A recent whitepaper created by NIR experts working at FOSS does a good job of comparing the strengths and weaknesses of different NIR spectrometer technologies: scanning grating, fixed grating DDA and FT-NIR. The paper also discusses the specifications used to define the different technologies and how these impact performance.
FT-NIR Interferometers, Scanning Grating Monochromators and Fixed Grating Detector Diode Array technologies -the three spectrometer technologies in this comparison all have their individual advantages and disadvantages.
Major distinctions between spectrometer technologies
|Scanning Grating:||+ Signal to Noise Ratio||– Measures needed to improve wavelength accuracy|
|+ Wavelength range|
|Fix Grating DDA:||+ Robustness||– Lower resolution|
|– Signal to Noise Ratio|
|FT-NIR:||+ Wavelength accuracy||– Sample heating|
|+ Resolution||– Vibration Sensitive|
Preferred use for the different technologies
|Scanning Grating:||Quantitative measurements for food and agricultural products.|
|Fix Grating DDA:||Process instrumentation|
|FT-NIR:||1. Qualitative measurements in the laboratory (Identification)|
|2. Possibly quantitative for narrow bandwidth absorbers|
Quantitative measurements for food and agricultural products
For routine analysis applications in the food and agricultural industries, for instance for benchtop analysis of cereal grain, feed, milk powder, ground meat or similar, a scanning grating spectrometer is a proven choice.
This type of spectrometer is ideal for quantative measurements across a broad spectrum of applications with reliable, repeatable accuracy. The broad wavelength range enables the use of this technology for a very wide range of applications including those such as colour in fish food or similar requiring the visible region. When using NIR transmis¬sion for measuring inhomogeneous grain or meat samples it is an advantage to use the Short Wave NIR range (850 – 1050nm) where the light penetration is good and the premium signal to noise ratio offered by a scanning grating monochromator is es¬sential. A further advantage is that there is little sample heating when the sample is illuminated by a monochromator.
On the minus side, wavelength accuracy is not given by the design as for FT-NIR. An accurate internal wavelength standard is required to achieve good wavelength accuracy. However, like all other aspects of NIR technology, scanning grating spectrometers are constantly evolving and many of the latest generation spectrometers are using this in¬ternal wavelength standard and can demonstrate good wavelength accuracy, ensuring that this option remains the optimal choice for many applications.
For measurements close to production processes, for example in a feed mill where an analyser is used close to the processing line or is mounted directly in the process for continuous in-line measurements, fixed grating DDA is the best option.
A fixed grating DDA spectrometer can be used to create robust and vibration-tolerant instruments ideal for use inside production plants. The simultaneous measurement of the full spectrum also makes it tolerant to sample movement. In other words, it is the preferred technology for in-line applications.
Downsides need to be considered however. In particular, a tradeoff between wavelength range, resolution and signal to noise ratio must be made. Inevitably, the wavelength range must be restricted to achieve an acceptable signal to noise ratio.
Qualitative measurements in the laboratory
For measurements where narrow instrument bandwidth is needed, FT-NIR has advan¬tages. Its unmatched resolution makes it ideal for pin-point qualitive measurements of substances having narrow absorption bands or for some quantitative measurement ap¬plications for samples having closely spaced narrow absorption bands. The wavelength axis accuracy makes it easy to transfer spectra and the resolution can be adjusted to obtain the best tradeoff between wavelength resolution and signal to noise ratio.
Disadvantages include a lower signal/noise ratio than a monochromator, particularly at short wavelengths and the omitted visible wavelength range (below 850 nm). FT NIR is also a vibration-sensitive technology and the design of an instrument must take this into account for applications in a production environment.