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pesticides residue analysis gc ms ms mass spectrometry lc ms ms codex efsa mrl m/z linear trap single ion monitoring collision focusing rf-only time-of-flight axial and radial ejection staple diagram c trap orbitrap collision cell quadrupole apci api esi c 18 solid phase selectivity peak separation residue analysis mobile phase column hplc flashback solvent condensation retention time gc column liner splitless injection split injection gas chromatography luke method sweden pesticide extraction methode quchers analyses mrm gap cxl usfda