The four pyrethroids, permethrin, phenothrin, fenvalerate and deltamethrin were applied to wheat which was stored for 52 weeks at 25 or 35"C, and either 12 or 15% moisture content. Rates of loss were calculated from residue analyses of the wheat at five intervals during storage. Calculated half-lives (weeks) for the pyrethroids at 25°C (12% moisture) and 35°C (15 % moisture) were: permethrin 252 and 44, phenothrin 72 and 29, fenvalerate 210 and 74, and deltamethrin 114 and 35, respectively.

The aim of this work was t o establish a method that would preferably afford simultaneous extraction and determination of nitrate and nitrite in vegetables. Both high-performance liquid chromatography (HPLC), with ultraviolet (UV) and conductivity detection, and automated spectrophotometry (AS) were available. The effects of ascorbic acid on nitrate and nitrite, as determined by AS, were investigated. Ascorbic acid significantly reduced the concentration of nitrite in solution, particularly when the solutions were heated. It was concluded that this interference was not occurring during colour development, but was occurring before the analytical determination. Nitrate could be quantitatively recovered from vegetables that were heated or blended with water or extracted under alkaline conditions. Nitrite could only be recovered from alkaline extracts because of the instability of nitrite under acidic conditions. These observations suggest that extraction of nitrite from vegetables with water, as carried out in some surveys, could lead t o underestimations of nitrite content. Use of HPLC with conductivity detection was impractical and error prone owing t o the long analysis times and interference from sulphate and phosphate, but HPLC with UV detection was about ten times more sensitive than with conductivity detection and AS, and hence has potential in ultra-trace analysis. However, the AS method was preferred for vegetables as it was more time and labour efficient.