Bran Luebbe InfraAlyzer 2000 NIR Spectrometer: Principle of Operation

How does the InfraAlyzer 2000 Work?

In the InfraAlyzer 2000, the sample is exposed to near infrared light of specific wavelengths, selected from up to 19 high precision interference filters. The light penetrates the sample, interacts with sample molecules and is partly absorbed and partly diffusely reflected. the reflected light is measured by a lead sulfide (PbS) detector mounted in a gold-coated integrating sphere located above the sample. The amount of Light absorbed by the sample at different wavelengths is directly related to the concentration of chemical function groups. 

Before the Instrument can be used to analyze samples, it must be calibrated, using a representative set of similar samples of the product to be analyzed and their known reference values. The values must have been obtained using standard laboratory methods of high accuracy. In a calibration, a correlation between the NIR absorbance values at the different wavelengths and the sample composition is established, and calibration constants are calculated for each wavelength.  

Bran Luebbe InfraAlyzer 2000

Light emitted by a tungsten halogen lamp is periodically interrupted by a constantly rotating chopper disk (3 in Figure) that is positioned in front of the filter wheel. The chopper disk has equally spaced windows that allow interrupted light to pass. The filter wheel contains up to 19 narrow-bandwidth near infrared filters that provide monochromatic light. The filter wheel successively rotates each filter selected for analysis into the light beam. 

The monochromatic light leaving the filter is directed through a lens and an aperture (5 and 6 in Figure) to a gold-coated mirror (7 in Figure). The mirror has tow operating positions: 

  • Sample position: In the sample position (7a in Figure), the light beam is directed through an opening in the integrating sphere onto the sample in the sample drawer, where it interacts with the sample. The  light diffusely reflected from the sample is collected in the gold-coated integrating sphere (8 in Figure) and measured by the lead-sulfide detector mounted in the sphere wall (9 in Figure). The resulting analog signal is sent to the analog board where it is amplified and digitized. 
  • Reference position: In the reference position (7b in Figure), the light beam is directed to the inside of the integrating sphere wall. The light measured  by the detector is only reflected from the inside of the sphere without interacting with the sample. It serves as an internal reference representing an almost totally reflecting sample.

From the ratio sample reading/reference reading the reflectance value is calculated, which provides the basis for calculating the property values of the sample.