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Protecting our environment is the most important strategy for maintaining a high quality of life for current and future generations.
Vital in the determination of hazardous substances
The environmental policy of the European community demands advanced standards which are flexible enough to accommodate local circumstances, and are continuously updated to meet current requirements. The goal is to avoid contamination of air, water and soil in the interests of health and safety of the European population and to define limits of maximum allowable concentrations of hazardous substances.
Since 1 January 2003 the new European drinking water regulation (TVO 2001) has been in effect in Germany. This regulation protects human health from the adverse effects of consumption of polluted water. The law applies to all types of water used for drinking, cooking, food and beverage preparation as well as for personal hygiene and cleaning of objects which come into contact with foods. When used for these purposes, water may not contain any chemical compounds in concentrations that can lead to human health hazards. For many classes of compounds, especially heavy metals, well-defined maximum contaminant levels exist which may not be exceeded and therefore require regular monitoring.
Atomic absorption spectroscopy (AAS) is an important technique for the quantitative determination of element concentrations. AAS in the flame and graphite furnace mode enables accurate determination of extremely low concentrations of metals down to the ultra-trace range. Fully automatic multi-element analysis of sample series for up to 20 elements, as well as the optimisation of system parameters is possible using atomic absorption spectrometers.
The use of sophisticated background compensation techniques guarantees high quality of the analytical results even for complex sample matrices and spectral interferences and enables secure monitoring during routine analyses.
In addition to the analytical procedures according to drinking water regulations (TVO), the guidelines for waste electrical and electronic equipment (WEEE) [1] and the restriction of the use of certain hazardous substances in electrical and electronic equipment (RoHS) [2] are lately in the focus of public interest. These guidelines have already been implemented in national law in most of the European member states. In Germany the electrical and electronic equipment act (ElektroG) was introduced 24 March 2005, and public collection of electrical and electronic waste began 13th August 2005.
1 July 2006 is an important deadline for the electro and electronics industries. As of this date, use of hazardous substances such as lead, cadmium, mercury, hexavalent chromium or polybrominated biphenyls and diphenyl ethers is banned in electro and electronic equipment. This restriction, discussed since 1991, has become more and more important every year when considering the volume of electronic waste releasing toxic substances which are accumulated in the environment, the food chain and human beings. In order to enforce the substances ban and the limitation or substitution of hazardous components, 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 compounds – for example cadmium, using an atomic absorption spectrometer in the flame atomisation mode up to 0.1 mg/ L, or using the digital graphite furnace technique for electrothermal atomisation even up to 0.1 ?g/L. For the determination of hexavalent chromium, UV-VIS spectrometry is the method of choice and can be carried out quickly and easily in routine analyses.
Polybrominated biphenyls as well as polybrominated diphenyl ethers are analysed using FTIR spectrometers. The development of routine analytical methods for accurate determination of all hazardous components will be explained, and the advantages of flexible system configurations will be discussed.
Determination of heavy metals in drinking water using AAS and UV-VIS spectrometry
Appendix 2 Part II of the drinking water regulation ratified on 1 January 2003 includes under toxicological aspects the important metals lead, copper and nickel. The presence of these metals needs to be monitored frequently. These parameters are much stricter now when compared with the previous version of the drinking water regulation. The maximum contaminant level for lead has been decreased in the new EC guideline from 0.05 to 0.01 mg/L. This is due to the known high toxicity of lead, especially for children and adolescents as well as for pregnant women. The assessment of copper in drinking water is also stricter under the new regulation. The previous maximum contaminant level of 3 mg/L was reduced to 2 mg/L. Even though copper is a widely distributed metal and is an essential trace element for humans, it can, after longer exposure, lead to severe health problems in infants and small children even at concentration levels of 10 mg/L.
Lowering the maximum contaminant level for nickel in the new drinking water regulation from 0.05 mg/L to 0.02 mg/L should prevent nickel pollution levels of drinking water that could lead to further increase of the already common nickel allergies in humans. For the quantitative determination of the above metals, the AA-6300 atomic absorption spectrometer was used, allowing the atomisation of these elements in the flame mode (copper) or the electrothermal atomisation in combination with the GFA-EX7i graphite furnace (lead and nickel). Table 1 shows the instrumental parameters used.
For the elements copper and nickel, background compensation was carried out using the deuterium technique. For the determination of lead, the high speed self reversal technique was used to compensate for spectral interferences. Calibration of the elements was carried out in the linear range for the element lead in concentrations of 0.005 up to 0.02 mg/L.
In addition to atomic absorption spectrometry, photometric methods also have an important analytical role in the quantification of toxic metals in drinking water. They are particularly useful due to the high precision, simple handling and short time needed for analysis. At the same time, a high level of operational safety is provided for a comparatively low investment.
Utilisation of such systems in routine analysis is often carried out in combination with commercially available cuvette tests, as is the case for chromium. Although chromium appears in various oxidation states, the hexavalent state is of specific importance as it is thought to be extremely toxic and potentially carcinogenic. Determination of hexavalent chromium is a typical application in the field of drinking water analysis and is normally analysed in a concentration range of 0.01 to 0.25 mg/L.
Screening of hazardous substances using energy-dispersive X-ray fluorescence spectrometry
The efficiency of X-ray fluorescence spectrometry as a fast screening method is demonstrated using the analysis of cadmium in Sicolen® (coloring for plastic products) 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 stabilisers. 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 figure 1.
The 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] 4d10 5s2): K? = 23.106 KeV. Figure 2 shows a typical signal profile of the K?-line of cadmium. The X-axis represents characteristic energy in KeV (kilo electron volts) and the Y-axis, intensity of the signal in cps/ ?A (counts per second per microAmpere).
An ideal sample for EDX measurement is flat, has a smooth surface, is relatively thick (> 3 mm) and is larger than the beam diameter. The beam diameter can be reduced in four steps by the use of collimators from 10 mm to 0.3 mm, significantly improving 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 (< 3 mm), 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 [3].
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 (table 2 – above).
These standards have been used for cadmium calibration showing a very good linearity in the concentration range from 40.9 mg/kg up to 407 mg/kg. All measurements have been performed using a primary molybdenum filter (standard), 10 mm 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 ± 2.1 mg/kg cadmium in Sicolen® has been measured in the same way as the standards, resulting in a concentration of 76.5 mg/kg (ppm) cadmium, well within the certified tolerance. Energy-dispersive X-ray fluorescence spectrometry 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.
An alternative method for the quantitative analysis of heavy metals such as lead, cadmium, mercury and chromium according to the RoHS directive is atomic absorption spectrometry (AAS). The most important difference in comparison with X-ray fluorescence is the sample preparation, since all measurements are performed against a calibration curve of aqueous standard solutions. Unfortunately problems can emerge when the composition of standards and samples is different.
Problems related to background absorption are classified as spectral interferences. Interferences which can be solved by background compensation methods include molecular absorption, particulate caused scattering and absorption line overlapping. During determination of heavy metals according to RoHS guideline, the spectral interferences by direct and indirect absorption line overlapping are to be expected for cadmium, lead and chromium [4], which explains why a sophisticated system configuration has been used for all determinations.
The determination of cadmium, lead and chromium has been carried out in the calibration range from 0.1 mg/L to 5 mg/L using flame atomisation, and 0.1 ?g/L to 20 ?g/L using electrothermal atomisation. Mercury determination is possible using electrothermal atomisation for concentrations from 5 ?g/L to 100 ?g/L and down to 1 ?g/L using the cold vapour method. A total volume of 5 ?L palladium modifier solution with a Pd (NO3)2 concentration of 1000 mg/L has been added to the 20 ?L injection volume of standards and sample solutions during the cadmium and lead analysis.
This analytical procedure allows a precise and reliable determination of heavy metals such as cadmium, lead, mercury and chromium using atomic absorption spectrometry.
X-ray fluorescence and atomic absorption spectrometry allow the determination of total chromium concentrations, but the RoHS directive only bans hexavalent chromium. This requires the use of a photometric method, to measure the absorption of the red-violet chromium (VI) colour complex.
Identification of PBB and PBDE using FTIR spectrometry
According to RoHS, the following compounds are considered as hazardous: pentabrominated diphenyl ether (PentaBDE) and octabrominated diphenyl ether (OctaBDE). OctaBDE has been used in polymers such as ABS and PS. Currently, decaBDE is being used largely as a flame retardant in PS, PE, ABS and polyester. DecaBDE has not yet been included in the RoHS directive.
However, commercial decaBDE consists of a mixture of approximately 97 % to 98 % decaBDE and 0.3 % up to 3 % of other BDEs. Therefore, when a polymer contains 10 % decaBDE (containing 1 % contamination of other brominated BDEs), the PBDE concentration will exceed the RoHS threshold value of 1000 ppm.
In order to comply with the requirements of the RoHS directive, total bromine concentration of a sample is first determined using the fast screening method. If this exceeds 5 % after preliminary examination using the EDX systems, infrared spectroscopy is recommended, as this will enable the identification of compounds.
This simple and non destructive method leads quickly to useful results. Compound identification is possible as the flame-retardants were present up to now in polymers in concentrations of higher than 5 %. This level is still detectable in polymer mixtures using FTIR. However, concentrations approaching the detection limit must be measured using other analytical methods. In this case GC/MS is highly suitable as all brominated compounds can be separated and detected down to the trace level. GC/MS, on the other hand, is more time consuming with respect to sample preparation and data analysis.
In general, it is recommended to carry out an overall pre-screening via energy dispersive X-ray fluorescence (EDX). Using this analytical method the total concentration of elemental bromine in the sample is detected, although it is not possible to determine which compound actually contains bromine. When more than 5 % of total bromine is detected, FTIR can be used for further identification of bromine compounds. When less than 5 % bromine is detected, GC/MS analysis will be the right tool for separation and identification.
Fast and straightforward IR-analysis of polymers is possible since brominated biphenyls exhibit very characteristic infrared spectra. The polystyrene example exhibits three spectra: DBDPE, PS with DBDPE and pure PS. The range in the IR fingerprint, where DBDPE in PS identification is possible, is clearly discernible.
Infrared spectrometry can therefore be regarded as a fast and simple alternative solution to the preselection of polymers. Minimal sample pretreatment is necessary and fast results are obtained via predefined methods.
The analysis of polybrominated flame retardants in concentrations of less than 5 % down to the ppm range is carried out using GC/MS, which allows quantitation and identification of ultra low sample concentrations by comparison with standard substances according to their specific retention time and mass spectra.
Conclusion
The system configurations and application examples discussed have been used to demonstrate an actual overview on the state-of-the-art technology for determination of hazardous substances according to European regulations such as TVO, WEEE, and RoHS.
References
1. Guideline 2002/96/EC for Waste Electrical and Electronic Equipment (WEEE)
2. Guideline 2002/95/EC for the Restriction of Hazardous Substances in Electrical and Electronic Equipment (RoHS)
3. Hesper J, Oppermann U, (2005) GIT Labor-Fachzeitschrift 113-115 4. Oppermann U, Schram J, Felkel D, (2003) Spectrochim. Acta B 1567-1572
Published: 10th Mar 2006 in AWE International
