Silicon in nature is typically bonded to oxygen, in a tetravalent oxidation state. The major forms of solid Si are silicate minerals and amorphous silica, whereas in aqueous solutions the dominant forms are orthosilicic acid and its dissociated species. There are three stable isotopes of Si, associated with the following mean natural abundances: Si– 92.23%, Si– 4.67%, and Si– 3.10%. The isotopic composition of Si is often formulated by the delta notation, as the following:
The reference material (standard) for defining the δSi of a sample is the National Bureau of Standards (NBS) 28 Sand Quartz, which has been certified and distributed by the National Institute of Standards and Technology (NIST), and is also named NIST RM 8546. Currently, there are four main analytical methods for the measurement of Si isotopes: Gas Source Isotope-Ratio Mass Spectrometry (GC-IRMS), Secondary Ion Mass Spectrometry (SIMS), Multi-Collector Inductively Coupled Plasma Mass Spectrometry (MC–IPC–MS), and Laser Ablation MC–ICP–MS.