Part II Safety and quality
R. Wood, Food Standards Agency, London
8.7 Standardised methods of analysis for contaminants
Collaborative trials
As seen above, all ‘official’ methods of analysis are required to include precision data. These may be obtained by subjecting the method to a collaborative trial conforming to an internationally agreed protocol. A collaborative trial is a procedure whereby the precision of a method of analysis may be assessed and quantified. The precision of a method is usually expressed in terms of repeatability and reproducibility values. Accuracy is not the objective.
Recently there has been progress towards a universal acceptance of collaboratively tested methods and collaborative trial results and methods, no matter by whom these trials are organised. This has been aided by the publication of the IUPAC/ISO/AOAC Harmonisation Protocol on Collaborative Studies.14That Protocol was developed under the auspices of the International Union of Pure and Applied Chemists (IUPAC) aided by representatives from the major organisations interested in conducting collaborative studies. In particular, from the food sector, the AOAC International, the International Organisation for Standardisation (ISO), the International Dairy Federation (IDF), the Collaborative International Analytical Council for Pesticides (CIPAC), the Nordic Analytical Committee (NMKL), the Codex Committee on Methods of Analysis and Sampling and the International Office of Cocoa and Chocolate were involved. The Protocol gives a series of 11 recommendations dealing with:
• the components that make up a collaborative trial
• participants
• sample type
• sample homogeneity
• sample plan
• the method(s) to be tested
• pilot study/pre-trial
• the trial proper.
8.6.6 Statistical analysis
It is important to appreciate that the statistical significance of the results is wholly dependent on the quality of the data obtained from the trial. Data that contain obvious gross errors should be removed prior to statistical analysis. It is essential that participants inform the trial co-ordinator of any gross error that they know has occurred during the analysis and also if any deviation from the method as written has taken place. The statistical parameters calculated and the outlier tests performed are those used in the internationally agreed Protocol for the Design, Conduct and Interpretation of Collaborative Studies.14
collaborative trial organised to conform to one of the internationally accepted protocols described previously. Such organisations will include AOACI, the European Organisation for Standardisation (CEN) and the Nordic Committee for Food Analysis (NMKL). Within Europe, the most important of these international standardising organisations is probably CEN. CEN has a technical committee dealing with horizontal methods of analysis in which both additive and contaminant methods of analysis are discussed (TC 275). The methods of analysis for contaminants within its work programme are outlined below. This is given by Working Group. The titles under the Working Group heading refer to the work item (topic area) of that Working Group. The Working Groups not listed (e.g. 1, 2, etc.) are concerned with additive methods of analysis.
Work programme of CEN TC 275 Working Group 3: Pesticides in Fatty Foods Work Item A: determination of pesticides and polychlorinated biphenyls (PCBs):
Part 1: general considerations
Part 2: extraction of fat, pesticides and PCBs and determination of fat content Part 3: clean-up methods
Part 4: determination, confirmatory tests, miscellaneous.
Work programme of CEN TC 275 Working Group 4: Pesticides in Non-Fatty Foods
Work Item A: multiresidue methods for the gas chromatographic determination of pesticide residues:
Part 1: general considerations
Part 2: methods for extraction and clean-up Part 3: determination and confirmatory tests.
Work Item B: determination of dithiocarbamate and thiuram disulfide residues:
Part 1 spectrometric method Part 2: gas chromatographic method Part 3: xanthogenate method.
Work Item C: determination of bromide residues:
Part 1: determination of total bromide as inorganic bromide Part 2: determination of bromide.
Work Item D: determination of N-methyl carbamate residues.
Work Item E: determination of benomyl, carbendazim, thiabendazole and thiophanate-methyl.
Work programme of CEN TC 275 Working Group 5: Biotoxins
Work Item A: determination of aflatoxin B1and/or the sum of B1, B2, G1and G2
in cereals, shell fruits and derived products – high-performance liquid
chromatographic method with postcolumn derivatisation and immunoaffinity column.
Work Item B: determination of ochratoxin A in cereals and cereal products:
Part 1: HPLC method with silica gel clean-up Part 2: HPLC method with bicarbonate clean-up.
Work Item C: determination of ochratoxin A in cereals and cereal products – HPLC method with immunoaffinity clean-up.
Work Item D: determination of patulin content.
Work Item E: determination of fumonisins.
Work Item F: criteria of analytical methods for mycotoxins – CEN-Report.
Work Item G: determination of domoic acid in mussels.
Work Item H: determination of aflatoxin B1 and total aflatoxins by immunoaffinity column clean-up and HPLC in fig paste, pistachios, peanut butter and paprika powder.
Work Item I: determination of okadaic acid and dinophysis toxin in mussels by HPLC.
Work Item J: determination of saxitoxin and dicarbamoyl saxitoxin in mussels by HPLC.
Work Item K: determination of aflatoxin M1 in liquid milk.
Work programme of CEN TC 275 Working Group 6: Microbiology Work Item A: enumeration of Staphylococcus aureus:
Part 1: colony count technique with confirmation of colonies (ISO/DIS 6888-1: 1997)
Part 2: colony count technique without confirmation of colonies (ISO/DIS 6888-2: 1997).
Work Item B: horizontal method for the detection of coagulase positive Staphylococci (Staphylococci aureus and other species).
Work Item C: horizontal method for the detection and enumeration of Listeria monocytogenes:
Part 1: detection method Part 2: enumeration method.
Work Item D: enumeration of Clostridium perfringens – colony count technique.
Work Item E: horizontal method for the detection of Salmonella.
Work Item F: general guidance for enumeration of Bacillus cereus – colony count technique at 30ºC.
Work Item G: detection of thermotolerant Campylobacter.
Work Item H: detection of Yersinia enterocolitica.
Work Item I: preparation of the test sample, of initial suspension and of decimal dilutions, for microbiological examination:
Part 1: general rules for the preparation of the initial suspension and of decimal dilutions
Part 2: specific rules for the preparation of the test samples and initial suspension of meat and meat products
Part 3: specific rules for the preparation of the test samples and initial suspension of milk and milk products
Part 4: specific rules for the preparation of the test samples and initial suspension of fish products
Part 5: specific rules for the preparation of the test samples and initial suspension of products other than milk and milk products, meat and meat products and fish products.
Work Item J: general guidance for microbiological examinations.
Work Item K: validation of alternative microbiological methods.
Work Item L: guidelines on quality assurance and performance testing of culture media:
Part 1: quality assurance of culture media in the laboratory Part 2: performance testing
Part 3: practical implementation of the general guideline on quality assurance of culture media in the laboratory.
Part 4: performance testing of culture media.
Work Item M: horizontal method for the detection of Escherichia coli O 157.
Work Item N: horizontal method for the enumeration of Bacillus cereus.
Work Item O: guidelines on quality assurance and performance testing of culture media (to be elaborated as European Prestandards C02/97, C03/97):
Part 1: quality assurance of culture media in the laboratory
Part 2: practical implementation of the general guidelines on quality assurance of culture media in the laboratory
Part 3: performance testing.
Work programme of CEN TC 275 Working Group 10: Determination of Trace Elements
Work Item A: determination of trace elements – general considerations.
Work Item B: determination of mercury by CVAAS after pressure digestion.
Work Item C: determination of lead and cadmium by ETAAS after dry ashing.
Work Item D: performance criteria and general considerations.
Work Item E: pressure digestion.
Work Item F: determination of lead, cadmium, chromium and molybdenum by ETAAS after pressure digestion.
Work Item G: determination of lead, cadmium, zinc, copper, iron, chromium and nickel after dry ashing.
Work Item H: determination of lead and cadmium by ETAAS after microwave digestion.
Work programme of CEN TC 275 Working Group 11: Genetically Modified Organisms
Work Item A: detection of genetically modified organisms and derived products – sampling.
Work Item B: detection of genetically modified organisms and derived products – nucleic acid extraction.
Work Item C: detection of genetically modified organisms and derived products – qualitative nucleic acid based methods.
Work Item D: detection of genetically modified organisms and derived products – protein-based methods.