Other Instrumental Methods

There are many other instrumental methods of analysis available that are of growing interest to environmental engineering and science because of the increasing complexity of the problems that must be addressed, and the expanding concern over health and environmental impacts of very low concentrations of organic and inorganic contaminants in the environment. Only a brief review of some of these methods is given in the following.

Figure 12.33

A gas chromatograph/mass spectrometer (GC/MS) system. (Courtesy of Agilent Technologies.)

1 Mass Spectrometry m

1- as indicated in Sec. 12.4, mass spectrometry when used in conjunction with gas 1; ^,romatography (GOMS), HPLC (LC/MS), or IPC (IPC/MS) can give positive pi ^ntification and quantification for a large number of individual organic and inor-p. ' njC compounds present in water and wastewater, soils, or air, A typical GC/MS pi system is illustrated in Fig. 12.33. A mass spectrometer is an instrument that will ll" jpjt out charged gas molecules or ions according to their masses. The substance to be analyzed is vaporized and converted to positive ions by bombardment with rapidly moving electrons. The ions formed are pulled from the gas stream by an pjectrical field. The ions are accelerated in some fashion depending upon the type of instrument and are separated by their mass-to-charge ratio. A suitable detector can then record the particles of different mass either qualitatively, quantitatively, or

Figure 12.33

A gas chromatograph/mass spectrometer (GC/MS) system. (Courtesy of Agilent Technologies.)

both. Mass spectrometry is useful for tracer experiments using stable isotopes such as 15N, or for determining isotope ratios such as for oxygen when determining the ^ age of water from different sources. S

When used for the positive identification of organic materials, such as those A emerging from a gas chromatograph, the bombardment of the organic molecule by '-S the rapidly moving electrons breaks the organic molecules into a number of charged fragments. The mass-to-charge (m/z) ratio of each fragment is measured as is the ' relative quantity of each fragment. Every organic molecule has its own pattern of fragmentation when bombarded under a given set of conditions as illustrated in Fig. 12.34. By comparing the mass-to-charge ratio and density of fragments of an unknown with that of known materials, positive identification can be made. MS is a powerful analytical method that is now widely used in environmental analysis. In : combination with GC, HPLC, and IPC, it is helping to solve many difficult analytical problems and is greatly adding to our knowledge of the nature and fate of organic materials in the environment, and in developing technical measures for their control.

Figure 12.34

Mass spectrum for ethylbenzene as obtained with a GC/MS system. The m/z value refers to the mass-to-charge ratio for molecular ions resulting from electron impact on the molecule. The base peak at 91 corresponds to the ion formed from loss of a methyl group (!2QH3 = iaCH2+), while the smaller peak at 106 represents !3C6H52C2Hj. The smaller peaks adjacent to the larger peaks are generally attributable to ions having the same chemical formula, but different isotopic compositions. For example, a l3C atom in the ion would increase the mfz by one unit.

Figure 12.34

Mass spectrum for ethylbenzene as obtained with a GC/MS system. The m/z value refers to the mass-to-charge ratio for molecular ions resulting from electron impact on the molecule. The base peak at 91 corresponds to the ion formed from loss of a methyl group (!2QH3 = iaCH2+), while the smaller peak at 106 represents !3C6H52C2Hj. The smaller peaks adjacent to the larger peaks are generally attributable to ions having the same chemical formula, but different isotopic compositions. For example, a l3C atom in the ion would increase the mfz by one unit.

X-Ray Analysis

X rays are high-energy electromagnetic radiations with short wavelength. They are uSed for analytical purposes much in the same way as radiations of longer wavelength, such as visible light. X-ray absorption follows the same absorption laws as for other radiation, except that the phenomenon is on an atomic, rather than a molecular) level. X-ray absorption is used for the measurement of the presence of heavy elements in substances composed primarily of low-atomic-weight materials. An example is the determination of the quantity of uranium in solution.

X-ray emission or X-ray fluorescence is used for the study of metals and other massive samples. A sample is bombarded with high-energy X rays that are absorbed by certain elements and re-emitted as X rays of lower energy. The bombarding X rays can dislodge an electron from an atom, leaving an unstable atom with a "hole" in one of the electron shells. Stability is regained by single or multiple transitions of electrons from outer shells to fill the hole. Each time an electron is transferred, the atom moves to a less energetic state and radiation is emitted at a wavelength corresponding to the energy difference between the initial and final states. The energy of the emitted radiation is characteristic for each emitting element and so can be used for analysis. This is similar to fluorimetry as previously discussed, except for the different energies of the respective radiations. The usefulness of X-ray fluorescence for rapid analysis for many heavy metals in water has been demonstrated. The heavy metals are first concentrated by passing a water sample through filter paper containing ionexchange resins; the filter paper is then subjected to X-ray fluorescence. This technique is likely to see wider usage for water and wastewater analysis in the future.

X-ray diffraction is primarily of value for the study of crystalline material. X rays are reflected off the surfaces of crystals, and by studying the patterns of reflection as the crystalline material is rotated in the path of the X rays, much information about the structure of the material can be obtained.

Nuclear Magnetic Resonance Spectroscopy

NMR is used to detect and distinguish between the nuclei of atoms in a molecule. It is based upon absorption by the nuclei of radiation in the radio-frequency range. In order for absorption to be measured, the compound of interest must first be placed in a fixed magnetic field that causes nuclei to develop the energy states required for absorption. This method can be used to carry out specific chemical analyses or for ascertaining the structure of both inorganic and organic species. It is a highly specialized instrumental method that is finding increasing applications in the environmental field.

Radioactivity Measurements

Instrumental methods of analysis are routinely used for the measurement of radioactivity in the environment or in research studies that make use of radioactive tracers. Various instruments are available to measure particular types of radiations, the fre quency of emissions, or both. A discussion of the many different instruments available is beyond the scope of this book.

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