Commission Decisionof 12 August 2002implementing Council Directive 96/23/EC concerning the performance of analytical methods and the interpretation of results(notified under document number C(2002) 3044)(Text with EEA relevance)(2002/657/EC)

2.4. U.K. CONFIRMATORY METHODS FOR ELEMENTS

Confirmatory analyses for chemical elements shall be based on the concept of unequivocal identification and accurate as well as precise quantification by means of physical-chemical properties unique to the chemical element at hand (e.g. element characteristic wavelength of emitted or absorbed radiation, atomic mass) at the level of interest.

The following methods or combinations of methods are considered suitable for the identification of chemical elements:

Table 7

Suitable confirmatory methods for chemical elements

Technique		Measured parameter
Differential pulse anodic stripping voltametry		Electric signal
Atomic absorption spectrometry
	Flame	Absorption wavelength
	Hydride generation	Absorption wavelength
	Cold vapour	Absorption wavelength
	Electrothermal atomisation (graphite furnace)	Absorption wavelength
Atomic emission spectrometry
	Inductively coupled plasma	Emission wavelength
Mass spectrometry
		Inductively coupled plasma	Mass-to-charge-ratio

2.4.1 Common performance criteria and other requirements for confirmatory methods U.K.

Reference or fortified material containing known amounts of analyte, at or near either the maximum permitted limit or the decision limit (non-compliant control sample) as well as compliant control materials and reagent blanks should preferably be carried through the entire procedure simultaneously with each batch of test samples analysed. The recommended order for injecting the extracts into the analytical instrument is as follows: reagent blank, compliant control sample, sample to be confirmed, compliant control sample and finally non-compliant control sample. Any variation from this shall be justified.

In general, most analytical techniques require complete digestion of the organic matrix to obtain solutions prior to determination of the analyte. This can be achieved by using microwave mineralisation procedures, which minimise the risk of loss and/or contamination of the analytes of interest. Decontaminated Teflon vessels of good quality shall be used. If other wet or dry digestion methods are resorted to, documented evidence shall be available to exclude potential loss or contamination phenomena. As an alternative to digestion, separation procedures (e.g. extraction) may under certain circumstances be chosen to separate analytes from matrix components and/or to concentrate analytes in order to introduce them into the analytical equipment.

As regards calibration, be it external or based on the standard addition method, care shall be taken not to exceed the working range established for the analysis. In the case of external calibration, it is mandatory that calibration standards are prepared in a solution that matches as closely as possible the composition of the sample solution. Background correction shall be also applied if required by specific analytical circumstances.

2.4.2 Additional performance criteria and other requirements for quantitative methods of analysis U.K.

2.4.2.1 Trueness of quantitative methods U.K.

In the case of repeated analyses of a certified reference material for elements, the deviation of the experimentally determined mean content from the certified value shall not lie outside the limit ± 10 %. When no such CRMs are available, it is acceptable that trueness of measurements is assessed through recovery of additions of known amounts of the element to the unknown samples. Attention is drawn to the fact that, unlike the analyte, the added element is not chemically bound in the real matrix and that therefore results obtained by this approach have lesser validity than those achieved through the use of CRMs. Recovery data are only acceptable when they are within ± 10 % of the target value.

2.4.2.2 Precision of quantitative methods U.K.

In the case of repeated analysis of a sample carried out under within-laboratory reproducibility conditions, the intra-laboratory coefficient of variation (CV) of the mean shall not exceed the following values:

Table 8

CVs for quantitative methods at a range of element mass fractions

Mass fraction	CV (%)
≥ 10 μg/kg to 100 μg/kg	20
> 100 μg/kg to 1 000 μg/kg	15
≥ 1 000 μg/kg	10

2.4.3 Specific requirements for differential pulse anodic stripping voltametry (DPASV) U.K.

Complete destruction of organic matter in samples prior to DPASV determinations is of the greatest importance. No broad signals due to the presence of organic materials shall be seen in the voltamograms. Inorganic matrix constituents may influence peak heights in DPASV. Therefore, quantification has to be done by the method of standard additions. Specimens of typical voltamograms of a sample solution shall be supplied with the method.

2.4.4 Specific requirements for atomic absorption spectrometry (AAS) U.K.

This technique is basically mono-elemental and requires therefore optimisation of the experimental settings depending on the particular element to be quantified. Wherever possible, results shall be checked qualitatively and quantitatively by resorting to alternative absorption lines (ideally, two different lines shall be selected). Calibration standards shall be prepared in a solution matrix that matches as closely as possible that of the sample measurement solution (e.g. acid concentration or modifier composition). To minimise blank values, all reagents shall be of the highest available purity. Depending on the mode chosen to vaporise and/or atomise the sample, various types of AAS can be distinguished.

2.4.4.1 Specific requirements for flame AAS U.K.

The instrument settings shall be optimised for each element. Especially the gas composition and flow rates have to be checked. A continuum source corrector shall be used to avoid interferences caused by background absorption. In the case of unknown matrices, a check shall be made as to whether or not background correction is required.

2.4.4.2 Specific requirements for graphite furnace AAS U.K.

Contamination in the laboratory often affects accuracy when working at ultra-trace levels in the graphite furnace. Therefore high purity reagents, deionised water and inert plastic ware for sample and standard handling should be used. The instrument settings for each element shall be optimised. Especially the pre-treatment- and atomisation-conditions (temperature, time) and the matrix modification have to be checked.

Working under isothermal atomisation conditions (e.g. transverse 0heated graphite tube with integrated Lvov platform (8) will reduce the influence of the matrix concerning the atomisation of the analyte. In combination with matrix modification and Zeeman-background correction (9), quantification by means of a calibration curve based upon measuring of aqueous standard solutions will be allowed.

2.4.5 Specific requirements for hydride generation atomic absorption spectrometry U.K.

Organic compounds containing elements such as arsenic, bismuth, germanium, lead, antimony, selenium, tin and tellurium can be very stable and require oxidative decomposition to obtain correct results for total element content. Therefore, microwave digestion or high-pressure ashing under strong oxidative conditions is recommended. The greatest care shall be devoted to the complete and reproducible conversion of the elements into their corresponding hydrides.

The formation of arsenic hydride in hydrochloric acid solution with NaBH₄ depends on the oxidation state of arsenic (As III: fast formation, As V: longer formation period). To avoid a loss of sensitivity for the determination of As V with flow injection technique, caused by the short reaction time in this system, As V has to be reduced to As III after the oxidative decomposition. Potassium iodide/ascorbic acid or cysteine are suitable for this purpose. Blanks, calibration solutions and sample solutions shall be treated in the same way. Working with a batch system allows determining both arsenic species without affecting accuracy. Due to the delayed formation of As V-hydride, calibration shall be performed by peak area integration. The instrument settings shall be optimised. The gas flow, which transfers the hydride to the atomisator, is especially important and shall be checked.

2.4.6 Specific requirements for cold vapour atomic absorption spectrometry U.K.

Cold vapour is used only in the case of mercury. Due to volatilisation and adsorption losses of elemental mercury, special care is necessary during the whole analysis. Contamination by reagents or the environment has to be avoided carefully.

Organic compounds containing mercury require oxidative decomposition to obtain correct results for total mercury content. For decomposition, sealed systems with microwave digestion or high pressure asher are to be used. Special care is required for cleaning the equipment that had contact with mercury.

Working with the flow injection technique is advantageous. For lower decision limits, adsorption of elemental mercury on gold/platinum adsorber followed by thermal desorption is recommended. Contact of the adsorber or the cell with moisture will disturb the measurement and shall be avoided.

2.4.7 Specific requirements for inductively coupled plasma atomic emission spectrometry (ICP-AES) U.K.

Inductively coupled plasma atomic emission spectrometry (10) is a multi-element method, which allows a simultaneous measurement of various elements. To use the ICP-AES, the samples first have to be digested to decompose organic matrices. Sealed systems with microwave digestion or high pressure ashing shall be used. For a meaningful ICP-AES analysis, the instrument calibration and element or wavelength selection play an essential role. For instrument calibration, in case of linear calibration curves, it is usually necessary to measure calibration solutions of only four concentrations, because ICP-AES calibration curves are generally linear over four to six orders of magnitude of concentration. Calibration of the ICP-AES system should normally be performed with a multi-element standard, which shall be prepared in a solution that contains the same acid concentration as the measurement solution. For the linear curve, the element concentrations shall be checked.

The selection of wavelengths for measurement of the emission from the analytes is appropriate for the concentrations of the elements to be determined. When the analyte concentration falls outside of the working range of an emission line, a different emission line shall be used. At first, the most sensitive emission line (not interfered) shall be chosen, then a less sensitive line. When working at or near the detection limit, the most sensitive line for the respective analyte is usually the best choice. Spectral and background interferences are causing the major difficulties in ICP-AES. Possible interferences are e.g. simple background shift, sloping background shift, direct spectral overlap and complex background shift. Each of these interferences has its own causes and remedies. Depending on the matrices, interference corrections and optimisation of operating parameters shall be applied. Some interferences can be avoided by dilution or by adaptation of the matrices. With each batch of test samples analysed, reference and fortified material containing known amounts of the analyte(s) as well as blank material shall be treated in the same way as the test samples. For testing for a drift, the standard shall be checked e.g. after 10 samples. All reagents and the plasma gas shall be of the highest available purity.

2.4.8 Specific requirements for inductively coupled mass spectrometry (ICP-MS)(11)) U.K.

The determination of trace elements of average atomic mass, such as chromium, copper and nickel may be subject to strong interference from other isobaric and polyatomic ions. This can be circumvented only when a resolution power of at least 7000-8000 is available. Difficulties associated with the MS techniques include instrumental drift, matrix effects and molecular ion interference (m/z < 80). Multiple internal standardisation covering the same mass range as the elements to be determined is required to correct instrumental drift and matrix effects.

Complete decomposition of organic matter in samples prior to ICP-MS measurements is required. As in the AAS, after digestion in sealed vessels, volatile elements e.g. iodine are to be transferred to a stable oxidation state. Most severe interference results from molecular ion combinations of argon (plasma gas), hydrogen, carbon, nitrogen and oxygen (dissolution acids, plasma gas impurities and entrained atmospheric gases) and the sample matrix. Complete digestion, background measurements, appropriate choice of analytical masses sometimes associated with a lower abundance (poorer detection limit) and of dissolution acids, e.g. nitric acid, are required to avoid interferences.

For the elements to be determined, interferences are to be excluded by the appropriate choice of specific analytical masses including confirmation of isotope ratios. Instrument response considering Fano-factors shall be checked for each measurement by the use of internal standards.