Some Common Chromophores

Most aromatics absorb around 240-260 nm, in addition to a higher intensity absorption around 200 nm. Aliphatics also absorb around 200 nm, while alkenes absorb around 215 nm. An additional conjugated double bond adds roughly 15-30 nm to the highest wavelength, depending on the other substituents. Phenols absorb around 260-280 nm, occasionally above 300 nm. 

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APPENDIX 8 P0LAR0GRAPHIC HALF-WAVE POTENTIALS 835 APPENDIX 9 RESONANCE LINES FOR ATOMIC ABSORPTION 837 APPENDIX 10 ELECTRONIC ABSORPTION CHARACTERISTICS OF SOME COMMON CHROMOPHORES 838... 

Table 16.2 lists some common chromophores and their approximate wavelengths of maximum absorption. 

A third class of compounds comprising aldehydes, ketones, aliphatic nitro compounds, and esters is characterized by absorption maxima in the near-UV region. But they have intensities so low that they are useful only under special circumstances i.e., acetone has a very low molecular extinction coefficient (a) at its wavelength maximum of 270 nm. By way of comparison, a moderately strong bond would have an e of 1000-10 0001 mol cm and values as high as 100 0001 mol cm are not uncommon. Table 1 lists some common chromophoric groups. 

Table 1 Examples of some common chromophoric groups...

TABLE 14-1 Absorption Characteristics of Some Common Chromophores... 

TABLE 2.2 Some Common Chromophores and Theii Properties... 

Table 7 lists some of the most common chromophores used in stereochemical studies by the exciton chirality method. 

The characteristic absorption spectrum of each carotenoid is determined by a series of conjugated double bounds, the so-called chromophore. Usually the spectrum shows three absorption bands, which are affected by the length of the chromophore, the nature of the double bound, and the taking out of conjunction of one double bond. Several absorption spectra of some common carotenoids are shown in Fig. 2. A change of solvent may, however, cause a shift of the absorption bands. Owing to the extensive double-bond system, carotenoids exist in many geometrical isomeric forms (Z or E isomers). In nature most carotenoids occur in the all-trans form (E isomers) cis isomers (Z isomers) are frequently present in small amounts (6). Cis isomers can be distinguished from trans isomers by a characteristic absorption band ( cis peak ) that appears at 300-360 nm (7). 

The mobile phase plays an important part in the fluorescence of a molecule. Unless chosen with care, the mobile phase can quench the fluorescence of the molecule of interest. Most of the non-halogen-containing solvents used in HPLC can be used with fluorescence detection. However, dissolved oxygen or other impurities in the eluent can cause quenching. The solvent polarity and the pH of the mobile phase can also affect the fluorescent process if they influence the charge status of the chromophore. For example, aniline fluoresces at pH 7 and at pH 12, but at pH 2, where it is cationic, it does not fluoresce. Table 3.5 shows wavelength selections for some common LC-fluorescence applications.30... 

Properties of Some Common Photo-redox-active Metal-Polypyridine Chromophores... 

Most UV absorption bands correspond to transitions of electrons from n - n, n - n, or n - o molecular orbitals. Table 4.3 lists some common organic functional groups with chromophoric (light absorbing) properties. 

A Note on Post-Column Reaction Techniques A post-column reaction unit is an online derivatization system that supplies reagents to the column eluent into a heated chamber to convert the analytes into more chromophoric forms for higher sensitivity detection. Some common applications of post-column reaction systems are amino acid analysis using ninhydrin (with visible detection), and carbamate pesticide analysis using o-phthaldehyde (with fluorescence detection).]... 

Polcin and Rapson published a comprehensive series of papers describing diffuse reflectance studies of the chromophores in spruce and hemlock groundwood pulps [89-91], and their response to some common bleaching agents [92,93]. Based on either reflectance spectra or differences in remission function, they concluded that... 

Fig. 2. UV spe,ctra of some common indole chromophores (not normalized for concentrations)...

Azo compounds bring two substituted aromatic rings into conjugation with an azo group, which is a strong chromophore. Therefore, most azo compounds are strongly colored, and they make excellent dyes, known as azo dyes. The diazo coupling synflieses of some common azo dyes follow. 

Resonance Raman Spectroscopy. If the excitation wavelength is chosen to correspond to an absorption maximum of the species being studied, a 10 —10 enhancement of the Raman scatter of the chromophore is observed. This effect is called resonance enhancement or resonance Raman (RR) spectroscopy. There are several mechanisms to explain this phenomenon, the most common of which is Franck-Condon enhancement. In this case, a band intensity is enhanced if some component of the vibrational motion is along one of the directions in which the molecule expands in the electronic excited state. The intensity is roughly proportional to the distortion of the molecule along this axis. RR spectroscopy has been an important biochemical tool, and it may have industrial uses in some areas of pigment chemistry. Two biological appHcations include the deterrnination of helix transitions of deoxyribonucleic acid (DNA) (18), and the elucidation of several peptide stmctures (19). A review of topics in this area has been pubHshed (20). 

Many of the common reagents for introducing nitrophenyl chromophores into a molecule for DV-visible detection are also suitable for use with the electrochemical detector [554,555,637]. Some further exeuaples aure the formation of p-aminophenyl derivatives of carboxylic acids [637], N-(4-anilinophenyl) aaleimide derivatives of sulfadiydryl compounds [639], and... 

Relatively few probes have been widely applied to FlAs. Fluorescein and rho-damine, their derivatives, and to some extent phycoerythrin have dominated immunoassays that are based on steady-state measurements. Dansyl, umbelliferone, coumarins, and pyrenes are among others that have been used. Several more commonly used dyes are shown in Figure 14.2. Shorter-wavelength dyes tend to be less useful since they excite at wavelengths where endogenous chromophores, fluorophores, and... 

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