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Near IR opens a window to rapid analysis.

Remember seeing all those peaks between 780 and 2,500 nm in the wavelength spectrum, commonly referred to as the near IR region, and ignoring them because of their complexity? Well, take another look. Near infrared (NIR) spectroscopy has found its way into many laboratories and industrial sites to provide information within seconds that once took hours or even days to obtain by classical analytical methods.

Used by agricultural chemists for years to determine the moisture content in grain, NIR is not a new technology. However, it has become a valuable analytical tool largely due to the availability of computer software designed to grind through the complex mathematical analyses necessary for spectral interpretation.

Techniques such as partial least squares, multiple linear regression, and discriminant analysis, nearly impossible to do manually, can now be performed easily by the laboratory computer. This software, together with fiber optic probes attached to cables up to several hundred meters in length, have made NIR more accessible to the researcher.

Industries that have greatly benefited from recent progress in near IR technology include the chemical, pharmaceutical, pulp and paper, food and beverage, petrochemical, and biomedical sectors.

"The true value of near IR spectroscopy is that it gives us a window right where we really want one, to watch the chemistry," says Jon Gethner, an infrared process analyzer consultant who founded his own company, Adaptive Analyzer Technologies, Westfield, NJ.

"In making real-time measurements, it allows us to understand and control the chemistry in a reaction where we had to make certain assumptions in the past. That's really going to assist us in improving the manufacturing processes," states Gethner.

"The biggest application for near IR today is in the petrochemical industry," says Allen Bickel, sales manager for LT Industries, Rockville, MD. "It is being used to measure octane number, aromatic and aliphatic ratios, and physical measurements such as Reid vapor pressure and boiling point, all of which are obtained in seconds. And these measurements can be performed in a multiplex configuration, meaning one unit is capable of monitoring several points simultaneously."

Bickel also describes the use of NIR spectroscopy in the food and beverage industry. "It's used with several on-line blending applications; for example, when syrup is mixed with water, and [CO.sub.2] is then added to make a soft drink. The exact blend ratios can be monitored in real time so that the blending process can be totally controlled. Beer production can also be managed in a similar way.

"And all it takes is just one or two saved batches to recover the investment of the NIR equipment. Typically, a bad batch would have to be dumped, and sewer taxes alone could be in excess of $100,000.

"In the polymer industry," Bickel continues, "the progress of a polymerization process can actually be followed. The forming polymer can be monitored as the monomer reacts and tracked to the end point of the polymerization process. The plant operator can be notified when it is time to stop the reaction and transfer the completed product to containers for shipment."

"Near infrared spectroscopy is terrific news for us in the pulp and paper industry," says Robert Horton, assistant professor at the Institute of Paper Science and Technology, Atlanta. Horton is focusing on the analysis of black liquor, which contains the spent chemicals used in the digestion of wood chips to make pulp fibers. The black liquor is concentrated, through evaporation, and burned to recover the energy value of the dissolved organic compounds. Residual ash from the inorganic chemicals can be rejuvenated and recycled for use in the pulp digesters, completing the cycle.

"There is a lack of acceptable on-line measurements for black liquor composition, which makes controlling the recovery processes a manual operation, based largely on trial and error," says Horton. "Laboratory analyses take four to five hours, and often overnight, to perform by standard methods. Near IR measurements require no sample preparation, and results can be obtained within two minutes."

Horton, enthusiastic about the future of NIR in the pulp and paper industry, is quick to list several other potential applications. These include chlorine monitoring, alkali strength determination, moisture sensing, hardwood/softwood ratio analysis, and identification of cellulose, hemicellulose, and lignin.

In today's manufacturing environment, one plant makes many different products. As a result, these plants are often in a state of transition, changing from one product to another. For economic reasons, the transition periods should be as short as possible. Cary Sohl, plastics industry research physicist at the DuPont Experimental Station, Wilmington, DE, describes how companies can save millions of dollars by improving the efficiency of such product transitions.

"In the past, when the plant manager initiated a product changeover, he sampled the material frequently during the transition period. These samples were analyzed in the laboratory, and results were obtained in about three hours. Due to the time lag, the plant operator had to make changes to the system based on conditions that existed several hours ago. It was a very inefficient process. By using an on-line NIR analyzer, information is obtained minute by minute. This allows a plant transition from one product to another to proceed quicker, safer, and more efficiently."

Often, the economies can be very large, as Sohl indicates: "If a plant is operating at 50,000-pounds capacity per hour, and the material sells for $2.00 per pound, $100,000 worth is produced in that plant per hour. If the transition period is shortened from six hours to three, the company saves $300,000 each time that transition is made.!"

Although NIR equipment is not inexpensive, cost justification for its purchase is very straightforward. A substantial savings will be realized with every transition.

At BASF, Wyandotte, MI, John Moote, research chemist, describes some of the uses for NIR spectroscopy in the basic chemicals field. "We use near IR to monitor the water content in our incoming chlorine supply. Water is a critical contaminant in our starting materials, and this is an easy way to check for moisture. Other applications include hydroxyl number determination of polyalcohols and measurement of residual [H.sub.2.O] following the distillation of our products," says Moote.

Linda Lane, principal chemist at Arco Products, Carson, CA, is investigating the use of NIR for on-line characterization of gasoline, including compositional and physical properties.

For example, methyl tertiary butyl ester (MTBE), a gasoline blending component of increasing importance to the oil refining industry, can be accurately measured within 30 seconds at a remote site with no sample preparation. This technique allows for on-line applications in refineries and provides for field checking capabilities by regulatory agencies.

Lane also heads the newly formed NIR study group, a body within the ASTM absorption spectroscopy Section OF, which is part of the D02.04 subcommittee on hydrocarbon analysis. This group is in the process of selecting its first method development.

"The wide range of refining and petroleum-related companies represented by this team of 45 members confirms the enthusiasm from the industry in establishing standards for the NIR technique," says Lane.

Mark Chavez and Joan Carducci, biomedical research scientists at the National Institutes of Health, Bethesda, MD, are using NIR to study various enzyme/substrate reactions. With a fiber optic probe, the kinetics of the ATPase/ATP reaction, for example, can be observed over a period of time.

Adenosinetriphosphatase (ATPase) maintains the concentration gradient of sodium and potassium across red blood cell walls. In doing so, ATP is utilized as the energy source and undergoes hydrolysis. Because ATP does not have an optical signature in the UV or visible region, the reaction could not be followed easily in the past.

By determining how a protein modulates a reaction, the researchers can characterize its structure as it performs. Hopefully, this will lead to an understanding of basic biochemical problems.

Chavez indicates that NIR may even have use as a noninvasive probe for the determination of pH and glucose levels in the human body.

Emil Ciurczak, chemistry department faculty member, Seton Hall Univ., South Orange, NJ, reports on the development of an HPLC/NIR detector.

"Its primary use lies in detecting analytes without conventional UV chromaphores," says Ciurczak. "It could be used to quantify ratios of enantiomers in racemic mixtures. The wavelength shifts of polar molecular sections caused by optically active solvents may be used to measure this ratio. Molecules which form true racemic crystals may also be identified by near IR from their solid-state spectra."

What is the future of near infrared spectroscopy? According to Vince Catalano, product manager at Guided Wave, El Dorado Hills, CA, "The challenge in applying this technology to any industry lies in the calibration. Ultimately, vendors will be pushed by their customers for precalibrated, turnkey instruments.

"However," Catalano continues, "it's not just the technology that's required for successful implementation. The key is the people--from research, analytical control, process engineering, plant maintenance, and the instrument vendor--all working together to obtain an implementation plan with realistic goals."

Don Webster, president of NIRSystems, Silver Spring, MD, sees the field expanding in two directions. "First, there will be a proliferation of turn-key systems developed for specific uses. And, because of the improvements in fiber optics and the ease of calibration, process engineers will be able to use a 'tool box' approach in selecting equipment to suit their particular applications.

"To meet the needs of applications in the field, instrumentation will become smaller, portable, and more rugged to withstand hostile environments.

"Based on the maturity of the technology and the interest levels of potential users, we would expect to see a minimum growth of 20% per year over the next five to 10 years," Webster concludes.
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Title Annotation:includes related article; near infrared spectroscopy
Author:Goldner, Howard J.; Kohn, Wolfgang; Griffin, Jennifer
Publication:R & D
Date:May 1, 1991
Previous Article:Lab of the year.
Next Article:Biosensor applications are springing up all over.

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