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.
- 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,
- 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.
- Determination of the isotopic composition of
ammonium-nitrogen at the natural abundance level from estuarine waters.
Marine Chemistry, 26: 351--361.