R. Cecchi· G. Ghermandi
proton beam is larger than the deposit, it allows the integration of the whole specimen surface: the diameter of the proton beam impinging on the target is generally
lower than 15 mm, while the commercially available air samplers collect a typically
47 mm diameter aerosol deposit on the backing.
An air sampler device may separate the aerosol in two or more size classes
(Johansson et al. 1995). The dichotomous aerosol samplers ("Gent" stacked filter unit,
IMPROVE dichotomous) separate by filtration coarse mode (2.0 flm < particle diameter < 10 flm) and fine mode fraction «2 flm).
The cascade impactors provide the collection of aerosol particles by impaction on
different stages. Some of these devices are especially suited for PIXE: the model 1-1
PIXE International Cascade Impactor (PCI), with seven impaction stages and a backup filter stage, and the Small Deposit area Impactor (SDI). The SDI (Maenhaut et al.
1996) is a 12-stage, multinozzle device, with a deposit area with diameter less than 8 mm.
The air flow rate is III min -1. The experimental aerodynamic cut diameters for the 12stages SDI range from 8.5 to 0.045 flm, with 7 values under 1 flm.
Other impactors frequently coupled with PIXE are the rotating version of the Micro Orifice Uniform Deposit Impactor (MOUDI) and the DRUM sampler, that collect
the aerosol particles on each stage along a rotating ring. The resulting specimen is a
strip that, to optimize the PIXE analysis, should be mounted on a movable frame that
adequately steps, on command, for proton irradiation of each increment.
PCI and SDI employ a 1.5 mm KIMFOL polycarbonate film as collection substrate,
coated with vaseline (higher diameter stages) or paraffin (smaller diameter stages),
or apiezon L in order to reduce particle bounce-off effects. The targets from all the
SDI impaction stages allow the detection of 22 elements (Maenhaut et al. 1996), from
Na to Pb (experimental conditions: 2.4 MeV proton beam, 150 nA beam current, 100 flC
collected charge, funny filter absorber).
The quantitative PIXE analysis of aerosol samples are generally performed by comparison with external standard specimens (very thin, certified layers of elements evaporated onto suitable membrane). The INFN Laboratory at Legnaro, Padova (Italy) performs standard target by vaporization of elements onto polycarbonate (PC) backings.
The PIXE determination of elements suspended and/or dissolved in fresh and sea
water involves greater difficulties in preparing targets, given the very low concentrations of several heavier elements of interest in presence of major lighter elements, and
the need of solid targets for PIXE analysis. Different problems arise with ground and
pore water. Suitable selective techniques of preconcentration are therefore necessary,
also reducing the matrix effects.
Among the numerous treatments that use chelants to extract trace metals, a methodology has been optimized which performs the extraction of elements as carbamates, coprecipitated with a collector element on thin membrane filters, also involving
minimal sample handling. The carbamate chelant complexes the trace elements in their
more stable state, as they generally are in natural waters, and in their reduced form as
ions, as they are carried by sample acidification (at the collection or during the pretreatment). The sample acidification also breaks chemical bonds of the majority of
organic compounds. Obviously this procedure is not recommended for aqueous liquid waste samples, where other complexing agents like ethylenediaminetetraacetic acid
(EDTA) may be discharged in significant concentrations, resulting in possible reduction of carbamate extraction efficiency.
proton beam is larger than the deposit, it allows the integration of the whole specimen surface: the diameter of the proton beam impinging on the target is generally
lower than 15 mm, while the commercially available air samplers collect a typically
47 mm diameter aerosol deposit on the backing.
An air sampler device may separate the aerosol in two or more size classes
(Johansson et al. 1995). The dichotomous aerosol samplers ("Gent" stacked filter unit,
IMPROVE dichotomous) separate by filtration coarse mode (2.0 flm < particle diameter < 10 flm) and fine mode fraction «2 flm).
The cascade impactors provide the collection of aerosol particles by impaction on
different stages. Some of these devices are especially suited for PIXE: the model 1-1
PIXE International Cascade Impactor (PCI), with seven impaction stages and a backup filter stage, and the Small Deposit area Impactor (SDI). The SDI (Maenhaut et al.
1996) is a 12-stage, multinozzle device, with a deposit area with diameter less than 8 mm.
The air flow rate is III min -1. The experimental aerodynamic cut diameters for the 12stages SDI range from 8.5 to 0.045 flm, with 7 values under 1 flm.
Other impactors frequently coupled with PIXE are the rotating version of the Micro Orifice Uniform Deposit Impactor (MOUDI) and the DRUM sampler, that collect
the aerosol particles on each stage along a rotating ring. The resulting specimen is a
strip that, to optimize the PIXE analysis, should be mounted on a movable frame that
adequately steps, on command, for proton irradiation of each increment.
PCI and SDI employ a 1.5 mm KIMFOL polycarbonate film as collection substrate,
coated with vaseline (higher diameter stages) or paraffin (smaller diameter stages),
or apiezon L in order to reduce particle bounce-off effects. The targets from all the
SDI impaction stages allow the detection of 22 elements (Maenhaut et al. 1996), from
Na to Pb (experimental conditions: 2.4 MeV proton beam, 150 nA beam current, 100 flC
collected charge, funny filter absorber).
The quantitative PIXE analysis of aerosol samples are generally performed by comparison with external standard specimens (very thin, certified layers of elements evaporated onto suitable membrane). The INFN Laboratory at Legnaro, Padova (Italy) performs standard target by vaporization of elements onto polycarbonate (PC) backings.
The PIXE determination of elements suspended and/or dissolved in fresh and sea
water involves greater difficulties in preparing targets, given the very low concentrations of several heavier elements of interest in presence of major lighter elements, and
the need of solid targets for PIXE analysis. Different problems arise with ground and
pore water. Suitable selective techniques of preconcentration are therefore necessary,
also reducing the matrix effects.
Among the numerous treatments that use chelants to extract trace metals, a methodology has been optimized which performs the extraction of elements as carbamates, coprecipitated with a collector element on thin membrane filters, also involving
minimal sample handling. The carbamate chelant complexes the trace elements in their
more stable state, as they generally are in natural waters, and in their reduced form as
ions, as they are carried by sample acidification (at the collection or during the pretreatment). The sample acidification also breaks chemical bonds of the majority of
organic compounds. Obviously this procedure is not recommended for aqueous liquid waste samples, where other complexing agents like ethylenediaminetetraacetic acid
(EDTA) may be discharged in significant concentrations, resulting in possible reduction of carbamate extraction efficiency.