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1 Introduction
Polychlorinated biphenyls (PCBs) are biphenyl compounds in which a hydrogen atom attached to a carbon atom is replaced by chlorine. It has 209 isomers and homologues and is one of environmental persistent organic pollutants. Listed as one of the 12 categories of POPs that are prioritized in the Stockholm Convention. PCBs are important endocrine disruptors, teratogenic, carcinogenic and mutagenic, and are important organic pollutants that endanger human health. Unlike conventional contaminants, such POPs are extremely difficult to degrade in the natural environment and can be transported through carriers such as water or air, resulting in global spread of pollution.
At present, the detection of PCBs is mostly performed by gas chromatography or GC/MS. However, the selectivity and specificity of gas chromatography detection are poor. The pretreatment process of GC/MS is cumbersome and the detection limit is high. Both of them can not meet the requirements of complex matrix trace residue detection. Therefore, it is necessary to study anti-interference ability, qualitative ability and The detection limit can meet the required trace detection technology.
This article describes the use of Thermo Fisher Scientific's new generation of triple quadrupole gas chromatography mass spectrometry to analyze and detect six PCBs in aquatic products. The secondary mass spectrometry scan has been used to reduce the number of complex matrix samples. The influence of background interference improves the detection sensitivity of the target compound. The method has the advantages of high sensitivity, good stability and wide linear range.
2. Experimental part
2.1 Instruments and Reagents <br>Mass Spectrometers: Thermo Scientific TM TSQ 8000 Evo Mass Spectrometer (Thermo Fisher Scientific, USA);
Gas Chromatograph: Thermo Scientific TM Trace1310 GC with AI l310 Autosampler (Thermo Fisher Scientific, USA);
Column: TG-5MS 30 m*0.25 mm*0.25 μm capillary column;
Reagents: dichloromethane, acetone, n-hexane (agricultural residue level);
2.2 Instrument Method <br> Gas Phase Method:
Column oven: 60 oC for 1 min, rise to 300 oC at 20 oC/min for 5 min;
Inlet: no split injection, no split time: 1 min;
Liner: inert without split (item number: 453A1925), inlet temperature is 270 oC;
Carrier gas: constant flow, 1 mL/min; transmission line: 300 oC.
Mass spectrometry method: The ion source temperature was 300 oC, using the Acquisition-Timed method, SRM scanning, and the specific detection ion pair is shown in Table 1: 
2.3 pre-processing methods
2.3.1 Extract accurately and weigh the homogenized fish sample 5.0 g (accurate to 0.01 g), place it in a 50 mL polypropylene centrifuge tube with a screw cap, add 8 mL of water, and vortex well on the rapid mixer. After mixing for 1 min, accurately add 15.0 mL of acetonitrile, vortex and mix for 2 min, add QuEChERS salting-out agent (6.0 g anhydrous magnesium sulfate, 1.5 g of sodium chloride), shake quickly, and cool in ice bath, 7000 Centrifuge for 10 min at r/min, and remove 8 mL of the supernatant into another 15 mL polypropylene centrifuge tube with a screw cap for purification.
2.3.2 Purification QuEChERS purification powder (600 mg anhydrous magnesium sulfate, 500 mg neutral alumina and 300 mg PSA) was added to a centrifuge tube containing 8 mL of extract, vortexed for 1 min, centrifuged at 7000 r/min. Min. Accurately transfer 5.0 mL of supernatant to a 10 mL stoppered test tube, concentrate to near dryness in a 45 ° C water bath, add 1 mL of dichloromethane to reconstitute, sonicate for 30 s, vortex to mix, pass 0.22 μm filter , to be measured on the machine.
3. Analysis of experimental results
3.1 Chromatographic Separation Results We can determine the SRM mass spectrometry conditions (parent ion-subion-collision energy) of each compound by optimizing the collision energy by Auto-SRM mode. See Table 1 under this condition, and 6 kinds of PCBs can be obtained by running the sample under this condition. The SRM chromatogram is shown in Figure 1. 
3.2 Standard curve and minimum limit of quantitation <br> According to EU standards, standard substances are added to the negative samples, and the concentration of PCBs is 0.1 μg/L, 0.5 μg/L, 2.5 μg/L, 5 μg/L, according to 2.2. Chromatography-mass spectrometry. Take S/N=3 as the detection limit and S/N=10 as the quantitative limit to calculate the signal-to-noise ratio. The quantitative limit spectrum is shown in Figure 2. As can be seen from Figure 2, the S/N of the six PCBs is greater than 10. Taking the standard solution concentration as the abscissa (X), the chromatographic peak area of ​​the quantitative ion pair is plotted on the ordinate (Y), and the regression equation is obtained. The linear relationship of the six PCBs in the range of 0.1-5 μg/L is good, the correlation coefficient is greater than 0.992, and the linear equation is shown in Fig. 3. The detection limit of this method is better than the reported value in the literature, which can ensure the qualitative and quantitative detection of the five PCBs in the sample.
3.3 Determination of method precision and spike recovery rate <br> The actual sample sold in the market was determined by this method, and no positive sample was found. A standard addition method was used to add a standard of 0.25 μg/L and 2.5 μg/L to the grass carp blank sample for recovery test. The chromatogram of the sample spiked spectrum and the six actual samples is shown in Figure 4. After the continuous injection of 6 needles at a concentration of 0.25 μg/L, the RSD values ​​are calculated. The results are shown in Tables 2 and 3. The data in Tables 2 and 3 show that the recovery of the six PCBs residues is 80%–128.9%, and the relative standard deviation (RSD) is 2.78%–4.89%. The recovery and precision of the method can meet the relevant regulations. 
4. Conclusions This method uses the new generation of triple quadrupole mass spectrometer TSQ 8000 Evo of Thermo Scientific to determine the residues of six polychlorinated biphenyls in aquatic products with high sample extraction rate and convenient operation. The instrument has the advantages of good selectivity, wide linear range and high sensitivity. At the same time, the ion pair scanning provided by the TSQ 8000 Evo can greatly eliminate the interference of false positives, thus making the detection results more accurate. At the addition levels of 0.25 μg/L and 2.5 μg/L, the recovery ranged from 80% to 128.9%. The relative standard deviation (RSD, n=6) at the concentration level of 0.25 μg/L was 2.78–4.89%. The method has a minimum limit of quantification of 0.1 μg/L, which fully meets the requirements for detection of PCBs in the European Union and the United States.
Determination of Six Polychlorinated Biphenyls in Aquatic Products by Triple Quadrupole Gas Chromatography-Mass Spectrometry
Li Chunli Thermo Fisher Scientific (China) Co., Ltd.