7 AnalyzerDetector227DetectorThe detector in the MSD analyzer is a high energy conversion dynode (HED) cou-pled to an electron multiplier (EM). The detector is located at the exit end of thequadrupole mass filter. It receives the ions that have passed through the mass fil-ter. The detector generates an electronic signal proportional to the number of ionsstriking it. The detector has three main components: the detector focus lens, thehigh energy dynode, and the electron multiplier horn.Detector focus lensThe detector focus lens directs the ion beam into the HED, which is located offaxis. The voltage on the detector focus lens is fixed at -354 V.High energy dynodeThe high energy dynode (HED) operates at -10,000 volts, attracting the positivesample ions exiting the quadrupole. The HED is located off-axis from the center ofthe quadrupole mass filter to minimize signals due to photons, hot neutrals, andelectrons coming from the ion source. When the ion beam hits the HED, electronsare emitted. These electrons are attracted to the more positive electron multiplierhorn.Electron multiplier hornThe electron multiplier horn carries a voltage of up to -3000 volts at its openingand 0 volts at the other end. The electrons emitted by the HED strike the EM hornand cascade through the horn, liberating more electrons as they go. At the far endof the horn, the current generated by the electrons is carried through a shieldedcable outside the analyzer to the signal amplifier board.The voltage applied to the electron multiplier horn determines the gain. The volt-age is adjustable from 0 to -3000 V dc. Use the electron multiplier voltage foundin autotune as a baseline for the electron multiplier voltage setting.• To increase signal strength, increase the electron multiplier voltage.• For concentrated samples where less signal strength is needed, decrease theelectron multiplier voltage.
