title: Direct DOC and nitrate determination in water using dual pathlength and second derivative UV spectrophotometry.
abstract: UV spectrophotometry is largely used for water and wastewater quality monitoring. The measurement/estimation of specific and aggregate parameters such as nitrate and dissolved organic carbon (DOC) is possible with UV spectra exploitation, from 2 to multi wavelengths calibration. However, if nitrate determination from UV absorbance is known, major optical interferences linked to the presence of suspended solids, colloids or dissolved organic matter limit the relevance of UV measurement for DOC assessment. A new method based on UV spectrophotometric measurement of raw samples (without filtration) coupling a dual pathlength for spectra acquisition and the second derivative exploitation of the signal is proposed in this work. The determination of nitrate concentration is carried out from the second derivative of the absorbance at 226 nm corresponding at the inflexion point of nitrate signal decrease. A short optical pathlength can be used considering the strong absorption of nitrate ion around 210 nm. For DOC concentration determination the second derivative absorbance at 295 nm is proposed after nitrate correction. Organic matter absorbing slightly in the 270-330 nm window, a long optical pathlength must be selected in order to increase the sensitivity. The method was tested on several hundred of samples from small rivers of two agricultural watersheds located in Brittany, France, taken during dry and wet periods. The comparison between the proposed method and the standardised procedures for nitrate and DOC measurement gave a good adjustment for both parameters for ranges of 2-100 mg/L NO3 and 1-30 mg/L DOC.
authors.affiliation: EHESP Rennes, Leres, Sorbonne Paris Cité, Avenue du Professeur Léon Bernard, CS 74312, 35043 Rennes Cedex, France; Irset-Inserm UMR 1085, 9 avenue du Prof. Léon Bernard, 35000 Rennes, France.
authors.forename: Jean
authors.initials: J
authors.lastname: Causse
authors.affiliation: EHESP Rennes, Leres, Sorbonne Paris Cité, Avenue du Professeur Léon Bernard, CS 74312, 35043 Rennes Cedex, France; Irset-Inserm UMR 1085, 9 avenue du Prof. Léon Bernard, 35000 Rennes, France.
authors.forename: Olivier
authors.initials: O
authors.lastname: Thomas
authors.affiliation: EHESP Rennes, Leres, Sorbonne Paris Cité, Avenue du Professeur Léon Bernard, CS 74312, 35043 Rennes Cedex, France; Irset-Inserm UMR 1085, 9 avenue du Prof. Léon Bernard, 35000 Rennes, France; School of Environmental Engineering, Campus de Ker-Lann, Avenue Robert Schumann, 35170, Bruz, France.
authors.forename: Aude-Valérie
authors.initials: AV
authors.lastname: Jung
authors.affiliation: EHESP Rennes, Leres, Sorbonne Paris Cité, Avenue du Professeur Léon Bernard, CS 74312, 35043 Rennes Cedex, France. Electronic address: marie-florence.thomas@ehesp.fr.
authors.forename: Marie-Florence
authors.initials: MF
authors.lastname: Thomas
mesh_terms: D002244:Carbon; D009566:Nitrates; D045483:Rivers; D013053:Spectrophotometry; D014867:Water
chemical_list: D009566:Nitrates; D014867:Water; D002244:Carbon
keywords: DOC; Dual optical pathlength; Freshwaters; Nitrate; Second derivative; UV spectrophotometry