Infrared analyzers applications

Simply stated, fiber-optic cable allows for the transfer of light from one point to another withont going in a straight line, and as such their use in process analytical apphcations is highly seductive. In a likely near-infrared process application there may be multiple sample stream take-offs, each with different sample characteristics, located in positions spread around a process unit, with the only apparently convenient analyzer location some distance away. 

This instrument has evolved from ihe laboratory spectrophotometer to satisfy the specific needs of industrial process control. While dispersive instruments continue to be used in some applications, the workhorse infrared analyzers in process control are predominantly nondispersive infrared (NDIR) analyzers. The NDIR analyzer ean be used for either gas or liquid analysis. For simplicity, the following discussion addresses the NDIR gas analyzer, hut it should be recognized that the same measurement principle applies to liquids. The use of infrared as a gas analysis technique is certainly aided by the fact that molecules, such as nitrogen (N ) and oxygen tO , which consist of two like elements, do not absorb in the infrared spectrum. Since nitrogen and oxygen are the primary constituents of air. it is frequently possible to use air as a zero gas. 

In the laboratories of BASF (Badische Anilin- and Soda-Fabrik) at Ludwigshafen, the importance of infrared spectroscopy for industrial purposes was realized as early as the 1930 s. The first IR instrument with a modulated beam was built by Lehrer in 1937 and modified to a double beam instrument with optical compensation in 1942. Luft described the first non-dispersive infrared analyzer in 1943. He used the gas to be analyzed as absorber in a photo-acoustic detector cell. Thus, the instrument was sensitive only to this gas. He also provided a survey of early industrial applications of infrared spectroscopy (Luft, 1947). 

In confined spaces it is necessary to keep the concentration of CO from automobile exhaust down to acceptable limits. Therefore, CO infrared gas analyzers have been installed on a large scale in such places (e.g., tunnels, parking areas, etc.). In medicine, respiratory units for lung-function investigations have used infrared analyzers. The use of infrared methods for qualitative and quantitative information on a variety of gases and vapors has been discussed earlier in this chapter in connection with anesthesiology and toxicology applications. 

E. V. Rouir, Infrared Analyzers, Industrie chim. beige 21, 221-234, 1956. Description of the principles and applications. 

Fourier transform infrared (FTIR) analyzers can be used for industrial applications and m situ measurements in addition to conventional laboratory use. Industrial instruments are transportable, rugged and relatively simple to calibrate and operate. They are capable of analyzing many gas components and determining their concentrations, practically continuously. FTIR analyzers are based on the spectra characterization of infrared light absorbed by transitions in vibrational and rotational energy levels of heteroatomic molecules. 

It is a commonplace that FTIR-based analyzers are the predominant technology for mid-infrared applications. This arises from a unique tie-in between the inherent advantages of the FTIR method and serious limitations in the mid-infrared range. The most serious problem for mid-infrared spectroscopy is the very low emissivity of mid-infrared sources combined with the low detectivity of mid-infrared thermal detectors. 

Infrared (IR) spectroscopy offers many unique advantages for measurements within an industrial environment, whether they are for environmental or for production-based applications. Historically, the technique has been used for a broad range of applications ranging from the composition of gas and/or liquid mixtures to the analysis of trace components for gas purity or environmental analysis. The instrumentation used ranges in complexity from simple filter-based photometers to optomechanically complicated devices, such as Fourier transform infrared (FTIR) spectrometers. Simple nondispersive infrared (NDIR) insttuments are in common use for measurements that feature well-defined methods of analysis, such as the analysis of combustion gases for carbon oxides and hydrocarbons. For more complex measurements it is normally necessary to obtain a greater amount of spectral information, and so either Ml-spectrum or multiple wavelength analyzers are required. 

Infrared Spectroscopy for Process Analytical Applications 191 6.6 Process IR Analyzers a Review... 

The objective of this chapter is to reduce the learning curve for the application of near-infrared (NIR) spectroscopy, or indeed any process analytical technology, to the chemical industry. It attempts to communicate realistic expectations for process analyzers in order to minimize both unrealistically positive expectations (e.g. NIR can do everything and predict the stock market ) and unrealistically negative expectations (e.g. NIR never works don t waste your money ). The themes of this chapter are value and... 

Rottenbacher, L., Behlau, L. and Bauer, W., The application of infrared gas analyzers for the fast determination of kinetic parameters for ethanol production from glucose, ]. Biotech., 2 (1985a) 137-147. 

Despite the distinct advantages of pneumatic nebulizers, ultrasonic nebulizers may alternatively be used, in some instances, with success. In a recent application, a variation of ultrasonic nebulizer called spray nozzle-rotating disk FTIR interface was successfully applied to confirm the presence of methyltestosterone, testosterone, fluoxymesterone, epitestosterone, and estradiol and testosterone cyp-ionate in urine, after solid-phase extraction and reversed-phase LC separation (151). Using a commercial infrared microscopy spectrometer, usable spectra from 5 ng steroid deposits could be readily obtained. To achieve success with this interface, phosphate buffers in the mobile phase were not used because these nonvolatile salts accumulate on the collection disk and their spectra tend to swamp out small mass deposits. Another limitation of the method was that only nonvolatile analytes could be analyzed because volatile compounds simply evaporated off the collection-disk surface prior to scanning. 

Mass Spectrometers as Process Analyzers. The use of mass spectrometers in process analysis lias not been widespread because nl its perceived complexity and high cost. Technological advancements over the past decade or two have reduced costs and simplified mass spectrometry to the point where it is suitable lor a number of process analytical applications in place of infrared absorption or gas chromatography mass spectrometer is well accepted in such applications hecause of low cosi. good reliability, and ease of cnmpuicr-controllcd interfaces. 

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