Geoscience Reference
In-Depth Information
M
1
M
2
M
3
Electromagnet
6000
V
Source slit
Faraday cups
(collectors)
R
=
10
11
Ω
Ion source
Voltmeters
Figure E.4
Mass spectrometer. A source emits ions that are accelerated by a high voltage and then separated
in a magnetic field according to their mass (
M
1
,
M
2
, etc.). Ion beams are collected in Faraday
cages, converted to voltages by a high-value resistor, and these voltages, which are proportional
to isotopic abundances, are analyzed by an array of voltmeters.
situ
analysis of isotopic compositions, in particular of oxygen in minerals and of argon
for geochronology, now allows the operator to select the domains to be analyzed so as
to avoid mineral rims and altered material.
2.
Thermal ionization
(TIMS) sources are based on the probability of ions rather than
neutral atoms being ejected from the surface of a sample placed on a filament - usually
rhenium, tantalum, or tungsten - and heated to a high temperature (1200-1800
◦
C) in a
vacuum. This method has dominated isotopic measurements of positive ions of Sr, Nd,
Pb, and Th, and negative ions of Os for decades. However, many elements, especially
those with high ionization potentials, cannot be analyzed in this way. In addition, the
isotopic compositions of elements such as lead that do not have at least two stable
isotopes cannot be corrected precisely for analytic mass bias.
3. The
inductively-coupled plasma
(ICP) sources have been described above. These most
successful instruments couple this type of source with a magnetic sector and an array of
multiple collectors (MC-ICP-MS). For many elements, this method, which emerged in
the mid 1990s, has met with considerable success for two main reasons: the capacity of
the plasma torch to ionize all elements, including those too refractory to be analyzed by
TIMS; and the possibility of correcting analytical bias precisely for all elements. The
outcome of this technique for radiogenic isotopes such as Hf, Nd, and Pb, as well as
the “new” stable isotopes (B, Fe, Cu, Zn) makes this method the successor to thermal
ionization.
4.
Secondary-ion sputtering
(SIMS), also known as the ion probe, uses a primary beam
of ions, typically negative oxygen ions or positive cesium ions, to spray the sample
surface and sputter the sample ions into the instrument. This very sensitive method is
well adapted to
in situ
isotope measurement and is particularly successful for
in situ
U-Pb dating of zircons (especially the famous SHRIMP of Canberra, Australia). It is
