Analytical Quality Assurance
A sample batch consists of approximately 60 samples. Within the batch, samples of the same matrix are grouped together, usually in sub-batches of 7 samples of the same matrix. Within each subgroup, one duplicate and one sample spike were analysed.
Where possible Certified Reference Materials (CRMs) are used to monitor recovery, eg NIST (National Institute of Science and Technology, USA) Bovine Liver, AGAL20 Bovine Liver. It is not possible to use CRMs for each matrix as they are not always available. Acceptance criteria had to be met for all sample spikes, duplicates and CRMs.
With every batch, standards were run at the beginning and end of the run and acceptance criteria had to be met for the results to be accepted. 3 reagent blanks were analysed. All abnormally high concentrations (eg, lead) samples were re-analysed to check reproducibility.
A sample batch consists of 10 samples of the same matrix. With every batch:
- Standards were run at the beginning and end of the batch. Acceptance criteria had to be met for these standards;
- A solvent blank was analysed, made by running the solvent used for extraction through the entire analysis procedure;
- A spike was run and recoveries were checked against performance criteria;
- All positive samples were re-extracted and re-analysed to check reproducibility; and
- The identity of all positives found was confirmed by full scan gas chromatography / mass spectrometry.
Summary of analytical methods used and limits of reporting achieved in the survey
Table 27. Analytical methods used in the 19th ATDS
Procedure and limit of reporting
Metal screen (antimony, arsenic, cadmium, copper, lead and mercury, selenium, tin, zinc)
Microwave digestion, with re-distilled nitric acid and determined using ICP- MS. Limits of reporting were 0.01 mg/kg except for cadmium (0.005 mg/kg), selenium (0.02 mg/kg) and tin (0.02 mg/kg).
Perchloric acid digestion at 80° C for 1 hr, filtered, reduced with HBr and hydrazine sulphate to As 3+ . Hydride form generated with NaBH 4 and quantified by ICP-AES. Limit of reporting 0.05 mg/kg.
Organochlorine pesticides, Organophosphorus pesticides, Synthetic pyrethroid pesticides, Fungicide residues and PCBs
Acetone extraction and partition into dichloromethane/hexane. Extraction, evaporation and clean-up by GPC. GC determination with nitrogen/phosphorus detector for organophosphorus pesticides and fungicides; further clean up of extract using florisil chromatography followed by GC/ECD for organochlorine compounds, pyrethroids and fungicides; additional clean up using silica mini columns followed by GC/ECD for PCBs. Limit of reporting is 0.01 mg/kg.
Extraction into methanol/water. Purified using affinity chromatography. Determined using HPLC. Limit of reporting is 0.001 mg/kg.
Organotin compounds are extracted their matrix using solid phase extraction and are then ethylated using sodium tetraethylborate. The derivatised organotin compounds are detected and quantitated using an internal standard procedure by gas chromatography and an atomic emission detector.
Benomyl and its metabolite, Carbendazim are extracted from the sample with acetonitrile. The filtered extract is purified using a solid-phase ion exchange cartridge and the filtrate is analysed by reverse-phase high performance liquid chromatography with isocratic elution and diode array detection over the wavelength 200 - 350 nm. The limit of reporting is 0.2 mg/kg.
Dithiocarbamates are analysed as a family by decomposing them using acidified stannous chloride. The carbon disulfide released is determined by an internal standard procedure using head-space gas chromatography and a sulfur-specific flame photometric detector. LOR = 0.1 mg/kg.< BR >
||Atomic Emission Spectroscopy |
||Gel Permeation Chromatography|
||High Performance Liquid Chromatography|
||Inductively Coupled Plasma|