358
R. Cecchi· G. Ghermandi
different oxidation states. Adequate sample preparation procedures may be required
in environmental PIXE applications, since solid samples have to be exposed to the
proton beam.
The environmental studies also take a great advantage from the development of
another variation of PIXE, called micro-PIXE, that in principle is analogous to the electron microprobe or Scanning Electron Microscope-Energy Dispersive X-ray Analyser
(SEM-EDAX), where a proton accelerator replaces the electron gun. The proton beam
can be scanned across the surface of the specimen, giving an elemental mapping of
the tested area. The micro-PIXE detection limits are better by a factor of about 100
than the electron microprobe ones, and result of the order of 10 ppm of the exposed
target: the small size of the irradiated area (a few of square micrometers) allows detection of absolute quantities of 10- 15 _10- 16 g.
The widest used application of PIXE analysis in the marine ecosystem consists of
the determination of trace elements of interest in the atmospheric aerosol. In fact the
membranes used in air samplers may be very suitable for immediate PIXE analysis,
without further handling. In addition, micro-PIXE may be a very powerful tool in the
investigation of major and trace elements in aerosol particles: the micro-PIXE maps
of a portion of an aerosol deposit on a thin backing allows one to determine the element associations and their relative abundances, providing an immediate characterization of the studied aerosol.
Marine aerosol is strongly involved in an exchange of substances between sea and
atmosphere, and further transfer to coastal areas. It is also well known that under
favourable conditions, the finest fraction of aerosols may be transported far from the
source, to remote areas: long-range transport of pollutants may follow.
Water sample analyses are more problematic. The seawater matrix may determine
problems in the treatement of samples that contain several elements of interest at very
low concentrations. Techniques of preconcentration are therefore necessary. In addition, solid targets have to be prepared for PIXE analysis. The preconcentration procedure has to be chosen, taking account of the main features of the samples: in sea water
the concentration of the so-called major lighter elements are generally higher than
the heavier ones; therefore it is necessary to employ selective preconcentration.
Bottom sediment samples may be analysed as such, or reduced to slices (if possible),
powders or solutions. Destructive and non-destructive sample preparation procedures
may be applied for bulk analysis by PIXE.
Other procedures are especially suited to extract the non reticular fraction from
sediment samples. Even if satisfactorily applied, PIXE can not completely explain its
capabilities in this field, given the high matrix effects. Pore waters extracted from bottom sediment cores may be instead usefully analysed by PIXE.
As it will be described in what follows, when coupled with adequate sample preparation methodologies, PIXE technique becomes a very powerful tool suitable for a wide
investigation of a marine ecosystem.
19.2
The PIXE Technique and the Experimental Set-up
PIXE (Johansson and Campbell 1988; Johansson et al. 1995) is an analytical technique
that uses the spectrometry of the characteristic X-rays emitted by target atoms ion-
R. Cecchi· G. Ghermandi
different oxidation states. Adequate sample preparation procedures may be required
in environmental PIXE applications, since solid samples have to be exposed to the
proton beam.
The environmental studies also take a great advantage from the development of
another variation of PIXE, called micro-PIXE, that in principle is analogous to the electron microprobe or Scanning Electron Microscope-Energy Dispersive X-ray Analyser
(SEM-EDAX), where a proton accelerator replaces the electron gun. The proton beam
can be scanned across the surface of the specimen, giving an elemental mapping of
the tested area. The micro-PIXE detection limits are better by a factor of about 100
than the electron microprobe ones, and result of the order of 10 ppm of the exposed
target: the small size of the irradiated area (a few of square micrometers) allows detection of absolute quantities of 10- 15 _10- 16 g.
The widest used application of PIXE analysis in the marine ecosystem consists of
the determination of trace elements of interest in the atmospheric aerosol. In fact the
membranes used in air samplers may be very suitable for immediate PIXE analysis,
without further handling. In addition, micro-PIXE may be a very powerful tool in the
investigation of major and trace elements in aerosol particles: the micro-PIXE maps
of a portion of an aerosol deposit on a thin backing allows one to determine the element associations and their relative abundances, providing an immediate characterization of the studied aerosol.
Marine aerosol is strongly involved in an exchange of substances between sea and
atmosphere, and further transfer to coastal areas. It is also well known that under
favourable conditions, the finest fraction of aerosols may be transported far from the
source, to remote areas: long-range transport of pollutants may follow.
Water sample analyses are more problematic. The seawater matrix may determine
problems in the treatement of samples that contain several elements of interest at very
low concentrations. Techniques of preconcentration are therefore necessary. In addition, solid targets have to be prepared for PIXE analysis. The preconcentration procedure has to be chosen, taking account of the main features of the samples: in sea water
the concentration of the so-called major lighter elements are generally higher than
the heavier ones; therefore it is necessary to employ selective preconcentration.
Bottom sediment samples may be analysed as such, or reduced to slices (if possible),
powders or solutions. Destructive and non-destructive sample preparation procedures
may be applied for bulk analysis by PIXE.
Other procedures are especially suited to extract the non reticular fraction from
sediment samples. Even if satisfactorily applied, PIXE can not completely explain its
capabilities in this field, given the high matrix effects. Pore waters extracted from bottom sediment cores may be instead usefully analysed by PIXE.
As it will be described in what follows, when coupled with adequate sample preparation methodologies, PIXE technique becomes a very powerful tool suitable for a wide
investigation of a marine ecosystem.
19.2
The PIXE Technique and the Experimental Set-up
PIXE (Johansson and Campbell 1988; Johansson et al. 1995) is an analytical technique
that uses the spectrometry of the characteristic X-rays emitted by target atoms ion-