A remote in situ monitor based on continuous flow analysis for the quantitation of sub-micromolar levels of hexavalent chromium in natural waters

Abstract

We describe the design, optimization, and application of a small, lightweight, deployable monitoring instrument for accurately measuring parts-per-billion levels of hexavalent Cr in surface waters at hourly intervals. The monitor quantifies Cr(VI) using a standard molecular absorbance spectroscopic method, i.e. by formation of a complex with 1,5-diphenylcarbazide (DPC). The continuous flow analysis (CFA) design uses narrow conduits (0.90 mm) that are hot-forged onto poly(methyl methacrylate) (‘Plexiglas’) plates based on the method of Jannasch et al. (Anal. Chem., 1994, 66, 3352). The sample stream is drawn through the manifold at 25 μl min⁻¹ using a mini-peristaltic pump; osmotic pumps (10 μl h⁻¹) are used to continuously inject reagent (2.0 mM DPC, 0.60 M HNO₃, 5.0% w/v acetone, and 0.10% w/v Brij-35) and to periodically introduce quality control standards and a cleaning solution (0.50 M HNO₃). The ‘Z-type’ optical cell uses a liquid-core waveguide (10 mm) to collimate the light-emitting diode source beam (λ_max 574 nm) to a broadband photodiode detector. Figures of merit are: 7 min cycle time, response within 28 min and clear-down within 31 min, low waste generation (<40 ml d⁻¹), detection limit (3σ) of 48.4 μg l⁻¹ as Cr(VI) or 0.411 μM as chromic acid, 1.54% relative standard deviation at 100 μg l⁻¹, and selectivity for dissolved Cr(VI) in authentic surface water samples containing moderate levels (>0.21% w/v) of total particulate matter. Using a test chamber containing Milwaukee Harbor water that was periodically fortified with Cr(VI) standards, continuous testing over a 15 day period (354 h) yielded results that were in excellent agreement (<5% variation) with measurements made using an ICP-MS reference method. Drift in the calibration model over the test period was 1.23% and the variation in a 0.50 mg l⁻¹ Cr(VI) standard was 3.8% (n = 11). Known interferences to the DPC chemistry (Mo, V, and Hg at >5 mg l⁻¹) were undetected in the harbor water by ICP-MS.