Mass Spectrometry Inorganic Trace Analysis by Isotope Dilution Mass Spectrometry-New Frontiers

Isotope dilution mass spectrometry (IDMS) is used extensively at NBS in the certification of elemental concentrations in Standard Reference Materials. It is regarded at NBS as a "definitive method," that is, a method of proven high accuracy. Since the theme of this symposium is accuracy in trace element analysis, it is appropriate to review the role IDMS plays in accurate inorganic analysis. The thesis of this paper is that mass spectrometry is a dynamic technique, marked by continuous productive activity and change, and that new mass spectrometric methods and new ionization techniques promise to make IDMS more general, more available, and more cost effective. Furthermore, I argue that the adoption of this technique by analytical laboratories outside of reference laboratories would do much to broaden the accuracy base of the world's measurements. In IDMS the quantity of an element present in a material is determined from the change produced in the isotopic composition of the element when a known amount of stable isotope (called a spike) is added. Thus, the technique is applicable to all elements with more than one stable isotope, or greater than 60 elements in the periodic table. In practice this number is reduced to those elements readily handled and ionized in the source of a mass spectrometer. In addition, some mononuclidic elements can be determined using radioactive isotopic spikes. For instance, procedures have been developed in our laboratory for iodine and thorium us-