Idealized schematics of QQQ MS used in SRM analysis. The first quadruple (Q1) filters out most co-eluting ions from the chromatographic system. However, interfering ions may pass Q1 and enter the second quadruple (Q2). Ions in Q2 are fragmented and form the input of the third quadruple (Q3). Ideally, the specific m/z selection in Q3 passes only fragments of the desired ion and eliminates interfering ions.