BRIMROSE
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BRIMROSE
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Near-infrared spectroscopy using acousto-optic tunable filters brings real-time process analysis and improved quality control of products as varied as prescription drugs, gasoline, and cheese. Spectroscopy speeds industrial process control By: Brimrose Corporation of America Near-infrared (NIR) spectroscopy based on an acousto-optic tunable filter (AOTF) is gaining popularity for industrial use. When used in combination with chemometrics, it is a quantitative tool for performing real-time calibrations during product manufacture. The instrument validates a known sample, analyzes an unknown sample, and calculates a concentration from the NIR optical spectrum of the sample (see "Fundamentals of NIR spectroscopy," p. 121). The vast array of sample presentation techniques, which includes transmission, reflectance, transflectance, and attenuated total reflectance, gives AOTF-NIR spectroscopy a significant advantage over technologies such as high-performance liquid chromatography and gas chromatography. The principle of the AOTF is based on the acoustic diffraction of light in an anisotropic medium (Fig. 1 and Laser Focus World, Aug. 1994, p. 87). The device consists of piezo-electric transducer bonded to a birefringent crystal. When the transducer is excited by an applied radio frequency (RF) signal, acoustic waves are generated in the crystal. The propagating acoustic wave produces a periodic modulation of the refractive index. The effect provides a moving phase grating that, under proper conditions, will diffract portions of an incident light beam. For a fixed acoustic frequency, a narrow band of optical frequencies satisfies the phase-matching conditions and can be cumulatively diffracted. As the exciting frequency is changed, the center of the optical bandpass is changed accordingly so that the phase-matching condition is maintained. The implementation of AOTF-NIR spectroscopy in a manufacturing process requires a sampling interface to the process (normally a remote fiber optic device), the AOTF-NIR analyzer, chemometric software to develop the calibration models, and an interface to the process distributive control system. This can take the form of a bi-directional interface between the control system and the AOTF-NIR device. With such a system in place, a manufacturing operation can respond to analytical data generated in real time. Based upon the analytical results, decisions are made whether to make changes to the manufacturing process, such as increasing the flow rate of a particular ingredient from a dosing pump or increasing the temperature of a dryer to lower the moisture content of a product.
Pharmaceuticals and AOTF-NIR Process control using AOTF-NIR spectroscopy is particularly beneficial in the pharmaceutical industry where it is applied to blending operations, reaction monitoring, 100% inspection of products, on-line testing of excipients, and solvent recovery. The manufacture of a dosage form involves blending the active component with the excipients to produce a homogeneous mixture. Instead of using traditional high-performance liquid chromatography (HPLC) to determine homogeneity and potency of the active ingredient, an on-line approach with AOTF-NIR analysis provides close monitoring of the blending process and easy determination of the blending end point. Testing every tablet, capsule, or other pharmaceutical product is important to the pharmaceutical industry. Correct dosage levels of active ingredients and appropriate tablet hardness are of primary interest. Currently, out of a batch of 100,000, only 100 may be subjected to HPLC analysis, and they may not be representative of the entire batch. The speed of the Luminar 2000 AOTF-NIR analyzer (Brimrose Corp. of America; Baltimore, MD) makes 100% inspection possible. Measurements can be taken at the rate of 30 tablets per second on the production line, and tablet hardness, dosage level, moisture content, and tablet identification are routinely measured. Additionally, a high-resolution line camera is used to trigger the spectrometer to make sure that the spectrum is collected at exactly the same table position each time. The camera can also be used to look for any physical defects in the table or capsule.
The AOTF-NIR technique has been successfully implemented in following pharmaceutical reactions and determining end points. A fiber optic probe is placed directly into a reactor to collect near-infrared spectra in the range from 800 to 2400 nm. On-line AOTF-NIR analysis provides an answer in seconds, resulting in an accurate endpoint monitor that can reduce cycle time and increase product yield (see Fig. 2). The traditional method is to remove a sample from the reactor and analyze it in the laboratory using HPLC. By the time this is complete, the reaction may have gone past the optimum end point, resulting in a yield decrease. The sensitivity of the AOTF-NIR to physical properties such as particle size means that differences can be monitored between chemically identical samples. An example of this is the commonly used excipient microcrystalline cellulose, which comes in two different particle sizes . They are both white powders with no visible difference in appearance and very little spectral difference. However, by using an AOTF-NIR device coupled with a handheld fiber optic probe, it is possible to discriminate between these tow products as they enter the factory and before they are used in the manufacturing process. On-line AOTF-NIR analysis may replace gas chromatography during solvent recovery (see Fig. 3.) A typical solvent recover plant consists of a number of stills, each with an associated condenser. The purity of solvent in the feed stream is typically around 50%, and the target purity for the distilled solvent is between 95% and 99.9%. Gas chromatography, the traditional method for determining solvent purity and the concentrations of minor impurities, is time-consuming, requiring three hours before results are reported to the process control room. In that time, the product may be out of specification. Alternatively, on-line AOTF-NIR readings can generate multicomponent results in real time, and changes to the reflux ratio, feed rate, temperature, and pressure can be made to optimize the process and keep the product within specification. This results in energy savings and a more consistent product quality.
The speed of the AOTF-NIR system gives the Luminar family of spectrometers advantages in the area of process control. A target for drug manufacturers is 100% inspection of pharmaceutical products, and this is now possible using acousto-optic technology. Conventional technology permits only 1% inspection. In the area of solvent recovery, the use of an optical multiplexer to measure up to 16 channels provides feed-forward as well as feedback control over the entire process resulting in energy savings and improved quality of product. Other industrial applications On-line AOTF-NIR analysis has been implemented in many other industrial processes as well, including refining, polymers, chemicals, pulp and paper, and the food and dairy industry. In the refining area, on-line analysis during gasoline blending enables less octane giveaway as well as optimizing the blending process by measurement of all of the component streams as well as the blend stream. Aromatics, olefins, parafins, oxygenates, benzene, and distillation points are all measure simultaneously with an analysis time of less than five seconds. Similar applications have been developed for diesel fuel and kerosene. Polymer measurements with an AOTF-NIR device include measurement of physical properties as well as chemical composition. The pulp and paper industry uses AOTF-NIR technology to measure kappa number in pulp and calorific values in black liquor, a corrosive liquid used in the pulp digestion process. Waste black liquor can be burned to provide energy for paper-processing plants. High speed applications include the measurement of polymer, silicone, or adhesive coating thickness on paper moving at up to 100 ft/s. In the food and dairy industry, protein, fat and moisture are routinely measured during production cycles. In the manufacture of cheese, it is important to standardize the incoming raw milk for fat and solids content. Closed-loop control based on AOTF-NIR measurements can enable the addition of cream or skimmed mild to increase or decrease the fat and solids content to the desired specification. The AOTF-NIR technology has taken routine analysis of many products out of the laboratory and into the process arena, resulting in better quality products, reduced cycle time, and substantial energy savings.
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