The physics and technology of quadrupole mass spectrometers

Abstract

The fundamental principles of quadrupole mass spectrometers (QMS) are discussed, concentrating on instruments used in vacuum applications.The mass-selective element is the quadrupole mass filter, first described in a 1956 patent. Although originally intended for isotope separation, the concept soon became applied to analytical chemistry and to residual gas analysis (RGA), also known as partial pressure analysis (PPA). The ion optical design of a typical general-purpose quadrupole RGA has altered little since the 1970s – although of course there have been major advances in electronics packaging, data capture, and automation. There is growing interest in miniaturised systems for high-pressure work.These instruments mostly use electron-impact ionization sources, based on principles established nearly 100 years ago. Those used for vacuum applications are generally of two types: an “open” design, rather like a miniature Bayerd-Alpert gauge, and a “closed” design, generally based on Nier's work.The simplest and most robust detector is a Faraday collector; alternatively an electron multiplier is used when the highest sensitivity is required.Often the quadrupole drive electronics and the amplifier are mounted directly on the vacuum flange, giving a compact assembly. Instrument control and data acquisition is almost always done via a computer interface. This gives a very versatile and powerful system. Nevertheless, to obtain the maximum benefit from any mass spectrometer it is often necessary to keep in mind the underlying physical principles.