Derivative-spectrum method

Erk [20] described a spectrophotometric method for the simultaneous determination of metronidazole and miconazole nitrate in ovules. Five capsules were melted together in a steam bath, the product was cooled and weighed, and the equivalent of one capsule was dissolved to 100 mL in methanol this solution was then diluted 500-fold with methanol. In the first method, the two drugs were determined from their measure d%/dk values at 328.6 and 230.8 nm, respectively, in the first derivative spectrum. The calibration graphs were linear for 6.2—17.5 pg/mL of metronidazole and 0.7—13.5 pg/mL of miconazole nitrate. In the second (absorbance ratio) method, the absorbance was measured at 310.4 nm for metronidazole, at 272 nm for miconazole nitrate and at 280.6 nm (isoabsorptive point). The calibration graphs were linear over the same ranges as in the first method. 

Chen used a second-derivative spectrophotometric method for the determination of miconazole nitrate in Pikangshuang [22]. Sample of miconazole nitrate was dissolved in anhydrous ethanol and the second-derivative spectrum of the resulting solution was recorded from 200 300 nm miconazole nitrate was determined by measuring the amplitude value between the peak at 233 nm and the trough at 228 nm. The recovery was 99.8% with a relative standard deviation (n = 6) of 0.2%. 

Electron spin relaxation in aqueous solutions of Gd3+ chelates is too rapid to be observed at room temperature by the usual pulsed EPR methods, and must be studied by continuous wave (cw) techniques. Two EPR approaches have been used to study relaxation studies of the line shape of the cw EPR resonance of Gd3+ compounds in aqueous solution, and more direct measurement of Tle making use of Longitudinally Detected EPR (LODEPR) [70]. Currently, LODESR is available only at X-band, and the frequency dependence of relaxation is studied by following the frequency dependence of the cw EPR line shape, and especially of the peak-to-peak line width of the first derivative spectrum (ABpp). 

Using absorbance data collected as a function of time, the distribution function H(k,t) was calculated, and plotted versus In(time) to yield a curve providing a maximum for each first-order process in the reaction. The position of each maximum yields an estimate of the rate constant (t = 2/k), and the area under the maximum provides an estimate of the amount of metal dissociating by that process. Due to concerns of using higher-order derivatives to calculate rate constants for distributions of pathways in multiple first-order mechanisms as described in the literature (16-20), the kinetic spectrum method was used only to obtain initial estimates for the appropriate rate equations. The actual rate parameters reported herein were obtained from a simplex non-linear regression (21) of the original experimental data. When dissociation occurred in both fast (ti/2 > 30 s) and slow (t 1/2 < 30 s) time domains, each data set was treated independently. 

Another method for the determination of salicylic acid in aspirin powder by second derivative ultra-violet spectrometry was done by Kitamura et al. (28). A differentiator with electronic derivative circuit and incorporating three circuits with differencial time constants of 27, 82 and 22 was connected to a double beam spectrophotometer derivative spectra were recorded with a slit width of 1 mm and scanning speed f 120 nm min 1. The second derivative spectrum of salicylic acid showed a trough at 292 nm and a satellite peak at 316 nm, in the presence of large amounts of aspirin, the trough disappeared, but the peak was unaffected. The relationship between the height of this peak and the concentration of salicylic acid was rectilinear for 1 to 10 //g ml 1. 

Various mathematical procedures may be employed to differentiate spectral data. We will assume that such data are recorded at evenly spaced intervals along the wavelength, X, or other x-axis. If this is not the case, the data may be interpolated to provide this. The simplest method to produce the first-derivative spectrum is by difference. 

The derivative spectrophotometry methods provide higher selectivity and higher sensitivity than do the methods based on normal (zero-order) absorption spectra. The increase in selectivity (with reduction or elimination of the effect of the spectrum of one substance on the spectrum of another one) results from reducing the band-width in the derivative spectra. An appropriate order of derivative spectrum may give complete separation of the spectra owing to the corresponding components of the system). 

The increase of selectivity in the derivative spectrophotometry methods results from the fact that the values of derivatives increase, in the case of basic spectra characterized by sharp peaks, and decrease in cases of broad-band zero-order spectra (Fig. 2.2). The sharp-peak spectra enable one to make determinations of analytes in the presence of considerable excess of elements having flat spectra. An example may be the direct determination of traces of manganese (as Mn04 ) in nickel salts, based on the fourth-order derivative spectrum [45]. An increase of selectivity may also be obtained by proper selection of the instrument setting parameters in recording the derivative spectra. 

The analytical value of the derivative is determined mostly by the zero-crossing method (determination of the derivative at the zero point of the derivative for the interfering component), by the peak-to-peak method (determination of the amplitude of the derivative spectrum in a point corresponding to the maximum difference between the derivatives of the... 

In other methods for determination of beryllium use is made of such organic reagents, as 2-phenoxyquinalizarin-3,4 -disulphonic acid [12], l-hydroxy-2-carboxyanthraquinone (the first-derivative spectrum is used) [50], 8-hydroxyquinaldine (e = 3.5-10 ) [51], and carminic acid [52]. 

Immobilized semiquinones often are detected in membrane systems. In most instances analysis of their ESR spectra has been limited to measurement of a g value and a linewidth. More detailed analysis [154,155] could provide additional structural information to help identify the radical, its ionization state, and its local environment. The g value usually measured is that for the cross-over in the first derivative spectrum. While certainly useful in providing an approximate indication of the radical structure, it is generally frequency-dependent because of the anisotropy in the radical spectrum. It cannot give the more precise information on structure and environment that is available from the isotropic g value. The latter can readily be obtained from spectra of immobilized species by the method of Hyde and Pilbrow [156]. Related information is available from linewidth considerations [154] in general, spectra for neutral semiquinones are approximately twice as broad as those for semiquinone anions and are sensitive to deuteration of solvent. Information of this kind can be of great help in assigning a spectrum of an unknown to a given radical species. The point has been... 

The numerical method involved first solving Equation 9 for u for given values of solving Equation 11 for du/day. The observed derivative spectrum is given by ... 

The simplest method to produce the first-derivative spectrum is by difference,... 

Since the absorption component of the FM signal is recorded as a derivative spectrum, a method for extraction of the Doppler profiles form the FM absorption signal is also needed. North et al. [69] first suggested using a resursive relationship in order to extract this information. In a more recent paper, they settled on a method that expands the FM absorption signal in a Taylor series [67], and then used both intergation and differentiation with respect to dto in order to obtain two solutions, one for <5(cu0),... 

In some cases, small amount of caffeine is added to soft drinks. Caffeine has a classic bitter taste that enhances other flavors. UV spectrometry, HPLC, and CE are used for the determination of caffeine in beverages. All of the above methods require sample preparation. FTIR spectroscopy with chemometrics, using partial least squares first derivative spectrum in the region between 2800 and 3000 cm have been developed to directly determine the caffeine in soft drinks. 

Savitzky-Golay method This method determines a derivative spectrum by moving a spectral window comprising 2 +1 measurement points over an absorbance spectrum. Then a polynomial of order m... 

Hgure 7 Influence of the modulation amplitude a [8] on the shape of a second-order derivative spectrum [19]. The example was determined by means of an aqueous sample containing approx. Imgl of each benzene, toluene, p-xylene. (Reprinted with permission from Vogt F, Tacke M, Jakusch M, and Mizaikoff B (2000) An ultraviolet spectroscopic method for monitoring aromatic hydrocarbons dissolved in water. Analytica Chimica Acta 422 187-198 Elsevier cf. Erratum (2001) Analytica Chimica Acta 431 167.)... 

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