Chapter 19
PIXE Analysis for Trace Elements in
Marine Environments
R. Cecchi· G. Ghermandi
19.1
Introduction
During the past decades considerable attention has been paid to the geochemical cycles
of major and minor elements (and substances), and consequently to cycling models
among different physically well-defined parts of the earth (or "boxes"), whose number depends on the previous knowledge of the way in which elements of interest are
distributed about the earth's surface. For example, a global cycling model subdivides
the earth in four boxes, namely land, atmosphere, ocean and sediments. From one box
to another a substance is transferred by a transport path, that is, a directional property of the system depending on the physicochemical characteristics of the substance
itself; for the ocean the transport paths are with the atmosphere (gases, dust and aerosol), from the land (rivers) and to the bottom sediments (sedimentation).
From this general point of view, to describe the behaviour of an element we need a
set of differential equations (the mass balance for each box) whose number depends
on the different phases (in the ocean the aqueous phase, the mineral suspension, the
plankton), on the speciation and on the processes of removal or production in the
system. One must also take into account the various transport agents controlling inputs and outputs of the element.
A very important point in such a cycling model is the knowledge of the above mentioned input and output terms, involving experimental evaluation of the fluxes. For
diffusive processes it is important to measure concentration gradients; for advective
ones, one must be able to determine the mean concentration of the substance under
investigation. Consequently, in order to test a cycling model for the ocean in its various features, the crucial problem is the very high number of measurements involved.
A fast analytical technique, with multi-elemental capability, covering a large concentration range, requiring generally little or no sample preparation and able to measure
concentrations in different phases such as aerosols, waters, suspended inorganic particles and sediments, is obviously very useful.
PlXE (Proton Induced X-ray Emission) is an analytical method based upon X-ray
spectrometry. Its intrinsic detection limits are not very much below 1 ppm (in weight)
in respect to a given bombarded specimen. It offers its maximum sensitivity in the
two atomic number (Z) regions (20 < Z < 35 and 75 < Z < 85). Measurement errors are
in the order of 10%, depending mainly on the target preparation procedure and on
the slight variability of the proton flux. The PIXE technique allows fast (a few minutes), non destructive, highly sensitive simultaneous determination of a wide group
of elements (12 ~ Z ~ 85), without high variations of sensitivity among different elements. The total element concentration is measured by PIXE, not distinguishing among
PIXE Analysis for Trace Elements in
Marine Environments
R. Cecchi· G. Ghermandi
19.1
Introduction
During the past decades considerable attention has been paid to the geochemical cycles
of major and minor elements (and substances), and consequently to cycling models
among different physically well-defined parts of the earth (or "boxes"), whose number depends on the previous knowledge of the way in which elements of interest are
distributed about the earth's surface. For example, a global cycling model subdivides
the earth in four boxes, namely land, atmosphere, ocean and sediments. From one box
to another a substance is transferred by a transport path, that is, a directional property of the system depending on the physicochemical characteristics of the substance
itself; for the ocean the transport paths are with the atmosphere (gases, dust and aerosol), from the land (rivers) and to the bottom sediments (sedimentation).
From this general point of view, to describe the behaviour of an element we need a
set of differential equations (the mass balance for each box) whose number depends
on the different phases (in the ocean the aqueous phase, the mineral suspension, the
plankton), on the speciation and on the processes of removal or production in the
system. One must also take into account the various transport agents controlling inputs and outputs of the element.
A very important point in such a cycling model is the knowledge of the above mentioned input and output terms, involving experimental evaluation of the fluxes. For
diffusive processes it is important to measure concentration gradients; for advective
ones, one must be able to determine the mean concentration of the substance under
investigation. Consequently, in order to test a cycling model for the ocean in its various features, the crucial problem is the very high number of measurements involved.
A fast analytical technique, with multi-elemental capability, covering a large concentration range, requiring generally little or no sample preparation and able to measure
concentrations in different phases such as aerosols, waters, suspended inorganic particles and sediments, is obviously very useful.
PlXE (Proton Induced X-ray Emission) is an analytical method based upon X-ray
spectrometry. Its intrinsic detection limits are not very much below 1 ppm (in weight)
in respect to a given bombarded specimen. It offers its maximum sensitivity in the
two atomic number (Z) regions (20 < Z < 35 and 75 < Z < 85). Measurement errors are
in the order of 10%, depending mainly on the target preparation procedure and on
the slight variability of the proton flux. The PIXE technique allows fast (a few minutes), non destructive, highly sensitive simultaneous determination of a wide group
of elements (12 ~ Z ~ 85), without high variations of sensitivity among different elements. The total element concentration is measured by PIXE, not distinguishing among