and Sampling and Analysis Protocols for the JGOFS EqPac Program

Mark A. Altabet and Roger Francois


Water column samples collected using Niskin bottles (carried out by J.J. McCarthy and J. Nevins) are first transferred to 20-l plastic jerricans. Suspended particles are isolated by pressure filtration (< 5 psi) through precombusted 47 mm Whatman GF/F filters placed in in-line filter holders (Altabet, 1988). After filtration, filters are immediately dried at 50 C and placed in stoppered vials. Water samples for N analysis of NO- are collected from the filtrate and stored frozen in 1-l cubitainers. For large volume underway samples, a large filter holder (250 mm diameter) is attached to the ship's clean seawater line. At every 0.5 latitude, sufficient volume is filtered to clog the precombusted GF/F filter and 1 to 4-l of filtrate is collected as described above.

J. Bishop has provided size fractionated particulate samples using the scheme of Altabet (1990) by placing 47 mm filter holders on sidearm ports to this MULVFS units. These samples are dried and stored in precombusted Al envelopes. Both he and M.P. Bacon will also provide large and small size particle samples collected by their MULVFS and LVPS systems. Samples of sinking particles have and will be obtained from floating (J. Murray and J. Newton) and moored (S. Honjo) sediment traps. For the former, `swimmers' are picked at sea and particles from a dedicated trap are filtered onto a precombusted GF/F filter and treated as described above.

Given the availability of sufficient sample mass for isotopic analysis, the principal quality control issue is to eliminate the potential for significant carbon and nitrogen contamination. The details of our methods are designed to prepare materials free of C and N by pre-cleaning and precombustion. Collection and handling of materials and samples at sea are designed to prevent contamination by particles in the ship's environment. For example, filter holders are loaded and unloaded in a plexiglass enclosure. Checks on these procedures are done by having a number of procedural blanks prepared at sea in which filters are subjected to all manipulations except actual exposure to sample.

Sample Preparation and Isotopic Analysis at WHOI

NH+ is extracted from solution using steam distillation under alkaline conditions (Velinsky, 1989). Samples of seawater NO- will be prepared similarly but will be first reduced to NH+ using Devarda's alloy (Cline and Kaplan, 1975). Modifications of the procedure (S. Horrigan, pers. comm.; Velinsky, 1989) using an ion sieve as a trap for the distilled NH+ permits subsequent application of the ampule combustion procedure (see below) on the dried solid sample. Particle samples are acid fumed in a glass desiccator overnight to remove inorganic carbon prior to placement in quartz ampules with CuO, Cu, and Ag metal as reagents (Minagawa et al., 1985). Following evacuation on a vacuum line, the ampules are flame sealed and combustion takes place at 850 C. N and CO are purified on a manifold attached to the mass spectrometer inlet. Isotopic analyses will be made using a Finnigan MAT 251 dual-inlet isotope ratio mass spectrometer. Sample sizes can be as low as 0.2 µmol for carbon and 0.5 µmol for nitrogen. Typical reproducibility is ±0.1/. Quality assurance and standardization is performed by carrying out these procedures on standard material of known isotopic content and by analysis of procedural and reagent blanks.

Literature Cited

Altabet, M.A. (1988).
Variations in nitrogen isotopic composition between sinking and suspended particles: implications for nitrogen cycling and particle transformation in the open ocean. Deep-Sea Research, 35: 535--554.

Altabet, M.A. (1990).
Organic C, N, and stable isotopic composition of particulate matter collected on glass-fiber and aluminum oxide filters. Limnology and Oceanography, 34: 902--909.

Cline, J.D. and I.R. Kaplan (1975).
Isotopic fractionation of dissolved nitrate during denitrification in the Eastern Tropical North Pacific Ocean. Marine Chemistry, 3: 271--299.

Minagawa, M., D.A. Winter and I.R. Kaplan (1985).
Comparison of Kjeldahl and combustion methods for measurement of nitrogen isotope ratios in organic matter. Analytical Chemistry, 56: 1859--1861.

Velinsky, D.J., J.R. Pennock, J.H. Sharp, L.A. Cifuentes and M.L. Fogel (1989).
Determination of the isotopic composition of ammonium-nitrogen at the natural abundance level from estuarine waters. Marine Chemistry, 26: 351--361.