Determination of Multi-residues of Organophosphorus Pesticides in Fruits, Vegetables and Cereals

This article introduces the determination of residues in fruits, vegetables, cereals and other crops that have used 20 pesticide preparations such as dichlorvos (according to the first method of GB/T5009.
20-2003)
.

1.
Principle

Containing organic phosphorus specimen in a hydrogen-rich combustion flame in the form HPO fragments characteristic of the light emitted is 526mm
.

After this light is selected by the filter, it is received by a photomultiplier tube, converted into an electrical signal, and amplified by a micro-current amplifier to be recorded

2.
Reagents

(1) Acetone
.

(2) Dichloromethane
.

(3) Sodium chloride
.

(4) Anhydrous sodium sulfate
.

(5) Celite545 filter aid
.

(6) Pesticide standard products: dichlorvos (purity ≥99%), sulfamephos (cis ≥60%, trans ≥40%), monocrotophos (≥99%), phorate (≥98%), Parhamidophos (≥99%), Diazinon (≥98%), Emetathion (≥97%), Methylmethionate (≥99%), Methylparathion (≥99%), Daofenjing (≥99%), hydrocarbophos (≥99%), oxyquinalphos (≥99%), Daofengsan (≥99.
6%), metqualofos (≥99.
6%), Kexian Phosphorus (≥99.
9%), Ethion (≥95%), Dimethoate (≥99.
0%), Quinalphos (≥98.
2%), Parathion (≥99.
0%), Spirothiophos (≥98.
5%) )
.

(7) Preparation of pesticide standard solutions: Weigh the standard substances in (6) accurately, dissolve them with dichloromethane , and prepare 1.
0 mg/mL standard stock solutions respectively
.

Store in a refrigerator (4°C), draw different amounts of standard stock solution according to the response of each pesticide species on the instrument, and dilute it with dichloromethane to form a mixed standard solution

3.
Apparatus

(1) Gas chromatograph: with flame photometric detector (FPD)
.

(2) Organization masher
.

(3) Crusher
.

(4) Rotary evaporator
.

4.
Operation steps

(1) Sample preparation

①Sampling: Grain samples are crushed by a pulverizer and passed through a 20-mesh sieve to make grain samples; the non-edible parts of fruit and vegetable samples are removed to prepare samples for analysis
.

② Extraction: Weigh samples (fruits and vegetables 50.
00g; grains 25.
00g), place them in a 300mL beaker, add 50mL water and 100mL acetone (the total volume of the extract is 150mL), and extract with a tissue masher for 1 to 2 minutes
.

The homogenate was filtered under reduced pressure through a Buchner funnel spread with two layers of filter paper and about 10 g of filter aid

③ Purification: Add 10-15g of sodium chloride to the filtrate to saturate the solution
.

Shake vigorously for 2 to 3 minutes, and let stand for 10 minutes to separate the acetone and water phase, and shake the water phase with 50 mL of dichloromethane for 2 minutes, and then stand for separation

④Concentration: The acetone and dichloromethane extracts are combined and filtered into a 250mL round bottom flask through a glass funnel containing 20-30g of anhydrous sodium sulfate, and then the container and anhydrous sodium sulfate are washed with about 40mL of dichloromethane several times.
, The washing liquid is also incorporated into the flask
.

Concentrate to about 2 mL with a rotary evaporator, transfer the concentrated solution quantitatively to a 5-25 mL volumetric flask, and add dichloromethane to make the volume up to the mark

(2) Gas chromatograph (FPD) measurement

① Chromatographic reference conditions:

Chromatographic column: a.
Glass column 2.
6m×3mm (id), filled with a support of Chromosorb W AW DMCS (80-100 mesh) coated with 4.
5% DC-200+2.
5% OV-17
.

b.
Glass column 2.
6m×3mm (id), filled with Chromosorb W AW DMCS (60～80 mesh) coated with 1.
5% (mass fraction) QF-1

Gas speed: nitrogen 50ml/min, hydrogen 100mL/min, air 50mL/min
.

Temperature: Column temperature 240℃, vaporization chamber 260℃, detector 270℃
.

② Chromatographic analysis: absorb 2 ~ 5uL mixed standard use liquid and sample purification liquid into the gas chromatograph, and record the retention time and peak height of the chromatographic peak
.

Qualitatively by retention time, and quantitatively by comparing the peak height or area of ​​the sample with the standard components

③Chromatogram: The chromatograms of 16 organophosphorus pesticides are shown in Figure 6-4; the chromatograms of 13 organophosphorus pesticides are shown in Figure 6-5
.

5.
Result calculation

Where x _i ——the content of organophosphorus pesticide of component _{i in the} sample, mg/kg

A _i ——the peak area of ​​component _{i in the} sample, the unit of integration

A _{si ——the} peak area of ​​component i in the mixed standard solution, the unit of integration

V ₁ ——Total volume of sample extraction solution, mL

V ₂ ——Total volume of extraction liquid for purification, mL

V ₃ ——Concentrated constant volume volume, mL

V ₄ ——Injection volume, uL

m ₁ —the mass of i standard component injected into the chromatograph, ng

m ₂ ——the mass of the sample, g

The calculation result retains two significant digits

Figure 6-4 Chromatograms of 16 organophosphorus pesticides
1—Dichlorvos, the lowest detection concentration is 0.

005mg/kg 2—Quamiphos, the lowest detection concentration is 0.
004mg/kg

Figure 6-5 Chromatograms of 13 organophosphorus pesticides