INTRODUCTION

Basic analytical measurements in isotope geochemistry are absolute and relative abundances of isotopes of an element available in a solid, liquid, or gaseous form. X-ray Fluorescence (XRF) Spectrophotometer, Atomic Absorption Spectrophotometer (AAS), Inductively Coupled Plasma Atomic Emission Spectrophotometer (ICP-AES), and Electron Probe Micro Analyzer (EPMA) are commonly used for elemental analysis (Potts, 1987). These instruments rely on the excitation of orbital electrons and detection of the resulting emission of radiation, or absorption of external radiation, characteristic of each element. Isotopes of an element have the same electronic structure (Chapter 1), and, hence, are not distinguished by the above instruments. Mass sensitive instruments, called mass spectrometers, have become the preferred instruments for isotope geochemistry (De Laeter, 1998). Many types of mass spectrometers have been developed over the last century, but most of them consist of three main components:

PRINCIPLES OF MASS SPECTROMETRY

Figure 6.1 is the schematic of a very old design of a mass spectrometer (Dempster, 1918) showing all the three basic components. Consider a beam of two singly charged isotopic ions ‘a’ and ‘b’ of mass ma and mb (ma + δm), respectively, produced with negligible kinetic energy in front of a stack of metal plates with narrow rectangular slits, known as the ion source. The beam is accelerated by falling through a potential of V volts distributed between the plates of the ion source, and collimated to emerge as a narrow rectangular beam from the final slit of width, ws, called the Source Slit (SS).