Effects of nitrogen blowing and rotary evaporation on the recovery rate of multi-pesticide residues

Preface

Have you ever encountered an unstable or very low recovery rate when doing more pesticide residues? The solid-phase extraction column and elution conditions may not be the main reasons. Improper handling during the concentration process will also lead to more pesticide residues. The recovery rate is reduced.

Introduction to the principle of nitrogen blowing and rotary evaporation

The principle of the nitrogen blower: is to blow nitrogen gas into the surface of the heated sample to concentrate the sample.

Features: Time-saving, easy to operate, easy to control, and get the expected results quickly. The solvent in the sample can be rapidly evaporated, typically by blowing nitrogen into the sample, increasing the surrounding air flow and raising the temperature.

Nitrogen is an inert gas that can isolate the air and prevent oxidation. The nitrogen blower uses the fast flow of nitrogen to break the gas-liquid balance above the liquid, thereby accelerating the volatilization of the liquid; and increases the temperature through dry heating or water bath heating (the boiling point of the target is generally higher than that of the solvent), so that To achieve the purpose of anaerobic concentration of the sample and keep the sample purer. Using a nitrogen blower instead of a common rotary evaporator for concentration can condense dozens of samples at the same time, greatly shortening the sample preparation time, and has the characteristics of time saving, easy operation and speed.

The working principle of the rotary evaporator is to make the distillation flask rotate at a constant speed at a suitable speed through the motor control to increase the evaporation area, and at the same time, the evaporation flask is in a negative pressure state through the vacuum pump to reduce the boiling point of the solution and speed up the evaporation rate.

Vacuum evaporators are used as a method of evaporation because reducing the pressure above the liquid will reduce the boiling point of the component liquids in it, thereby speeding up the evaporation rate to a certain extent. The evaporating flask is placed in a water bath and heated at a constant temperature while it is rotating, and the solution in the flask is heated, diffused and evaporated under the condition of negative pressure in the rotating flask. The evaporation system can be sealed and decompressed to 400 to 600 mmHg, the solvent in the distillation flask is heated with a heating bath, and the heating temperature can be close to the boiling point of the solvent, and it can also be rotated at a speed of 50 to 160 rpm to form a film of the solvent Increase the evaporation area, in addition, under the action of the condenser, the hot vapor can be quickly liquefied to speed up the distillation rate.

GB 23200.13-2016
Determination of 448 pesticides and related chemical bottle residues in tea

Add about 2 cm high anhydrous sodium sulfate to the Welchrom® TPT column, and place the column on a holder with a heart flask. Before adding the sample, pre-wash the column with 5 mL of acetonitrile-toluene solution. When the liquid level reaches the top of sodium sulfate, quickly transfer the sample extract to the purification column, and replace it with a new chicken heart bottle. Add a 50mL reservoir to the TPT column, elute pesticides and related chemicals with 25mL acetonitrile-toluene solution, combine them in a heart flask, and rotate and concentrate to about 0.5mL in a 45°C water bath, blow nitrogen at 35°C After drying, the residue was dissolved in 1 mL of acetonitrile-water solution, filtered through a 0.2 μm microporous membrane, and used for liquid chromatography-tandem mass spectrometry determination.

According to the national standard method, we have conducted preliminary research on hexythiazox, thiazide, dimethoate, diafenthiuron, simazine, thiamethoxam, difenoconazole, carbofuran and chlorothiline.

100 μg/L mixed standard was evaporated to 0.5 mL by rotary evaporation at 45 °C, nitrogen was blown to dryness at 35 °C, and 1 mL of acetonitrile-water solution was dissolved and put on the machine.

100 μg/L mixed standard was blown to 0.1 mL with nitrogen at 40°C, and the volume was adjusted to 1 mL with acetonitrile-water solution.

In conclusion

1. Because the elution solution contains toluene with a boiling point of 110.6°C, the rotary evaporator can concentrate the elution solution faster, but the pressure pump of most rotary evaporators does not contain a pressure control unit, and the increasing vacuum may cause Pesticide residues volatilize together, resulting in lower recovery rates.
2. Compared with the rotary evaporator, nitrogen blowing has a slower volatilization speed of toluene, but its stable effect and the characteristics of batch operation make it able to make up for the lack of speed.
3. After the above research, 33 pesticide residues were spiked and recovered in tea matrix using Welchrom® TPT special column, which met the requirements.