Abstract: This experiment combined with laboratory conditions, using solid-phase extraction technology, established a gas chromatography-mass spectrometry method for organophosphorus pesticide residues in bean sprouts to provide technical support for further standardizing the market.
In recent years, the detection technology of pesticide residues in vegetables has been continuously developed, and the methods of using gas chromatography and gas chromatography-mass spectrometry to detect organophosphorus pesticide residues are very mature. New technologies such as QuEChERS have also appeared in pre-processing, but there are currently few practical applications in China.
1 Materials and methods 1.1 Instruments and reagents Shimadzu GC2010 gas chromatography mass spectrometer, rotary evaporator, solid phase extraction device, nitrogen blower, centrifuge; C18 column: 2 000 mg (Varian); PSA column: 500 mg (Varian ); N-hexane (chromatographic grade), acetone (chromatographic grade).
1.2 Preparation and storage of the solution Take pesticide standard methylphenidate, methyl parathion, methyl pyrimidine, malathion, chlorpyrifos, quetiaphos, triflufon, chlorfenapyr, profenofos, ethiophen (Provided by the National Standards Research Center, the concentration of standard materials is 100ug / ml) Each 1 ml, dilute to 10 ml with n-hexane to obtain a standard solution with a concentration of 10 ug / ml, and store it at 4 ° C as a standard stock solution . Dilute (concentrate) with n-hexane to 1, 2, 3, 4, 5 ug / ml as standard working solution. Preparation of eluent: acetone: n-hexane = 2: 81.3 Sample preparation 1.3.1 Sample preparation Weigh sample 10 9, stir, add 20 ml acetonitrile, ultrasonic extraction for 15 min, add sodium chloride 2.5 g, 4 000 r / rain centrifuge for 5 min. The supernatant is ready for use.
1.3.2 Purification and concentration C18 column purification: pre-equilibrate the C18 column with acetonitrile (5 ml x 2) and pure water (5 ml x 2), transfer the supernatant to the C18 column, rinse the column with 12 ml acetonitrile water (31), vacuum Drain. Make up to 1 ml with eluent.
PSA column purification: PSA column was pre-equilibrated with eluent (5 ml x 2), the above 1 ml sample solution was passed through the column, and 3 ml eluent was eluted 5 times, evaporated at 40 ° C water bath, and the volume was blown to 1 ml with nitrogen, ready for use .
1.4 Chromatography mass spectrometry conditions Chromatography column: capillary column Rtx -5, 30 mx0.25 mmx0. 25 um.
Gas chromatography conditions: column temperature (programmed temperature increase): 60 ° C for 3 rain. Raise to 200 ° C at 20C / min, 220 ° C at 2 ° C / min, then 280C at 20 ° C / min for 9 minutes; inlet temperature: 260C; carrier gas: helium ≥99.995%, 1 ml / min; injection volume: 2ul.
Mass spectrometry conditions: GC-MS interface temperature: 280 ℃; ion source temperature: 200 ℃; mass spectrometry detection mode: SIM mode (select ion detection).
Qualitative confirmation: Within the retention time window, a peak consistent with the monitoring ions of the corresponding target compound and the ratio with the monitoring ions of the target are consistent, which is regarded as suspected detection. If necessary, further confirmation by SCAN is required.
1.5 Qualitative analysis standards and methods The chromatographic peaks are characterized by a combination of full-scan library search and extraction of corresponding retention time mass chromatograms for qualitative characterization. Qualitative standards: determine 2 characteristic ions and 1 quantitative ion in each standard substance (see Table 1); at the corresponding retention time, the selected ions should all appear, and the ratio of the selected ion abundance to that of the standard sample The ion abundance ratio is consistent (with a tolerance of ± 20%).
1.6 Quantitative analysis method adopts the external standard method for single ion quantitative determination, that is, each base peak ion is used as the quantitative ion, and after the blank is subtracted, the peak area is compared with the standard curve for quantitative analysis. Calculated as follows. In order to reduce the influence of the matrix, the standard solution was prepared with a blank solution for quantification, and the concentration of the standard solution was close to the concentration of the test compound.
X = Ai`C`V / As-in where: X-sample target compound content, mg / kg; Ai-sample target compound peak area; As-standard solution target compound peak area; C-standard The concentration of the target compound in the working solution, ug / ml; v--the volume of the final sample solution, ml; m--the amount of sample represented by the final sample solution, g.
2 Results and analysis 2.1 The selection of characteristic ions This test adopts the selective ion detection mode (SIM). The selection of characteristic ions for detection can effectively reduce background interference and improve the measurement sensitivity. According to the sequence of pesticide peaks, the detection mode is divided into time periods, and one or several substances to be tested are detected in a certain time period, so that the number of times the characteristic ions are scanned in a unit time increases, background interference is reduced, and the signal-to-noise ratio is improved. The component segmentation is completed. The selection of characteristic ions follows the following principles: (1) Specificity: The mass of characteristic ions that characterize a particular component should not be the same as the mass of characteristic ions that characterize other components. (2) Sensitivity: The abundance of fragment ions should be as large as possible. (2) Reproducibility: Select ions with large mass, high symmetry and good reproducibility. According to this principle, the selected characteristic ions are shown in Table 1.
2.2 Preparation of standard curve Take the prepared standard solution, inject, qualitatively determine the organophosphorus pesticide varieties and retention time, and then inject in SIM mode to obtain the standard substance map (Figure 1). The peak areas of standard quantified ions at different concentrations were measured and plotted as a standard curve (Table 2).
The correlation coefficients of the standard curves of 10 kinds of organophosphorus pesticide residues are between 0.9901-0.9988; the lower limit of quantitative detection method (S / N = 10, n = 3) is between 2.6-3.8 ug / kg; the absolute error of the measurement results Less than 5.2% -13.5% of the arithmetic mean. 2.3 Market sampling test results In order to understand the pesticide residues of bean sprouts on the market, we randomly selected 26 bean sprout samples from 13 farmers' markets on the Taian market to test the above 10 organophosphorus pesticide residues. 2. The recovery rate is 60% to 85%.
The test results (Table 3) show that none of the above-mentioned 10 kinds of organophosphorus pesticide residues were detected in the bean sprout samples tested, and the single pass rate was 100%. Although all the 10 indicators tested in this test are qualified, the possibility of using other drugs is still not ruled out.
3 Conclusion In this experiment, a gas chromatography-mass spectrometry method for 10 organophosphorus pesticide residues in bean sprouts was established. The method has simple pretreatment, less sample impurities, better separation, no obvious peaks, and the correlation coefficient of the standard curve of 10 pesticide residues is between 0.9901 -0.9988; the lower limit of quantitative detection method (S / N = 10, n = 3 ) Between 2.6 -3.8 ug, / kg; the absolute error of the measurement result is less than 5.2% -13.5% of the arithmetic mean, and the recovery rate is 60% ~ 80%. 26 batches of bean sprouts in 13 farmers ’markets in Tai’an were tested, and 10 organophosphorus pesticide residues were all undetected, with a single pass rate of 100%.
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