A Comparison of the Extraction Methods used in the UK Nitrate Residues Monitoring Program

Derek Farrington ^(I), Andrew P. Damant ^(II), Karen Powell ^(II), James Ridsdale ^(II) , Margaret Walker ^(I) and Roger Wood ^(III)

(I) Direct laboratories, Woodthorne, Wergs Road, Wolverhampton, WV6 8TQ; to whom correspondence should be addressed, derek.farrington@directlabs.co.uk
(II) The Food Standards Agency, Aviation House, 125 Kingsway, London, WC2B 6NH;
(III) The Food Standards Agency, c/o Institute of Food Research, Norwich Research Park, Colney, Norwich, NR4 7UA.

Summary

It has been reported that significantly different nitrate levels may be obtained from the same crop at the same harvest when analysed by different laboratories. These differences may be due to the methods of analysis used by the laboratories, and in particular the extraction procedures that may be applied to extract nitrates from the crops. In this project a suitable homogenisation procedure for the preparation of samples was developed for preparation of samples to reduce this variability. Several nitrate extraction methods, including that described in BS EN 12014-2:1997, were evaluated. The BS method was subjected to a robustness test and then a collaborative trial involving 9 laboratories. Of the extraction methods compared, the hot water extraction method (BS EN 12014-2:1997) was found to produce the most reliable results. Cold water extraction of nitrate from lettuce and spinach samples was found to give significantly low, and variable, recoveries of nitrate when the sample had not been previously frozen. Laboratories should therefore adopt a hot water extraction method.

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