Measuring 14C-isotope ratios
The 14C analysis group supports scientists with measurements of 14C isotope ratios in soil, sediment, wood, water and air samples. The measurement of 14C in natural samples is a large effort because the 14C/C ratio is only 10-12 even in modern samples. In our laboratory, the sample size is 0.7 mg carbon for normal samples and 0.25 mg carbon for small samples. The first step in measuring 14C isotope ratios of a sample is the sample preparation, which can be very sample specific and even the more costly part of the whole measurement.
Often only one fraction of carbon (or sometimes one fraction not) should be measured in the samples. Therefore, in the first preparation step, the desired and undesired fractions of carbon are separated by chemical or physical methods. For example, if "organic carbon" is to be measured in soil samples, the inorganic carbon, if present, must be removed first. The next step is to extract the carbon from the samples. In the example of a soil sample it consists mainly of the sand and clay and there are a few percent of carbon (mostly less). This extraction is done by burning the carbon at high temperatures to CO2, for which we use an elemental analyzer. The resulting CO2 is separated from the other gases and graphitized with hydrogen and an iron catalyst at 550°C. This graphite, derived from the sample material, is pressed into aluminum holders (called targets or cathodes), which are then loaded into magazines for the MICADAS ion source.
Measurement of 14C isotope ratios
The measurements are performed with the method of AMS (AMS from Accelerator Mass Spectrometry). To measure the 14C isotope ratios, we use the AMS system MICADAS from the company Ionplus (Dietikon, Switzerland). The graphite in the targets becomes atomized and ionized to negative charge in the ion source of the AMS system. The ions are accelerated with electrical voltages to an energy of 400 keV and mass-separated with magnetic fields. The AMS system acts simultaneously as an accelerator and also as a filter for other unwanted ions (background), so that individual 14C ions can be counted in the detector at the end of the apparatus. From these count rates, along with the measured 12 C and 13 C current values, the 14C isotope ratios are determined.
A magazine contains 40 positions, which are loaded up with targets. In our laboratory, there are usually 6 standards and 6 blank standards for every 27 samples to be measured. (1 position must remain free.) The standards are needed because a 14C measurement is a relative measurement, i.e. the 14C isotope ratio of a sample is determined in relation to that of a standard. The 6 blank standards are used to determine the background. A (trouble-free) measurement runs automatically after the start and a full magazine is measured within 26 hours. Afterwards, another magazine with new samples is inserted and started again.