Geoscience Reference
In-Depth Information
return to later, excites virtually all the elements. The large number of energy lev-
els per element makes it easier to select rays with no interference with the matrix
elements. The method is therefore relatively sensitive, but the interference problems
between elements are complex.
2.
Atomic absorption spectroscopy
(AAS) relies on the opposite effect. Monochromatic
light sources illuminate a vapor produced by heating the solutions in a graphite oven so
as to excite electrons to higher energy levels. Excitation is particularly effective when
the incident radiation energy corresponds to the transition energy (resonance). For a
given element, the relative absorption energies of the sample and the standards are used
to determine concentrations.
3.
X-ray fluorescence spectroscopy
(XRF) relies on the exposure of the sample powder to
X-rays. The sample re-emits X-rays of lower energy by fluorescence and their wave-
lengths are analyzed. Each element appears as a peak whose intensity can be calibrated
against standards. Measurements are precise, but below 100 ppm the method is not
sensitive enough to produce good results.
4.
Inductively-coupled plasma mass spectrometry
(ICP-MS) relies on ionizing the solu-
tions to be analyzed by inductive coupling of the solution nebulized in a stream of argon
within a plasma torch. The ICP acronym, standing for inductively-coupled plasma, is a
buzzword that the to-be geochemist must remember. Argon is chosen for its high ion-
ization energy: once ionized as Ar
+
by induction in a coil, it captures the least firmly
bonded electrons of the other ions vaporized in the plasma. The different masses are
separated in the ion beam by a simple, lightweight mass spectrometer known as a
quadrupole: the trajectory of ions injected between four bars with crossed potential
is disturbed by radio frequencies so that only the ions of a specific mass cross the
device and are counted by the detector. This type of device has no magnets and so
allows very fast scanning across the mass range. Masses close to those of Ar, ArO, ArN
(40-56) or to the atmospheric gases (N, O, C) cannot be measured, as the detector is
saturated by the gas stream. For other masses, the respective heights of the signals are
compared for the sample and the standard solutions. This method is both very sensi-
tive (detection level of less than 10
−
11
g
10 pg), and very precise (1-2%). Even so,
it yields only poor isotopic ratios (at best 2%), as the tops of the mass peaks are not
flat and the counting statistics marginal (see below). As an alternative to dissolution, a
spot of the rock or mineral sample may be illuminated with a laser beam, preferably
operated in the ultraviolet wavelength range to minimize elemental and isotopic biases
due to partial evaporation, and the ensuing fine dust and vapor sprayed into the argon
stream. The remainder of the protocol (ionization in the plasma torch and mass analy-
sis) remains unchanged. This method, known as laser ablation, allows
in situ
analysis
of concentrations with a beam size of less than 30 micrometers and a remarkably low
detection level.
=
Isotope dilution
is a complex, but accurate, reference method operating on a different prin-
ciple; the equations will not be set out in detail here. The element in question must have
more than one isotope, which, in particular, excludes phosphorus, aluminum, manganese,
cobalt, etc. To cut a long story short, let us use another animal analogy and take as our job
