Platform for interdisciplinary isotopic research by means of multi-collector inductively coupled plasma-mass spectrometry (MC-ICP-MS)

Isotopic analysis, or the identification of the isotopic signature of chemical compounds, has a widespread applicability in the natural sciences, i.e. biological, earth and environmental sciences. Often, the main focus is on the analysis of light elements, e.g. H, C, N, O and S, as it is well-known that those light elements show natural variation in their isotopic composition as a result of fractionation effects. For heavier elements, it was for a long time believed that, except for those elements containing (a) radiogenic nuclide(s), such as strontium (Sr), neodymium (Nd) or lead (Pb), the isotopic composition is invariant. Due to the improved precision resulting from technical advances in mass spectrometric instrumentation, this view had to be recently adapted. It is now generally accepted that all elements with ≥ 2 isotopes can undergo natural isotope fractionation, i.e. as a result of their difference in mass, the isotopes of an element do not necessarily take part with exactly the same efficiency to physical processes and (bio)chemical reactions. Both kinetic and thermodynamic effects have been documented. Kinetic effects dictate that the lighter of any two isotopes reacts faster than the heavier one, while thermodynamic effects refer to the preference of the heavier of any two isotopes for the “stronger bonding environment”. This can lead to very small variations in the isotopic composition of those elements, which can only be quantified by means of highly precise isotopic analyses.
While for a long time, thermal ionization mass spectrometry (TIMS) has been the technique of choice for the measurement of those tiny differences in the isotopic signature, the introduction of multi-collector inductively coupled plasma – mass spectrometry (MC-ICP-MS) in the 1990s was a major breakthrough in isotopic analysis. MC-ICP-MS combines a highly efficient ion source, operating at atmospheric pressure, with simultaneous monitoring of the intensities of the ion beams of interest. As a result, also for elements with a higher ionization energy (IE), such as Fe (IE = 7.9 eV), an isotope ratio precision, down to 0.002% relative standard deviation (RSD) can be achieved under ideal circumstances.
In geo- and cosmochemistry, MC-ICP-MS was welcomed with open arms and many “non-traditional” isotope systems were exploited, which definitely broadens the application field.
Recently, the capabilities of studying natural variation in the isotopic composition of essential mineral elements in biomedicine have been discovered and such approaches are now being developed and their merits and limitations assessed.