Introduction
In Summer 2000, the EU commission approved drafts of the guidelines for waste electrical
and electronic equipment (WEEE) [1] and the guidelines for the restriction of the
use of certain hazardous substances in electrical and electronic equipment (RoHS)
[2]. These were subsequently presented to the European Parliament and the Member
States for discussion and decision-making. Meantime, the German Mediation Committee
between the Houses of Parliament accepted the papers. On 13 February 2003, the WEEE
and RoHS guidelines were ratified, and finally in January 2005, they have been adapted
into the national legislature.
The guidelines establish that consumers can return waste electrical
and electronic equipment to the manufacturers free of charge. Manufacturers and
importers will, at the end of a transition period, finance the treatment, reuse
and environmentally safe disposal of waste equipment. For waste equipment already
purchased before adoption of the guidelines, all manufacturers will share
responsibility. Bilateral agreements can be made for capital goods.
The minimum weight for collection is four kilograms per inhabitant
per year. For reuse and recycling the minimum weight applies. 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. Exceptions are provided for certain applications.
System configurations for determination of hazardous substances
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 0.1 mg/L, or using the digital graphite furnace for
electrothermal atomisation even up to 0.1 m g/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).
Energy-dispersive X-ray fluorescence spectrometry
The efficiency of X-ray fluorescence spectrometry as a fast screening
method for RoHS related samples 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 stabilizer. 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.
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. These standards have been used for the 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. The quantitative analysis results in a concentration of 76.5 mg/kg
(ppm) cadmium. The result is therefore within the certified tolerance – without
further sample preparation and after only 300 seconds of measurement time!
Energy-dispersive X-ray fluorescence spectrometry using the Shimadzu
EDX-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.
Atomic absorption spectrometry
For the quantitative analysis of heavy metals such as lead, cadmium,
mercury and chromium according to the RoHS directive, atomic absorption spectrometry
(AAS) is the method of choice. AAS is a relative measuring method for quantitation
of element concentrations down to the ppt level of liquid or solid samples, using
the relationship between concentration and absorbance according to Lambert-Beer's
law. The measurement of unknown samples is performed against a calibration curve
of aqueous standard samples. 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]. Since the
deuterium lamp method is not able to compensate for these interferences, the high
speed self reversal method has been selected here as a universal technique covering
the entire wavelength range from 190 nm to 900 nm in flame and furnace atomization
and compensating interferences for both, direct and indirect line overlapping. All
determinations have been performed using the Shimadzu atomic absorption spectrometer
AA-6800 which is equipped with both deuterium lamp and high speed self reversal
background compensation methods as a standard configuration, enabling proper compensation
of all interferences.
For electrothermal atomization the GFA-EX7 high sensitivity graphite
furnace with digital control has been used in all cases [5]. In fully automatic
measurement sequences the calibration curves have been prepared from multi-element
stock standard solutions using the autosampler ASC-6100 in combination with an ASK-6100
autodiluter. The experimental results have been obtained from standard solutions,
diluted samples and certified reference material. 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 into solution.
The recommended sample preparation for polymers is a dry ashing method or a microwave
acid digestion using nitric acid with hydrogen peroxide and hydrofluoric acid.
UV-VIS spectrometry
X-ray fluorescence and atomic absorption spectrometry allow the determination of
total chromium concentration of a sample with or without sample preparation. Since
the RoHS directive requires determination of hexavalent chromium, the photometric
method using 1.5-diphenylcarbazide is used for this purpose. The method is suitable
for the determination of hexavalent chromium which is used as a coating to protect
metallic surfaces of electric and electronic equipment against corrosion. It is
also used as a coating for screws, washers and fittings.
In the first stage the sample material has been eluated during a defined time in
the reaction vessel, after which the blank value is measured and finally the reagent
is added to the sample solution. The hexavalent chromium oxidates 1.5-diphenylcarbazide
to 1.5-diphenylcarbazone, which forms a red-violet colored complex. The absorption
of the color complex measured at a wavelength of 540 nm is directly related to the
concentration of hexavalent chromium.
The determination of PBB and PBDE using FTIR and GC/MS
The RoHS directive regulates the restriction of the use of brominated
flame retardants in electrical and electronic equipment as of July 2006. 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 largely being used 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 content will exceed the RoHS threshold value of 1,000 ppm.
In order to comply with the requirements of the RoHS directive, the
total bromine content of a sample is first determined. If this exceeds 5 % after
the preliminary examination using the EDX systems, infrared spectroscopy is recommended
as this will enable identification of compounds.
The determination of PBB and PBDE using FTIR and GC/MS
This simple and non-destructive method quickly leads 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 distinguish 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 can
be implemented for separation and identification.
This system consisting of GCMS-QP2010 and the pyrolyzer Py-2020iD is an ideal configuration
for a high sensitivity determination of flame retardants according to RoHS.
