Page 19 - An Introduction to Analytical Atomic Spectrometry - L. Ebdon

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Mass analysis is a relatively simple technique, with the number of ions detected being directly
proportional to the number of ions introduced into the mass spectrometer from the ion source. In
atomic mass spectrometry the ion source produces atomic ions (rather than the molecular ions formed
for qualitative organic analysis) which are proportional to the concentration of the element in the
original sample. It was Gray who first recognized that the inductively coupled plasma would make an
ideal ion source for atomic mass spectrometry and, in parallel with Fassel and Houk, and Douglas and
French developed the ion sampling interface necessary to couple an atmospheric pressure plasma with
a mass spectrometer under vacuum.

1.2 Basic Instrumentation

1.2.1 Optical Spectroscopy.

Figure 1.2 shows the basic instrumentation necessary for each technique. At this stage, we shall define
the component where the atoms are produced and viewed as the 'atom cell'. Much of what follows will
explain what we mean by this term. In atomic emission spectroscopy, the atoms are excited in the atom
cell also, but for atomic absorption and atomic fluorescence spectroscopy, an external light source is
used to excite the ground-state atoms. In atomic absorption spectroscopy, the source is viewed directly
and the attenuation of radiation measured. In atomic fluorescence spectroscopy, the source is not
viewed directly, but the re-emittance of radiation is measured.

Current instrumentation usually uses a diffraction grating as the dispersive element and a
photomultiplier as the detector, although solid-state detectors are becoming more widespread.

Historically, data collection and manipulation were effected by means of analogue meters, chart
recorders, digital displays and paper print-outs. However, the advent of the microcomputer now allows
data to be stored electronically, calibrations performed and concentrations calculated and reported on a
user-defined form.

1.2.2 Mass Spectrometry

Figure 1.2 shows the basic instrumentation for atomic mass spectrometry. The component where the
ions are produced and sampled from is the 'ion source'. Unlike optical spectroscopy, the ion sampling
interface is in intimate contact with the ion source because the ions must be extracted into the vacuum
conditions of the mass spectrometer. The ions are separated with respect to mass by the mass analyser,
usually a quadrupole, and literally counted by means of an electron multiplier detector. The ion signal
for each