DETECTORS

This chapter describes detectors. Detectors for mass spectrometers fall into two main categories: direct measurement detectors, which detect the charge arriving at the detector, and multiplier detectors, which use electron multiplication to give a signal of sufficient intensity to enable single-ion detection and rapid scanning. The choice of detector depends on the application. Direct measurement methods are essential for high-accuracy isotope ratio measurements, as the gain of electron multiplier vanes not only with mass but also with the structure of the ion. The simplest way to detect an ion beam is to place a metal plate in its path and measure the charge falling on the plate by connecting it to an amplifier using a large feedback resistor. However, when an ion strikes a surface with the sort of energy typically used in a sector-field mass spectrometer, secondary electrons are produced. The charge associated with an electron leaving a surface has the same magnitude and polarity as a positive ion arriving at the surface, and, typically, several electrons are produced for each incident ion. © 1990, Elsevier Inc. All rights reserved.