Contamination may also be caused by humic acids circulating throughout the soil. Humics are the decayed remnants of dead plants. They may exchange carbon or adhere to samples that have large surface areas and make any radiocarbon results too young. This surface exchange is termed 'adsorption' and is especially common in samples such as peats, charcoals and muds. Certain samples, especially shell, may show evidence of isotopic exchange or recrystallisation.
Isotopic exchange occurs when shells exchange carbon with percolating ground acids.
This alters their isotopic ratios and affects their 'true' age. An analysis of the carbon isotope ratios using a mass spectrometer will reveal the extent of any exchange. A correction factor may be applied or the sample rejected on this basis. Recrystallisation refers to the modification of shell aragonite to calcite, often involving an exchange with modern calcite and a subsequent altering of isotopic ratios. In investigating the extent and effect of these types of sample contamination, the radiocarbon laboratory is faced with two major problems.