Transflectance

Entries on new materials, including re-cyclate plastics, fullerenes, hard-surfaced polymers, dendrimers, transflective materials, rapid prototyping materials, silicone nitride, supercritical fluids, bulk molding compounds, conversion coatings, folic acid, replacements for chloro-fluorocarbons ... 

Nonetheless, near-IR is the most widely used IR technique. Less intense water absorptions permit to increase the sampling volume to compensate, to some extent, for the lower near-IR absorption coefficients and the inferior specificity of the absorption bands can for many applications be overcome by application of advanced chemometric methods. Miniaturised light sources, various sensor probes, in particular based on transmission or transflectance layouts, and detectors for this spectral range are available at competitive prices, as are (telecommunications) glass or quartz fibres. 

Figure 5. Single-point IR sensor head layouts a transmission probe with fibre coupling b transflectance probe with variable pathlength and single fibre coupling c (diffuse) reflection probe with single illumination fibre and collection fibre bundle d two-reflection ATR probe with fibre-optic coupling e multi-reflection ATR probe (DiComp -type) f ATR fibre...

Solid samples can also be measured in transmission, although reflection or transflection measurements are more common. Open arrangements with the source on one side of the sample and the spectral analyser on the other side are prevalently used, e.g. in industrial process control. For absolute quantitative analysis the thickness of the object must either be constant or be measured. 

For some applications, especially such involving solid samples or fluids containing suspended particles, reflection spectroscopic systems are better suitable than transmission sensors. Apart from specular reflection, which provides comparatively little information and is of hardly any practical importance for IR sensing, two reflectrometric methods can be used to gain spectroscopic information about a sample diffuse reflection and transflection, a combination of transmission and diffuse reflection. 

Transflection is essentially a cross between transmission and mirror reflection. When light is shined onto a reflective surface covered by an optically clear sample, either in liquid or in solid form, two processes occur -... 

In practice, very few applications of FEWS sensors can be found outside laboratory applications and demonstration systems. In the near-IR, suitable fibres are readily available but usually there is no real necessity to use them. Possible transmission pathlengths are sufficiently large to allow using standard transmission probes, while turbid samples can be measured using transflection or diffuse reflection probes. In the mid-IR, high intrinsic losses, difficulties in fibres handling and limited chemical and mechanical stability limit the applicability of optical fibres as sensor elements. 

Like the filtered flow cell, the filtered immersion probe (either transflection or transmission) may be used in sample environments in which bubbles or particles make the use of unfiltered samples impossible. 

The procedures used to record NIR spectra for samples are much less labor-intensive than those involved in other spectroscopic analytical techniques. NIR spectral information can be obtained from transmission, reflectance and transflectance measurements (Figure 14.1) this allows the measurement process to be adapted to the physical characteristics of the sample and expedites analyses by avoiding the need for sample preparation. 

Figure 14.1 Configuration for spectral acquisition (transmittance, reflectance and transflectance).

The samples used to construct the models should be similar to the production samples also, their spectra should be recorded in the same mode (reflectance, transmission or transflectance) as those for the samples to be subsequently predicted, and include all potential sources of variability. Although such sources are relatively limited - pharmaceutical samples usually have a well-defined qualitative and quantitative composition from raw material to end product, and production processes are solidly established and reproducible - their... 

Semisolid samples. As with liquid samples, methods (B) and (C) are the best choices for this type of sample. The specific choice will depend on fhe rheological properties (viscosity, density, air retention) of the particular preparation. These samples are best measured in the transflectance mode. Liquid and semisolid samples may contain a mixture of solvents of disparate volatility which may evaporate separately during the measurement process. Differences in solvent volatility can alter the sample matrix and lead to errors in the determination which are best avoided by using a set of calibration samples spanning an expanded range of solvent proportions. ... 

Dexketoprofen Gel Non-destructive drug quantitation method of a hydrogel in transflectance mode 35... 

NIR spectroscopy is probably the most successful technique for the development of qualitative and quantitative methods in the pharmaceutical industry. NIR spectra contain both chemical and physical information from samples (solid and liquid). Spectra can be acquired off-line in three different modes transmittance, reflectance and transflectance. In all cases, the spectra are obtained in a few seconds without or minimum sample pretreatment. Multivariate data analysis techniques are usually needed for the development of the... 

M. Blanco and M.A. Romero, Near infrared transflectance spectroscopy. Determination of dexketoprofen in a hydrogel, J. Pharm. Biomed. Anal., 30, 467-472 (2002). 

The use of a transflectance probe, rather than a transmission probe, allowed good quality spectra to be obtained despite the low light transmission due to the high solid levels. 

Beyers et aV° in the Polymer Research Division of BASF-AG used in-line transflectance NIR to monitor methyl methacrylate (MMA) and iV,7V-dimethylacrylamide (DMAAm) monomers in a copolymerization reaction. The work in this paper is of interest as it illustrates an example of calibration development done off-line with a very limited number of prepared calibration samples. The value of the measurement is to control the end properties of the products resulting from the copolymerization reaction. The end properties are related to many parameters including the intramolecular chemical composition distribution (CCD). The... 

FIGURE 10 Diffuse reflectance, transflectance, and transmittance measurements. 

An in situ probe is a slender probe of either transflectance or transmission design that is permanently inserted into each dissolution bath in an apparatus. This has the advantage of allowing measurements at the same physical location in the vessel where the sipper tube had been positioned, according to USP guidelines. A disadvantage is the disturbance of the laminar flow in the stirred vessel - thus there has been an effort to make this probe with as small a diameter as possible (1/8" or less). 

Sennosides Senna pods PLS Transflectance mode used. Method validated using ICH-adapted guidelines 140... 

Figure 13.1 Schematic diagram of different measurement configurations (a) transmission, (b) diffuse reflectance, (c) transflectance, and (d) photoacoustic.

As the reflected radiation is emitted from the sample in a random direction, diffusely reflected radiation can be separated from, potentially sensor-blinding, specular reflections. Common techniques are off-angle positioning of the sensor with respect to the position(s) of the illumination source(s) and the use of polarisation filters. Application restrictions apply to optically clear samples with little to no scattering centres, thin samples on an absorbing background and dark samples. In either of these cases, the intensity of radiation diffusely reflected off such samples is frequently insufficient for spectral analysis. While dark objectives remain a problem, thin and/or transparent samples can be measured in transmission or in transflectance. 

Transflection is essentially a cross between transmission and reflection. When light is shined onto a reflective surface covered by an optically clear sample, two processes occur - reflection off the top surface of the sample and transmission and reflection off the mirror, followed by a second transmission. As for most materials the surface reflection is low in comparison to the reflection off the mirror, the total collected radiation corresponds to a transmission measurement with double pathlength. Instead of a transparent support, transflection systems require only a spectrally neutral, or at least constant, broadband reflector under the sample. 

The first published analysis for powders was written in 1981 by Becconsall et al. It described the analysis of propranolol and magnesium carbonate mixes. In 1987, Ciurczak and Torlini published on the analysis of solid and liquid dosage forms using reflectance and transflectance, respectively. 

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