# Residual Level of Chlorpyrifos in Lettuces Grown on Chlorpyrifos-Treated Soils
## 3 Results and Discussion  
Analytical method validation. The recovery, reproducibility, MLOQ, and linearity of the calibration curve were prepared to evaluate the analytical performance. A calibration curve of chlorpyrifos exhibited good linearity ranging from 0.1 mg/L to 10 mg/L with the regression equation of y = 428022.4x + 35931.7. The value of the correlation coefficient (r\({}^{2}\)) was 0.9998. According to the analysis of standard solutions, 0.1 ng was determined as the LOQ, which was satisfactory for the residual analysis of chlorpyrifos. The MLOQ value of chlorpyrifos was calculated as 0.02 mg/kg for both soil and lettuce. These values met the criteria of the Korea Food and Drug Administration as it was below 0.05 mg/kg, or half of the maximum residue limit [36]. For obtaining the recovery rate, the control samples were spiked with chlorpyrifos standard solution at two different concentrations and were analyzed according to an established analytical method. Average recovery rates from the two types of soils ranged from 102.3 to 109.1%,while recoveries from lettuce ranged from 90.2% to 92.3% with low coefficients of variation of less than 3.6% and 4.6%, respectively (Table 2). In the gas chromatography chromatograms of soil and lettuce samples, there were no interfering peaks in the untreated sample and the retention time of chlorpyrifos was 4.8 min. Thus, the residual analytical method employed in this study was adequate to quantify residual chlorpyrifos in samples. Dissipation of chlorpyrifos in the treated soil. Chlorpyrifos residue in field 1 soil treated with 1.0 mg/kg was 0.86 \(\pm\) 0.09 mg/kg on the day of application, and decreased to 0.06 \(\pm\) 0.01 mg/kg at 36 DAT. Meanwhile, the initial concentration of chlorpyrifos with an application of 2.0 mg/kg was 2.31 \(\pm\) 0.09 mg/kg, which decreased to 0.18 \(\pm\) 0.05 mg/kg after the same period (Figure 1). The regression equations of dissipation were C = 0.8437e\({}^{-0.0825}\) (r\({}^{2}\) = 0.9596) and C = 2.3877e\({}^{-0.0766}\) (r\({}^{2}\) = 0.9789) with \(DT_{50}\) values of 8.4 d and 9.0 d, respectively. In field 2 treated with applications of 10.0 mg/kg and 20.0 mg/kg of pesticide, the residue levels were reduced to 1.20 \(\pm\) 0.05 mg/kg at 43 DAT from 11.10 \(\pm\) 0.68 mg/kg with a regression equation of C = 12.2685e\({}^{-0.0370}\) (r\({}^{2}\) = 0.9359), and 22.59 \(\pm\) 1.95 mg/kg at 0 DAT was degraded to 3.04 \(\pm\) 0.47 mg/kg at 43 DAT with an equation of C = 20.4078e\({}^{-0.0499}\) (r\({}^{2}\) = 0.9185), respectively. The \(DT_{50}\) values were 18.7 d and 13.9 d with the application of 10.0 mg/kg and 20.0 mg/kg, respectively (Figure 1). The degradation of chlorpyrifos in a field where cucumbers were grown showed a \(DT_{50}\) of 25.7-73 d. The \(DT_{50}\) values of chlorpyrifos in soils were reported to be 0.4-2.1 d in a cabbage field and 175-1576 d in five different soils treated with 1000 mg/kg of pesticide. The dissipation of pesticides is affected by climate conditions, treatment amount, soil conditions, and cultivation method [36]. The high log \(K_{\rm ow}\) and low water solubility values of chlorpyrifos can result in long retention in soil with a long \(DT_{50}\). Concentration of chlorpyrifos in lettuce. Chlorpyrifos residue in lettuce grown in the field treated at the 1.0 mg/kg application rate ranged from 0.04 \(\pm\) 0.00 mg/kg at 22 DAT to 0.02 \(\pm\) 0.00 mg/kg at 32 DAT, while chlorpyrifos in the lettuce grown in the field with an application rate of 2.0 mg/kg ranged from 0.02 \(\pm\) 0.01 mg/kg at 22 DAT to 0.05 \(\pm\) 0.00 mg/kg at 32 DAT. In field 2 treated with application rates of 10.0 mg/kg and 20.0 mg/kg, residue levels were 1.47 \(\pm\) 0.20 mg/kg to 0.79 \(\pm\) 0.03 mg/kg and 1.03 \(\pm\) 0.10 mg/kg to 0.66 \(\pm\) 0.07 mg/kg, respectively (Table 3). The residue decreased in proportion to the time after treatment. The lettuce cultivated in field 2 was larger than that grown in field 1 at harvest, and showed higher residual chlorpyrifos levels (Figure 2).