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How Technology Choice Affects NIR Performance

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How Technology Choice Affects NIR Performance

NIR analysis entered into the mainstream of analytical techniques in the 1970's and 1980's following on pioneering work of Dr. Karl Norris with the USDA and many others following. Over time, a number of technologies have become commercially available for the routine analysis of samples.  

A common question from people investigating NIR is What Instrument or Technology is best?  As with many such questions, the answer is "It depends."

There are 4 main types of instruments currently available.  While there are differences across manufacturers that can make one instrument more suitable for your application, the choice of technology has a major impact on the ultimate performance of your NIR solution. The discussion below introduces each major technology type and describes the benefits and disadvantages of each. 

Filter based instruments

Some of the earliest instruments on the market were based on a filter technology due to simplicity of the design and mechanism. In these instruments, light passes through a filter allowing a particular wavelength or band of light to pass on to the sample.  The reflected (or transmitted) energy is collected by a detector.  One or more filters are used to collect the intensity signal for each filter, and a simple linear equation consisting of a coefficient at each filter (wavelength) and an offset is created to predict the constituent of interest. 

Filter based NIR diagram

Diode Array or Photo Diode Array (PDA) Instruments

Diode array instruments are based on a linear array of independent detectors on a chip.  In this design, white light from the source is directed on to the sample, and the reflected (or transmitted) light is collected and diffracted by a fixed holographic grating. The dispersed light is collected on a the diode array detector, with the linear dispersion of the energy corresponding to the various wavelengths. A calibration is made with a coefficient from each wavelength along with an offset. 

PDA based NIR diagram

FT-NIR

Most FT NIR instruments are based on the Michelson interferometer which was first developed in the late 1800's.  FT intruments create 2 light paths of unequal distances so that when combined, the various wavelengths (frequencies) destructively and constructively interfere with each other.  Using a mathematical technique known as a Fourier Transformation, the raw signal (interferogram) can be converted into a spectrum of absorbance or transmittance values.  

FT based NIR diagram

Scanning Monochromators

Scanning monochromator became commercially available in the 1980's and have become the most widely used technology for laboratory and at-line NIR analysis. A scanning grating monochromator uses a moving grating to diffract the light into individual wavelengths, and a slit limits the wavelengths illuminating the sample at any one point in time.  A detector measures the intensity of the reflected signal and calculates the reflectance at each datapoint.

Scanning monochromator NIR diagram

Which is best?

The answer to this question depends on your application and sampling requirements.  Unity Scientific has chosen the scanning monochromator technology as it provides the highest quality, most repeatable, full spectrum NIR data available, but this technology is not suited for difficult environments such as on a combine or in an extreme production environment.  The table below outlines the advantages and disadvantages of each technology. 

NIR Technology Comparison Chart
 

TYPE ADVANTAGES DISADVANTAGES
Filter
  • Low Cost
  • Robust design good for difficult environments
  • Few, fixed wavelengths
  • Limited consituents
Diode Array (PDA)
  • No major moving parts
  • Very fast
  • Low data resolution and limited wavelength range 
  • Difficult to transfer calibrations
  • Post-dispersive design heats sample
FT-NIR
  • Very high wavelength accuracy
  • Adjustable resolution
  • Lower signal to noise ratio 
  • Sensitive to vibration
  • Compromise between resolution and signal to noise in NIR region
Scanning Monochromator
  • High signal to noise ratio
  • High data resolution
  • Broad wavelength range
  • High ability to transfer calibrations
  • Wavelength scale requires external standard for transferability