Vegetables were collected near peak harvest from the main production regions in Queensland and were analysed for residues of nitrate and nitrite. A small sample of hydroponic produce was also included in the survey. Nitrite-N from 1 to 4 mg kg-' was found only in dwarf beans and in lettuces. Levels of nitrate in potatoes, cabbages and beets were higher than those reported in other surveys and exceeded threshold limits set in one other country. The median nitrate-N concentration measured in hydroponic lettuce (465 mg kg-nitrate-N) was more than twice the median concentration for field-grown lettuce. Poor correlation between total N and nitrate in vegetables raises doubts about the use of total N alone as an indicator of N status.

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.