In Summer 2000, the EU commission approved drafts of the guidelines for waste electrical and electronic equipment (WEEE) and the guidelines for the restriction of the use of certain hazardous substances in electrical and electronic equipment (RoHS).
These were subsequently presented to the European Parliament and the Member States for discussion and decision-making. Recently, these guidelines are in the focus of public interest, since their implementation into national law in most European member states has been delayed. In Germany the Electrical and Electronic Equipment Act (ElektroG) was introduced in March 2005, and public collection of electrical and electronic waste has started since August 2005.
The guidelines establish that users can return waste electrical and electronic equipment to the manufacturers free of charge. This regulation applies to electrical household equipment, electrical tools, consumer electronics, IT and telecommunication equipment, lamps and lights, toys, medical equipment, monitoring and control instruments as well as slot or vending machines. As of July 2006 lead, mercury, cadmium, chromium (VI), polybrominated biphenyls (PBB) and polybrominated diphenyl ethers (PBDE) are prohibited. For certain applications there are exceptions.
In order to enforce the substances ban and the limitation or substitution of hazardous substances such as mercury, cadmium and lead, elemental analysis is obviously the most important control measure for monitoring limiting values. This requires precise analytical systems such as X-ray fluorescence, ICP and atomic absorption spectrometers. These instruments are able to detect trace concentrations of hazardous substances – for example cadmium, using an atomic absorption spectrometer in the flame atomisation mode up to 0.1mg/L, or using the digital graphite furnace for electrothermal atomisation even up to 0.1µg/L.
For the determination of hexavalent chromium,
UV-VIS spectrometry is the method of choice and can be carried out quickly and easily using a routine spectrophotometer such as UVmini-1240. Polybrominated biphenyls as well as polybrominated diphenyl ethers are analysed using FTIR spectrometers such as the
IRPrestige-21 or in the lower concentration range with GCMS systems (QP2010).
The efficiency of X-ray fluorescence spectrometry as a fast screening method is demonstrated using the analysis of cadmium in Sicolen following the directive for restriction of the use of hazardous substances in electrical and electronics equipment. Red, orange and also green polymers can contain organic cadmium compounds as pigments or stabiliser. In particular, ‘older’ materials can include cadmium concentrations up to the percent range. Cadmium and other hazardous substances according to RoHS, as well as all elements from 6C/11Na to 92U can be determined quantitatively using energy-dispersive X-ray fluorescence spectrometers such as Shimadzu’s EDX series (EDX-700HS/ -800HS/ -900HS) in a fast and reliable way, often without any sample preparation. For the determination of heavy metals in plastic components such as cases or cable insulations down to the ppm range the samples are positioned directly in the large sample compartment as shown in Fig.1.
The following experimental work on the investigation of cadmium-containing polymers demonstrates the efficiency of the EDX technique as a screening method according to the RoHS guidelines. Cadmium as a toxic heavy metal shows the most intensive fluorescence signal at Cd([Kr]4d105s2):Ka=23.106KeV. Figure2 shows a typical signal profile of the Ka-line of cadmium. The
X-axis represents characteristic energy in KeV (kilo electron volts) and the Y-axis, intensity of the signal in cps/µA (counts per second per microampere).
An ideal sample for EDX measurement is flat, has a smooth surface, is relatively thick (>3mm) and is larger than the beam diameter. The beam diameter can be reduced in four steps by the use of collimators from 10mm to 0.3mm, which significantly improves the analysis results of small samples. The use of energy-dispersive X-ray fluorescence spectrometers is suitable also for the analysis of thin, curved or small samples (<3mm), unlike wavelength dispersive systems. In these cases, the background intensity of the X-ray tube (Rhodium anode) and the fluorescence radiation of the sample itself are used to correct changes in the absolute intensity of the signals, caused by thickness or shape of the sample. This internal background correction can be used comfortably via the EDX software. For experimental work, cadmium reference standard material has been used which has been prepared and certified by the Institute of Reference Materials and Measurements (IRMM), Geel, Belgium (Table1).
These standards have been used for the cadmium calibration (Fig.3) showing a very good linearity in the concentration range from 40.9mg/kg up to 407mg/kg. All measurements have been performed using a primary molybdenum filter (standard), 10mm collimator and
300 seconds measurement time. In order to evaluate the calibration curve, another certified cadmium standard has been analysed using the same method.
Sicolen orange (ref no 28/16494) containing 75.9±2.1mg/kg cadmium in Sicolen has been measured in the same way as the standards. The quantitative analysis results in a concentration of 76.5mg/kg (ppm) cadmium. The result is therefore within the certified tolerance.
Energy-dispersive X-ray fluorescence spectrometry using the ShimadzuEDX-700HS is a fast and non-destructive method for quantitative determination of heavy metals in polymers. The experimental results of cadmium are also representative for other heavy elements such as lead, mercury, chromium and bromine. Depending on the system configuration, even measurement of the complete element range from 6C/11Na to 92U is possible.
Alternatively the quantitative analysis of heavy metals like lead, cadmium, mercury and chromium according to the RoHS directive, can be done using an atomic absorption spectrometer as well. The AA-6800 is equipped with both deuteriumlamp- and high speed self reversal background compensation methods as a standard configuration so in fact all interferences will be properly compensated.
In contrast to the EDX screening method which does not require any sample preparation, the determination of heavy metals using AAS needs the digestion procedure to bring all samples in solution. The recommended sample preparation for polymers is a dry ashing method or a microwave acid digestion using nitric acid with hydrogenperoxide and hydrofluoric acid.
Uwe Oppermann and Dr Johannes Hesper are with Shimadzu Deutschland GmbH, Duisburg, Germany. www.shimadzu.de"