Simplifying and improving the extraction of nitrate from freshwater for stable isotope analyses

Abstract

Determining the isotopic composition of nitrate (NO3_) in water can prove useful to identify NO3_ sources and to understand its dynamics in aquatic systems. Among the procedures available, the ‘ionexchange resin method’ involves extracting NO3_ from freshwater and converting it into solid silver nitrate (AgNO3), which is then analysed for 15N/14N and 18O/16O ratios. This study describes a simplified methodology where water was not pre-treated to remove dissolved organic carbon (DOC) or barium cations (added to precipitate O-bearing contaminants), which suited samples with high NO3_ ($400 mM or 25 mg L_1 NO3_) and low DOC (typically <417 mM of C or 5 mg L_1 C) levels. % N analysis revealed that a few AgNO3 samples were of low purity (compared with expected % N of 8.2), highlighting the necessity to introduce quality control/quality assurance procedures for silver nitrate prepared from field water samples. Recommendations are then made to monitor % N together with % O (expected at 28.6, i.e. 3.5 fold % N) in AgNO3 in order to better assess the type and gravity of the contamination as well as to identify potentially unreliable data

Diversification of Nitrogen Sources in Various Tundra Vegetation Types in the High Arctic

“…The stable isotope composition of NO 3 in snow from NW Spitsbergen (Ny-Ålesund, Kongsfjorden) was reported as –8.6±1.0‰ ( δ 15 N) and +70.8±3.0‰ ( δ 18 O) [ 53 ]; but no values have been reported for rainwater or NH 4 because of extremely low concentrations. Rainwater samples collected during the present study also contained extremely low concentrations of NO 3 and NH 4 (<0.001 mg/L), which prevented precise analyses using the adopted precipitation method with preconcentration on resin [ 56 ]. Only one measurement using the “denitrification method” was successful.…”

Combining stable isotopes with contamination indicators: A method for improved investigation of nitrate sources and dynamics in aquifers with mixed nitrogen inputs

“…Groundwater samples were analysed for thirteen physico-chemical and microbial parameters: ), Na + , K + , magnesium (Mg 2+ ), calcium (Ca 2+ )), coliforms (total TC and faecal FC). After NO 3 concentrations were measured, an aliquot of each groundwater sample containing 100 μmol of NO 3 was portioned off and extracted according to a simplified ion-exchange resin method best suited for freshwater samples with high NO 3 -(> 25 mg L -1 ) and low dissolved organic carbon (DOC) levels (typically < 5 mg C L -1 ) (Minet et al, 2011…”

Determining the Isotopic Composition of Nitrate in the River Daugava and Loads of Nitrogen to the Gulf of Riga

“…This steps completed in the laboratory within 48 hours of collection [7]. The NO 3 is stripped from the columns by five 3 mL increments of 3 M HCl passed through the column (elutant kept chilled to minimise volatilisation of HNO 3 ), each increment kept in the column for 30 seconds before slowly blowing the column dry, rinsing (whole step carried out in the dark) [9]. Elutant immediately neutralised by slow addition of about 6 g of silver oxide (three different batches of high-grade Ag 2 O heavily contaminated with nitrate were previously washed [7]) until pH reached 5-6 (checked with pH-paper), filtration through a 0.2 µm nylon membrane to remove excess Ag 2 O and silver chloride (AgCl), rinsing (whole step carried out in the dark).…”

“…In our time, δ 15 N-NO 3 and δ 18 O-NO 3 in nitrate can be measured according to three analytical methods [6]: two 'denitrifier methods' requiring Isotope Ratio Mass Spectrometry (IRMS) analysis of N 2 O gas generated by bacterial or chemical means, and a procedure known as the 'ion-exchange resin method' whereby NO 3 is extracted from freshwater and converted into solid silver nitrate (AgNO 3 ) that is analysed by IRMS [7]. Minor modifications have been adopted by some to ease the sample preparation [8], one of the most significant changes being the precipitation of O-bearing contaminants (mainly sulphate and phosphate) with barium chloride (BaCl 2 ) prior to passing water through an anion-exchange resin so that AgNO 3 is ready for both δ 15 N and δ 18 O analyses [9]. The objectives of the research proposal are to use isotope data for nitrate as a tool for determining nitrate sources in river systems, while also evaluating how natural variation in water discharge can influence seasonal changes of nutrient loads and isotopic values.…”