# Residual Level of Chlorpyrifos in Lettuces Grown on Chlorpyrifos-Treated Soils
## 2 Materials and Methods
\begin{table}
\begin{tabular}{c c} \hline Instrument & Shimadzu GC-2010 \\ Detector & ECD (electron capture detector) \\ Column & DB-5 (0.25 mm \(\times\) 30 m, 0.25 \(\upmu\)m film thickness) \\ Oven temperature & 200 \({}^{\circ}\)C (2 min); 20 \({}^{\circ}\)C/min; 280 \({}^{\circ}\)C (5 min) \\ Detector temperature & 300 \({}^{\circ}\)C \\ Injection port temperature & 250 \({}^{\circ}\)C \\ Gas flow rate & Carrier (N\({}_{2}\)): 1.3 mL/min; Make up gas (N\({}_{2}\)): 60.0 mL/min \\ Injection mode & Split (20:1) \\ Injection volume & 1.0 \(\upmu\)L \\ Retention time & 4.8 min \\ \hline \end{tabular}
\end{table} 
Table 1: Instrumental parameters for the analysis of chlorpyrifos. extracted with acetone (100 mL) by shaking at 200 rpm with a mechanical mixer for 1 h, followed by filtering through Whatman filter paper No. 2 under a vacuum. The container and the filtrate were washed again with acetone (30 mL), and the extracts were transferred to a 500 mL separatory funnel. Then, a saturated NaCl solution (10 mL), water (90 mL), and DCM (70 mL) were put into the separatory funnel and the funnel was shaken. After shaking, the lower DCM layer was drained into a 500 mL flask through anhydrous sodium sulfate. This partitioning was repeated two times using 70 mL of DCM. After this procedure, the extracts were collected, pooled, and concentrated to dryness using a rotary vacuum evaporator at 40 \({}^{\circ}\)C. The residue was re-dissolved in 2 mL of a mixture of acetone and hexane (20/80, \(v/v\)). The concentrated extract was loaded into a florisil SPE cartridge, which was prewashed with 5 mL of 20% acetone in hexane. Then, chlorpyrifos was eluted with 8 mL of 20% acetone in hexane. The eluted fraction was concentrated using a nitrogen evaporator, and the residue was re-dissolved in 4 mL of acetone (2 mL of acetone was used for lettuce) for analysis. Sensitivity of the analytical method. The method limit of quantitation (MLOQ) is a practical limit of quantitation (LOQ) for the total analytical method using samples. It is calculated using the LOQ, injection volume, final solution volume, and sample weight in the analytical procedure [11]. MLOQ (mg/kg) = (LOQ \(\times\) final solution volume)/(injection volume \(\times\) sample weight)  
MLOQ (mg/kg) = (0.1 ng \(\times\) 4 mL)/(1.0 \(\mu\)L \(\times\) 20 g) = 0.02 mg/kg (soil)  
Dissipation and half-life in soil. The dissipation of chlorpyrifos in the soil was determined using a first-order kinetics regression analysis with the following model [34]:  
\[C_{t}=C_{0}e^{-kt}\]  
where \(C_{t}\) is the concentration of the pesticide at any time \(t\), \(C_{0}\) is the initial concentration, and \(k\) is the rate constant (d\({}^{-1}\)). The half-life (\(DT_{50}\)), which is the time at which the concentration of the initial deposit reaches 50%, was determined by the following equation:  
\[DT_{50}=\ln 2\times k^{-1}=\frac{0.693}{k}\]  
Calculation of the crop bioconcentration ratio (CBR). The crop-to-soil bioconcentration ratio is used to relate the concentration of the chemical measured in the crop to the concentration in the soil supporting the crop. The CBR was calculated by dividing the residue concentration of the pesticide in the crop by the concentration in the soil at 0 DAT according to the following equation [35]:  
\[\text{CBR}=\frac{\text{Concentration in crop}}{\text{Initial concentration in soil}}\]