Quantitative PCB and OCP Analysis by GC-MS/MS

Polychlorinated biphenyls (PCBs) and organochlorine pesticides (OCs) were analyzed through gas chromatography coupled with tandem mass spectrometry (GC-MS/MS). Forty-one PCBs (CB-17, 18, 28, 31, 33, 44, 49, 52, 70, 74, 82, 87, 95, 99, 101, 105, 110, 118, 128, 132, 138, 149, 151, 153, 156, 158, 169, 170, 171, 177, 180, 183, 187, 191, 194, 195, 199, 205, 206, 208 and 209) and eighteen OCPs (α-HCH, β-HCH, δ-HCH, γ-HCH, Heptachlor, Aldrin, Heptachlor epoxide, Endossulfan-I, p-p’DDE, Dieldrin, Endrin, Endossulfan-II, p-p’DDD, Endrin aldeide, Endossulfan sulphate, p-p’DDT, Endrin cetone and Metoxychlor) were determined using certified standards (AccuStandard ®, Quebec Ministry of Environment Congener Mix (C-QME-01) and Pesticide Mix (Z-014C-R), respectively).
Standard working mixtures of all compounds were prepared from stock solutions by dilution to obtain the following concentrations: 0.50, 1.0, 2.0, 5.0, 8.0, 10, 15, 20 and 25 ng m L⁻¹. The analyses were performed using a Thermo Scientific Trace GC Ultra model coupled to a Thermo Scientific TSQ Quantum XLS tandem mass spectrometer. Chromatographic conditions are detailed in Table 1.
The GC-MS/MS analysis, using a triple-quadrupole analyzer (QqQ), was performed in the selective reaction monitoring mode (SRM). This method results in higher selectivity than traditional mass spectrometry, as it monitors the fragmentation pattern between the precursor ion (Q1) and the product ion (Q3) obtained in the second quadrupole (q) [28 (link),29 (link),30 (link)]. The GC-MS/MS method is widely applied to samples containing low compound concentrations and to complex matrices, as it reduces spectral interferences being, therefore, more selective [30 (link)]. The triple quadruple fragmentation patterns (two precursor-products for each analyte) used for identification and quantification are detailed in Table 2. The method limit of quantification (LOQ) corresponds to ten times the standard deviation of background signal to noise of the lowest level of the curve. The LOQ ranged from 0.12 to 0.38 ng g⁻¹.
The PCBs, Pesticides, surrogates and internal standards were obtained from Accustandard ® (New Haven, CT, USA). Sorbent materials used for column chromatography comprised silica gel 60 (0.063–0.200 mm) (CAS-No 112926-00-8; Supelco, Saint Louis, MO, USA) and aluminum oxide 90 active neutral (CAS-No 1344-28-1; Supelco, Saint Louis, USA), both acquired from Merck (Rio de Janeiro, Brazil). Sodium sulfate (CAS-No 7757-82-6) was acquired from Sigma-Aldrich (Saint Louis, USA).
Method precision was estimated based on the Residual Standard Deviation (RSD) $⩽20\%$ obtained by analyzing seven replicate samples fortified with 5 ng of all targeted compounds (RSD—9 ± 6 %). Samples of the IAEA-435 certified material—Tuna Homogenate—were extracted and quantified as part of the analytical control process. The evaluation of IAEA-435 results was performed through normalized error (equation 1), a relation between the mean and uncertainty of the certified reference material ( ${\overline{x}}_1\pm u_1$ ) and the achieved results ( ${\overline{x}}_2\pm u_2$ ). Normalized error values lower than 1 are considered conforming or passing, and outside of this value (≥1) are considered nonconforming or failing. All values for normalized error in this research were within an acceptable range (<1; 0.61 ± 0.17).
$$\frac{|{\overline{x}}_1-{\overline{x}}_2|}{\sqrt{u_1^2+u_2^2}}$$