A method of targeted mass spectrometric analysis is provided for analyzing trace compounds at sub-ppb level compared to sample matrix. Sample is chromatographically separated at standard conditions to employ a map of target mass (M) versus retention time (RT). Small mass ions under M(RT) are rejected by RF field, and remaining ions are accumulated for pulsed injection into a multi-reflecting TOF MS, either directly from EI source, or from linear RF trap or via a heated RF only quadrupole with axial ion trapping. In combination with EI source the method provides sub femtogram sensitivity at matrices loads in microgram range.