Ionization difference spectra

Ionization of phenolic hydroxyl groups in lignin results in bathochromic and hyperchromic alterations of the absorption spectra maxima. These spectral changes are often used to determine the type and frequency of phenolic hydroxyl units in lignin samples. 

Determination of Phenolic Hydroxyl Content by Ultraviolet Difference Spectroscopy 

The difference spectra in Fig. 5.1.4 have ADmax values of 12.8 and 5.3 at 250nm and 298 nm, respectively, for spruce lignosulfonate, and 15.2, 5.7, and 8.1 at 250, 300, and 371 nm, respectively, for pine kraft lignin. Using the information available in Fig. 5.1.3, the types and quantities of phenolic hydroxyl groups are determined as follows  

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In preparing an alkaline solution (for ionization difference spectra measurement), the alkali is added to a neutral solution immediately before recording the spectrum. Do not stir alkaline solutions in air for an extended period of time. 

Fig. 5.1.4. Ionization difference spectra (AD,-curves) of spruce lignosulfonate and pine kraft lignin Solvents water (lignosulfonate), methyl cellosolve water (8 2, v/v) (kraft lignin)...

Adler and Marton [39] nsed a combination of ionization difference spectra and NaBH4 reduction to estimate the amounts of the four conjugated carbonyl groups shown in Table 3.5 in spruce milled-wood lignin. Note that, although the difference... 

Ionization of phenolic hydroxyl groups in lignin with alkali causes a batho-chromic shift and a hyperchromic effect in the absorption spectrum. An alkaline ionization difference spectrum is obtained by subtracting the spectrum of the solute in a neutral solution from the corresponding spectrum measured in an alkaline medium. The difference spectrum may be measured directly with a spectrophotometer according to the following procedure ... 

It is well known that the electron-impact ionization mass spectrum contains both the parent and fragment ions. The observed fragmentation pattern can be usefiil in identifying the parent molecule. This ion fragmentation also occurs with mass spectrometric detection of reaction products and can cause problems with identification of the products. This problem can be exacerbated in the mass spectrometric detection of reaction products because diese internally excited molecules can have very different fragmentation patterns than themial molecules. The parent molecules associated with the various fragment ions can usually be sorted out by comparison of the angular distributions of the detected ions [8]. 

It is important to note that each of these ionization sources, either a laser in MALDI-TOF or the high voltage ionization of droplets in ESI-MS/MS or LC-MS/ MS, will each produce a different spectrum of detectable ions and intensities because the effectiveness and nature of peptide ionization is quite different for each source. In addition, the presence of multiple peptides that influence each other s ionization potential notably through ion suppression makes most peptide ion measurements only semiquantitative. 

Fig. 9. Spectra of tyrosine in various states of ionization. Above, spectra of isoelectric tyrosine (pH 6), and the divalent anion (pH 13) which is dominated by the phenolate ion. Center, the spectral change from pH 6 to pH 13 represented as a difference spectrum. Bottom, for comparison with center, the difference spectrum generated by measuring tyrosine at pH 6 versus tyrosine at pH 0.5. Note the disparity of ordinate scales for the three levels of the figure. The scale for the bottom figure above 2430 A is on the right. (S. Malik, 1961.)...

The solvent polarity effect was examined by employing acetonitrile as the solvent. The difference spectrum of 4T in acetonitrile at 20 ps showed a broad absorption band at 650 nm, which is essentially the same as that observed in toluene. At 150 ps, on the other hand, a sharp absorption band was confirmed at 640 nm, which can be attributed to 4T +, indicating the ionization from 4T(T ) [105-108]. In the kinetic trace of AO.D. at 600nm, bleaching of 4T(Ti) showed incomplete recovery, indicating that 38% of 4T(T ) changed to 4T +. The radical cation formation in acetonitrile seems to be an adequate process since polar solvents stabilize the radical ion. The generation of a radical cation from the T state in acetonitrile was also confirmed for 5T in acetonitrile at 700 nm [105-108]. 

As an example for the soft ionization possibility of MUPI in contrast to electron impact, both the El and the MUPI mass spectra of a biological sample are displayed in figure 10. Here the mass spectra of native chlorophyll a, obtained by methanolic extraction of the cyano-bacterium Spirulina geitlerie without further purification, are shown. The soft ionization mass spectrum shows three different molecular ions, the chlorophyll a at mass 892, 10-hydroxy-chlorophyll a at mass 908 and phaeophytin a at mass 870. This mass spectrum again demonstrates the abilities of laser ionization, since by choosing the right wavelength only the porphin structures in the mixture are ionized. 

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