～1 min per sample. This work demonstrates that μSI-LOV is capable of attaining detection limits that are close to those needed for open ocean determinations of Zn(2+) without preconcentration or separation of the analyte from the seawater matrix. The low reagent consumption (50 μL per sample), full automation and minimal maintenance requirements of μSI-LOV make it well suited for shipboard analysis and, eventually, for development to meet the pressing need for trace element measurements in unattended locations.