1. INTRODUCTION
The development of the AMS technique as well as the construction
of the facility at WHOI have greatly increased the number of individuals
and laboratories in the field of oceanography interested in obtaining
14C analyses, and have greatly increased the importance
of collecting the sample in a clean, 14C-free environment.
Here, we discuss the special precautions necessary for collecting
and handling samples for the measurement of natural levels of
14C. Analysis by AMS requires a sample that is orders
of magnitude smaller than that for a conventional radiocarbon
analysis (1 mg. vs 5 g). While this greatly increases the scope
of radiocarbon studies, it also means that a much smaller amount
of contaminant is required to ruin a sample. For most users,
the sample that is submitted will be prepared for AMS analysis
in the NOSAMS sample-preparation laboratory. Virtually every
AMS laboratory has experienced down time because of the clean-up
required after preparing and analyzing a "hot" sample.
We have prepared this document to help users of the NOSAMS facility
ensure that they are providing us with a clean sample whose preparation
will not compromise the operation of our laboratory. Many oceanographic
research projects use radiocarbon as a spike in experiments at
sea and in the laboratory, e.g. the measurement of oceanic productivity,
and inadvertent spills can leave isolated spots that are severely
contaminated. The levels typically used in tracer experiments
can be several million times modern levels and very small residual
amounts can ruin the measurement of natural levels of 14C.
Contamination of the sample container and, thus, the sample,
can arise from collecting and handling the sample on a contaminated
surface. Therefore, we recommend the following procedures to
ensure collection and preparation of a "good" 14C
sample.
Laboratory Operations
Collection of natural samples for radiocarbon analysis usually requires the preparation of sample containers and sampling apparatus in a laboratory. All apparatus that is to be used for sample collection should be cleaned and prepared in a laboratory that is known to be 14C-free. Because it is difficult in many laboratories to be certain that 14C has never been used as a spike, we recommend conducting low-level swab tests whenever there is the slightest question about the history of a laboratory. The Tritium Laboratory at the University of Miami is one laboratory which has developed a well-established routine and protocol for testing ships and laboratories and which performs swab analysis for a fee. We encourage investigators new to the natural radiocarbon field to consider swab tests specifically designed to measure low levels of contamination in the laboratory as the first step in establishing a radiocarbon research program. In laboratories where a problem is found, corrective action must be taken. When an area is determined to be 14C-free, we recommend that this area be isolated from general laboratory operations and access be restricted to those familiar with the precautions necessary for handling natural-level radiocarbon samples.
Shipboard Operations
Swab tests of oceanographic research vessels have shown that
the use of radiocarbon to measure productivity leaves areas that
are severely contaminated. These cannot be used for collection
of natural-level radiocarbon samples. We recommend that surfaces
where samples are collected or handled should be covered with
fresh, disposable sheets of plastic or garbage bags and that disposable
gloves (changed often) should be worn during sampling. Sample
containers should be handled as little as possible and removed
from their packing crates only when necessary. When sample bottles
or containers are removed from shipping crates, they should not
be placed in direct contact with any surface on the ship either
on deck or in the laboratory. A data sheet(s) should be kept
for each crate of sample bottles. Information regarding the history
of each crate should be recorded on this sheet. This information
should include identification of the laboratory in which the bottles
were prepared, the shipping and storage history (dates and location)
of each crate, information regarding the condition of laboratories
and storage facilities (e.g., refrigerated or not) and should
identify other sampling programs in progress on the ship. In
general, it is not advisable to consider collecting radiocarbon
samples on a cruise on which 14C-spiking experiments
will be performed. If laboratory analyses are required at sea,
precautions similar to those discussed in the previous section
must be observed.
More detailed information can be obtained by contacting NOSAMS,
(508) 289-2513. We have published a detailed protocol for the
collection of seawater inorganic carbon samples* and
will be happy to discuss the collection of other types of samples
with interested researchers.