Abstract: The field of elemental speciation has grown over the years, although the focus has shifted considerably throughout this time period-from developing hyphenated techniques (e.g. GC-ICP-MS, HPLC-ICP-MS, SFC-ICP-MS) to today’s challenge o identification of new element species using molecular mass spectrometry (e.g. electrospray ionization mass spectrometry (ES-MS)) and Speciated Isotope Mass Spectrometry (SIDMS). The fundamental theory of SIDMS in environmental systems is established and documented. In nature, an element may exist in different chemical forms (species) that are characterized by oxidation state and/or molecular structure. The toxicity and bioavailability of some elements is often directly related to the chemical form (i.e. species), for example trivalent Chromiun (Cr III) is essential for glucose metabolism in animals whereas hexavalent Chromium (Cr VI) is carcinogenic. Many other elements, such as mercury, arsenic, tin, lead and selenium, display similar “inter-species” toxicit differences. The different molecular forms of arsenic can in fact exercise different toxicity and it has long been recognized that not all arsenic compounds are toxic. Hence, it became necessary to identify the molecular form of arsenic in food and drink (i.e. the speciation of arsenic). Obviously, it is of concern to have available sensitive and accurate analytical method for qualitative and quantitative elemental speciation in any given sample.