engleski

Chromatography Separation and Detection od Nitrate ions in Lettuce (Lactuca sativa)

Lettuce (Lactuca sativa) is an annual plant of the daisy family Asteraceae. It is most often grown as a leaf vegetable. It can accumulate higher amount of nitrate that may harm the health of the consumer as it can be converted to nitrite causing methaemoglobinaemia or carcinogenic nitrosamines. Depending on growing conditions and harvesting time of lettuce NN (146/12) prescribed maximum level of nitrates is 3000 mg NO3-/kg in fresh plant material. According to Statistical Yearbook of the Republic of Croatia average annual production of lettuce in period from 2000–2014 was 3.64 t/ha and it was in the range form 1.77 t/ha in 2014 up to 6.54 t/ha in 2007. In order to determine the effect of different nitrogen [KAN N(MgO) 27% (4.8%)] fertilization levels on the accumulation of nitrate content in lettuce field trial was set up at Miroševac location in Zagreb. The trial involved a monofactorial field experiment with five increasing N-fertilization levels (0, 50, 100, 150, 200 kg N ha-1) in randomized block design with five replication (1 m2 plot area for each replication). In each plot twelve plants were planted in April 2014. Sampling of plan material was conducted in July 2014. From each plot four randomly selected heads of lettuce were taken. Samples were dried at 70 °C for 72 h and weighed. Nitrate concentration was measured by extracting 30 mg of grounded dry plant material in 30 mL of deionized water at 90 °C for 2 h. After the extraction, samples were centrifuged and filtrated in two steps: firstly through filter paper and then through cartridge (OnGuard IIP, 1cc, Dionex, Sep-Pak Vac and OASIS HLM, Waters) to prevent contamination of the separation column with organic molecules. Nitrate separation and detection in extract was performed on Dionex ICS-1000 system: separator column [Ion Pac AS 17 (4×250 mm) Dionex]. Eluens concentration (KOH) was changed by eluens generator module (EGC-KOH, RFC-30) with on-line eluens preparation: the concentration was 10 mmol/L for the first 5.5 minutes and then was increased to 35 mmol/L, which was reached in 16.5 minutes. Total time of anions analysis was 18 minutes. After 18 minutes all seven anions left the column and were detected on a detector, but due to simultaneous analysis of cations, which takes 30 minutes, for the last 12 minutes the concentration of anion eluens was 10 mmol/L. Flow rate was 1.0 mL min-1 and for detection suppressed conductivity was used. Observed data were subjected to analysis of variance (ANOVA) using SAS Institute 9.1.3 and mean values were separated by Fisher’s LSD test at P ≤ 0.05. Results revile that excessive mineral nitrogen fertilization did not significantly effect on enhancement of accumulated nitrates (NO3-) in the lettuce leaf. Average values in fresh plant material were in the range from 697.9 mg NO3-/kg at treatment with applied 50 kg N/ha to 878.9 mg NO3-/kg at treatment with applied 100 kg N/ha. It is important to emphasize that the determined values do not exceed the maximum levels prescribed by NN (146/12). Additionally, contents of fluoride, chloride, sodium, potassium, ammonium, magnesium and calcium were also separate and detected by means of ion chromatography system and average values, for all five treatments in fresh plant material, were: 5.3 g F-/kg ; 36.4 g Cl-/kg ; 13.5 g Na+/kg ; 155.3 g K+/kg ; 18.9 g N-NH4+/kg ; 10.7 g Mg2+/kg and 13.1 g Ca2+/kg.

Chromatography Separation ; Mineral Fertilization ; Lettuce

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