Radiocarbon dating is a technique for determining the age of materials based on their content of the radioisotope carbon- 14. Carbon- 14 decays to the isotope nitrogen-14 with a half-life (the time taken for half of the atoms to decay) of 5730 years. Measurement of the amount of radioactive carbon remaining in the material thus gives an estimate of its age. A mass spectrometer records and measures the types of isotope present in a material and their relative amounts, rather than counting the radioactive decay. This method involves much smaller samples than the usual radiocarbon method and for this reason is generally suitable for dating steel or cast-iron objects, except where fossil fuel (coal or coke) has been used in their production.