IRMS & ICPMS

Isotopes

A nuclide is characterized by a given number of protons (Z), number of neutrons (N) and an energy state. All nuclides being charactrised by a fixed Z belong to the same element. An element (Z=constant) comprises also nuclei with different N hence having different masses.

These nuclides are called isotopes and having the same chemical properties but different masses are naturally subject to light and measurable discriminations affecting their natural observed relative ratio.

Both stable and radioactive nuclides occur in nature as shown in the framework of the nuclide chart (Figure 1).

Figure 1: Nuclide Chart with nuclides coloured according to their radioactive decay mode. stability “Magic”numbers and N=Z line.

Black line dots identify stable nuclides, the last (i.e. heavy) element having a stable nuclide is Pb. Heavier elements do not have stable nuclides such as Th, U and Pu, they are called radioactive. Nuclides being far from the stability valley (i.e. the valley shaped by the stable nuclides) are also subject to radioactive decay.

The isotopic composition of the light elements H, C, N, O, and S is typically studied via gas source mass spectrometry (IRMS), for medium lived radionuclides including 14C, accelerator mass spectrometry (AMS) is replacing radiometric techniques to an increasing extent. For isotopic analysis of metals and metalloids, thermal ionization mass spectrometry (TIMS) and inductively coupled plasma mass spectrometry (ICP-MS) are the methods of choice. Noble gasses isotope ratios are measure by Static Vacuum Mass Spectrometers (SVMS).

Conventionally i

Isotope Ratio Mass Spectrometry

Isotope Ratio Mass Spectrometry is a branch of the widely diffused technique serving different kinds of analytical sciences.

Mass Spectrometry history has seen a calendar filled of different important events such as Nobel Prizes, among them it is important citing (adapted from a presentation of Lionnel Mounier):

  • Thompson received the Nobel Prize for his work on charged particles in 1906;
  • Aston received the Nobel Prize for the discovery of 212 of the naturally occurring isotopes using three generations of mass spectrographs in 1922;
  • Urey developed the theory of isotope geochemistry during the 30’s and received the Nobel Prize for the discovery of Deuterium in 1934;
  • Nier developed the first IRMS, a double-focusing double inlet MS during the 40’s.

The name mass spectrometry relies on the possibility of dispersion of charged molecules or elements based upon their mass/charge ratio. A mass spectrometer is basically based on a working block scheme (Figure 2):

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