Transflectance probe

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...

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. 

De Thomas etal. [Ill] studied the production of polyurethanes and showed that NIRS can be used successfully to monitor the course of the reaction in real time. Spectral data were obtained with a dispersive instrument, using standard transflectance probes. An MLR model was derived for the quantitative determination of isocyanate concentrations during the urethane polymerization reaction. Model predictions were used to build statistical process control charts and to detect trends along the polymerization reaction. The authors suggested that the integration of NIRS with process control routines could lead to improvements of product quality and consistency, while minimizing reaction time. However, model predictions were not used as feedback information for any sort of correction of the process trajectory. Similar studies were performed by Dallin [112] for prediction of the acid number during the production of polyesters. 

Santos et al. [55, 148] studied the styrene suspension polymerization using NIRS. A dispersive NIR instrument equipped with a transflectance probe was used to monitor the reaction course and very good PLS calibration models were developed for the final average particle sizes. Calibration models were then used as references for the implementation of an in-line procedure for control of average... 

Process Control Vieira et al. [176, 185, 186] studied copolymerizalions of butyl acrylate and MMA in anulsion reactors and reported for the first time the use of the NIR spectral signal to perform the feedback control of an anulsion polymerization reactor. Vieira et al. [176, 185, 186] used a dispersive instrument equipped with a transflectance probe to collect the NIR spectra and showed that robust and reliable PLS caUbralion models could be developed for the prediction of the major constituents of the emulsion. A detailed process model was used as reference to provide estimates of unmeasured process variables, such as average particle sizes and the polymer average molecular weight. The NIR model predictions were corrected in-line with the... 

Very little has been reported about the use of spectroscopic methods for monitoring and control of other polymerization systems. Lenzi et al. [191] reported that the NIR spectra collected in a dispersive instrument with a transflectance probe may contain very useful information about the structure of core-shell polystyrene beads produced through simultaneous semibatch emulsion/suspension polymerizations. Lenzi et al. [192] developed a polymerization technique that combines recipes of typical emulsion and suspension polymerizations to produce core-shell polymer beads. More interesting, the appearance of the core-shell structure always led to qualitatively different NIR spectra that could not have been obtained with polymer suspensions, polymer emulsions, or mixtures of polymer suspensions and emulsions. As described by Lenzi et al. [191], different spectral peaks could be detected in the wavelength region constrained between 1700 and 1900nm when the core-shell structure developed. 

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. 

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. 

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). 

Fortunately, automated fiber-optic probe-based dissolution systems have begun to appear for these solid dosage-form applications. One such system uses dip-type UV transflectance fiber-optic probes, each coupled to a miniature photodiode array (PDA) spectrophotometer to measure drug release in real time. This fiber-optic dissolution system can analyze immediate- and controlled-release formulations. The system is more accurate and precise than conventional dissolution test systems, and it is easier to set up than conventional manual sampling or automated sipper-sampling systems with analysis by spectrophotometry or HPLC. 

Probing At-Une/in-line ATR Transmission, transflection, diffuse-reflection... 

FT-MRS using transflectance and diffuse reflectance probes was particularly successful in morphology identification and differentiation (using... 

Atline/in-line probes ATR probes Transmission transflection diffuse reflection probes... 

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