Isotope Ratio Mass Spectrometers

Five different mass spectrometers (MS) are used at the Center of Isotope Research to determine stable isotope ratios. The MS are optimized for different types of samples and the way that the samples are supplied, as explained below. The MS are named: ZwOptima, AirOptima, DI-Optima, MM10 and Pyroh-Heka.

Analysis of ¹³C isotope ratio is needed for a correction on ¹⁴C isotope ratio to calculate the age of a sample. The ¹³C and ¹⁵N ratios are determined with the IsoPrime100 before a sample is analyzed on ¹³C and ¹⁴C ratios using the Acceleration Mass Spectrometer (AMS). Thus, ¹³C isotope ratios are measured both by IsoPrime100 and AMS and the results are compared to check the quality of both instruments. In case of good agreement, the data of IsoPrime100 are used for further analysis, because the IsoPrime100 is most accurate for the stable isotope ¹³C whereas the AMS is specially designed for an accurate determination of the ¹⁴C isotope ratio. The ¹⁵N isotope ratio is a supplementary measurement that informs on animal diet.

The isotope ratios of atmospheric CO₂: ¹³C and ¹⁸O, in particular samples from our Lutjewad monitoring station, are determined using the DI-Optima MS in combination with the AMS. As above, results of ¹³C measurements from both instruments are also used for quality control. The ¹⁸O isotope ratio informs on the temperature and the isotope ratios of C inform on fossil fuel fraction of atmospheric CO₂.

Changes in atmospheric CO₂ concentration –as measured with the DI-Optima- are commonly accompanied by changes in O₂ concentrations because CO₂ and O₂ are inversely exchanged in important global processes: fossil fuel combustion, photosynthesis and respiration. However, oceanic uptake of CO₂ is not related to O₂ uptake as solubility of CO₂ largely exceeds O₂ solubility, due to equilibrium of CO₂ with (bi)-carbonate in water. The Air-Optima is specially designed to measure the atmospheric O₂/N₂ concentration ratio to determine global oceanic CO₂ uptake. The measurements are performed with very high precision as relative changes in O₂/N₂ are very small.

All other ¹³C and ¹⁸O isotope ratios, for example from water samples are measured with the general purpose instrument MM10 MS. The water is equilibrated with CO₂ of known isotopic composition. The MM10 MS is then used to measure changes in the CO₂ isotopic composition before and after equilibration and this result in the determination of the isotopic composition of CO₂ in the water sample.

Isotope ratios of small (from 0.12 µl onwards!) water samples: ²H and ¹⁸O are measured using the Pyroh-Heka. The Pyroh reduces water on a chromium oven and the Heka pyrolysis water to get H₂ and CO for analysis at the MS. The instrument is mainly used for measurements on artificially enriched isotope ratios in biological samples, but also enriched ice from Greenland has been measured.