Qualitative Structural Analysis

As described at the beginning of this chapter, the types of compounds that absorb UV radiation are those with nonbonded electrons (n electrons) and conjugated double bond systems (it electrons) such as aromatic compounds and conjugated olefins. Unfortunately, such compounds absorb over similar wavelength ranges, and the absorption spectra overlap considerably. As a first step in qualitative analysis, it is necessary to purily the sample to eliminate absorption bands due to impurities. Even when pure, however, the spectra are often broad and frequently without fine structure. For these reasons, UV absorption is much less useful for the qualitative identification of functional groups or particular molecules than analytical methods such as MS, IR, and NMR. UV absorption is rarely used for organic structural elucidation today in modem laboratories because of the ease of use and power of NMR (Chapter 3), IR and Raman (Chapter 4), and MS (Chapters 9 and 10). 

When UV spectra are used for qualitative identification of a compound, the identification is carried out by comparing the unknown compound s absorption spectrum with the spectra of known compounds. Compilations of UV absorption spectra in electronic formats can be found from commercial sources such as the Informatics Division, Bio-Rad Laboratories (www.bio-rad.com), or the American Petroleum Institute (API) indices. Computer searching and pattern matching are the ways spectra are compared and unknowns identified in modern laboratories. Some academic libraries still maintain the printed spectra collections, which must be searched manually. 

Another approach that can be taken is the use of derivative spectra, that is, plotting the first, second, or even higher derivatives of the absorbance spectra. Derivative spectra can enhance the differences among spectra, resolve overlapping bands, and reduce the effects of interference from other absorbing compounds. The number of bands increases with higher orders of the derivative. The increased complexity of the derivative spectrum may aid in compound identification. For example, the absorbance spectrum of testosterone shows a single broad peak centered around 330 nm the second-derivative spectrum has six distinct peaks (Owen). 

UV/VIS spectrophotometry is a widely used spectroscopic technique. It has found use everywhere in the world for research, clinical analysis, industrial analysis, environmental analysis, and many other applications. Some typical applications of UV absorption spectroscopy include the determination of the concentrations of phenol, nonionic surfactants, sulfate, sulfide, phosphates, fluoride, nitrate, a variety of metal ions, and other chemicals in drinking water in environmental testing, natural products such as steroids or chlorophyll, dyestuff materials, and vitamins, proteins, DNA, and enzymes in biochemistry. 

For example, the purity of acetaminophen, C8H9NO2, can be determined by measuring the absorbance of an aqueous solution of the drug at 244 nm and comparing it to a solution of acetaminophen of known purity and concentration. See the suggested experiments at the end of this chapter for details. 

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Interestingly, many of the current LC/MS approaches for pharmaceutical analysis are extensions of gas chromatography/mass spectrometry (GC/MS) (Foltz, 1978), mass spectrometry (Garland and Powell, 1981), and MS/MS (McLafferty, 1983)-based methods. With the introduction and widespread use of LC/MS-based methods, these fundamental approaches for quantitative and qualitative structure analysis became more routinely applicable to a wider scope of pharmaceuticals. 

Fig. 8 Schematic of a tandem quadrupole MS/MS instrument. A tandem quadrupole MS/MS instrument consists of two quad-rupole MS filters, MSI and MS2, separated by a collision cell. Each quadrupole MS filter consists of four cylindrical or hyperbolic shaped rods. A unique combination of direct current (dc) potential and radiofrequency (rf) potential is applied to each pair of rods (one pair 180° out of phase with the other). A mass spectrum results by varying the voltages at a constant rf/dc ratio. A variety of scan modes (e.g., full scan, product ion, precursor ion, neutral loss) provide unique capabilities for quantitative and qualitative structure analysis. (Courtesy of Micromass, Manchester, UK.)...

This section deals with some general principles of tandem mass spectrometry and its applicability to quantitative analysis. The MS/MS acronym is used in this book as a general term for aU tandem mass spectrometry techniques. More detailed descriptions of how the principles are exploited in practice for the various instrumental types are given in later sections of this chapter. The general concept of tandem mass spectrometry in qualitative (structural) analysis is that additional chemical information, over and above that contained in a conventional onedimensional mass spectrum, can be obtained by examining the connectivity relationships among some or all of the ions in that mass spectrum. The connectivities arise as a result of the dissociation reactions that lead to the fragment ions in a mass spectrum, e.g. ... 

In qualitative (structural) analysis, so-called accurate (sometimes exact , although that can be a misleading... 

One difficulty was that the apparatus, simple to a physicist, appeared very complex to a chemist. The application of mass spectrometry of chemistry had to await the commercial production of instruments. The impetus came in the 1940s when the war effort demanded rapid and accurate hydrocarbon analysis in connection with aviation fuels. The next big step came in the 1950s when it was realized that in addition to quantitative analysis the technique could be used for the qualitative (structural) analysis of organic compounds. A certain resistance had to be overcome to induce mass spectrometrists to put dirty compounds into their instruments rather than clean hydrocarbons. This gave mass spectrometer manufacturers a further impetus to develop more and more advanced instruments and led to a new discipline - organic mass spectrometry. 

The photoelectron spectrum of nitrogen is shown in the second illustration. There are several peaks, corresponding to electrons being ejected from orbitals of different energy. A detailed analysis shows that the spectrum is a good portrayal of the qualitative structure (as depicted in 44). 

The different ways of species analysis - qualitative and quantitative - are well known. However, in structure analysis, they can also be differentiated between qualitative and quantitative ways according to the type and amount of information obtained (Eckschlager and Danzer [1994]). Identification of a sample or a given constituent may have an intermediate position between species and structure analysis. In any case, identification is not the same as qualitative analysis. The latter is the process of determining if a particular analyte is present in a sample (Prichard et al. [2001]). Qualitative analysis seeks to answer the question of whether certain components are present in a sample or not. On the other hand, identification is the process of finding out what unknown substance(s) is or are present (Eckschlager and Danzer [1994]). In Sects 9.1 and 9.3 it will be shown that there is a... 

When an examination is restricted to the identification of one or more constituents of a sample, it is known as qualitative analysis, while an examination to determine how much of a particular species is present constitutes a quantitative analysis. Sometimes information concerning the spatial arrangement of atoms in a molecule or crystalline compound is required or confirmation of the presence or position of certain organic functional groups is sought. Such examinations are described as structural analysis and they may be considered as more detailed forms of analysis. Any species that are the subjects of either qualitative or quantitative analysis are known as analytes. 

The first steps in unravelling the details of an unknown system frequently involve the identification of its constituents by qualitative chemical analysis. Follow-up investigations usually require structural information and quantitative measurements. This pattern appears in such diverse areas as the formulation of new drugs, the examination of meteorites, and studies on the results of heavy ion bombardment by nuclear physicists. 

A qualitative structural model of the reconstructed c(2 x 2) W(1(X)) surface was first proposed by Debe and King on the basis of symmetry arguments. Figure 39 shows this reconstruction model. The surface atoms exhibit only inplane displacements along diagonal directions. A subsequent LEED structure analysis of Barker et al. ° supported this picture. In a more recent quantitative LEED analysis, Walker et a/ deduced a lateral displacement of 0.16A at 200K. 

Synthetic operations involving ozonolysis lead to formation of aldehydes, ketones or carboxylic acids, as shown in Scheme 16, or to various peroxide compounds, as depicted in Scheme 7 (Section V.B.5), depending on the nature of the R to R substituents and the prevalent conditions of reaction no effort is usually made to isolate either type of ozonide, but only the final products. This notwithstanding, intermediates 276 and 278 are prone to qualitative, quantitative and structural analysis. The appearance of a red-brown discoloration during ozonization of an olefin below — 180°C was postulated as due to formation of an olefin-ozone complex, in analogy to the jr-complexes formed with aromatic compounds however, this contention was contested (see also Section V1I.C.2). 

The results of this crystal structure analysis should then be viewed as having a reliability intermediate between standard single crystal structures where R is often 0.1, and crystalline polymer structures where only qualitative agreement is expected between observed and calculated structure factors. 

In this section the methods which have been used to gain structural information are briefly summarized. The term structure is used in this context in its broadest sense, including more qualitative observations concerning the skeleton of the bridging atoms. As a general rule, the hydroxo-bridged polynuclear complexes of chromium(III) and cobalt(III) can be isolated as well-defined crystalline salts and it is therefore quite natural that single-crystal X-ray structure analysis has... 

Fourier-Transform Infrared (FTIR) spectroscopy as well as Raman spectroscopy are well established as methods for structural analysis of compounds in solution or when adsorbed to surfaces or in any other state. Analysis of the spectra provides information of qualitative as well as of quantitative nature. Very recent developments, FTIR imaging spectroscopy as well as Raman mapping spectroscopy, provide important information leading to the development of novel materials. If applied under optical near-field conditions, these new technologies combine lateral resolution down to the size of nanoparticles with the high chemical selectivity of a FTIR or Raman spectrum. These techniques now help us obtain information on molecular order and molecular orientation and conformation [1],... 

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