Derivative spectroscopy, advantages

The extent of homogeneous mixing of pharmaceutical components such as active drug and excipients has been studied by near-IR spectroscopy. In an application note from NIRSystems, Inc. [47], principal component analysis and spectral matching techniques were used to develop a near-IR technique/algorithm for determination of an optimal mixture based upon spectral comparison with a standard mixture. One advantage of this technique is the use of second-derivative spectroscopy techniques to remove any slight baseline differences due to particle size variations. 

The use of second derivative spectroscopy allows us to observe the effects of pressure on this enzyme, to quantitate the changes occuring (we can use the values of r to calculate Keq as a function of pressure and hence AG and AV for dissociation) and to interpret those changes in terms of the known structure of the enzyme. Second derivative spectroscopy has both advantages and disadvantages vis-a-vis fourth derivative spectroscopy. With second derivatives, one uses the ratio of the amplitudes, which is independent of the absolute absorbance. Therefore, one does not have to correct the spectra for compression effects and can easily compare samples studied at different times. The changes, however, are dominated by the contributions of tyrosines and provide much less information about the tryptophan residues. 

The UV-vis spectrum is a display of absorbance as a function of wavelength A = / (X). At the absorbance maximum, the derivative dA/dX = 0 the second-order derivative d Ajdl is a negative maximum. For all even-order derivatives, the spectra show well-defined peaks, and these can be used for analyte determination. The advantages of derivative spectroscopy are as follows. 

For bands with similar widths, it is not possible to take advantage of the discriminative power of the derivatives. One can then use the zero crossing method. To the maximum or minimum of each band, there is a correspondence with a zero derivative value the concentration value of the compounds do not matter. To the inflexion point of the usual spectrum, the second derivative is zero, etc. To these particular points, the value of the derivative is due to the only contribution of the second compound and, in this way, the interference of the first one may be removed. This methodology was used at the very beginning of derivative spectroscopy and was mainly applied to those compounds that had close spectra. One has to note, however, that this methodology requires the use of very reproducible wavelength-positioning spectrophotometers. 

Even derivative spectra do not produce an increase in information compared to zero order absorption spectra. Differentiation can visualise the wavelength regions in which spectral changes are exceptional. For this reason derivative spectroscopy has been applied to kinetic analysis [87]. Fig. 4.20 demonstrates an example of a derivative reaction spectrum given for the above mentioned photoreaction of stilbene. The second derivatives are shown. The advantage in selection of characteristic wavelengths improves the kinetic evaluation as discussed in Chapter 5. However, in each application the optimum between increased noise and extracted information has to be found. 

The advantage of this derivative spectroscopy [2] with a frequency-modulated laser is the possibility for phase-sensitive detection, which restricts the frequency response of the detection system to a narrow frequency interval centered at the modulation frequency Q. Frequency-independent background absorption from cell windows and background noise from fluctuations of the laser intensity or of the density of absorbing molecules are essentially reduced. Regarding the signal-to-noise ra-... 

Derivative spectroscopy is quite common. This involves calculating mathematical derivatives of the spectrum. The advantage is that close peaks can be resolved out. Derivatives rely on the assumption that pure peaks have only one maximum. The summation of two peaks results in a turning point that can be resolved out. However, the problem is that noise is enhanced, so derivatives are often combined with smoothing functions. These methods are described in more detail in this encyclopedia. 

Derivative filters The main advantage of derivative spectroscopy lies in the enhancement of the spectral fine structures combined with a reduction of broad baseline effects. Unfortunately, derivative spectroscopy requires a high SNR, which is sometimes hard to achieve, particularly if the IR microspectra are acquired with high spatial resolution. The example of Figure 6.8, panels C and D, demonstrates that the application of a first derivative Savitzky-Golay filter with nine smoothing points to the raw spectral data in combination with vector normalisation dramatically enhances the number of discriminative spectral features. We consider this combination to be the most effective and robust combination of pre-processing routines for classification analysis. ... 

Further, using a combination of X-ray crystallography and mass spectroscopy, Knox et al. [73] has firmly established a central role for Ser-130 in the inhibition of SHV-1 /1-lactamase (class A) by tazobactam. Many additional modifications (Table 3) were carried out on tazobactam with the aim of increasing inhibitory activity against AmpC enzymes, but none of these derivatives (e.g., 13c, 13d, and 13e) had any advantage over tazobactam [74— 77]. Renewed interest in the modification at the C-2 position of sulbactam was developed when scientists from Hoffmann-La Roche disclosed a series of 2/J-alkenyl penam sulfones that possess the ability to simultaneously inactivate both class A penicillinase as well as class C cephalosporinase. Compound... 

This method suffered from sensitivity problems initially as the bile-acid molecules lack a chromophore, but did offer the distinct advantage that conjugated bile acids could be determined without hydrolysis. The sensitivity issue was addressed by use of fluorescent derivatives such as dimethoxycoumarin esters with a C18 reverse phase column and were able to resolve endogenous mixtures of bile acids. The combination of hplc and mass-spectroscopy detection has further improved the sensitivity along with providing specific identification, important as the resolution of bile acids by hplc is not as good as capillary column glc. ... 

The physical chemist of today has a wide variety of methods at his disposal for the experimental investigation of electronic structure and all of them have been used in attempts at obtaining evidence of the participation of outer d-orbitals in bonding. One such group of methods is constituted by the various techniques of radiofrequency spectroscopy, which have the advantage that they yield information about the molecule in its ground state. In this they have a distinct superiority over, say, electronic absorption spectra where it is necessary to consider both ground and excited states. Moreover much of the data derived from radiofrequency spectroscopic methods concerns essentially just one part of the molecule so that attention can be concentrated on those atoms of interest in whatever study happens to be under way. 

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