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    Home > Chemicals Industry > Chemical Technology > GC/MS actual detection spectrum analysis

    GC/MS actual detection spectrum analysis

    • Last Update: 2021-09-07
    • Source: Internet
    • Author: User
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    GC/MS has two ion acquisition modes, full scan (SCAN) and selective ion scan (SIM)
    .
    In order to determine the retention time of each compound before method development, a full scan method is used to determine the respective peak positions

    .
    However, if the full scan method of GC/MS is used for the routine detection and analysis of pesticides, the detection limits of most of the pesticides tested will not meet the requirements

    .
    In order to meet the detection limit requirements, GC/MS pesticide residue analysis usually adopts the selective ion method, with fewer ions selected, and the background ions are not detected, the signal-to-noise ratio is greatly improved, and the detection limit is lower

    .
    Therefore, it is necessary to divide the adjacent compounds into the same group according to the peak situation.
    Each compound can scan three ions to reduce the signal of background ions.
    Figure 2-44 shows the selected ion scanning mode of 52 pesticide standards.
    Total ions Flow chromatogram

    .

    Figure 2-44 Total ion current chromatogram of 52 pesticides mixed standard

    The grouping of the selected ion scan mode is carried out according to the following principles: each pesticide selects a quantitative ion (target ion), and selects 2 to 3 reference ions according to the number of compounds in each group
    .
    It must be avoided that it is the same as the background ions of interfering substances such as sample matrix, column bleed or solvent.
    Generally, ions with a larger number of ions and a higher response value should be selected comprehensively.
    The test shows that the mass-to-charge ratio (m/2) is below 100.
    Choose carefully

    .
    For example, the
    phorate -based peak is 75 and the dimethoate peak is 87, but many sample co-extracts will produce these fragment ions, so these ions cannot be selected, so 75 can be replaced with 231, and dimethoate 87 can be replaced with 229; Dichlorvos 145 is easily interfered by ions in the matrix and can be changed to 220
    .
    If there are many compounds in a group, in order to have enough cycles, the residence time of each ion will be less, or the number of ions selected for each compound will be reduced (each compound cannot be less than 2 ions), so that The number of cycles per second of the chromatographic peak is greater than 2, so that there are enough sampling points

    .
    Ideally, select the ion scan mode to have the largest number of ion groups, so that the number of ions in each group is the least, and each ion group can increase the number of scans per unit time, so as to obtain more accurate quantitative results and good peak shapes

    .
    However, there should be a certain time interval between different groups, at least 0.
    2 to 0.
    5 min

    .
    Similar isomer compounds are best classified into the same group because of the same fragment ions; halogen-containing compounds try to choose their specific isotope ion group

    .
    For quantitative ions, you need to choose carefully.
    If the base peak ion is greater than 100, it should be the preferred choice, but it must be ensured that it is not interfered by other source ions, otherwise it must be replaced

    .
    Table 2-14 lists the main fragment ions and possible sources of contaminants that must be avoided by the selective ion scanning method

    .

    Table 2-14 Selected ion scanning mode The main fragment ions and pollutant sources that must be avoided by the selected ions

    The following shows a qualitative and quantitative example of chlorpyrifos by a GC/MS gas- mass spectrometer .
    Figure 2-45 is the standard information.
    In one interface, the total ion current graph, ion overlap graph, target substance spectrum graph, and standard curve are included.
    And quantitative results

    .
    Figure 2-46 shows the qualitative and quantitative detection results of chlorpyrifos in the sample (each ion peaks accurately and the ratio meets the requirements)

    .

    Figure 2-45 Standard GC/MS processing spectrum of poisonous cicada

    Figure 2-46 Qualitative and quantitative detection results of chlorpyrifos in the sample (each ion peaks accurately and the ratio meets the requirements)

    Single-stage mass spectrometry is more prone to false positive results
    .
    A positive sample means that the target compound is detected in the sample (or exceeds the limit standard), and system contamination and other conditions that are not contained in the sample to be tested are excluded

    .
    False positive samples are mainly due to matrix interference, contamination, system residues, qualitative errors and other reasons that misjudge the sample to detect the target compound

    .
    False positive samples such as sample contamination and system residues are mainly related to the testing techniques of testers, and these situations should be avoided

    .
    The qualitative error samples are obtained by mass spectrometry analysis, which mainly involves comparing the mass spectrum of the target compound detected in the sample with the mass spectrum of the target compound of the standard product

    .
    The comparison method is mainly to see whether the mass-to-charge ratio of each ion is consistent, and it is also necessary to check whether the relative ion abundance is within the allowable range

    .
    If it is cascade mass spectrometry or ion trap mass spectrometry, it is possible to compare secondary ions or even tertiary ions

    .
    Figure 2-47 shows the positive detection result of
    permethrin in lotus root samples ; Figure 2-48 shows the false positive identification map of kungfu pyrethroid in spinach samples, where the deviation of m/z208 ion ratio in the sample is out of range; Figure 2-49 It is a screenshot of the false positive identification of pp'-DDT in cucumber, where the deviation of the ratio of m/z165 and 237 ions in the sample is out of range
    .

    Figure 2-47 Positive detection results of permethrin in lotus root samples
    a.
    Total ion current chromatogram b.
    Comparison of sample mass spectra with standard library spectra c.
    Qualitative and quantitative results


    Figure 2-48 False-positive identification map of gongfu pyrethroid in spinach (large deviation of the ratio of m/z208 ion)
    a.
    Total ion current color harmonic map b.
    Comparison of sample quality harmonic map and standard library map c.
    Qualitative and quantitative results

    Figure 2-49 Screenshot of pp'-DDT false positive identification in cucumber (m/z165, 237 ion ratio deviation is large)

    Related links: Analysis of the actual detection spectrum of liquid chromatography

     

     

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