Sample preparation

B.A.M. Kers, J.J. Spriensma, B.M.A.A. Verstappen, T.F. van Dijk-Dijkstra, H.G. Jansen, H.A.J. Meijer

Samples are prepared for accurate measurement of isotope concentrations by cleaning and -where necessary- extracting the measurable fraction out of the supplied sample. Extracting is necassary for watery samples as water itself cannot be measured in the mass spectrometers. Preparation of samples for ¹⁴C determination is described in the section on isotope dating and below the preparation of other samples is described.  

Natural water samples: ¹⁸O

The small -usually 0.6 ml- water samples are first freed from environmental air. To prevent escape of water vapour, the water is freezed  before vacuum pumping. Next, the initial CO₂ is removed from the water by quickly warming the water so that CO₂ removes from the water and is pumped away. This step is executed quickly to reduce loss of water vapour. A precisely known amount of CO₂ from a cylinder with known CO₂ concentration is brought to the pure water  and equilibrium between the water sample and CO₂ is obtained during 48 hours at a temperature of exactly 25 ⁰C. As a result, the ¹⁸O of the water sample is transferred to the CO₂. The CO₂ is finally vacuum extracted and cryogenically collected for measurement on a Mass Spectrometer.  

Enriched water samples: ¹⁸O

Three systems exist, all based on the H₂O-CO₂ oxygen exchange principle. One system is reserved exclusively for enrched samples in order to minimize the risk of contamination. The labs for natural and enriched water are therefore separate. The other systems are designed for small quantities of water (≈ 10 m l), that arrive in small capillaries. In order to keep the so-called "water-correction" (that is the correction for the amount of oxygen in the added CO₂ gas) as low as possible, the systems are small, and the amount of added CO₂ is kept to a minimum.

Enriched water samples: ²H

The -mostly small bio-medical- samples are vacuum distilled to obtain pure water. The water is reduced in the Pyroh and pyrolysed in the Heka before measurement of isotope concentrations in the MM10 mass spectrometer (see Isotope Mass Spectrometers).

Organic carbonates like lime or shell: ¹³C and ¹⁸O

The sample preparation for carbonates is based on the reaction with oversaturated phosphoric acid. The acid-carbonate reaction takes place while the sample is in the water bath (at 25.0 °C). The reaction is given about eight hours of time to complete. The formed CO₂ is removed from the sample tubes by means of vacuum extraction and cryogenic collection.

Inorganic carbon in water: ¹³C

In this system, the water is brought in an evacuated system, of which the pump line runs via a water trap (using dry ice-alcohol) and a CO₂ trap (liquid nitrogen). In order to remove all organic carbon rapidly, phosphoric acid is added to the water.