What is the AFP?
The Analytical Fingerprint (AFP) is a method that uses intrinsic information of minerals such as mineralogy and geochemistry to evaluate by the means of applied statistics whether or not the origin of tin, tungsten and tantalum ore as declared in accompanying documents can be verified. As such, AFP does not represent an alternative to everyday mineral traceability techniques (e.g., tagging) but seeks to complement existing Chain-of-custody systems in order to strengthen the integrity of the applied traceability scheme and thus the credibility of the whole certification system.
How does AFP work?
AFP compares a sample from a shipment in question to reference samples of the declared originstored in a database. This is done by analysing characteristic geochemical features preserved in mineral concentrates combined with applied statistics. As a result, AFP evaluates whether or not the origin given in the documents of a shipment in question is plausible.
AFP Method
The following key parameters are used to verify the documented origin of an ore concentrate:
- Geological age of ore minerals
- Geochemical composition (major and trace elements) of ore minerals
- Mineralogical composition of ore concentrates.
Sampling
AFP reference sampling of ore concentrates is done on-site by accredited samplers according to specific standards. The precise position of the sample location is determined by GPS and detailed site information is documented. Those concentrates provide a sufficient number of ore mineral grains for analysis, still reflecting the characteristic features of a deposit including deposit- specific gangue or accessory minerals. A single AFP sample ideally consists of 50-200g of ore concentrate material depending on the grain size. One or more samples need to be taken at each individual production site.
Sample preparation and analysis
A streamlined analytical protocol including the following steps has been established to obtain the analytical data necessary for AFP:
- Sample preparation
- An aliquot of the AFP sample is mounted in epoxy resin. After the resin is cured, a polished section is prepared by grinding and polishing until a uniformly flat, scratch and relief - free, highly reflective surface is obtained. This polished section is subsequently used for mineralogical and chemical analysis.
- Identification of grain mineralogy:
Scanning electron microscopy (SEM) combined with a software for automated mineralogy is used to determine the mineralogy of each individual grain on the surface of the polished section. 
Above: Image showing characteristic mineralogy of a given cassiterite- coltan concentrate obtained through SEM- coupled automated mineralogy analysis.
Major and trace element analysis
About 50 ore mineral grains previously identified by SEM are analysed by laser ablation inductively coupled plasma- mass – spectrometry (LA- ICP- MS) for about 50 elements to obtain their chemical compositions and an age estimate of the sample. The analytical results (geochemical, geochronological, and mineralogical data) of reference and control samples are stored in a constantly extended database
Data evaluation
AFP data of a sample in question are compared to reference sample data of the mine site as declared in the sample’s documentation. The similarity of these two data sets is examined and evaluated by statistical methods developed for the unique features of ore mineral concentrates.
Above: Exemplary cumulative frequency curves of MnO content of two concentrates from the same mine site and one with different origin; arrows
indicate the Kolmogorov-Smirnov distance (KS-D) used as a measure of similarity.
Organization of AFP
AFP has been developed by the Federal Institute for Geosciences and Natural Resources (BGR) and is funded by the German Ministry for Economic Cooperation and Development (BMZ) as part of the regional German cooperation framework with the International Conference on the Great Lakes Region (ICGLR). AFP is managed by the AFP Management Unit (AMU) within ICGLR’s Natural Resources Unit located at the ICGLR headquarter in Bujumbura. AMU hosts the reference sample database and evaluates the AFP data (supported by BGR). Three local sample preparation laboratories in Bukavu, Bujumbura, and Kigali together with the AFP analytical laboratory at the African Minerals and Geosciences Centre (AMGC) in Dar es Salaam provide the necessary data.
Above: Location of AFP entities in the Great Lakes Region. AFP management unit (AMU) is part of the Natural Resources/ Technical Unit of ICGLR, sample preparation laboratories in the related countries. The analytical laboratory is situated on the premises of AMGC.
The Analytical Fingerprint Laboratory
What is the Analytical Fingerprint laboratory?
The Analytical Fingerprint (AFP) Laboratory is an analytical laboratory funded by the German Federal Ministry for economic cooperation and development (BMZ) and was implemented by the German Institute for Geosciences and Natural Resources (BGR). In context with the Regional Certification Mechanism of the International Conference of the Great Lakes Region (ICGLR), the focus of the facility is to provide data for the Analytical Fingerprint and to analyze geological samples on a commercial basis. The laboratory is equipped with a scanning electron microscope (TIMA, Tescan Integrated Mineral Analyzer) and an inductively coupled plasma-mass spectrometer (ICP-MS, Agilent 7900) with a sample introduction system for solid samples (laser ablation unit, ESI NWR213).
Location
Above: Location of the African Minerals and Geosciences Centre (AMGC), Kunduchi Beach area.
Scanning electron microscopy (TIMA)
Above: TIMA automated particle anlaysis applying the high resolution dot mapping mode.
Analytical capabilities
- High resolution imaging (back scattered (BSE) and secondary electron (SE) imaging
- Fast and quantitative mineral analysis (applying EDX detectors)
- Bright particle searches for platinum group minerals and gold-silver ores
- Grain size distribution measurements (total amount of a particular mineral with a specified grain size)
- Liberation grade analysis (extent to which a mineral is surrounded/enclosed by another mineral)
Above: TIMA automated particle analysis applying the high resolution dot mapping mode.
Applications
- Quantitative mineral analysis of rocks, ores, concentrates, tailings, leach residues or smelter products
- Mineral separation efficiency tests
- Assessment of enclosed minerals to adjust the grinding fineness
- Porosity and homogeneity measurement of cement
Laser ablation inductively coupled plasma mass spectrometer (LA-ICP-MS)
Analytical capabilities
- Quantitative mineral analysis of rocks, ores, concentrates, tailings, leach residues or smelter products
- Mineral separation efficiency tests
- Assessment of enclosed minerals to adjust the grinding fineness
- Porosity and homogeneity measurement of cement
Right: Agilent 7900 inductively coupled mass spectrometer (ICP-MS) Left: ESI 213 nm laser ablation system (LA)
Both systems combined represent the LA-ICP-MS for direct analysis of solid materials without previous digestion.
Applications
- Trace element analysis of small mineral particles (>30 µm)
- REE analysis of solid samples (applied on fused beads after XRF analysis)
- Detection of secondary gemstone treatment
- Detection of element zonation in solid samples, e.g. building materials like cement or steel
Preparation Laboratory
For the preparation of solid samples prior to analysis and microscopy, a sample preparation facility was built. The equipment can be used to produce perfect polished specimen of high quality from various geological or even building materials. Polished sections or thin sections can be prepared for reflected and transmitted light microscopy or SEM and LA-ICP-MS applications.
Sample preparation services
- Mounting in epoxy resin; roundish samples with either 25 or 30 mm diameter (polished section). Subsequent grinding and polishing. Final polishing is performed with either 9 or 3 μm polycrystalline diamond suspension.
- Preparation of thin sections.
Above: Left: automatic grinding and polishing machine Buehler Ecomet 250 which allows six roundish or three rectangular samples to be prepared at the same time.
Right: thin section cutting and grinding machine Buehler Petrothin.