Traditionally, absorption spectroscopy has been used in laboratories to perform precise analyses. For production environments, a very robust portion of the electromagnetic spectrum—UV/Vis/NIR—emerged for process analytics.

• Absorption spectroscopy is a proven analytical technique

Although NIR data was once considered a challenge to interpret because of broader, overlapping peaks than in the IR region, the NIR region is information-rich and has many practical advantages when applied in a process environment. One of these advantages is the reliable performance of online fiber-optic coupled analyzers. The analyzer can be located in a safe environment away from the sample interface (sampling point) which is located in the process stream … thus giving the process engineer or operator simultaneous real-time information for multiple parameters as it occurs within the process. In addition, software tools evolved to make NIR interpretation routine and are performed automatically to provide the actionable information the operator needs. Process analyzer professionals and reliability engineers are now well-experienced in NIR-based analyzer system deployment throughout their plants.

• Online, fiber-optically coupled NIR Analyzer Systems provide simultaneous real-time compositional and physical parameter information for better process control

A key benefit of an NIR spectrometer analyzer system includes the ability to quickly and accurately measure samples while keeping the main analyzer away from any potentially dangerous processes. Fiber optic cables connecting the sample interface (insertion probe, flow cell, vapor cell, etc.) to the analyzer make this possible. Carrying light energy over distance through a light-conduit (optically efficient fiber optic cables) can potentially introduce error sources for light measurement because the light must travel from the spectrometer to the optical probe and then back to the spectrometer for analysis. To perform optimally, it is especially important that all the components within the total analyzer system be tuned to work efficiently with each other. At Guided Wave all fiber optic cables, spectrometers, photometers, and optical probes (sample interfaces) are all designed and tested to ensure optimal transmission and compatibility. The improved transmission causes the NIR data to be more accurate and thus more accurately analyzed.

• Optically-matched and optimized components to increase the performance and reliability of the total analyzer system

The basic optical element of the Analyzer is a plane (flat) diffraction grating. A diffraction grating is essentially an aluminum-coated mirror with thousands of parallel and equally spaced grooves etched into its surface. (This surface is very delicate and should never be touched or cleaned.) As such, a grating is one of the most precise objects ever made. How gratings work is described in all good introductory college physics textbooks. Let us review a few of these facts, as they are important to the correct operation of the Analyzer. The operation of the grating is defined by the grating equation:

mλ = 2d(sin θ + sin φ)
m = the grating order integer
λ = the wavelength
d = the grating constant (lines per millimeter)
θ = the angle of the incident light (measured from the perpendicular to the grating)
φ = the angle of the diffracted light (also measured from the grating perpendicular)

This equation describes how white light is dispersed into its fundamental wavelengths (e.g., color for visible light). White light enters the monochromator through an entrance fiber in the multiplexer housing. The light is dispersed onto the output module fiber array by the grating, where it is directed to the detector module. A spectrum is recorded by rotating the grating and measuring the intensity of the light impinging on the detector.

The Analyzer is programmed to calibrate itself via the insertion of a NIST traceable rare-earth standard filter mounted on the filter wheel. Thus, the grating angles are accurately translated into the wavelength axis that is presented in a normal spectral scan.

It is essential, however, that the user understands the significance of the grating order integer, m. For zero order, m = 0, the angles of incidence and diffraction are equal but have opposite signs. This is the condition for a mirror – no dispersion occurs and white light is present at the exit fiber. Zero-order is only useful during the initial calibration process and, therefore, is of no concern. The spectrometer is designed to work in the first order, m = 1. However, under certain conditions, second order, m = 2, light may reach the detector. This second order light will have a negative impact on the photometric linearity of the resulting absorption measurements. An example of this is that 1000 nm light will appear naturally at 1000 nm in the first order but also at 2000 nm in the second order and again at 3000 nm in the third order. If the instrument is equipped with an extended range InGaAs detector that is sensitive from 900 – 2150 nm, second order light will begin to appear at about 1950 nm since there is a 975 nm long pass filter permanently mounted in the lamp housing. This second order light is undesirable and needs to be filtered out. To prevent second order radiation from interfering with first-order analytical measurements, it is customary to insert an order sorting (long pass) filter into the optical path. This filter is provided in the lamp assembly in the form of a 1550 nm long pass filter (LP1550). For spectra recorded beyond 1900 nm, it is recommended to use the long pass option in the setup screen of the analyzer control software and insert the LP1550 filter at a point in the spectra that have no, or little, spectral information. The insertion point can be anywhere between 1550 nm and 1950 nm.

Near-infrared (NIR) spectroscopy is a non-destructive technique that can provide fast, accurate results for your process without the high maintenance costs or the extensive upkeep considerations associated with gas chromatography (GC). We use our optical technology to bring light to the sample, eliminating the costly sample systems and fast loops needed for process gas chromatography while providing faster results to you.