INFLUENCE OF DIFFERENT EXTRACTION CONDITIONS AND SAMPLE PRETREATMENTS ON QUANTIFICATION OF NITRATE AND NITRITE IN SPINACH AND LETTUCE

Abstract

Different sample pretreatment and extraction techniques are often used for analysis of nitrates and nitrites, however, the effects of these variables have not been properly examined. Comparative investigations were carried out with the objective of finding the most suitable conditions for quantification of nitrate and nitrite in spinach and lettuce. A rapid and cost effective RP-HPLC/UV method was validated and used to select the most appropriate extraction procedure to eliminate chromatographic interferences and to evaluate the influence of different sample pretreatments on the accuracy and reproducibility of the results obtained. Similar nitrate concentrations were obtained for fresh and two weeks frozen samples. Freeze drying and oven drying pretreatment of the spinach and lettuce material was inappropriate. No nitrite was detected in either fresh, freeze dried, oven dried, and frozen spinach or lettuce.

Keywords:

• lettuce
• nitrate
• nitrite
• RP-HPLC/UV
• spinach

Notes

^a Intra-day: running three times within 24 hours.

^b Inter-day: running six times within successive 7 days with at least 24-hour intervals.

^c Reproducibility was evaluated by the relative standard deviation (RSD, %).

^a The content of nitrate in unspiked lettuce was 2.91 mg L⁻¹; nitrite was not detected.

^b The content of nitrate in unspiked spinach was 3.79 mg L⁻¹; nitrite was not detected.

^c Average of triplicate assays.

S – fresh spinach; L – fresh lettuce; SFd – freeze-dried spinach; LFd – freeze-dried lettuce; SO – oven dried spinach; LO – oven dried lettuce; SF – Frozen spinach; LF – frozen lettuce.

Letters ^a–c indicate significant differences at p < 0.05 in the Duncan test.

Nitrate quantification by HPLC/UV, extraction variables held constant (extraction temperature 80°C and extraction time 20 min) except sample size that was 0.25 for fresh and frozen samples and 0.025 for freeze-dried and oven dried samples, to fall within the linear range of calibration curve.

Letters ^a–c indicate significant differences at p < 0.05 in the Duncan test.

Nitrate quantification by HPLC/UV, extraction variables held constant (extraction temperature 80°C and extraction time 20 min) except sample size that was 0.25 for fresh and frozen samples and 0.025 for freeze-dried and oven dried samples, to fall within the linear range of calibration curve.