Spectral turbidity

Beckman DU 7500 spectrophotometer has been used to determine the size distribution of monodisperse latex in the size range 200 to 800 nm, to 

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Figure 3. Average distribution of spectral global irradiance observed in Barcelona during 1989-92 for cloudless conditions and for the four turbidity classes considered (from figure 1 of Lorente at al. 1993).

Equation (16.1) is an approximation it assumes that the measured spectral contribution from Pa is independent of the overall composition of the sample. In reality, the bulk absorption and scattering properties of different samples will cause variations in how much Raman signal is collected. In the case of urine and blood serum specimens, the variation is often negligible. For whole blood and turbid tissue specimens, the influence of bulk optical properties is more important. Methods of correcting for their effects and returning to the simple linearity assumptions of (16.1) are mentioned briefly in the next section. 

Sample variability is a critical issue in prospective application. For optical technologies, variations in tissue optical properties such as absorption and scattering coefficients can create distortions in measured spectra. This section provides a brief overview of techniques to correct turbidity-induced spectral and intensity distortions in fluorescence and Raman spectroscopy, respectively. In particular, photon migration... 

The same general principle that applies for intrinsic fluorescence should hold true for Raman spectroscopy as well. Unlike in fluorescence spectroscopy, spectral distortion owing to prominent absorbers is less of an issue in the NIR wavelength range. However, for quantitative analysis the turbidity-induced sampling volume variations become very significant and usually dominate over spectral distortions. 

Turbidity and light scattering caused by undissolved drug or excipients interferes with spectral measurement and increases noise. The interference should be limited to 2%-3% and if this is not achievable, conventional discrete sampling and off-line measurement may be required. 

We are now going to calculate the attenuation of direct (directional) solar radiation through scattering and absorption in the atmosphere. The atmosphere is assumed to be cloudless for details on the complicated effect of clouds, [5.34] is suggested. We will consider a bundle of rays that goes through an optically turbid, namely absorbent and scattering medium, Fig. 5.42. The reduction dLx of its spectral... 

Fig. 5.42 Reduction of the spectral intensity L in transit through a turbid (scattering and absorbing) medium...

The computer-controlled system acquires raw spectral data within the range of 200-400 nm at specified time intervals, calculates the results with the use of validated software, and stores the data in a secured computer database. Except for the dissolution apparatus itself, the system contains no moving parts. The spectral data can be corrected for turbidity-related scatter with the use of either a simple baseline subtraction or a second-derivative-based algorithm, and test results and/or profiles can be viewed in real time. 

FIGURE 21 Dissolution profiles for buffered aspirin tablets obtained with the fiber-optic dissolution system (260-350 nm) and manual sampling with HPLC analysis. The dissolution was performed with USP apparatus 2 at 75 revolutions per minute. A second-derivative baseline correction was performed on the fiber-optic raw spectral data to correct for scattering due to the turbid solution. 

An important application of derivative spectroscopy lies in the determination of analytes in turbid media. Turbid solutions usually present a continuous increase of the absorbance towards shorter wavelengths and, as a consequence, do not produce any sudden spectral change either in the first or in the second derivative spectrum. Phenol determination in wastewater was one of the first practical applications of the derivative spectroscopy in turbid media [19], Another study, more recent, deals with the evaluation of the second-derivative determination of nitrate and total nitrogen [20],... 

Regarding exploitation of the isosbestic point in DWS for the analysis of turbid samples, a good example is the determination of hydrogen peroxide in wood pulp bleaching streams relying on peroxide association with the molybdate ion [22]. The influence of dispersed lignin in the pulp bleaching stream, a critical spectral interferent in the analytical procedure, was quantitatively circumvented. 

The influence of monochromaticity of the incident radiation beam in turbidimetry is not as critical as in spectrophotometry because scattering is involved. Linearity of the analytical response curve is generally not influenced by the spectral bandwidth of the incident beam in a significant manner because the turbidance versus wavelength function is characterised by broader bands. Consequently, relatively large wavelength bandwidths (typically 10—100 nm) can be used. This favours the exploitation of LED-based technology [53]. 

Such double beam instruments can be controlled more easily in processes by relays or microprocessors. No mechanics for automatic exchange of sample and reference cells have to be included. The energetic efficiency of the light paths is lower. A double monochromator supplies higher quality photometry. The spectral resolution can be increased and the amount of stray light is drastically decreased. The slit in-between the two monochromator parts is essential. A high performance instrument is shown in Fig. 4.3. Such spectrometers are rather expensive but are very useful in the examination of complex photoreactions as well as in the measurement of problematic samples such as turbid solutions, viscous samples, or thin films. 

Infrared spectroscopy can be used in turbid suspensions, such as membranes or with big proteins, but the methods for studying the spectrum are impaired by the difficult interpretation of the composite bands obtained from proteins. Thus, more powerful methods of spectral analysis are needed... 

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