TABLE OF CONTENTS Once ions leave the mass analyser they pass to the ion detector (except in the case of the ICR) where they generate an image current. The earliest mass spectrometers detected ions by means of photographic paper onto which line images were visualised. Many different types of detectors are now in use, the choice of which depends in part on the nature of the mass analyser. To begin, the simplest detector used in mass spectrometry, the so-called Faraday cup or cage will be considered.
The Faraday cup or cage consists of a long thin rectangular box arranged such that the incoming ion beam strikes the base of the box at the collector. The collector plate is usually angled steeply with respect to the ion beam in order to prevent reflected ions and secondary electrons emitted from the plate from escaping. Each collector box is typically enclosed by a second box (held at ground) that serves to shield the detector from electrical noise. A slot on the front of this box acts as a resolving slit. When positive ions strike the collector plate they are neutralised by electrons drawn from ground across a resistor. This current is amplified and recorded. Faraday cup detectors are extremely simple in their design and are an inexpensive and reliable alternative to other detectors. They are best used for quantitation and accurate mass measurements where ion currents do not change appreciably during the course of a measurement. The smallest ion...
