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Hodgson E. Modern toxicology [2004].pdf
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460 ANALYTICAL METHODS IN TOXICOLOGY

respect to one another by bending or stretching covalent bonds. The vibrational motions are very useful in identifying complex molecules, because functional groups (e.g., OH, C, O, SH) within the molecule have characteristic absorption bands. The principle functional groups can be determined and used to identify compounds in cases in which chemical evidence permits relatively few possible structures. Standard IR spectrophotometers cover the spectral range from 2.5 to 15.4 Nm (wave number equivalent to 4000–650cm1) and use a source of radiation that passes through the sample and reference cells into a monochromator (a device to isolate spectral regions). The radiation is then collected, amplified, and recorded. Current instruments use microprocessors, allowing a number of refinements that have increased the versatility of IR instruments so that more precise qualitative and quantitative data can be obtained.

Ultraviolet/Visible–Spectrophotometry(UV/VIS). Transitions occur between electronic levels of molecules producing absorptions and emissions in the visible (VIS) and UV portions of the electromagnetic spectrum. Many inorganic and organic molecules show maximum absorption at specific wavelengths in the UV/VIS range, and these can be used to identify and quantitate compounds. Instruments designed to measure absorbance in the UV/VIS portions of the spectrum (190–700 nm) have been used in many specific purposes, such as detectors in HPLC and CE. These detectors use small flow cells having short path lengths (approximately 10 mm) and hold small volumes (e.g., 10.0 µL) through which light at a specific wavelength passes. Basic spectrophotometers have the same components as the IR instruments described previously, including a source (usually a deuterium lamp) monochromator, beam splitter, sampler and reference cells, and detector.

Nuclear Magnetic Resonance (NMR). Nuclear magnetic resonance (NMR) detects atoms that have nuclei and possess a magnetic moment. These are usually atoms containing nuclei with an odd number of protons (charges). Such nuclei can exist in two states: alow-energystate with the nuclear spin aligned parallel to the magnetic field and ahigh-energystate with the spin perpendicular to the field. Basically the instrument measures the absorption or radiowave necessary to change the nuclei from a lowto ahigh-energystate as the magnetic field is varied. It is used most commonly for hydrogen atoms, although13C and31P are also suitable. Because the field seen by a proton varies with its molecular environment, such molecular arrangements as CH3, CH2, and CH give different signs, providing much information about the structure of the molecule in question.

25.2.7Other Analytical Methods

The instruments discussed earlier are the primary ones used in toxicant analysis, but an enormous number of analytical techniques are used in the field. Many of the instruments are expensive (e.g., Raman spectrometers, X-rayemission spectrometers) and few laboratories possess them. Many other instruments are available, however, such as thespecific-ionelectrode, which is both sensitive and portable.Specific-ionelectrodes have many other advantages in that sample color, suspended matter, turbidity, and viscosity do not interfere with analysis; therefore many of the sample preparation steps are not required. Some of the species that can be detected at ppb levels are ammonia,