Sampling for Isotopic Measurements

The nature of isotopic data is discussed in later chapters, but sample collection techniques are discussed here. 

Fifty-milliliter samples are adequate. Evaporation of samples has to be avoided by prompt closing of the bottles and storage in the shade. Normally each laboratory prefers its own bottles. 

One-liter bottles are desirable to allow for repeated measurements and preconcentration (in the case of low tritium concentrations). Glass bottles are preferred, as tritium can diffuse through PVC bottles and equilibrate with the air. 

Fifty- to two-hundred-liter samples are needed (according to the HC03 concentration and the laboratory requirements). These may be collected in large containers with minimum exposure to air. The detailed procedure should be discussed with the laboratory. Samples have to be delivered to the laboratory within days to avoid equilibration with air. 

Precipitation of the dissolved carbon species and their extraction from 50 200 1 is preferably done in the field. This procedure requires experience (guided by the laboratory staff or, preferably, carried out by a trained hydrochemist). Precipitation in the field is time consuming, but it reduces the volume and weight of samples that have to be transported to the laboratory. 

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Sea cores offer a data base which should in principle allow deduction of the history of the local sea surface temperature immediately above the deposition site of the core, for there is enough organic material in sea cores to provide the necessary samples for isotope measurement at frequent intervals versus depth in the core, but the time resolution is far less accurate than in varves and tree rings because burrowing sea bottom animals smear the record of the layers. 

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