Quantitative analysis baseline

However, these absorption spectra can be employed as an aid to charaeterization, particularly when authentic reference substanees are ehromatographed on a neighboring track. The use of differential speetrometry yields additional information [64]. Quantitative analysis is usually performed by scanning at the wavelength of greatest absorbance (2m ). However, determinations at other wavelengths can sometimes be advantageous, e.g. when the result is a better baseline. An example is the determination of scopolamine at 2 = 220 nm instead of at =... 

Until fairly recently, IR spectroscopy was scarcely used in quantitative analysis owing to its many inherent shortcomings (e.g. extensive band overlap, failure to fulfil Beer s law over wide enough concentration ranges, irreproducible baselines, elevated instrumental noise, low sensitivity). The advent of FTIR spectroscopy, which overcomes some of these drawbacks, in addition to the development of powerful chemometric techniques for data processing, provides an effective means for tackling the analysis of complex mixtures without the need for any prior separation of their components. 

A constituent is characterised by its retention time. tR, defined by the time taken between the moment of injection into the chromatograph and the peak maximum recorded on the chromatogram. In an ideal case, the retention time fR is independent of the quantity injected. A compound not retained will elute out of the column at time tM, called the void time or the dead time1 (sometimes designated by t0). The separation is complete when as many peaks are seen returning to the baseline as there are components in the mixture. In quantitative analysis, it suffices to separate only the components that need to be measured. 

For quantitative analysis, the diffusion current in the plateau region is proportional to the concentration of analyte. Diffusion current is measured from the baseline recorded without analyte in Figure 17-16b. The residual current in the absence of analyte is due to reduction of impurities in solution and on the surface of the electrodes. Near — 1.2 V in Figure 17-16, current increases rapidly as reduction of FI+ to H-, commences. 

E. Resolution of two peaks depends on the column plate number, N, and the separation factor, -y, as given by Equation 23-30. Suppose you have two peaks with a resolution of 1.0 and you want to increase the resolution to 1.5 for a baseline separation for quantitative analysis (Figure 23-10). 

Several gas chromatographic detectors are sensitive to changes in the flowrate of the carrier gas. Any changes in flow rate cause the baseline to be displaced. These displacements make quantification quite difficult especially since the response of certain detectors such as thermal conductivity also changes with changes in flowrate. When an accuracy of 1% in quantitative analysis is required, the flowrate should not fluctuate more than 0.2 percent (see Chapter 4). 

High resolution l3C NMR is also used in the determination of the composition of the dispersed phase in cured rubber modified epoxies in order to analyze the chemical structure of the mobile segments 152). In this case quantitative analysis is possible because the areas under each peak are approximately equal to the number of carbons contributing to the peak, and the intensities of the broad lines from the rigid phase are very low, almost indistinguishable from the baseline noise. The structure of crosslinked networks based on poly(3,4-pyrrolidinediethylene), synthesized by different methods, was determined from gels swollen in water and chloroform 153). 

There are a variety of other factors that influence the accuracy of quantitative analysis. Noise, in the form of baseline disturbances and baseline drift, affects area more than it does height, as it can cause area to be lost at the tailing edges of the peaks where they are widest. Peak asymmetry and detector saturation or nonlinearity, however, have a more detrimental effect on peak height. Figure 7.6 shows a calibration curve comparing peak height measurements with peak area measurements.13... 

Quantitative analysis is possible with good accuracy if the 33S spectra are characterized by a good S/N and if the signals of the species to be determined are well resolved. It must be considered that in 33S NMR spectra, baseline distortions can seriously endanger the final results. 

In summary, peak area is the preferred measurement especially if there are any changes in chromatographic conditions, such as partition ratio, temperature, or sample introduction method, that can cause changes in peak height or width (but not area). However, peak height measurements are less affected by overlapping peaks, noise, and sloping baselines. It must also be remembered that the commonly used concentration detectors are flow sensitive and prone to errors if areas are used for quantitative analysis. 

The energy dispersive x-ray fluorescence spectrometer, which had been developed recently as a qualitative analysis instrument, showed promise of meeting the goals of the new laboratory (1). Its unique features, which earned it the name, The Curators Dream Instrument, are The measurements require neither sampling nor alteration of the object in any way. Systems for obtaining quantitative analysis data are now operational (I). Concentrations of up to thirty elements above chlorine (Z = 17) can now be printed out simultaneously. Techniques have been developed that minimize errors caused by sample size, shape, position, overlapping spectral peaks, matrix effects, and baseline compensation. Interpretative procedures have been established that recognize the shallow depth of penetration of the excitation radiation (2). 

In recent years several normal-phase HPLC methods have been reported for the quantitative analysis of tocopherols and tocotrienols (Table 11.5). The best of these methods have been able to achieve baseline separation of all four tocopherols and all four tocotrienols, as shown in Figures 11.2 and 11.3. Kamal-Eldin et al. (2000) reported the optimal baseline separation of all eight common tocols using a Diol-bonded phase column and an isocratic mobile phase of hexane/methyl tert-butyl ether (MTBE), 96 4, v/v (Figure 11.2). Similar separations were reported by Moreau et al. (2007) using the same type of column and mobile phase. Schwartz et al. (2008) reported that, with a normal-phase silica column, plastochromanol-8 in rapeseed oil eluted between y-tocopherol and 5-tocopherol. 

Several capillary electrochromatography (CEC) methods have also been reported for the quantitative analysis of tocopherols and tocotrienols (Table 11.7). Three of these report baseline separation of all four tocopherols, but none has investigated the separation of tocotrienols. 

Variations in wave-number and or intensity of the lines can be used for quantitative analysis. The intensity of a band is determined by measuring the maximum intensity of the peak above a baseline or the integrated intensity (Fig. 12.11) and should always be expressed in units of absorbance. If interferences are encountered, it is possible to decompose spectra with Gaussian profiles, for example. 

Nobuhara et al. separated opium alkaloids on octadecyl and cyanoalkyl phases using mixtures of acetonitrile and 1% aqueous ammonium acetate (pH 5.8) as mobile phase. Both types of column enabled baseline separation of the major opium alkaloids (Mg. 7.4 and 7.5, TABLE 7.2). However, the cyanoalkyl stationary phase gave the best separation for quantitative analysis. 

As shown in Fig. 2-4, these parameters can be obtained directly from the chromatogram. A resolution of R = 2.0 (corresponding to an 8quantitative analysis, if the peaks exhibit a Gaussian peak shape. The two peaks are thus completely baseline resolved, since the peak width at the base is given by... 

The data generated from a NIR or Raman spectrum do not immediately provide the concentrations of the species at any time, so there is no predictive capability. Construction of a calibmtion set requires an independent measure of the property, e.g. by HPLC or by NIR of known mixtures of the components. Two such methods are principal-component regression (PCR) and partial least squares (PLS). As soon as quantitative analysis is considered, the question of noise and reproducibility of the data set becomes important. It is therefore necessary to treat the mw data to remove the drift in baseline etc. which will occur over a long period of spectml acquisition. 

It is apparent that there has been significant offset of the baseline between samples and hence that the data are unsuitable for quantitative analysis in the raw form. Three different methods of correction of these data have been compared (Shimoyama et al, 1998) ... 

Fourier Transform Infrared (FT-IR) Spectroscopy With the introduction of commercial FT-IR spectrometers, the application of oil analysis by IR became relatively commonplace for production oil analysis laboratories. The mathematically intensive infrared data analysis techniques that were difficult or impossible to perform on the earlier IR systems became easy on these systems. In addition, quantitative analysis measurement techniques such as peak height, peak area, local baselines and more sophisticated matrix methods could be easily employed in the analysis, and the automation of lubricant analysis became commercially viable. 

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