In the second installment of this tutorial, the authors explain the instrumentation for measuring naturally occurring stable isotopes, specifically the magnetic sector mass spectrometer. This type of instrument remains unrivaled in its performance for isotope ratio mass spectrometry (IRMS), and the reader is reminded of its operation and its technical advantages for isotope measurements.

The first installment of this series focused on the theoretical aspects of stable isotopes and the calculation of their distribution patterns. In this second chapter, we will look at the technical measurement of stable isotopes using low-resolution magnetic sector mass spectrometers. We focus on magnetic sector instruments primarily because this mass spectrometer remains unrivaled in its performance for isotope ratio mass spectrometry (IRMS). Gas isotope ratio measurements require a much greater precision than that obtainable from other low-resolution instruments such as quadrupoles. The sector mass spectrometer also still prevails in other areas such as process control gas analysis (that is, blast furnace top gas monitoring or coke oven gas analysis), which demand exceptional long-term stability and a high tolerance of changes in the nature of the sample gas from oxidizing to reducing behavior (1).

Furthermore, in their most sophisticated form, magnetic sector instruments remain the gold standard for petroleum analysis, ultratrace level determination of organic pollutants such as polychlorinated dibenzo-para-dioxins (PCDD) and polychlorinated dibenzofurans (PCDF) by gas chromatography (GC)–MS. The second reason for concentrating on magnetic instruments in this tutorial series is the increasing difficulty of finding detailed descriptions of the sector instrument's theory and construction, particularly because the manufacturers' manuals devote less and less space to these topics. Furthermore, different classes of mass analyzers, such as quadrupoles and their derivatives, have increased in popularity to the extent that now there is a whole generation of MS practitioners, many of whom have never considered the sector instruments. Therefore, the present authors found it worthwhile to remind mass spectrometrists of the technical principles of an MS technique that still dominates particular areas of science. Readers interested in the technical descriptions of the other mass analyzers (for example, quadrupoles, ion traps, time-of-flight instruments, ion cyclotron resonance, and orbitraps) are referred to references 2 and 3.

With this in mind, this short tutorial seeks to collate details of the low-resolution magnetic sector instrument with the view of offering current and potential users of these instruments a description of their components in greater depth than is offered by elementary resources. For these, the components of the mass spectrometer will be taken as the gas inlet system, the ion source, the magnetic sector, and the detector. The construction of the vacuum chamber and its associated components, such as pumps and vacuum gauges, will not be discussed, nor will any detailed description of the electronics be given.

Gas Inlet Systems

Before we delve into the inner workings of the magnetic sector instrument, it is useful to spend a few moments on the design of the interface between the gaseous sample and the vacuum of the mass spectrometer. To understand the transport and pressure reduction elements involved requires familiarity with vacuum techniques, and in particular the characteristics of gas flow over a wide range of working pressures. A full discussion of the vacuum science and technology required is outside the scope of this tutorial; however, the salient points for our purposes are that the rate of material flow through a vacuum pump (throughput) is given by

where S is the pump speed (usually quoted in liters per second) and P the working pressure of the pump (usually quoted in millibar). Similarly, the rate of material flow though a tube or other element of the vacuum system is given by

where C is the conductance of the element, which is determined by both its physical dimensions and the operating pressure, and ΔP is the pressure drop across the element. For a more comprehensive discussion, the reader is referred to http://www.vacuumlab.com/, and the references that are cited there.