Effect of chloride on the chemical conversion of nitrate to nitrous oxide for δ¹⁵N analysis

Abstract

We investigate the influence of chloride concentration on the performance of the chemical reduction method for measurement of the nitrogen isotopic ratio (δ 15 N) in NO 3 -in natural waters (McIlvin and Altabet, 2005). In this method, NO 3 -is first reduced to NO 2 -using activated cadmium metal, with further reduction to N 2 O using sodium azide in an acetic acid buffer. N 2 O is introduced into an isotope ratio mass spectrometer (IRMS) for isotopic measurement. Previously, it was recognized that the presence of halides was necessary for the speed and efficiency of the second step but not thought to be important for the first step. Whereas quantitative Cd reduction of NO 3 -to NO 2 -had been noted for seawater samples, here we report, for freshwater and low-salinity (S < 30) samples, a variable conversion efficiency (both under-and overreduction were observed) and significant variation in δ 15 N determination. Addition of 5 M NaCl to all samples resulted in rapid (<4 h) and quantitative (>99%) reduction of NO 3 -to NO 2 -as well as stable δ 15N values that closely matched expected values for standards (within 0.3‰ of standard value). The positive effect of NaCl is likely due to a decrease in free Cd 2+ produced over the course of the reaction due to formation of CdCl 2 .*Corresponding author: E-mail: eryabenko@ifm-geomar.de AcknowledgmentsThe authors thank Frank Malien, Annette Kock, and Gert Petrick for technical assistance. The work was supported by the DFG-funded "Future Ocean" Excellence Cluster and Sonderforschungsbereich 754 "Climate-Biogeochemistry Interactions in the Tropical Ocean." Limnol. Oceanogr.: Methods 7, 2009, 545-552 © 2009, by the American Society of Limnology and Oceanography, Inc. LIMNOLOGY and OCEANOGRAPHY: METHODSdeveloped by McIlvin and Altabet (2005), which uses azide for quantitative nitrite conversion to N 2 O for the isotopic analysis of seawater and freshwater. The method allows separate analysis of nitrite without interference from the isotopic signature of nitrate and has a standard deviation of less than 0.2‰ for δ 15 N in nitrate samples ranging in concentration from 40 to 0.5 µM. We refer to the McIlvin and Altabet method as MA (2005).For NO 3 -concentration measurements (e.g., by an autoanalyzer), a quantitative (100%) reduction is not necessary, because NO 3 -standards are run under exactly the same conditions as the samples, so that sample concentrations are corrected for any N analysis, on the other hand, a quantitative (100%) reduction of NO 3 -is essential to avoid potentially large and variable isotopic fractionation.Application of the MA (2005) method at IFM-GEOMAR includes analysis of samples collected from the Atlantic and Pacific Oceans and the Baltic Sea covering a wide range in salinity (equivalent to 0-0.5 M NaCl). A salinity effect on NO 3 -reduction yields has been discussed in several articles, which examined the effect of Cd column methods for NO 3 -concentration measurements (Gal et al. 2004, Nydahl 1976. For example, Nydahl (1976) argued: "Conside...

Modifications to the azide method for nitrate isotope analysis

“…In the fourth experiment (Fig. ), the performance of Cd powder in the whole reaction was compared with that of the original method (spongy Cd) and the modified method (acid‐activated Cd powder) . In this experiment (experiment 4), 5‐mL aliquots of 15 μM solution of four international standards and of laboratory NaNO 2 were pipetted into 10‐mL centrifuge tubes.…”

“…Briefly, a zinc stick was inserted into a cadmium chloride solution, and the reduced Cd crystals that were formed were scraped off the stick. Activated Cd powder was prepared by rinsing Cd powder with HCl (6 M) and then repeatedly with deionized water until the pH became neutral . In this experiment the produced NO 2 − was later converted into N 2 O using the original azide buffer and the N 2 O isotope ratios were determined using an automated purge and cryogenic trap system coupled to an isotope ratio mass spectrometer (for details, see below).…”

“…This method needs specialized denitrifier cultures to be maintained, which may be impractical for some isotope laboratories. The azide method is based on chemical conversion of NO 3 − into NO 2 − by spongy Cd or activated Cd powder and finally to N 2 O with the azide and acetic acid buffer ('azide buffer' hereafter) . This method is easy to set up and suitable for the laboratory where NO 3 − isotopes are not regularly determined.…”

Carbonate system parameters of an algal-dominated reef along West Maui

“…, with precisions of 0.6-3.0, 0.6-0.8, 0.9-1.3, and 0.3-1.0 % relative standard deviations, respectively. Select samples were collected and analyzed for nitrate isotope (δ 15 N and δ 18 O) analyses at the University of California at Santa Cruz using the chemical reduction method (McIlvin and Altabet, 2005;Ryabenko et al, 2009) and University of California at Davis' Stable Isotope Facilities using the denitrifier method (Sigman et al, 2001). The isotope analysis was conducted using a Thermo Finnigan MAT 252 coupled with a GasBench II interface; isotope values are presented in per mil (‰) with respect to AIR for δ 15 N and Vienna Standard Mean Ocean Water (VSMOW) for δ 18 O with a precision of 0.3-0.4 and 0.5-0.6 ‰ for δ 15 Nnitrate and δ 18 O-nitrate, respectively.…”

An eddy-stimulated hotspot for fixed nitrogen-loss from the Peru oxygen minimum zone

“…NaCl was added to ensure consistent quantitative yield (Ryabenko et al, 2009). Pre-existing NO − 2 was removed at the time of collection by addition of sulfanilic acid (Ryabenko et al, 2012).…”