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NIR spectrometer technology

NIR Spectrometer Technology Compared

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.

Read the paper using or if you would just like the overall conclusion we have included it below. Don’t forget to comment if you have views on this topical subject.

Conclusions

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.

Process instrumentation

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.

4 comments
gabiruth
gabiruth

How come you didn't include the AOTF technology in your revuew - this technology is being used all over the world for inprocess measurements from pharma to polymer to food to agro - every application you can think of


Gabi Levin Ph.D.

Brimrose Corp. of America

Haakan Wedelsbaeck
Haakan Wedelsbaeck

@gabiruth

Thank you for your feedback on the whitepaper.

The reason for not including AOTF is that we chose to limit the document to the most common spectrometer technologies that are used by many different manufacturers.

For the future we are considering to extend the document to cover some other technologies such as AOTF, LVF, MEMS Fabry Perot filters and LED arrays. We are grateful for tips on good literature describing these technologies.

Håkan Wedelsbäck

FOSS

gabiruth
gabiruth

@Haakan Wedelsbaeck @gabiruth 

Now I am really surprised - by which yard stick did you measure  most common spectrometers - because if I count the numbers of our spectrometers used by pharma, chemical, food, agro (including the only automated single seed sorting machine) pulp and paper, and many more from south America, US, Canada, Europe, Japan, India etc. I would dare say that diode arrays aren't more popular than ours.

Besides - there isn't even one type that is real time, all the time REAL dual beam like the AOTF - a fact that by itself gives it the right to be in the forefront of any discussion.

Haakan Wedelsbaeck
Haakan Wedelsbaeck

@gabiruth 

Since this site is a resource center for NIR feed analysis knowledge I used the agricultural analysis yardstick for my comment above.

I agree that what you call real dual beam configuration is advantageous for process applications with demands on high and continuous sampling rate. For other applications, at least as good analytical performance can be achieved with the technologies described in the whitepaper.

Doing instrument design is about making the best tradeoffs for a given measurement situation. The real time referencing tradeoff is having two separate detectors (one for reference and one for sample) that need to be matched and temperature controlled. The other configuration with one detector for both sample and reference has the advantage of real detector reference but the tradeoff is speed and higher requirements for short time stability.

You are welcome to contact me at hwk@foss.dk for a more in depth discussion about pros and cons for different NIR-technologies. As I wrote before I’m also grateful if you would like to share some knowledge about AOTF-technology.

Håkan Wedelsbäck

FOSS