The ¹⁴CO₂ sampling consists in pumping continuously ambient air over two weeks through a CO₂ absorbing sodium hydroxide (NaOH) solution. In order to enlarge the NaOH reaction surface, the solution is held in a rotating glass tube filled with a packed bed of glass Raschig rings. Using this method, the atmospheric CO₂ is absorbed nearly quantitatively in the NaOH solution. The sample is then sent to the ICOS CAL CRL in Heidelberg, Germany for analysis. Currently, only the integral ¹⁴CO₂ sampler produced by ICOS-CRL is eligible for the ICOS network. The level of ¹⁴C within the sampled CO₂ is a tracer for the fossil fuel emissions.