Onboard Determination of Submicromolar Nitrate in Seawater by Anion-Exchange Chromatography with Lithium Chloride Eluent

Abstract

Nitrogen is an essential element for phytoplankton growth in aquatic environments. Nitrate is the main nutrient supporting this production in the open ocean, and the supply of nitrate often controls primary production in oligotrophic areas. Excluding the high-latitude and eastern equatorial regions of the Pacific, the surface waters of the North and South Pacific Ocean from about 40˚ N to 40˚ S are relatively depleted in nitrate. In the North Pacific subtropical gyre, nitrate concentrations are typically below 10 nmol/l.1 To understand the nitrogen cycles in the open ocean ecosystem, a sensitive analytical method to determine the nanomolar levels of nitrate in seawater is essential.The standard method of nitrate determination in seawater is based on a flow-injection technique with spectrophotometric detection. [2][3][4] In this method, nitrate is first reduced to nitrite in a copperized cadmium column.Nitrite is coupled with sulfanilamide to form a diazonium cation. This diazonium reacts with N-1-naphtylethylenediamine dihydrochloride, forming a purple azo dye whose absorbance is measured at 550 nm. Next, the nitrite is measured independently according to the same method without reduction. Nitrate is then calculated from the difference in the concentration between [nitrate + nitrite] and nitrite. This method can be conventionally applied to the determination of nitrate and nitrite on board; its detection limit is about 50 nmol/l as [nitrate + nitrite].A more sensitive method of nitrate analysis has been developed by utilizing the chemiluminescence of nitrogen monoxide. [5][6][7] In this method, both nitrate and nitrite are reduced to nitrous oxide in a highly acidic solution of sulfuric acid containing iron(II) ammonium sulfate and ammonium molybdate. Nitrous oxide in the solution is carried to a drying tube and reacts with ozone, resulting in chemiluminescence. The detection limit is around 2 nmol/l. This measurement method also requires nitrate reduction to nitrite. By this method, Cavender-Bares et al. 8 measured 2 to 10 nmol/l concentrations of [nitrate + nitrite] in surface seawater that were below the detection limit of the standard method using azo dye formation. They revealed the vertical gradients of [nitrate + nitrite] in the North Atlantic Ocean.As a simple method, direct UV determination 2,4,9 has also been examined, but often encounters interference by bromide or organic compounds, such as humic substances, which also have UV absorbance in a wavelength range similar to that of nitrate. Capillary electrophoresis 10,11 is very useful because it gives high efficiency in the separation of analytes, and enables rapid analysis and the direct injection of seawater. However, the small injection volume and short path length of the detector results in a poor detection limit for nitrate and nitrite.Ion chromatography (IC) 12 is advantageous in the determination of nitrite and nitrate because these anions can be separated from other ions, such as bromide and organic substances. However, with highly ionic mat...

Microbial Community Stratification Controlled by the Subseafloor Fluid Flow and Geothermal Gradient at the Iheya North Hydrothermal Field in the Mid-Okinawa Trough (Integrated Ocean Drilling Program Expedition 331)

“…Porewater was obtained from 10-to 20-cmlong WRCs; the total alkalinity and the ammonium, sulfate, and methane concentrations were determined previously (26). The nitrate concentration was measured by ion chromatography using a high-capacity anion exchanger (TSK-gel SAX column; Tosoh) with UV detection (LC-10Ai and SPD-10A; Shimadzu) (29,30). The lower detection limit of the nitrate concentration was 0.3 M, and the reproducibility was better than 10%.…”

Quantification of Microbial Communities in Hydrothermal Vent Habitats of the Southern Mariana Trough and the Mid-Okinawa Trough

“…The analytical precision was generally estimated to be within 3 % for the seawater analysis. The nitrate concentration was determined onboard using a liquid chromatograph (Shimadzu, LC10AI) with a UV detector (λ ¼ 220 nm, Shimadzu, SPD10AVP) following a modified protocol (Maruo et al 2006). To eliminate interference from major anions such as Cl À and SO 4 2À , a highcapacity anion exchange column (Tosoh, TSK gel-SAX, 4.6 Â 50 mm) was used with a 1 M sodium chloride eluent at a flow rate of 0.8 ml/min.…”

Unique Elution Behavior of Bromide and Nitrate in Anion-Exchange Chromatography Using Aqueous Potassium Chloride Eluent Containing Cadmium or Zinc Ion

“…8 By using alkaline and alkaline earth metal chlorides, the selectivity for each analyte was enhanced by decreasing the concentration of the eluent anion, but the order of elution could not be changed.…”

“…[5][6][7] Our laboratory has reported that the separation factor between bromide and nitrate can be modified through the use of alkaline metal cations along with a high-concentration eluent and high-capacity anion-exchange resin. 8 This method was used to determine nitrate in seawater. When using alkaline and alkaline earth metal cations in the previous study, the separation factor of nitrate relative to bromide was found to depend on both the countercation used and its concentration.…”