SUMMARY: Seawater is collected from known depths using CTD-rosette sampling procedures. Subsamples are drawn into acid-washed, HDPE bottles. Soluble reactive silica (SRSi) is measured colorimetrically following the formation of silico-molybdic acid from the reaction of ammonium molybdate and silica at acidic pH.
Silicon is the second most abundant element in the Earth's crust. Subaerial weathering processes produce orthosilicic acid Si(OH)4 which eventually is deposited in the oceans. In seawater, various groups of organisms (diatoms, radiolarians, silicoflagellates, sponges and some fungi) utilize silica primarily as a structural component.
The analysis of soluble reactive silica (SRSi) is based upon the formation of yellow silico-molybdic acid from the reaction of ammonium molybdate and silica at acidic pH. Phosphate also reacts to produce a positive interference due to the formation of molybdophosphoric acid. The addition of oxalic acid eliminates the phosphate interference. The sensitivity of the analysis is increased by a further reduction of the yellow silico-molybdic acid using ascorbic acid, in order to produce "molybdenum blue."
Contamination is a primary concern in oligotrophic seawaters especially for samples collected from the euphotic zone where SRSi concentrations approach detection limits of the standard autoanalysis assay procedure. To avoid contamination, all sample bottles must be cleaned with dilute HCl and rinsed with DIW before use. It is important to realize that silica is leached from glass at seawater pH. Therefore, plastic should be used for all sample handling and storage. Finally, special care must be taken when performing SRSi analyses on frozen seawater samples especially for deep-water sample where SRSi concentrations exceed 40 µM.
| 3.1. | Rinse the nutrient sample bottle (acid-washed, 125 ml HDPE bottle) 3 times with sample before filling. Fill to approximately ⅔ full, tighten cap and freeze upright. Separate Silica samples are collected from depths greater than 450 m and are stored refrigerated (rather than frozen) to avoid polymerization (Macdonald et al., 1986). |
| 3.2. | Record cruise, cast and bottle number on the bottle and data sheet. |
SRSi analyses are performed using a 4-channel SEAL Analytical AA3 continuous flow system. The automated wet chemistries generally follow the standard methods of seawater analysis provided by the manufacturer. These involve a reaction of the sample with oxalic acid, ammonium molybdate, and ascorbic acid to produce a colored product. The absorbance is read at 820 nm using a 10 mm path length flowcell.
| 5.1. | Calibration stocks and regression standards |
The calibration of dissolved inorganic nutrient determinations in the autoanalysis of seawater samples is performed using standard solutions containing dissolved N, P and Si salts. A nutrient stock solution is prepared by dissolving dried (65°C, 72 hr) analytical grade reagent chemicals with DIW in 1 L glass volumetric flasks containing 1 ml of chloroform. Once dissolved, this stock solution is immediately transferred into 1 L HDPE bottles and stored at room temperature in the dark (see Reagents section for details). The daily regression standards are prepared by diluting the stock standards with low nutrient natural seawater (LNSW). LNSW is 0.2 µm filtered open ocean surface seawater stored in a carboy at room temperature in the dark for at least 6 months prior to use. This technique provides a mixed standard solution of N, P and Si that is matrix-matched with the seawater samples and any cross-nutrient interference effect should also be accounted for. | |
| 5.2. | Blank corrections
All seawater standard absorbance peaks were corrected for the absorbance of the seawater (LNSW). All seawater sample peaks were corrected for the refractive index absorbance for each unique nutrient detection system. The refractive index corrections represent the increase in absorbance that is due strictly to the presence of dissolved salts in seawater when compared to the DIW baseline. These corrections are determined by running alternating sample cups of DIW and LNSW through the autoanalyzer with only non-color developing reagents on-line. The difference in peak height between DIW and LNSW is calculated and an average is taken to obtain the refractive index correction. This refractive index run is done once a week. |
The detection limit for SRSi is approximately 0.05 µM. The coefficient of variation of field-collected replicates is 0.451 ± 0.641 μmol L-1 based on 2019 data (9 cruises).
acid-washed, 125 ml high-density polyethylene bottles
SEAL Analytical AA3 and accessories
pipettes and volumetric flasks
Since Si is the principle component of glass, all solutions must be made up and stored in plastic.
DIW
Ammonium molybdate solution: Dissolve 10 g of ammonium molybdate into ~800 ml of DIW containing 2.8 mls of concentrated sulfuric acid. Dilute to 1 L. Store in an amber plastic container. Last two weeks unless white precipitate seen on the rim of the bottle.
Oxalic acid (0.56 M): Dissolve 50 g of oxalic acid into 900 ml of DDW and dilute to 1 L. Store in an amber plastic container. Lasts one month.
Ascorbic acid solution: Dissolve 17.6 g of ACS grade ascorbic acid in 500 ml of DDW containing 50 ml of acetone. Mix until dissolved, then add 1 g of sodium dodecyl sulfateand dilute to 1 L. Store in an amber plastic container and in refrigerator when not in use. Lasts one week.
Grasshoff, K., M. Ehrhardt, and K. Kremling. 1983. Methods of Seawater Analysis. Verlag Chemie.
Macdonald, R. W., F. A. McLaughlin and C. S. Wong. 1986. The storage of reactive silicate samples by freezing. Limnology and Oceanography, 31, 1139-1142.
Strickland, J. D. H. and T. R. Parsons. 1972. A Practical Handbook of Seawater Analysis. Fisheries Research Board of Canada, 167 p.
analyte-compound class:soluble reactive silica (SRSi)
method: automated, colorimetric analysis
precision: - laboratory: 0.057 ± 0.132 µmol L-1 based on 2019 data (9 cruises, 309-317) - field: 0.451 ± 0.641 µmol L-1 based on 2019 data (9 cruises, 309-317)
accuracy: See QC Charts
reference standard: - primary: Sodium hexafluorosilicate Na2SiF6 - secondary: WAKO, OSIL and SCOR-JAMSTEC
analysis history for HOT program:
HOT 1-12 cruise samples only were filtered through combusted GF/F filters prior to storage.
Deep water (>200 m) samples for SRSi analysis have been stored at 4°C rather than -20°C
Up until February 2000, analyses were conducted at room temperature on a four-channel Technicon Autoanalyzer II continuous flow system at the University of Hawaii Analytical Facility.
Between 2001 and 2005, the HOT nutrient program underwent substantial changes, including switching analysts twice, eventually establishing an analytical nutrient laboratory centered around a 4-channel SEAL Analytical AA3.
During this transition period, samples from > 200 m depth from HOT 127-166 were shipped to Oregon State University (OSU) for analyses. The OSU nutrient facility uses an AutoAnalyzer II manifold with 5 cm flow cell for PO4 analyses, and an Alpkem RFA 300 system for analyses of NO3+NO2.
Notes, comments or additions: