Method spectroscopic

Spectroscopic characterization is a very important tool in the study of HS here, the more important techniques are reviewed. 

Spectroscopic methods have been applied to the elucidation of the structures in coal from very early in the development of the methods (Speight, 1978). In the initial stages of the evolution of the spectroscopic methods, the data derived by their application to coal were more of a diagnostic nature as, for example, determination of functional entities or carbon-hydrogen bonds by means of infrared spectroscopy or determination of aromatic and aliphatic hydrogen by proton magnetic resonance spectroscopy. However, virtually all of the methods have at one time or another been applied to coal as a means of deriving more detailed information about coal structure with special emphasis on the 

FIGURE 10.3 Structures of aromatic units in coal based on oxidation studies. (From Hayatsu, R. et al.. 

However praiseworthy this may be, it is still difficult to portray the various maturation paths of coal (or, in other instances, petroleum) as occurring regularly so that, for example, the heteroatoms are scattered evenly throughout the model structure. Nature does not always behave in such regular fashion and there may be a tendency to concentrate, say, the heteroatoms only in certain parts of a molecule. Recognition of this fact alone detracts markedly from, and even nullifies, many of the structures that have been proposed for coal. For the present purposes, it is sufficient to note that spectroscopic methods of estimating the structure of coal also (as do chemical methods of structural determination) suffer from several, sometimes severe, limitations. Thus, while some useful information can be derived about the structure of coal, the concept that the data will eventually lead to an average or even to a representative structure can only be cautiously (if at all) accepted. 

Spectroscopic methods are often used as in-line or off-line analytical tools to identify chemical species or determine chemical concentrations. Optical spectroscopy may cover the entire spectral range of wavelengths from the ultraviolet (UV, 1 10 run) to the infrared (IR, A 1 mm). Spectra can be recorded in either absorption (UV, Vis, NIR, IR) or emission (IR, Raman, fluorescence). These methods are used in supercritical media and especially, but not exclusively, in supercritical carbon dioxide (SCCO2). 

There are many methods of this kind in the arsenal of chemical emalytics. Most important are 

With the exception of MS, these methods are based on changes of the energy states of the analyte s molecules or atoms, and detection of the energy absorbed or emitted. 

Ion pairing study via regular spectroscopic techniques (UV-vis, IR, Raman, NMR) does not involve major theoretical development, since ion-pairing is detected by noticing modifications of the spectra of free ions. When the absorbance of an electrolyte solution is measured in the UV-vis range, and apparent deviation from the Lambert-Beer law is attributed to ion-pairing, we can write for unit cell length  

In the previous chapter, we introduced methods to analyze the structure of surfaces. Now we discuss methods to analyze the chemical composition and the electronic energies of molecules at the surface. 

A variety of spectroscopic methods are available to investigate properties of semiconductors and oxides with relevance to the electrochemistry of these materials. These methods are often divided into in situ and ex situ methods. Some methods are described in other chapters (Chapters 7 and 11). Some aspects that are more closely related to oxides and semiconductors will be described in this chapter. Detailed descriptions exist and summaries of reviewing papers are found in the literature. -  

Infrared and other spectroscopic methods usually measure the distribution between a small number of possible conformations. Different conformations can have different atomic vibrational energies, so the frequency of a C—X stretch associated with the trans conformation may differ from that of the gauche form. So the relative intensity of the two infrared bands reflects the equilibrium distribution of the molecules between the two conformations. Changing the temperature will alter this ratio, reflecting changes in the population of each state. Then application of the Van t Hoff isochore allows evaluation of the energy difference between the conformations. 

Examining in more detail the conformation change occurring by rotation about backbone bonds, we find that the process can actually give rise to a quantised transition — a torsional vibration.This torsional frequency will usually lie in the far infrared range with wave numbers 60—200 cm f 

This resonance absorption will have a significant intensity only for the lowest energy conformer. In other words, it is looking at an oscillation at the bottom of the potential energy well. If it is assumed that the barrier to internal 

Raman spectroscopy, although it has different selection rules, similarly portrays atomic vibration frequencies that can be sensitive to conformational changes. The infrared and Raman transitions take place in times that are of the order of 10 to lO i s and so essentially freeze the distribution on this timescale, i.e. they show the nature of the interchanging states and the energy difference between them, but not the actual interchange rates. 

The magnetic absorption timescale for the NMR transition is typically of the order of 10 to lO s, and this can be relatively long compared to the time required for some molecular movements. 

One may divide the use of spectroscopic methods into the three categories of product identification, quantitative estimation of reactants or products at the end of a run, and in situ measurement of the concentrations of reactants or products during a reaction. Of these the last is the most important, because it presents an opportunity to follow the production and disappearance of transient species as well as those already mentioned. This is particularly true for very fast reaction techniques such as flash photolysis where the concentrations of the very reactive intermediates are likely to be high. 

Of the different regions of the spectrum involved, that concerning nuclear mag- 

Measurements were made at 500, 350, 257, 245 and 235 mp, 350 mp is an optical window for this system and therefore a reference point. Calibrations were made with the substances concerned. Beer s law was obeyed up to 30 torr. I2 was directly determined from the measurement at 500 mp. The absorbance for Ij was subtracted at the other wavelengths, leaving values from which the pressures of Mel and HI could be determined. In this way, 10 pmole of each could be detected. 

Other spectrophotometric estimations are cited in ref. 22c. Morse and Kaufman have measured the concentrations of oxygen, nitrogen and hydrogen 

IR spectroscopy appears to be a very promising tool for oxidation studies. The rate of formation of carbon monoxide, carbon dioxide and formic acid and the disappearance of ozone was measured this way with the O3/O2 + CH4 system . The system used by Burt and Minkoff for the combustion studies is shown in Fig. 61. Light from a Nernst filament is split in two and passed alternately through two heated cells and F2 containing either fuel + N2 or fuel + Oj. The beams are rejoined and fed into a Wadsworth monochromator containing a CaF2 prism and finally focussed onto a thermopile, from which a particular signal may be amplified and recorded. 

Infrared Spectroscopy Infrared spectroscopy has been one of the most frequently used instrumental analysis methods to characterize qualitatively the surface functionalities in coals [224,225], carbon blacks [226], charcoals [227], activated carbons [80,228-233], activated carbon fibers [234,235], and carbon films [236,237]. Fourier analysis (FTIR) provides an improvement over dispersive IR spectroscopy in signal-to-noise (S/N) ratio, energy throughout, accuracy of the frequency scale, and a capacity for versatile data manipulation. 

One of the major sample-handling problems in FTIR analysis of carbonaceous materials is that many of them are effective blackbody absorbers and thus are too opaque for direct transmission analysis in the midinfrared spectral region. Addition of KBr intensifies the signal to obtain transmission infrared spectra. It is time consuming, and grinding conditions and moisture are known to affect the spectrum of the sample [238]. Alternative techniques such as specular reflectance, diffuse reflectance (DRIFT), photoacustic spectroscopy (FTIR-PAS), and total 

C-0 stretch of ethers Ether bridge between rings Cyclic ethers containing COCOC groups Alcohols 1000-1300 1230-1250 1025-1141 1049-1276 3200-3640 

Source From ref. 229, with permission from Elsevier. 

The FTIR method was also used to determine the sulfur compounds present on the surface of activated carbons after H2S adsorption or oxidation. Using an approach proposed by Dandekar and co-workers [230] in which the spectrum for an initial sample is subtracted from that of a modified sample, Adib and 

A number of interesting problems on vibrational and electronic energy transfer have been attacked by means of flash photolysis (Volume 1, p. 118). The spectroscopic record of the individual quantum states permits direct observation of the relaxation steps. However the method is limited in application to simple mol- 

Spectra are usually recorded photographically, and for kinetic measurements, a set of exposures at different delay times can be displayed on a single plate. After each strip has been exposed, the reaction vessel is refilled, the condensers recharged and the plateholder shifted according to the length of the spectrograph slit. 

Some other experimental methods also require brief discussion here. The technique of microwave-pulse flash-spectroscopy is similar to that of flash photolysis, except that excitation is achieved by means of a powerful single pulse of microwave radiation from a magnetron19. The gas is contained in a quartz reaction vessel placed along the axis of a cylindrical cavity, tuned to the frequency 

Millikan20 has described an ingenious fluorescence technique for measuring relaxation rates of the CO(t = 1) molecule. In a flow tube at 5-20cm.sec-1, CO is excited to (o = 1) at the inlet with infrared emission from the CO fundamental (2143 cm-1), a suitably intense source being a CH4(rich)/02 flame. There are two competing processes by which the vibrational excitation can decay 

Analysis of the fluorescence from electronically excited molecules in a conventional static gas system21 provides a way of investigating vibrational relaxation of such molecules, and is also a means of studying selection rules for rotational relaxation22. It is now well established that multiple quantum rotational jumps can occur with high probability (see Section 6). 

Infrared red spectroscopy is based on the ability of the substances to absorb light of a given wavelength. Infrared spectroscopy is today one of the most important spectral analytical methods in the crude oil chemistry, because of its high information content and the variety of possibilities for sample preparation. 

The direct analysis of the structure of sample components without calibration with reference substances is impossible. However, defined chemical groups in the sample absorb infrared light in defined areas of the spectra. The direct prediction of the structure of the sample or components, in this case, is possible with the use of special empirical tables for infrared spectroscopy. 

In order to predict the structure of the sample analyzed, it is important to understand the principle of analysis. Infrared spectroscopy is based on the measurement of the absorbed infrared light by the sample analyzed. 

When a beam of infrared light of intensity I0 is passed through a sample, it can either be absorbed or transmitted, depending upon its frequency and the structure of the molecules. The final intensity I of the infrared light that passes through the sample can be calculated by the Lambert-Beer law (2.4) which is applicable to all types of electromagnetic radiation. 

Marcel Dekker, Inc. 270 Madison Avenue. New York, New York 10016 

Complete structure elucidation of individual resin glycoside constituents is now achieved readily by the use of a combination of high-resolution mass spectrometry and NMR spectroscopy. These methods are applicable to the isolated natural products or to their peracetylated and methylated derivatives. 

Due to extensive overlap within the region 5 3.0-4.5 ppm, H NMR spectra of oligosaccharides in many cases produce complex patterns. In one-dimensional NMR analysis, the solvent pyridine-ds improves signal dispersion better than methanol- /4 or acetone-dg. For structural analysis, the following steps are suggested  

Identification of constitutive monosaccharides two-dimensional homonuclear NMR techniques such as DQF-COSY and TOCSY are used to assign chemical-shift values for all C-bonded protons in each individual monosaccharide (96). One-dimensional NMR spectra provide useful information about the chemical shifts and scalar couplings of such well-resolved signals as methyl groups for 6-deoxy monosaccharides (fucose, quinovose, and rhamnose) at 6 1.1-1.3 ppm. 

If a surface, typically a metal surface, is irradiated with a probe beam of photons, electrons, or ions (usually positive ions), one generally finds that photons, electrons, and ions are produced in various combinations. A particular method consists of using a particular type of probe beam and detecting a particular type of produced species. The method becomes a spectroscopic one if the intensity or efficiency of the phenomenon is studied as a function of the energy of the produced species at constant probe beam energy, or vice versa. Quite a few combinations are possible, as is evident from the listing in Table VIII-1, and only a few are considered here. 

The various spectroscopic methods do have in common that they typically allow analysis of the surface composition. Some also allow an estimation of the chemical state of the system and even of the location of nearest neighbors. 

Subsequent to the investigation of haemocorin other phenalenones and phenalenone derivatives were isolated from other plants. Their structures were determined partly by conversion to known derivatives but largely by spectroscopic methods, using as models the many phenalenone derivatives prepared during the study of haemocorin. 

All the natural phenalenones, as derivatives of 2-hydroxy-IH-phenal-enone, have distinctive ultraviolet and visible absorption spectra. The 

6- Dihydroxy-5-methoxy-9-phenyl-l H-phenalen-1-one (Haemocorin aglycone) 

In the mass spectra of 9-arylphenalenones the occurrence of the (M-1) ion is a useful diagnostic feature. When the structure of the phenalenone is fixed the ion gives the base peak and presumably has a structure of type (53). However when tautomerism is possible, as in 6-hydroxy-9-arylphenalenones, this peak is of lower intensity. 

An investigation of carbon-13 nmr spectra of phenalenones has been reported (67) and the results have been applied in labelling experiments to study the biosynthesis of the natural arylphenalenones (68). 

It has been shown in Chapter 3 that electrodes (metals and semiconductors) interact with the electrolyte which strongly influences the optoelectronic properties of the junction. Frequently, it is very difficult to identify the microscopic and molecular nature of the states at the interface. Better scientific understanding demands a spectroscopic identification of the surface and the interface states. Several spectroscopic methods are available which allow the analysis of the chemical, structural and, also, electronic properties of the surface. 

Infrared spectroscopy (IR) is a fairly simple in situ method. Since the absorption coefficients of molecular vibrations are rather low, it is impossible to detect the IR absorption of a molecule adsorbed or bonded to the semiconductor surface, merely by an ordinary vertical transmission measurement. This problem was solved by using attenuated total reflection (ATR) spectroscopy, as introduced by Harrick [17], and first applied to semiconductor-liquid junctions by Beckmann [18,19]. In this technique, the incident IR light beam is introduced via a prism into a semiconductor, at such an angle that total internal reflection occurs at the semiconductor-liquid interface, as illustrated 

UV photoelectron spectroscopy (UPS), x-ray photoelectron spectroscopy (XPS) and low energy electron diffraction (LEED) are most commonly applied in this context. In the first method (UPS) electrons are excited by UV light (sources He I = 21.22 eV He II = 40.82 eV) and information on the electronic structure of the valence band region is obtained. The second method (XPS) provides information about the 

Tliere is a small number of reports on ex situ surface analysis of semiconductor electrode surfaces. Usually products formed during an electrochemical reaction have been determined. On the other hand, there are only very few systematic studies and all of these were performed with transition metal chalcogenides. It would be beyond the scope of this book to describe these investigations and the reader is referred to the literature [23, 24]. 

Steady state emission polarization measurements reflect the time over which the excited species remains rigidly bound to DNA, yielding some idea of the movement and orientation of the luminophore in the DNA microenvironment. 

Linear dichroism data with DNA oriented by an electric field [53, 54] or a linear flow [55, 56], under linearly polarised light, lead to determinations of the angle between the absorbing transition dipole moment of the chromophore in the molecule and the DNA helix axis conclusions concerning intercalation may thus be drawn from this technique. Finally, with chiral compounds, circular dichroism is also an attractive method to determine the enantioselectivity in the binding of the molecule [48, 57,58]. 

NMR spectroscopy. NMR is a powerful tool for the analysis of the structure and dynamics of drug-nucleic add complexes [59], and has been widely used for characterising the binding modes of organic molecules with oligonucleotides. It has been less applied so far for metal complexes [60-63]. 

EPR spectroscopy. Molecules can also be derivatised by stable nitroxides used as spin probes [64] which allows one to draw conclusions on the microenvironment of the interacting molecule, thus on its binding. 

Three main types of spectroscopy have been used to study hydrates. These are described below. 

Structure identification, quantifying relative cage occupancies. 1II NMR has been used for ethane, propane, and isobutane hydrates (Davidson et al., 1977 Garg et al., 1977), while 2H, 19F, 31P, and 77 Se NMR have been used for several si guests (Collins et al., 1990). 13C cross-polarization and magic angle spinning (MAS) NMR techniques have been applied to study hydrates of carbon dioxide, methane, and propane (Ripmeester and Ratcliffe, 1988, 1999 Wilson et al., 2002 Kini et al., 2004). 

Water mobility from molecular reorientation and diffusion. Evidence for the motion of the water molecules in crystal structures is typically provided by XH NMR (Davidson and Ripmeester, 1984). At very low temperatures ( 50 K) molecular motion is frozen in so that hydrate lattices become rigid and the hydrate proton NMR analysis suggests that the first-order contribution to motion is due to reorientation of water molecules in the structure the second-order contribution is due to translational diffusion. 2H NMR has been also used to measure the reori-entational rates of water and guest molecules in THF hydrate (Bach-Verges et al., 2001). Spin lattice relaxation rates (fy) have been measured during THF hydrate 

However, IR spectroscopy has not been widely used for hydrate studies. This is largely due to the technical problems associated with sample preparation (e.g., vapor deposition of thin films) to avoid the high IR absorptivity of water, and the difficulties of performing in situ and high pressure measurements. Therefore, this technique will not be further discussed here. 

Structure identification and relative cage occupancies. The hydration number and relative cage occupation for pure components and guests were measured by Sum et al. (1997), Uchida et al. (1999), and Wilson et al. (2002). Raman guest spectra of clathrate hydrates have been measured for the three known hydrate crystal structures si, sll, and sH. Long (1994) previously measured the kinetic phenomena for THF hydrate. Thermodynamic sl/sll structural transitions have been studied for binary hydrate systems (Subramanian et al., 2000 Schicks et al., 2006). 

In this method a known weight of sample mixture is chromatographed on a thin-layer plate, usually coated with silica gel, by normal procedures. The test compound (or compounds) is extracted from the plate with a suitable solvent and the extract diluted to a standard volume with solvent prior to spectrophotometric examination. If the compound contains a 

A portion of the adsorbent, equal in area to that of the sample, is removed from an unoccupied part of the chromatogram. It is processed in the standard manner and the extract is used as the blank solution in the spectrophotometer. The contribution of the blank in the UV region of the spectrum is by no means a negligible proportion of the actual measurement and it is also rather variable. 

To diminish further both the magnitude and the variability of the blank, the film may be pre-washed by allowing suitable solvents to ascend the film as in the development of a chromatogram (see Section 5.9). 

The coefficient of variation for the methyl and propyl esters of p-hydroxy benzoic acid were 3.2% (13 determinations) and 4.8% (14 determinations). 

In a simple case, when only one vibrational state Vq is excited primarily, the collisions lead to a decrease in intensity I of the bands Vq v and induce new bands v v different from Yq - v. In the stationary state, intensity I of a band Vq - v is related to coUisiqnless intensity Iq by 

Of wide use are now various kinetic spectroscopic methods which permit to follow the time evolution of the intensity of a particular optical or infrared transition. This provides direct measurements of the relaxation rate constants. Also, the double resonance technique should be mentioned. It uses one laser for the population of a particular vibrational molecular level and another (of a much lower intensity and of a quite different frequency) to monitor the population change of this and the neighbouring levels caused by relaxing transitions. 

Infrared spectroscopy (IR) is a fairly simple in situ method. Since the absorption coefficients of molecular vibrations are rather low, it is impossible to detect the IR absorption of a molecule adsorbed or bonded to the semiconductor 

UV photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS), and low-energy electron diffraction (LEED) are most commonly applied in this context. In the first method, UPS, electrons are excited by UV light (sources He I = 21.22 eV He ii = 40.82 eV) and information on the electronic structure of the valence band region is obtained. The second method, XPS, provides information about the elemental composition and the valence states of the elements. Here, X-ray excitation is used (possible radiation sources MgK = 1253.6eV or A K = 1486.6 eV). In both methods, the emitted electrons are analyzed as current densities in dependence of their kinetic energy. Since the XPS signals depend not only on elemental composition but are also sensitive to the chemical environment of specific atoms, valuable information on a molecular structure can be obtained (see Chapter 8). LEED is used for the analysis of the geometric structure of the surface. Details of these and other methods applicable in combined electrochemical/UHV systems are very well discussed in a review article byjagermann [23]. 

This is based on Ohm s law and Faraday s laws of electrolysis. In an electrolytic cell, a d.c. current, I, is applied between two electrodes dipping in one or more solutions. However, Ohm s law is modified to allow for the back galvanic emf, Eb, which opposes the applied emf, E., so that  

In an electrogravimetric determination, usually 2 gauze electrodes (Pt, Pt/Ir or Pt on Ti) dip in a solution of a salt of the metal to be determined, with the inner electrode rotating and the outer one fixed. The cell is connected to a d.c. supply via a fixed resistance and an ammeter. A small variable resistance and voltmeter in parallel with the cell are included in the circuit. In practice usually, the solution is that of a complex of the metal whose concentration and temperature are adjusted to ensure the separation of the metal as an adherent and smooth form which can be readily washed to remove the adhering electrolyte before drying and weighing. The technique can be adapted to separate two metals, provided their Ej s are sufficiently different. 

Classification of these methods depends on the part of the electromagnetic spectrum to which they apply and also on the type of spectrum studied. In all cases, Planck s equation  

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The spectroscopic methods, NMR and mass spectrometry for predicting cetane numbers have been established from correlations of a large number of samples. The NMR of carbon 13 or proton (see Chapter 3) can be employed. In terms of ease of operation, analysis time (15 minutes), accuracy of prediction (1.4 points average deviation from the measured number), it is... 

Conventional spontaneous Raman scattering is the oldest and most widely used of the Raman based spectroscopic methods. It has served as a standard teclmique for the study of molecular vibrational and rotational levels in gases, and for both intra- and inter-molecular excitations in liquids and solids. (For example, a high resolution study of the vibrons and phonons at low temperatures in crystalline benzene has just appeared [38].)... 

Williams D H and Fleming I 1995 Spectroscopic Methods in Organic Chemistry (London McGraw-Hill) ch 3... 

As discussed in more detail elsewhere in this encyclopaedia, many optical spectroscopic methods have been developed over the last century for the characterization of bulk materials. In general, optical spectroscopies make use of the interaction of electromagnetic radiation with matter to extract molecular parameters from the substances being studied. The methods employed usually rely on the examination of the radiation absorbed. 

Theory and experimental teclmiques for study of chemical reaction dynamics with ultrafast spectroscopic methods. 

For many reaction products and for the detection of molecules in their ground vibrational level, some laser-based spectroscopic method must be employed, rather than observation of spontaneous emission. The simplest spectroscopic method for detemiining concentrations of specified product internal states would involve the... 

Optical metiiods, in both bulb and beam expermrents, have been employed to detemiine tlie relative populations of individual internal quantum states of products of chemical reactions. Most connnonly, such methods employ a transition to an excited electronic, rather than vibrational, level of tlie molecule. Molecular electronic transitions occur in the visible and ultraviolet, and detection of emission in these spectral regions can be accomplished much more sensitively than in the infrared, where vibrational transitions occur. In addition to their use in the study of collisional reaction dynamics, laser spectroscopic methods have been widely applied for the measurement of temperature and species concentrations in many different kinds of reaction media, including combustion media [31] and atmospheric chemistry [32]. 

Perturbation or relaxation techniques are applied to chemical reaction systems with a well-defined equilibrium. An instantaneous change of one or several state fiinctions causes the system to relax into its new equilibrium [29]. In gas-phase kmetics, the perturbations typically exploit the temperature (r-jump) and pressure (P-jump) dependence of chemical equilibria [6]. The relaxation kinetics are monitored by spectroscopic methods. 

Far-infrared and mid-infrared spectroscopy usually provide the most detailed picture of the vibration-rotation energy levels in the ground electronic state. However, they are not always possible and other spectroscopic methods are also important. 

Improved spectroscopic methods showed that the spectrum of hydrogen contained many more lines than was originally supposed and that some of these lines were split further into yet more lines when... 

The equilibrium constants obtained using the metal-ion induced shift in the UV-vis absorption spectrum are in excellent agreement with the results of the Lineweaver-Burke analysis of the rate constants at different catalyst concentrations. For the copper(II)ion catalysed reaction of 2.4a with 2.5 the latter method gives a value for of 432 versus 425 using the spectroscopic method. 

He therefore wanted to synthesise TM 29 to check. Even with modem spectroscopic methods the quickest way to check the identity of a compound will often be to synthesise it by an unambiguous route and compare the n.m.r. and fingerprint i.r. spectra. How then would you make TM 29 ... 

These methods are now obsolete in comparison with spectroscopic methods. Werbel has shown that the structures of these isomers are easily determined by NMR (125) (see also Table VI-5). Furthermore. 2-imino-4-thiazoline derivatives are characterized by their stretching C=N vibration at 1580 cm , absent in their 2-aminothiazole isomers, and by the stretching NH vibration that appears in the range of 3250 to 3310 cm for the former and between 3250 to 3340 cm" for the latter (131). Ultraviolet spectroscopy also differentiates these isomers (200). They can be separated by boiling in ethanol the thiazoline isomer is usually far less soluble in this solvent (131),... 

Taurins reported that nitration of 2-nitramino-5-nitrothiazole yields the fully nitrated 2-imino-4-thiazoline (184) (Scheme 117) (87). This interesting compound should be studied by spectroscopic methods. 

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

With a regioselectivity opposite to that of the Zaitsev rule the Hofmann ehmma tion IS sometimes used in synthesis to prepare alkenes not accessible by dehydrohalo genation of alkyl halides This application decreased in importance once the Wittig reac tion (Section 17 12) became established as a synthetic method Similarly most of the analytical applications of Hofmann elimination have been replaced by 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. 

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