Detection of Pesticide Residues (3)

4.
Concentration

Since the amount of analyte in the sample in pesticide residue analysis is very small, and the amount of solvent used in conventional solvent extraction is relatively large, the concentration of residual pesticide solution extracted from the sample is generally very low , During purification and detection, it must first be concentrated to make the analyte in the detection solution reach a concentration above the sensitivity of the analytical instrument
.
At present, some new extraction methods, such as solid phase extraction, purge and trap method, can simultaneously achieve sample concentration

Commonly used concentration methods include reduced pressure rotary evaporation method, KD concentration method, nitrogen drying method, etc.
, which can be selected according to actual needs
.

5.
Detection

Pesticide residue determination methods mainly use chromatography and mass spectrometry
.
Chromatography has a strong ability to separate the components of different compounds

The gas chromatograph is composed of five parts: gas supply system, sampling system, separation system, detection system and recording system
.

The gas supply system includes components such as high-pressure steel cylinders, pressure reducing valves, purge pipes, stabilizing valves, pressure gauges, and flow meters
.
The sampling system includes a sampler and a gasification chamber

Figure 6-1 shows a typical gas chromatograph
.

Figure 6-1 Schematic diagram of a typical gas chromatograph

The detector is a key component of the gas chromatograph.
It is a device that measures the composition and concentration changes of the effluent material after the column.

Gas chromatograph detectors can be divided into general-purpose detectors and selective detectors according to their measuring ranges
.
General-purpose detectors respond to most substances, while selective detectors only respond to certain substances, and have no response or little response to other substances

The electron capture detector is a highly selective detector
.
High selectivity means that it only responds to substances containing elements with strong electronegativity, such as compounds containing halogens, S, P, N, etc.

Figure 6-2 Electron capture detector

When the pure carrier gas (typically with high purity N ₂ to enter the detection chamber), the beam irradiation, ion positive ionization (N by ^{2 +} ) and electrons E ^- , the generated positive ions and electrons respectively moving to the poles in the electric field, A base current of about 10 ^-8 A is formed
.
After adding the sample, if the sample contains a certain electronegative element that is easy to combine with electrons, these low-energy electrons will be captured and negatively charged anions (electron capture) will be generated.
These anions and the carrier gas are ionized.

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