Other Instrumental Methods in Environmental Analysis - Fundamentals of Environmental Sampling and Analysis - page 313

isotopes of an element, and then the ratio of two or more isotopes can readily be
determined. The isotope ratio information can be used in geological dating of rocks,
nuclear applications, determining the sources of a contaminant, and biological tracer
studies. In isotope dilution, an unknown sample is spiked with an enriched isotope
(usually of minor abundance) of the element of interest. Isotope dilution mass
spectrometry provides high accuracy and precision because the spiked isotope
serves as both a calibration standard and an internal standard.
12.1.2 Molecular Mass Spectrometry (GC-MS
and LC-MS)
Unlike the atomic mass spectrometric ICP-MS methods, GC-MS and LC-MS are
molecular mass spectrometric methods used for the determination of molecular
structure. As we learned in Chapter 10, the GC or HPLC technique alone
(without mass spectrometer) provides retention time as the only information to
help examine a compound's identity. The structural information provided by GC-
MS and LC-MS, on the contrary, are definite. They are also complementary to
two other major techniques for structural identification, that is, IR (Chapter 8)
and NMR (Section 12.2). For example, GC-MS and LC-MS are able to provide
analysts with unique information about the molecular weight. In the following
condensed discussion, we describe the general principles and instrumental
components of both GC-MS and LC-MS. The readers should, however, be aware
that although these two molecular mass spectrometric techniques share some
striking similarities in general, the key components, operational principles, and
applications areas are fundamentally different.
General Principles and Instrumental Components
of GC-MS and LC-MS
Regardless of many variations, the block diagrams shown in Figure 12.3 can be used
to illustrate the general principles and instrumental components of GC-MS and LC-
MS. Both techniques are based on the chromatographic separation of molecules from
the mixture, the generation of gaseous ions from analyte molecules, the separation of
ions according to their m/z ratios, and the subsequent detection of these ions. Hence
the major components include: (a) sample introduction, (b) analyte ionization, (c)
interface between chromatographic and mass spectrometric components, (d) vacuum
system, (e) mass analysis (mass spectrometer), (f) ion detection, (g) data handling and
system controller. Note that in GC-MS and LC-MS, the chromatographic parts are
used only for compound separation (sample introduction to the point of ionization).
These major components, to a certain extent, are also similar to an ICP-MS system
described previously. For example, the quadrupole mass filter is the same as that
which can be used in GC-MS and LC-MS, although the m/z ranges may be different
among various mass spectrometric techniques.
The discussions that follow focus only on the interface and the ionization. The
scope of this text prevents us from a detailed discussion on all ionization techniques