The LABEC (laboratory of nuclear techniques for the environment and the Cultural Heritage) hosts the new ECAC-CAIS unit Elemental Mass Calibration Centre (EMC2). The main goal of EMC2 is to expand the duties to ACTRIS atmospheric observatories, in particular providing: proficiency tests of individual laboratory analysis through the organization of inter-comparison and round-robin exercises; operation support for quality assurance and quality control for the measurement of mass concentration of particulate heavy metals and inorganic elements; hands-on training of operators and scientists to NF and ACTRIS users. EMC2 will work with the aim of harmonizing the analyses of the elemental composition of atmospheric aerosol samples, in order to make data from different observatories fully comparable. The analysis of the elemental mass gives only part of the information with regard to the chemical composition, as it has to be complemented by measurements for, e.g., important ionic species (such as ammonium, nitrate), for organic carbon and elemental or black carbon. Nevertheless, the analysis of the elemental mass gives important information on several markers that may help in disentangling the contributions of different atmospheric aerosol sources, e.g. through the application of positive matrix factorization (PMF) analysis. The implementation of EMC2 services has been achieved through an infrastructural upgrade of LABEC thanks to the financial support of PER-ACTRIS-IT—“Potenziamento della componente italiana della Infrastruttura di Ricerca Aerosol, Clouds and Trace Gases Research Infrastructure” through the PON Ricerca e Innovazione FESR 2014–2020 project of the Italian Ministry of University and Research.
LABEC is located in the scientific campus just outside Florence (Italy), and it is jointly managed by researchers of INFN (National Institute for Nuclear Physics) and of the Department of Physics and Astronomy of the University of Florence. LABEC is a high-qualified centre for the development of new technologies based on particle accelerators and ionizing radiations, and for their applications in environmental contexts and in heritage science. The laboratory is equipped with a “small” particle accelerator, namely a 3MV Tandetron produced by High Voltage Engineering Europe (see following picture). Without aiming at a comprehensive description of all the activities performed at LABEC, in this short contribution we aim at giving a short introduction of the laboratory in the field of the characterization of the elemental composition of aerosol samples collected on filters by Ion Beam Analysis (IBA) techniques (please visit the laboratory website for a wider look to LABEC, https://web.infn.it/labec/en).
IBA are a suite of analytical techniques that exploit the detection (and analysis) of radiation such as X-rays and gamma-rays, or charged particles, as emitted in the interactions obtained by the bombardment of the sample with an accelerated ion (usually proton) beam. Among IBA, the particle-induced X-ray emission (PIXE) technique has been widely used since its birth for the study of the aerosol composition. For a long period, it has been the dominating technique for its elemental analysis because up to 20 elements (from Na to Pb) including important anthropogenic elements (S, V, Ni, Cu, Zn, As and Pb) and all the crustal elements (Al, Si, K, Ca, Ti, Mn and Fe) can be detected in only few minutes. However, recently, other competitive techniques, such as those based on atomization by induced coupled plasma and detection by atomic emission spectroscopy (ICP-AES) or mass spectrometry (ICP-MS), have been developed. Furthermore traditional X-ray fluorescence (XRF) systems have been replaced by more efficient modern devices and synchrotron radiation XRF has started to be used for elemental analysis.
Nevertheless, PIXE technique can and indeed does continue to provide an invaluable contribution to the determination of the elemental composition of aerosols thanks to the following advantages:
- PIXE is a multi-elemental analysis: up to 20 elements (from Na to Pb) can be quantified in a single measurement
- Very short measuring time, about 60 s (to be compared with 1 h for typical ED-XRF analysis. ICP techniques analysis time is only a little longer, but ICP methods need long times for sample preparation). This can be a crucial aspect in the analysis of thousands or hundreds of samples collected in big sampling campaigns.
- PIXE gives data for major, minor and trace elements (PIXE is sensitive to concentrations down to μg/g)
- PIXE is highly quantitative (few %) and traceable
- It is a non-destructive technique, so further measurements with other complementary techniques can be carried out on the same samples
- No sample preparation or extraction is necessary, thus reducing the contamination from chemical reagents and possible loss of volatile elements in the sample. For instance, the sample mineralization by concentrated HF prevents the determination by ICP techniques of Si, one of the most relevant crustal markers.
A proper experimental setup is necessary to fully exploit the benefits of PIXE in the analysis of aerosol samples. Since 1989 at LABEC an external beam line has been fully dedicated to the analysis of aerosol samples. With the moving of the laboratory to the actual hangar and the installation of a new accelerator in 2004, a new line has been installed and continuously developed to be always at the state of the art (see following picture).
Without going into details, we would like to recall here that LABEC features an external beam set-up, this means that the accelerated particle beam is extracted in air through a thin window and the samples are placed in air at about 1 cm from it. The main advantages in working with an external beam set-up can be summarized as:
- direct analysis of filters having any size and shape (no need of sub-sampling);
- no charging effects, no need of sample pretreatment (i.e. conductive coating);
- no heating, thus drastically reducing the risk of selective loss of some more volatile aerosol components (like Cl, Br and ammonium) under beam irradiation during in-vacuum measurements;
- easy sample positioning;
- fast and efficient.
More information and references on PIXE and the LABEC set-up can be found in the document “Measurement Guidelines: Elemental analysis of atmospheric particulate matter samples with Ion Beam Analysis techniques”, available at this link https://www.actris-ecac.eu/pmc-elements.html)
In the following picture, PIXE spectra of a PM10 sample collected on Teflon in an industrial area taken by a two-detector system are reported as an example. Every element emits characteristic X-ray, i.e. X-rays with a known energy. The number of the detected X-rays is proportional to the number of flown particles (measured) and to the elemental sample concentration.
Still curious about PIXE, LABEC and EMC2? For any further questions please do not hesitate to contact us!
Further, please get informed on the Opportunity to access EMC2 services through RADIATE Transnational Access (TA). Access is granted free of charge to the users who submit positively evaluated proposals; all the useful information can be found here: https://www.actris-ecac.eu/blog/?emc2--opportunity-to-access-emc2-services-through-radiate-transnational-access--ta-