Near IR Spectroscopy in Process Analysis (Click on the title to view the complete Application Note)
The purpose of this discussion is to provide the reader with an overview of fiber optic NIR process analysis, including instrumentation, fiber optics, probes and mathematical methods.
NIR has been traditionally viewed by spectroscopists trained in laboratory methods as a less-than-useful spectral region that exists to fill the gap between the UV/VIS and IR regions. NIR peaks are broad, overlapped and much weaker than their IR counterparts. So, why has NIR become so widespread in process analysis?
Interest in NIR grew quickly in the 1980s due to the advent of fiber optics, bright light sources and sensitive detectors. NIR also solved some of the difficult sampling techniques characteristic of IR, because NIR optical paths through liquid samples may be millimeters or even centimeters, rather than microns. As a result, first commercial fiber optic spectrophotometers were introduced in the 1980’s.
The importance of these developments lies in the ability to safely bring the sampling interface, a fiber optic probe, to the process while remotely placing the electronics (ignition sources) away from hazardous environments. This also meant that the traditional, time‑consuming method of collecting (“grab”) samples and bringing them back to the laboratory for analysis could be replaced by continuous “on-line” measurements without exposing workers to potentially dangerous chemicals. Continuous on-line measurements, in turn, mean more responsive process control for better production yield, less rework of off-target material, improved product quality and enhanced worker safety. It began to change the paradigm by measuring chemistry directly, rather than inferring it from temperature, pressure, flow and fluid level. For NIR to be accepted, it had to provide reliable instrumentation requiring minimal maintenance. Otherwise, there would be little advantage over the much more widespread, but more maintenance-intensive methods, such as process gas chromatography (GC). Thus, considerable advancements occurred over the next ten years or so to meet the requirement of instrument reliability.