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NIR Spectrophotometer

NIR Systems at Celignis

We have two FOSS XDS NIR spectrophotometers in our laboratory. These both contain an XDS monochromator which is attached, for one system, to a rapid solid content analyser (RCA) module and, in the other system, to a rapid liquid analyser (RLA) module. Specifications of the XDS monochromator and each of these modules are detailed below:

XDS Monochromator

This system uses a tungsten halogen filament lamp as the radiative source. Light then enters the monochromator enclosure via the lamp housing exit slit and is directed onto the grating by a mirror after the entrance slit.

The grating is a concave holographic grating, ruled with fine lines to separate the white light into individual wavelengths. It is rotated by a positioning motor, and hence the rotational position is correlated with the wavelength. The resolution of the grating is specified as 0.5 nm. An effect of the diffraction grating is that many orders of radiation are produced, each one comprising a spectrum over a certain angle. There occurs a partial overlap of the different orders of radiation which will result in different wavelengths from different orders appearing in the same position. For example, the wavelengths 2400 nm, 1200 nm and 800 nm from the 1st, 2nd and 3rd orders, respectively, might be superimposed. The XDS only uses first order radiation, since this is the most intense. Hence, it is necessary to block the radiation from the other orders. This is accomplished with order-sorter filters which are mounted in a paddle whose position is kept in sync with the grating position motor. Hence only first order radiation is let through. The four filters are: 400-700 nm, 700-1100 nm, 1100-1700nm, and 1700-2500nm.

The light leaves the grating and is reflected towards the order sorter filter off of a second mirror. The light then passes through the order sorter and fills the exit slit with the selected light to pass, via optical fibre, to the detector module. Other relevant specifications of the XDS monochromator are detailed below:

  • Spectral Resolution - This can be determined as the measurement of the separation of closest pair of absorption lines that can be separately identified. It is reported to be 2 nm for the FOSS XDS.
  • Spectral Data Interval - The gaps over which data points are collected, in the XDS it is every 0.5 nm.
  • Wavelength accuracy - The difference between the measured wavelength of a wavelength standard and the wavelength reported for that standard by the equipment. This is reported as being less than 0.05 nm for the FOSS XDS.
  • Wavelength precision - Repeatability of wavelengths on the same instrument, given as < 0.005 nm on the XDS. Wavelength precision can also be measured based on a group of instruments in which case the value for the XDS is reported as <0.02 nm.
  • Scan speed - Time taken to cover all the analytical wavelengths. The XDS has a data acquisition rate of 2 scans per second.
  • Stray light - This is the sum of any energy/light, other than the chosen wavelength, that reaches the detector. It is reported to be < 0.1% at 2300 nm for the XDS.

Rapid Content Analyser

The Rapid Content Analyser (RCA) has a reflectance detector that incorporates 4 silicon detectors (for the wavelength range 400-1100nm) and 4 lead sulphide detectors (for the wavelength range 1100-2500 nm). These detectors are mounted at a 45 degree angle to the sample surface (which is irradiated by the light at normal incidence). The signal from the detector is then sent to the amplifier where it is amplified and, digitised, and sent to the computer associated with the system.

In a conventional set-up an iris is locked in place in the RCA module and used to focus a small circular cell over the scanning window so that spectra can be collected from the sample contained within this cell. The capacity and surface area of this cell are very small and would preclude the analysis of anything but well comminuted and homogeneous samples. This would prevent the development of NIR analysis methods for more hetereogeneous samples that have larger particle sizes. The RCA transport assembly solves this problem by allowing material within a much larger sample cell, the 'coarse sample cell', to be scanned. This rectangular cell is placed in the locking mechanisms of the wheeled solids-transport device and spectra are taken through the window. The transport assembly then moves, by means of a motor device, to a second position, which represents a separate part of the coarse cell to that which was previously scanned, and further spectra are collected. This process repeats six more times and at the final iteration the last part of the window of the coarse sample cell has been scanned and the transport assembly then returns the cell to its starting position.

Rapid Liquid Analyser

This system is designed for the analysis of liquids and includes a transmission detector and the capacity to include a variety of analytical cells of differing path-lengths. The system also includes a heater to allow for a set temperature to be reached before the spectrum is taken.

Further Information on NIR

We have additional pages on our website detailing the background to near infrared spectroscopy and the services that make use of our NIR equipment. Included below is a short animated video detailing the advantages of our NIR analysis package for the rapid analysis of biomass for lignocellulosic composition.

Analysis Packages that Use Our NIR Spectrophotometer

The Celignis Analysis Package(s) that use our NIR Spectrophotometer are listed below:

P11   NIR Prediction Package
Total Sugars, Glucose, Xylose, Mannose, Arabinose, Galactose, Rhamnose, Lignin (Klason), Lignin (Acid Soluble), Acid Insoluble Residue, Extractives (Ethanol-Soluble), Ash, Ash (Acid Insoluble)

Further Details

Constituents Determined Using Our NIR Spectrophotometer

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