Other Instrumental Methods in Environmental Analysis - Fundamentals of Environmental Sampling and Analysis - page 310

their general principles and environmental applications. At the end of this chapter,
further readings are provided for readers at both the elementary and the advanced
levels.
12.1 HYPHENATED MASS SPECTROMETRIC
METHODS AND APPLICATIONS
A hyphenated method is a combination of two instrumental techniques that offer
superior analytical results than either of the original individual methods. Three
major hyphenated mass spectrometric methods that are of importance in
environmental analysis are described below. They are inductively coupled plasma-
mass spectrometry (ICP-MS), gas chromatography-mass spectrometry (GC-MS),
and liquid chromatography-mass spectrometry (LC-MS). Recall from two previous
chapters that we have discussed ICP-OES, GC, and HPLC. The hyphenated methods
bear some similarities to these methods, but they are also fundamentally different
since a different detector (mass spectrometer) is used. In the following discussion,
we therefore focus on the general principles of mass spectrometer and the unique
features of hyphenated instrumental components. Major environmental applications
and the U.S. EPA methods using ICP-MS, GC-MS, and LC-MS will then be
described.
12.1.1 Atomic Mass Spectrometry (ICP-MS)
General Principles In ICP-OES (Chapter 9), elements are ionized by ICP and then
measured based on the emission of elemental ions by an optical device at a
wavelength characteristic of the element in the UV-VIS range. In ICP-MS, elements
are ionized by ICP in the same way as in ICP-OES. However, rather than separating
emission light according to their wavelengths, the mass spectrometer separates ions
according to their mass-to-charge ratios (m/z). The m/z of an atomic ion or
molecular ion is the mass of an ion (m) divided by the number of charge (z). For
example, the m/z of 24 Mg þ (atomic ion) is 24, whereas the m/z of 12 CH 4 (molecular
ion) is 16. Since most of the ions in mass spectrometers are singly charged, the m/z
ratio is simply equal to the mass of the ion. Because ions are counted in ICP-MS, the
numbers (abundance) of ions are used as the basis for quantitative measurement. It is
also important to note that the mass spectrum is, therefore, a plot of abundance vs.
m/z rather than the optical UV-VIS spectrum in ICP-OES which is a plot of light
emission vs. wavelength. While the atomic mass spectrum in ICP-MS is used for
element determination, the molecular mass spectrum in GC-MS and LC-MS is of
prime importance for organic structural identification. Molecular mass spectra will
be introduced in Section 12.1.2.
Instrumental Components A schematic diagram of an ICP-MS system is
shown in Figure 12.1. A typical ICP-MS instrument consists of the following:
(a) sample introduction system, (b) ICP torch (making ions), (c) interface (sampling
ions), (d) vacuum system (pumps), (e) lens (focusing ions), (f) quadrupole