Individual closed chamber: an alternative method for quantifying 14C in both labeled organic and inorganic carbon substrates

Incubation of 14C-labeled substrates continues to be a widely used procedure in soil organic matter (OM) research due to its sensitiveness. When the labeling is found in liquid fractions (soil extracts, hydrolysates), 14C can be easily quantified by using an aliquot for scintillation counting. For this reason, converting a solid carbon sample into liquid form is a typical step for accurate 14C analysis. We have developed an alternative method to carry out this step, which uses standard glass hardware and does not require complex laboratory facilities. Carbon (both in organic or inorganic forms) is converted into CO2 within a reaction vessel connected to a Twisselmann’s extractor with an alkali trap inside. This forms an individual closed chamber (ICC) for each sample, thus eliminating the risk of cross-contaminations. The alkali solution adsorbs the evolved CO2 within the closed system, and the excess of pressure is easily overcome by the use of a balloon. We tested the procedure on a set of substrates and two contrasting soils, checking also the effect of different sample loads (from 20 to 160 mg C) on the CO2 recovery of the process. The percentage of carbon recovered into the alkali (i.e. the efficiency of the process) ranged from 92% for the inorganic C to 93–95% for the organic C method, the latter being sensitive to the amount of sample used for analysis. The ICC method can be successfully applied to analyze 14C-labeling in both carbonates and OM from solid samples, thus representing an alternative method to some established protocols, and it is suitable for substrates with low or very low 14C contents, in which high volumes of sample must be analyzed in order to guarantee representative results.