Equipment and Techniques
Nutrient analyses (phosphate, silicate, nitrate plus nitrite, and nitrite) are performed on a Seal Analytical continuous-flow AutoAnalyzer 3 (AA3). After each run, the charts are reviewed for any problems and final concentrations (in micromoles per liter) are calculated using SEAL Analytical AACE 6.07 software.
The analytical methods used are described by Gordon et al. [Gord92], Hager et al. [Hage68] and Atlas et al. [Atla71]. The details of modification of analytical methods used for this cruise are also compatible with the methods described in the nutrient section of the GO-SHIP repeat hydrography manual [Hyde10].
Nitrate/Nitrite Analysis
A modification of the Armstrong et al. [Arms67] procedure is used for the analysis of nitrate and nitrite. For nitrate analysis, a seawater sample is passed through a cadmium column where the nitrate is reduced to nitrite. This nitrite is then diazotized with sulfanilamide and coupled with N-(1-naphthyl)-ethylenediamine to form a red dye. The sample is then passed through a 10mm flowcell and absorbance measured at 540nm. The procedure is the same for the nitrite analysis but without the cadmium column.
Phosphate Analysis
Ortho-Phosphate is analysed using a modification of the Bernhardt and Wilhelms [Bern67] method. Acidified ammonium molybdate is added to a seawater sample to produce phosphomolybdic acid, which is then reduced to phosphomolybdous acid (a blue compound) following the addition of dihydrazine sulfate. The sample is passed through a 10mm flowcell and absorbance measured at 820nm.
Silicate Analysis
Silicate is analyzed using the technique of Armstrong et al. [Arms67]. Acidified ammonium molybdate is added to a seawater sample to produce silicomolybdic acid which is then reduced to silicomolybdous acid (a blue compound) following the addition of stannous chloride. The sample is passed through a 10mm flowcell and measured at 660nm.
Sampling Requirements and Preservation
Freezing is the preferred method of preservation. 20-30 mL of sample are required for analysis. Polypropylene screw-capped centrifuge tubes are recommended. Tubes should be sterile or washed thoroughly with 10% HCl and rinsed with sample at least 3 times prior to filling. Leave a headspace for expansion during freezing. Samples are thawed overnight at ~1.7 C and brought to room temperature prior to analysis. The centrifuge tubes fit directly onto the sampler.
Data collection and processing
Data collection and processing is done with the software (ACCE ver 6.07) provided with the instrument from Seal Analytical. After each run, the charts are reviewed for any problems during the run, 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 ( Na2 SiF6 ), nitrate ( KNO3 ), nitrite ( NaNO2 ), and phosphate ( KH2 PO4 ) are obtained from Johnson Matthey Chemical Co. and/or Fisher Scientific. The supplier reports purities of >98%, 99.999%, 97%, and 99.999 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 (DIW).
Standardizations are performed at the beginning of each group of samples.