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    Home > Chemicals Industry > Chemical Technology > Development of Gas Chromatography Method for Pesticide Residue Detection

    Development of Gas Chromatography Method for Pesticide Residue Detection

    • Last Update: 2021-09-09
    • Source: Internet
    • Author: User
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    The multi-residue detection of pesticides has always been the goal pursued by analytical chemists, and is the research focus and development trend of pesticide residue analysis
    .
    Modern pesticide residue analysis is mainly for multi-residue analysis, which needs to meet the requirements of strong specificity, high sensitivity, good reproducibility and wide linear range

    .
    Modern pesticide residue analysis should be based on accurate analysis, and conduct fast and efficient analysis

    .
    Like conventional detection and analysis, multi-residue detection also includes sample pretreatment and instrument determination

    .
    In terms of technical points, the multi-residue analysis methods in various countries are basically the same as the single-component method.
    The difference is that different pretreatment operations are selected according to the different substrates

    .
    Using pesticide multi-residue technology, there are hundreds of pesticides that can be analyzed simultaneously in the world

    .
    Due to the complex composition of the food matrix, the physical and chemical properties of different pesticides are quite different, and the same pesticide has homologs, isomers, degradation products, metabolites and conjugates, so there is currently no multi-residue analysis method.
    Can cover all types of pesticides at the same time

    .

    In order to provide a legal basis for pesticide residue limits in agricultural products, China has successively formulated relevant pesticide residue standards since the mid-1980s
    .
    With the improvement of the living standards of our residents and the competition after China's entry into the WTO, our country has put forward higher requirements for the safety of agricultural products

    .
    In the establishment of pesticide residue testing methods, we mainly refer to the standards of the International Codex Alimentarius Commission.
    At the same time, we also attach great importance to the testing methods of major exporting countries

    .
    A relatively complete pesticide residue standard system has been preliminarily formed, which basically meets the requirements for agricultural residue testing

    .

    (1) Gas chromatography and method development

    Gas chromatography is currently the most widely used method for the analysis of organophosphorus and organochlorine pesticide residues, and it can also be used for the analysis of other organic substances
    .
    The principle is to use the difference in adsorption and desorption capacity of each component in the sample in the chromatographic column, that is, to use the difference in the partition coefficient of each component in the stationary phase and mobile phase of the chromatographic column to achieve the purpose of separation.
    The detector detects and scans the gas chromatogram, which is qualitatively determined by retention time and quantified by peak area

    .

    Gas chromatograph is a chemical analysis instrument used to separate compounds in complex samples
    .
    In the gas chromatograph, there is a slender hollow quartz capillary channel, which is the gas chromatographic column

    .
    In a chromatographic column, different samples have different physical and chemical properties, and have different interactions with the fixed phase on the inner wall of the column, and are driven by the gas flow (carrier gas) at different rates

    .
    When the compounds flow out from the end of the column, they are detected by the detector, and the corresponding signal is generated, which is converted into an electrical signal output

    .
    The role of the stationary phase in a chromatographic column is to separate different components so that different components flow out from the end of the column at different times (retention times)

    .
    Other factors that affect the sequence and retention time of substances flowing out of the column include the flow rate and temperature of the carrier gas

    .

    In gas chromatography, a certain amount (known amount) of gas or liquid analyte is injected into the injection port at one end of the column [usually a micro (automatic) sampler is used, or solid phase microextraction fiber (solid phase microextraction fiber) microextraction fibres)]
    .
    When the analyte is driven by the carrier gas to pass through the chromatographic column, the molecules of the analyte will be adsorbed by the stationary phase on the column wall, reducing the speed of passing through the column

    .
    The rate of molecules passing through the chromatographic column depends on the strength of adsorption, which is determined by the type of analyte molecule and the type of stationary phase

    .
    Since each type of molecule has its own passing rate, various components in the analyte will arrive at the end of the column at different times (retention time), thereby being separated

    .
    The detector is used to detect the effluent flow of the column to determine the time for each component to reach the end of the chromatographic column and the content of each component

    .
    Generally speaking, people characterize different substances by the order in which they flow out of the column (eluted) and their retention time in the column

    .

    Currently, there are more than 20 detectors that can be used in gas chromatographs
    .
    Among them, the commonly used flame photometric detector (FPD), nitrogen phosphorus detector (NPD), electron capture detector (ECD), hydrogen flame ionization detector (FID), thermal conductivity detector (TCD) and mass spectrometer ( MS) and the like

    .
    Commonly used detectors in organophosphorus analysis include ammonium phosphorus detector (NPD) and flame photometric detector (FPD)

    .
    Among them, NPD is used for the detection of nitrogen and phosphorus compounds, and FPD is mainly used for the detection of sulfur and phosphorus compounds

    .
    Organochlorine and pyrethroid compounds are detected by electron capture detector (ECD)

    .
    For example, the commonly used detector in NY/T761-2008 is the most typical example

    .
    FID, TCD and MS are universal detectors

    .
    At present, capillary columns have replaced packed columns as the dominant chromatographic column for compound separation

    .

    1.
    The main working principle of the detector

    (1) Thermal conductivity detector (TCD)
    .
    The thermal conductivity detector is a universal non-destructive concentration detector, which can theoretically be applied to the detection of any component

    .
    But because of its low sensitivity, it is generally used for constant analysis, mainly for inorganic gas and organic matter analysis

    .
    The main principle of TCD: It works based on the principle that different components and carrier gas have different thermal conductivity

    .
    Thermal conductivity detector sensitive element is a hot wire, such as gold-plated tungsten wire, platinum, gold and the like

    .
    When the measured component enters the thermal conductivity cell together with the carrier gas, since the thermal conductivity of the mixed gas is different from that of the pure carrier gas (usually lower than the thermal conductivity of the carrier gas), the heat transfer from the hot wire to the pool wall also occurs.
    The change causes the temperature of the hot wire to change, and its resistance also changes accordingly, which in turn causes an unbalanced potential at the output of the bridge to be output as a signal, and the signal is recorded to obtain the chromatographic peak

    .

    (2) Flame ionization detector (FID)
    .
    FID is a multi-purpose destructive quality universal detector with high sensitivity and wide linear range.
    It is widely used in the constant and trace detection of carbon-containing organic matter

    .
    The main principle: hydrogen and air burn to generate flames.
    When organic compounds enter the flame, due to the ionization reaction, ions that are several orders of magnitude higher than the base current are generated; under the action of an electric field, these positively charged ions and electrons are directed to the negative electrode and the electrons respectively.
    The positive electrode moves to form an ion current; after this ion current is amplified by the amplifier, it can be detected

    .

    (3) Flame-photometric detector (FPD)
    .
    FPD is a mass selective detector, mainly used to determine sulfur and phosphorus compounds

    .
    When in use, the amount of hydrogen introduced must be more than the amount of hydrogen required for normal combustion, that is, a flame can be obtained by burning under hydrogen-rich conditions

    .
    It is widely used in the analysis of trace sulfur compounds in petroleum products and organophosphorus compounds in pesticides

    .
    Main principle: When the components burn in a hydrogen-rich flame, the components become fragments or molecules to varying degrees, and are excited due to the collision of outer electrons; when the electrons return from the excited state to the low energy state or the ground state, they emit characteristic wavelengths The characteristic spectrum is measured after passing through the filter

    .
    For example, sulfur produces a spectrum of 350-430nm in the flame, and phosphorus produces a spectrum of 480-600nm, of which 394nm and 526nm are the characteristic wavelengths of sulfur-containing and phosphorus-containing compounds, respectively

    .

    (4) Electron capture detector (ECD)
    .
    ECD is a concentration-type selective detector, which can give a very significant response signal to electronegative components, and is used to analyze electrophilic compounds such as halogen compounds, some metal superfluous compounds and steroids

    .
    Main principle: The radioactive source in the detection chamber emits β-rays (primary electrons), which collide with the carrier gas passing through the detection chamber to generate secondary electrons and positive ions.
    Under the action of an electric field, they move to the electrodes with opposite polarities to form a base current.
    ; When the negatively charged component (that is, the component that can capture electrons) enters the detection chamber, it captures the electrons in the detection chamber and becomes negatively charged ions.
    Because the electrons are captured by the component, the base flow of the detection chamber is reduced , Generate chromatographic peak signal

    .

    (5) Nitrogen-phosphorus detector (NPD)
    .
    NPD is a highly selective mass detector that can be used to determine organic compounds containing nitrogen and phosphorus

    .
    The response mechanism mainly includes gas phase ionization theory and surface ionization theory.
    It is generally believed that gas phase ionization theory can better explain the working principle of NPD

    .
    The gas phase ionization theory believes that nitrogen and phosphorus compounds are first thermally and chemically decomposed in the gas phase boundary layer to produce an electronegative group; the electronegative group then undergoes a chemical ionization reaction with the gas atom (Rb) to generate Rb+ and anion , The negative ion releases an electron in the collector, and reacts with the hydrogen atom, and outputs the composition signal at the same time

    .

    2.
    The main scope of application of each detector

    (1) TCD has strong versatility, stable performance, maximum linear range, high quantitative accuracy, simple operation and maintenance, low cost, easy to popularize, suitable for constant and semi-micro analysis, especially suitable for permanent gas or relatively pure sample analysis
    .
    TCD is not suitable for environmental monitoring and trace analysis of food and pesticide residues.
    The main reasons are: TCD detection limit is large, sample selectivity is poor, that is, the ability to resist interference with non-detected components is poor; easy to be contaminated, and baseline stability changes Bad

    .

    (2) FID is particularly suitable for the macro-to-micro analysis of organic compounds, and is a conventional method for testing the content of active ingredients in pesticide products
    .
    Its anti-pollution ability is strong, the detector has a long life, and daily maintenance work is less

    .
    Since the FID response has a certain regularity, in the quantitative analysis of complex mixtures and multi-components, especially for general routine analysis, it is not necessary to calibrate the pure compound, which simplifies the operation and improves the work efficiency

    .

    (3) FPD is a highly sensitive and selective detector, which is particularly sensitive to phosphorus and sulfur.
    It is mainly used for the trace or trace analysis of phosphorus and sulfur in organic compounds and gas sulfides.
    the organophosphorus pesticides, Water pollution mercaptan

    .

    (4) ECD is particularly suitable for the analysis of complex multi-component and multi-interfering substances in environmental monitoring and biological samples, but some interfering substances have similar sensitivity (almost no selectivity) to the components to be quantified and quantitatively analyzed, especially in When doing trace analysis, the necessary pre-treatment of the sample should also be carried out, or the column separation should be improved to prevent qualitative errors
    .
    ECD analysis has high sensitivity to electronegative samples; ECD is sensitive to almost all operating conditions, and its high sensitivity to interfering substances and targets makes the operation of ECD more difficult, and only a small concentration of sensitive substances May cause interference to the analysis

    .

    (5) NPD has good selectivity and high sensitivity to nitrogen and phosphorus compounds, and is suitable for trace and trace analysis of nitrogen and phosphorus in samples
    .
    The sensitivity of NPD is related to the molecular structure of the compound.
    For example, when detecting nitrogen-containing compounds, it has the highest sensitivity to easily decomposed into cyano
    groups (-CN).
    Other structures, especially
    nitrates and amides, have little response
    .

    Related Links: Selection of Pesticide Residue Testing Instruments

     

     

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