Sources, Fluxes, and Biogeochemical Cycling of Silver in the Oceans - Reviews of Environmental Contamination and Toxicology - page 41

3.3 Qualifi cations

A major problem in understanding the biogeochemistry of silver in the world oceans,
apart from the paucity of data, is the diffi culty in comparing the different data sets.
The few measurements of truly trace concentrations of silver in the oceans have been
conducted by different investigators using different sampling, treatment, and analyti-
cal methods (Table 1 ). For example, onboard collections have been conducted using
different sampling systems, and have included—or not—a fi ltration of the samples on
various types and sizes of fi lters. The disparities extend to the laboratory, wherein two
types of methods have primarily been used to separate and pre-concentrate silver:
solvent-extraction and chelating resin column partitioning (Table 1 ). One method,
developed in the 1970s (Danielsson et al. 1978 , Bruland et al. 1979 ), uses a combina-
tion of 1-pyrrolidinedithiocarbamate and diethyldithiocarbamate (APDC/DDDC) to
chelate silver, followed by a double extraction into chloroform, and back-extraction
into nitric acid. Another method, developed more recently (Ndung'u et al. 2006 ),
consists of separating silver on a mini-column packed with a strong anion exchange
resin (Dowex 1-X8) connected to a fl ow-injection system that allows on-line deter-
mination by high resolution inductively coupled mass spectrometry (HR ICP-MS).
Different analytical methods have also been used for the analysis of silver concen-
trations, including thermal ionization mass spectrometry (TIMS), atomic absorption
spectrometry (AAS), and HR ICP-MS.
In addition, the use—or not—of ultraviolet (UV) irradiation prior to the analysis
may introduce some disparity among silver measurements. Ndung'u et al. ( 2006 )
showed that a digestion with UV radiation is necessary prior to silver analysis with
anion exchange resin to liberate any metal bound in refractory organic complexes.
Analysis of waters from the San Francisco Bay estuary and the North Pacifi c Ocean
showed a 10-70% increase in silver concentrations measured after exposure to UV
radiation. Similarly, comparable measurements on the Safe D2 and GEOTRACES
GDI intercalibration samples showed an increase of silver following UV irradiation
(unpublished data). Whether such discrepancies apply to the use of APDC/DDDC
extraction is not clear.
Comparison of the silver data generated by different laboratories is further compli-
cated by the lack of a certifi ed value for silver in any reference seawater material.
The National Research Council of Canada (NRC-CNRC) certifi ed reference material
for trace metals in estuarine water (SLEW-3) has an “information” value for silver,
but it is not certifi ed. To date, only two laboratories (including ours) have reported
silver concentrations in the GEOTRACES North Pacifi c (SAFe Reference Samples)
and North Atlantic (GEOTRACES Reference Samples) intercalibration waters, and
these measurements have not been published (Ken Bruland, personal communication).
But a few authors have reported numbers for the NRC-CNRC reference materials for
trace elements in nearshore seawater (CASS-4) and estuarine water (SLEW-3), which
are listed in Table 5 .
Finally, there have been a few measurements of silver isotopes in the marine
environment, but they have not been substantiated by intercalibration. Folsom et al.