Absorbance ratios, peak height

In transmission IR spectroscopy, using a constant path length, absorbance is proportional to concentration this relationship (the Beer-Lambert law) will only be true for DRIFTS if the nature of dispersion of filler particles in the diluent does not change with the level of surface treatment. Such an ideal condition will rarely be realised. Therefore it is essential to use internal standards within spectra and ratio variable absorptions to these internal standards. Use of absorbance peak area is preferable to the absolute absorbance value (peak height) as area takes the overall energy of the absorbance into account and thus accounts for changes in peak width. 

Figure 2-6 An FTIR spectrum of hydrous rhyolitic glass in the near-IR region of 600- to 375-mm wavenumbers. The absorbance A is defined as -log(7/7o), where Iq is the infrared beam intensity without the sample, and I is the intensity with the sample in the beam path. This sample contains about 0.8 wt% H20t. Both 523-and 452-mm peaks are combination modes. There are more hydrous peaks outside the region The fundamental stretch of OH (the strongest peak) is at 355 mm and the absorbance is usually off the scale. The overtone of the fundamental stretch is at 710mm, which is weak. As H20t content increases, the ratio of 523-mm peak height to 452-mm increases.

One further comment regarding noise in absorption spectral data, it is the signal-to-noise ratio that affects the quality of the results, not the peak-height -to-noise or information-to-noise ratio. This statement assumes that the data to be deconvolved are principally 10-30% absorbing and that the signal-to-noise ratio satisfies the requirements of Eq. (40). That this is a reasonable observation follows from the physically meaningful constraints that are imposed and the deconvolution process as discussed in Chapters 4 and 7. 

The 70 p-liter size of the flow cell contributed to the broadening of the chromatographic peaks observed in the fluorescence mode and could account for some loss in selectivity. The second variable, detector design, could account for the unexpected ratio in peak heights (compared with data in Table 2) obtained in Figure 4 for diphenyl and naphthalene the glass envelop that houses the photomultipler tube may absorb some of the fluorescence of biphenyl (emission maximum 305 nm) while the fluorescence of naphthalene (emission maximum 322 nm) passes through. 

Wavelength repeatability is a measure of the precision of wavelength measured. The bandwidth refers to the width of an emission band (from the monochromator) at half peak height. This value, normally provided by the manufacturer is accepted. Using a mercury vapor lamp one can also check the spectral width. A number of well defined emission lines at 243.7, 364.9, 404.5, 435.8, 546.1, 576.9, and 579 nm can be used to check spectral bandwidth. However, the accuracy of the absorbance measured is dependent on the ratio of spectral bandwidth to the normal bandwidth (NEW) of the absorbing species. Most active pharmaceutical compounds have a normal bandwidth of approximately... 

HPLC has also been used to determine econazole in plasma21 where the samples were made alkaline with potassium hydroxide and extracted in diethyl ether. The extracts were evaporated to dryness and the residue dissolved in methanol. Chromatography was performed on a Partisil 10 ODS column using a mobile phase consisting of methanol-aqueous potassium dihydrogen phosphate (0.01 M, 70 30, v/v) adjusted to pH 4.5 at a flow rate of 2.0 ml/min. Detection was by UV absorbance at 220 nm and quantitation was by peak height ratio against an internal standard (miconazole). The recovery of econazole from plasma was 84% 9.2, SD., n -25). 

Baker et al. reported the identification of various drugs by means of their relative retention times in combination with the absorbance ratio, calculated from the peak heights observed with 254 and 280 nm UV-detection. 

The carbon and oxygen content of silicon wafers can be determined at room temperature.53 The procedure is to record the spectrum of the sample wafer and ratio it against a background (empty cell) spectrum. The resulting absorbance spectrim is stored on the spectrometer data system. A similar absorbance spectrum of a pure wafer is recorded. A spectral subtraction is performed between the two spectra and only the impurity bands of the sample wafer remain. The peak height can be correlated to impurity concentration. This process can detect oxygen and carbon down to a few parts per million. This method has been shown to be very reliable and can be... 

A rarely seen graph, allowing errors in peak height measurements to be predicted, is shown in Rgure 12. This shows the dependence of the absorbance peak magnitude on the ratio of the spectral bandwidth to the natural band-... 

Sensitivity for any NIR method can be evaluated as the slope of the calibration line for concentration of analyte (y-axis), vs. change in optical response (jc-axis), between samples of varying concentration. Sensitivity, from a purely instrumental point of view, is expressed as signal-to-noise or the ratio of peak height for a particular compound vs. peak-to-peak noise at some absorbance... 

The absorbances (measured as the peak areas, which are proportional to the peak heights) are in the same ratios as the concentrations. Thus the third peak is twice as high as the second, and the second peak is twice as high as the first. 

Many of us instinctively want to measure peak heights at the peak top. Why are things done this way The peak shown in Figure 5.6 consists of 40 absorbances measured at different wavenumbers. In theory, the absorbance at any of these wavenumbers could be used to create a calibration line since all of them respond to changes in analyte concentration. However, we use the data point at the top of the peak for two reasons. First, assuming the noise level across the peak is constant, the maximum absorbance will have the best signal-to-noise ratio of all the data points in the peak. Second, the slope of the spectrum at the top of a peak is close to or equal to zero, so a small error in measured wavenumber will produce a smaller absorbance error than on the steep sides of the peak. 

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