Quantitative analysis of pesticides in QuEChERs extracts using APGC/MS/MS
Dominic Roberts and Antonietta Wallace
purpose
The pesticides in vegetables and fruits were extracted by the QuEChERs method, and the components separated by GC were detected and quantified. Demonstrate the detection limits and linearity that can be achieved using APGC/MS/MS.
background
Pesticides are widely used in agricultural production around the world. In order to ensure the safety of food supplies, the pressure on regulators and food producers is increasing. The problem of pesticide residues has received a lot of attention from consumers. Therefore, laboratories need to be able to screen out as many pesticides as possible from the sample in a proper time by a single analysis method. Pesticide residues are clearly defined in most countries. Legislation stipulates that the determination of the maximum residue of pesticides in food commodities is based on sensitive, accurate and reliable analytical techniques.
Multi-residue analysis is difficult because of the need for lower detection limits for multiple pesticides in a variety of food products to meet maximum residue requirements. At present, more than 1,000 pesticides are known to be put into use, and the pressure on laboratories is increasing in order to expand the scope of detection of conventional monitoring and analysis methods. Typically, such analysis is performed using a combination of GC-MS/MS (with EI source) and LC-MS/MS. EI is a "hard" ionization technique that produces more in-source pyrolysis fragments.
APGC/MS/MS can sensitively and accurately detect a variety of pesticide residues extracted by QuEChERs in fruits and vegetables.
Atmospheric pressure gas chromatography (APGC) is a "soft" ionization technique that produces fewer fragments, thus increasing the sensitivity and selectivity of molecular ions. The APGC source is easily interchangeable with the ESI source to form a single MS platform for LC and GC pesticide analysis, providing a complete pesticide residue analysis solution.
In this technical brief, we present a new analytical strategy for targeted analysis of trace pesticide residues in strawberries, pears, spinach and tomatoes.
Solutions <br> using DisQuE ™ QuEC hERS (CEN) method strawberries, pears, tomatoes and spinach samples were extracted to obtain the extract matrix blank acetonitrile. A pesticide mix was added to each matrix to make a nine-point calibration curve ranging from 0 to 50 ng/mL (μg/Kg). Prior to analysis, an internal standard at a fixed concentration of 2 ng/mL was added to each vial as the injection standard. All standards were analyzed in parallel three times using a Waters® Xevo TQ-S equipped with an APGC source and a 7890A GC. Two MRM channels for each pesticide were monitored, the most abundant ions were used for quantification, and the less abundant ions were used for confirmation. Under EI conditions, it is difficult to analyze 20 pesticides due to excessive fragmentation. Depending on the source operating conditions, the analyst can choose to promote proton transfer or charge transfer as the primary ionization mode. In the presence of a proton donor (under wet conditions), APGC typically produces a spectrum containing predominantly [M + H] + ions. If the parent ion intensity is high, a specific and sensitive MRM channel will be produced. In contrast, for the EI source, many pesticide MRM channels use low molecular weight, low specific fragment ions as precursor ions. These properties of APGC ensure reliable identification and quantification of pesticides in a variety of fruit and vegetable matrices.
Figure 1 shows a typical calibration curve and residual deviation plot of methyl azinphos-methyl obtained by three injections of a substrate spike of strawberry extract. The calibration curve was linear over the range of 0.05 to 50 ng/mL with a correlation coefficient R 2 of 0.997. All residues are less than 20%, indicating that the APGC system has good linearity and high reproducibility. The detection limits and linearity of all 20 pesticides analyzed using APGC-Xevo TQ-S are summarized in Table 1. The detection limits for all 20 pesticides ranged from 0.01 to 0.5 ng/mL with good linearity and R 2 >0.99. Figure 2 shows the APGC-MS/MS spectrum obtained with 1 ng/mL epoxy heptachlor B solvent and an extract from four different sample matrices. It was observed that there was no significant matrix effect and retention time, peak shape and response showed good reproducibility. The response of this compound was observed to be slightly enhanced only in some sample extracts (tomatoes and pears). These data indicate that the use of APGC with Xevo TQ-S has observed an increase in the strength of the parent ion, allowing sensitive routine detection of pesticides suitable for GC analysis in fruit and vegetable matrices.
to sum up
APGC is a soft ionization technique that produces abundant [M + H] + ions that help pesticides produce highly selective and sensitive MRM channels. Waters offers a universal ionization source that enables APGC, UPLC® or ACQUITY UPC 2 ® to be quickly and easily combined on a single MS platform to maximize MS system utilization in the lab. When used in conjunction with Xevo TQ-S, APGC is able to accurately quantify low levels of pesticides in a variety of sample matrices.
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