Dissolved Inorganic Nutrients

The ODF chemistry laboratory offers colorimetric determination of the following dissolved inorganic nutrients: nitrate, nitrite, phosphate, silicate, and ammonia. Contact Susan Becker for details and pricing. 

Sampling Requirements and Preservation

20-30 mL of sample are required for analysis. We recommend these Sarstedt tubes, which fit directly on the sampler. These tubes are sterilzed and do not require cleaning before sampling.   If the Polypropylene screw-capped centrifuge tubes are not sterile or are being re-used they should washed thoroughly with 1.2N HCl and rinsed with sample at least 3 times prior to filling. 

Samples not being analyzed right away (at sea) should be frozen and stored upright at < 0°C. Leave a headspace for expansion during freezing. Samples are thawed in a 50°C heating bath and brought to room temperature prior to analysis. 

Equipment and Techniques

Nutrient analyses are performed on a Seal Analytical continuous-flow AutoAnalyzer 3 (AA3). After each run, charts are reviewed for any problems and final concentrations (in micromoles per liter) are calculated using SEAL Analytical AACE software. 

Details about methodology, including data ranges, detection limits, and reagents can be found here. The general analytical methods used are described by Gordon et al. (1992), Hager et al. (1972), and Atlas et al. (1971). The modification of analytical methods used are compatible with those described in the nutrient section of the GO-SHIP repeat hydrography manual (Becker et al., 2019).  

Data collection and processing

Data collection and processing is done with the software provided with the instrument from Seal Analytical. After each run, the charts are reviewed for any problems, any blank is subtracted, and final concentrations are calculated, based on a linear curve fit. Next, a text file is created. This is reviewed for possible problems and then converted to an output file with only sample identifiers and nutrient concentrations that can be merged with other bottle data.

Standards and Glassware calibration

Primary standards for silicate (Na2SiF6), nitrate (KNO3), nitrite (NaNO2), phosphate (KH2PO4) and ammonia ((NH4)2SO4) are obtained from  Johnson Matthey Chemical Co. and/or Fisher Scientific. The supplier reports purities of >98%, 99.999%, 97%, 99.999% and 99.99%, respectively.

All glass volumetric flasks and pipettes are gravimetrically calibrated. The primary standards are dried and weighed out to 0.1 mg. When primary standards are made, the flask volume at 20°C, the weight of the powder, and the temperature of the solution are used to buoyancy correct the weight, calculate the exact concentration of the solution, and determine how much of the primary is needed for the desired concentrations of secondary standard. New standards are compared to the old before use.

All the reagent solutions, primary and secondary standards are made with fresh distilled deionized water.

Standardizations are performed at the beginning of each group of samples.

References:

Atlas, E.L., Hager, S.W., Gordon, L.I., and Park, P.K., "A Practical Manual for Use of the Technicon AutoAnalyzer in Seawater Nutrient Analyses Revised," Technical Report 215, Reference 71-22, Oregon State University, Department of Oceanography, pp.49 (1971).

Becker, S., Aoyama, M., Woodward, E.M.S., Bakker, K., Coverly, S., Mahaffey, C., and Tanhua, T., "The precise and accurate determination of dissolved inorganic nutrients in seawater; Continuous Flow Analysis methods and laboratory practices," IOCCP Report No. 14, GO-SHIP Repeat Hydrography Manual: A Collection of Expert Reports and Guidelines, ICPO Publication Series No. 134 (2019).

Gordon, L.I., Jennings, J.C., Ross, A.A., and Krest, J.M., "A suggested Protocol for Continuous Flow Automated Analysis of Seawater Nutrients in the WOCE Hydrographic Program and the Joint Global Ocean Fluxes Study," Grp. Tech Rpt 92-1, OSU College of Oceanography Descr. Chem Oc. (1992).

Hager, S.W.,  Atlas, E.L., Gordon L.I., Mantyla, A.W., and Park, P.K., " A comparison at sea of manual and autoanalyzer analyses of phosphate, nitrate, and silicate ," Limnology and Oceanography, 17, pp.931-937 (1972).