Once the ions have been separated by their mass-to-charge ratio, they must then be detected or counted by a suitable detector. The fundamental purpose of the detector is to translate the number of ions striking the detector into an electrical signal that can be measured and related to the number of atoms of that element in the sample via the use of calibration standards. Most detectors use a high negative voltage on the front surface of the detector to attract the positively charged ions to the detector. Once the ion hits the active surface of the detector, a number of electrons is released which then strike the next surface of the detector, amplifying the signal. In the past several years, the channel electron multiplier (CEM), which was used on earlier ICP-MS instruments, has been replaced with discrete dynode type detectors (see Figure 6). Discrete dynode detectors generally have wider linear dynamic ranges than CEMs, which is important in ICP-MS as the concentrations analyzed may vary from sub-ppt to high ppm. The discrete dynode
type detector can also be run in two modes, pulse-counting and analog, which further extends the instrument’s linear range and can be used to protect the detector from excessively high signals.