In mass spectrometers, secondary electron multipliers are commonly used as the detector for detecting ions (see Patent Literature 1). A secondary electron multiplier has a plurality of dynodes arranged in a cascade form. When an incident ion hits the first dynode, secondary electrons are released from this dynode, which are subsequently and repeatedly multiplied. The secondary electrons thus increased in quantity are made to enter a collector and be converted into a current signal. This signal is extracted as the detection signal.
To apply an ion-accelerating voltage to each of the dynodes, a certain level of detection voltage is normally applied to the secondary electron multiplier. However, even if this detection voltage is maintained at the same level, the detection sensitivity of the secondary electron multiplier changes depending on the mass-to-charge ratio of the ion; in particular, the sensitivity becomes low at high mass-to-charge ratios. The reason for this is as follows: When introduced into an ion detector, every ion is given the same amount of kinetic energy E (=eV0). Meanwhile, the speed v of the ion is given by v=(2E/m)1/2, which means that an ion having a higher mass m travels at a lower speed v than an ion having a lower mass m. Since the efficiency of conversion from ions to secondary electrons depends on the speed v of the ion, an ion having a high mass m can produce a smaller amount of secondary electrons than an ion having a low mass m, which results in a decrease in the detection sensitivity.
To address this problem, and particularly, to improve the sensitivity to the high-mass ions, a technique is often adopted in which a conversion dynode with a high level of voltage applied is placed in front of the secondary electron multiplier so that ions initially impinge on this conversion dynode and the thereby produced secondary electrons are introduced into and detected by the secondary electron multiplier. Applying a higher level of voltage to the conversion dynode improves the detection sensitivity. Accordingly, the voltage applied to the conversion dynode (which is hereinafter called the “CD voltage”) is normally set at the highest possible level within the range where no abnormal electric discharge occurs.
Basically, the CD voltage is maintained at the same level, although it is possible to lower the CD voltage and decrease the gain when the detector may possibly be saturated due to an excessive amount of ions, as described in Patent Literature 2.