Instruments Spectroscopic methods

Finally, analytical methods can be compared in terms of their need for equipment, the time required to complete an analysis, and the cost per sample. Methods relying on instrumentation are equipment-intensive and may require significant operator training. For example, the graphite furnace atomic absorption spectroscopic method for determining lead levels in water requires a significant capital investment in the instrument and an experienced operator to obtain reliable results. Other methods, such as titrimetry, require only simple equipment and reagents and can be learned quickly. 

Before the advent of NMR spectroscopy, infrared (IR) spectroscopy was the instrumental method most often applied to determine the striaeture of organic compounds. Although NMR spectroscopy, in general, tells us more about the structure of an unknown compound, IR still retains an important place in the chemist s inventory of spectroscopic methods because of its usefulness in identifying the presence of certain functional groups within a molecule. 

One of the main tasks of physical organic chemistry is to study the mechanisms of chemical reactions by instrumental methods. The rapid development of various techniques and new spectroscopic methods in recent years has attracted attention to the investigation of elementary steps of reactions and the intermediates involved. In accordance with modern requirements, the description of reaction mechanisms should include the participation of relatively stable species. 

In the preceding section, we presented principles of spectroscopy over the entire electromagnetic spectrum. The most important spectroscopic methods are those in the visible spectral region where food colorants can be perceived by the human eye. Human perception and the physical analysis of food colorants operate differently. The human perception with which we shall deal in Section 1.5 is difficult to normalize. However, the intention to standardize human color perception based on the abilities of most individuals led to a variety of protocols that regulate in detail how, with physical methods, human color perception can be simulated. In any case, a sophisticated instrumental set up is required. We present certain details related to optical spectroscopy here. For practical purposes, one must discriminate between measurements in the absorbance mode and those in the reflection mode. The latter mode is more important for direct measurement of colorants in food samples. To characterize pure or extracted food colorants the absorption mode should be used. 

However, since fluorometric methods require sophisticated instrumentation, their applicabihty is limited because of cost. In conclusion, spectroscopic methods usually enable crude estimates of chlorophylls in an extract, but in most cases accurate and detailed analysis of a specific composition requires separation of the mixture into individual compounds using methods such as HPLC. 

Conventional rubber compound analysis requires several instrumental techniques, in addition to considerable pretreatment of the sample to isolate classes of components, before these selected tests can be definitive. Table 2.5 lists some general analytical tools. Spectroscopic methods such as FTIR and NMR often encounter difficulties in the analysis of vulcanised rubbers since they are insoluble and usually contain many kinds of additives such as a curing agent, plasticisers, stabilisers and fillers. Pyrolysis is advantageous for the practical analysis of insoluble polymeric materials. 

Both the infrared spectroscopic method and the polarographic method require special instruments. When instruments for both are available, the latter method seems to be preferred. Neither method has been found to be applicable to spray residues. 

The following section presents a variety of instrumental spectroscopic techniques for the determination either of molecular structure or of parameters related to molecular structure. The applicability of each method, its particular advantages as well as its limitations, are presented. It is not an exhaustive list. The spectroscopic methods are discussed in order of increasing excitation energy. 

Direct measurement of soil is most often carried out on air-dried soil and involves spectroscopic instruments and methods. For example, X-ray dispersion (XRD), X-ray fluorescence (XRF), infrared (IR) spectroscopy,... 

Why would you guess that NIR instruments are so varied when compared with other spectroscopic methods ... 

The physico-chemical changes induced in polymers following exposure to radiation can be studied by a range of spectroscopic techniques. Recent developments in instrumentation and data analysis procedures in electronic, vibrational and magnetic resonance spectroscopies have provided considerable new insights into polymer structure and behaviour. The application of these spectroscopic methods in polymer studies are reviewed with emphasis on their utility in investigations of radiation effects on macromolecules. 

The variety of spectroscopic methods now available can be used to provide considerable information on radiation effects on polymeric materials. These applications are summarized in Table I. Improvements in instrumentation and data analysis procedures are continuing and the development of new spectroscopic techniques promise new insights into polymer structure and behaviour. 

Although the instrumental techniques described here give detailed mechanistic information, they do not provide an insight into the structure of intermediates. If we, however, combine electrochemical and spectroscopic methods, this is advantageously accomplished (spectroelectrochemistry) [73]. Various spectroscopies have been coupled with electrochemical experiments, among them ESR [74], optical [75], and NMR spectroscopy [76, 77], as well as mass spectrometry [78, 79]. 

Vibrational spectroscopy, in the form of mid-IR, NIR and Raman spectroscopy has been featured extensively in industrial analyses, both quality control (QC), process monitoring applications and held-portable applications [1-6]. The latter has been aided by the need for advanced instrumentation for homeland security and related HazMat applications. Next to chromatography, it is the most widely purchased classihcation of instrumentation for these measurements and analyses. Spectroscopic methods in general are favored because they are relatively straightforward to apply and to implement, are rapid in terms of providing results, and are often more economical in terms of service, support and maintenance. Furthermore, a single spectrometer or spectral analyzer, in a near-line application, may serve many functions, whereas chromatographs (gas and liquid) tend to be dedicated to only a few methods at best. 

The instrumentation and skills involved in the use of all five major spectroscopic methods are now widely spread, but the ease of obtaining and interpreting the data from each method under real laboratory conditions varies. 

Mercury is most accurately determined by the cold vapor atomic absorption spectroscopic method. The instrument is set at the wavelength 253.7 nm. The metal, its salts and organic derivatives in aqueous solution can be measured by this method. The solution or the solid compounds are digested with nitric acid to convert into water-soluble mercury(ll) nitrate, followed by treatment with potassium permanganate and potassium persulfate under careful heating. The excess oxidants in the solution are reduced with NaCl-hydroxylamine sulfate. The solution is treated with stannous chloride and aerated. The cold Hg vapor volatdizes into the absorption cell where absorbance is measured. 

As a result, while such methods have been very useful in the past and continue to be applied for initial surveys of air quality in areas in which measurements have not been made in the past, they have generally been abandoned in favor of instrumental methods of analysis. As a result, this chapter focuses on the most commonly used instrumental, often spectroscopic, methods for measuring air pollutants, trace gases, and particles in air (e.g., see Roscoe and Clemitshaw, 1997). The focus is on tropospheric measurements, although, in most cases, the same techniques are used in the stratosphere. 

Infrared spectroscopy was the province of physicists and physical chemists until about 1940. At that time, the potential of infrared spectroscopy as an analytical tool began to be recognized by organic chemists. The change was due largely to the production of small, quite rugged infrared spectrophotometers and instruments of this kind now are virtually indispensable for chemical analysis. A brief description of the principles and practice of this spectroscopic method is the topic of this section. 

D Skoog, F. Holler, and T Nieman, Principles of Instrumental Analysis, 5th ed (1998), Saunders (Philadelphia) An introductory coverage of analytical spectroscopy E Solomon and K Hodgson, Spectroscopic Methods in Biomorganic Chemistry (1998), Oxford University Press (New York) An excellent, specialized book L Stryer, Biochemistry, 4th ed (1995), Freeman (New York), pp 52-53, 66-68, 457-458 Application of NMR and MS to biochemistry... 

Instrumental Methods. Engineers in the IC industry prefer to use X-ray or FTIR spectroscopy to determine the quantities of phosphorus in thin films because of the speed of these methods. These spectroscopic methods are satisfactory for a relative indication of the dopant level in thin films or additives to metallization layers, but they do have serious drawbacks. X-ray spectroscopy is seriously affected by matrix effects and can easily be off by 15-20% of the actual concentration of dopant in thin films if the equipment is not properly calibrated against a material that has been analyzed by wet techniques. X-ray spectroscopy is further affected by the film thickness and the dopant profile throughout the film. 

Instrumentation developments in the 1920 s and 30 s led to a rapid expansion of spectroscopic methods in the laboratory (28, 34-39). These included further penetration into the infrared regime and some applications to infrared transmission in the atmosphere. Additional equipment was developed during World War II as a result of military requirements. This period was a fruitful one for the science of spectroscopy, and saw the first applications of infrared equipment as gas measurement tools (40-41) and as routine process controllers (42). 

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