Analyzers fiber-optic probes

With the appropriate fiber-optic probe and data processing techniques, UV-vis spectroscopy may be used to determine the optical thickness of a transparent thin film. It is possible to simultaneously measure thickness of different layers in a multilayer structure as long as each layer falls within the analysis range of the instrument. Typically, this means layers in the 0.5-150/rm range. A further constraint on this technique is that the layer structure of the film must be smooth on the scale of the spot size of the fiber-optic probe. Neighboring layers must have different indices of refraction in order for them to appear as distinct layers to the analyzer. 

There are many examples of second-order analyzers that are used in analytical chemistry including many hyphenated spectroscopic tools such as FTIR-TGA, IR-microscopy, as well as GC-MS, or even two-dimensional spectroscopic techniques. Another hyphenated technique that is being developed for the study of solid-state transitions in crystalline materials is dynamic vapor sorption coupled with NIR spectroscopy (DVS-NIR).26 DVS is a water sorption balance by which the weight of a sample is carefully monitored during exposure to defined temperature and humidity. It can be used to study the stability of materials, and in this case has been used to induce solid-state transitions in anhydrous theophylline. By interfacing an NIR spectrometer with a fiber-optic probe to the DVS, the transitions of the theophylline can be monitored spectroscopically. The DVS-NIR has proven to be a useful tool in the study of the solid-state transitions of theophylline. It has been used to identify a transition that exists in the conversion of the anhydrous form to the hydrate during the course of water sorption. 

Figure 3.40 shows the layout of a typical Raman analyzer that uses fiber optics for process application. In a Raman process system, light is filtered and delivered to the sample via excitation fiber. Raman-scattered light is collected by collection fibers in the fiber-optic probe, filtered, and sent to the spectrometer via return fiber-optical cables. A charge-coupled device (CCD) camera detects the signal and provides the Raman spectrum. To take advantage of low-noise CCD cameras and to minimize fluorescence interference, NIR diode lasers are used in process instruments. 

Bruckner and Kondratenko (2006) used a similar approach to characterize VOx/Ti02 catalysts. In a separate TPR experiment carried out with a quartz reactor equipped with a UV-vis fiber optical probe, the relationship between the "absorbance" at 800 nm and the degree of reduction as determined from H2 consumption via mass spectrometry was established. The absorbance at 800 nm increased with increasing reduction of the vanadium, but not linearly. During the catalytic reaction experiment, the absorbance at 800 nm was then used to determine the average valence of vanadium. Because contributions of reduced titanium species in the analyzed spectral range could not be excluded, only a lower limit of the vanadium oxidation state could be determined, which was 4.86 at 523 K and C3H8/02 = 1 1. 

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. 

With the ever-increasing need to improve quality and productivity in the analytical pharmaceutical laboratory, automation has become a key component. Automation for vibrational spectroscopy has been fairly limited. Although most software packages for vibrational spectrometers allow for the construction of macro routines for the grouping of repetitive software tasks, there is only a small number of automation routines in which sample introduction and subsequent spectral acquisition/data interpretation are available. For the routine analysis of alkali halide pellets, a number of commercially available sample wheels are used in which the wheel contains a selected number of pellets in specific locations. The wheel is then indexed to a sample disk, the IR spectrum obtained and archived, and then the wheel indexed to the next sample. This system requires that the pellets be manually pressed and placed into the wheel before automated spectral acquisition. A similar system is also available for automated liquid analysis in which samples in individual vials are pumped onto an ATR crystal and subsequently analyzed. Between samples, a cleaning solution is passed over the ATR crystal to reduce cross-contamination. Automated diffuse reflectance has also been introduced in which a tray of DR sample cups is indexed into the IR sample beam and subsequently scanned. In each of these cases, manual preparation of the sample is necessary (23). In the field of Raman spectroscopy, automation is being developed in conjunction with fiber-optic probes and accompanying... 

The ideal situation for a process analyzer is to have the detector/electrical components in an area physically isolated from the sample stream and any liquid components required for calibration or instrument operation (e.g., solvents for liquid chromatography or gel-permeation chromatography). In this arrangement, isolation of the electrical components ensures that a spark source or hot surface (such as a spectroscopic light source) will not cause a problem with flammable components (both sample stream components and solvents). Also, this isolation wiU limit instrument damage in case of a sample system leak or solvent leak. Although the ideal situation is often not realized in a practical application, one should make every effort to protect the delicate sections of the instrument. In one application of the authors, a fiber optic probe having a quartz window sealed on the end of the probe was installed in the reactor of a process that contained... 

Figure 4.58 Antaris II FTNIR analyzer, Thermo Scientific, equipped with a fiber-optic probe for remote measurement, transmission, and integrating sphere and a tablet analyzer for direct analysis of pharmaceuticals. ( Thenao Fisher Scientific (www.thermofisher.com). Used with permission.)...

Process analyzer measurements, e.g., spectra or chemical images, typically require a mathematical transformation, e.g., multivariate data analysis, to correlate the process analytical data to a more relevant critical product attribute for design space definition. For brevity, throughout this section the measurement system that yields process analytical data is noted as a PAT method and the subsequent mathematical transformation is described as a model. To forego a debate regarding what constitutes a process analyzer, i.e., temperature sensor versus a Raman fiber optic probe, herein focuses on process analyzers that yield multivariate data. 

As for beef, the texture of salmon is important for the consumer. Fifty-three farmed Atlantic salmons were measured on the inside of the fillets, using a NIR fiber-optic probe as described above (53) and a texture analyzer (26). The texture was measured by placing a 20 X 20 X 80-mm muscle in a five-blade Kramer shear force cell (HDP/KS5, Stable Micro systems, Surrey, UK) in duplicates. The area under the profile from the... 

Nufiez-Sanchez et al. (37) evaluated NIR calibration equations for the main constituents of ewe s cheese under two different sample preparation methods (homogenized and intact) and under reflectance and fiber-optic probe. The SECV values obtained for the homogenized cheeses and for both analysis modes were comparable for fat, protein, and dry matter. The calibration statistics for the intact cheese analyzed by fiber-optic probe were higher than those obtained with homogenized cheese. 

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