Finding the right fit: FT vs Dispersive NIR

The longest continuously running mountain biking series in the United States is hosted in southwest Pennsylvania. Every year, hundreds of locals show up to test themselves in the 5-race series and vie for the title of “local hero.”

Races each take place at different venues, and each venue has a signature “wrench” to throw at riders, ranging from long, strenuous climbs to short but incredibly steep kickers, jagged rock gardens, jostling roots and log jumps, hair-raising descents, and (sometimes) even some smooth, flowing singletrack.

So what’s the point and how does this have anything to do with NIR?

The series champs are the best all-around riders. Top riders are often the most physically fit, yes, but they also are the most prepared. Oftentimes, they have to bring different bikes to best-match the terrain of any given race to get an extra advantage. They bring their fat-tired bike to glide over the rock gardens, or their narrow-tired, lightweight bike to pull away from the competition in the cyclocross race on the dry, grassy field. They bring their full-suspension bike to the downhill race, and their favorite geared bike to the climbing course.

The point is… the equipment (i.e. bike, NIR technology) you bring to the table should best meet the challenge (i.e. race course, NIR application) to perform at the best possible level and achieve the best possible result.

BUCHI offers four different NIR products, the NIRFlex N-500, NIRMaster, ProxiMate and NIR-Online, to meet different challenges or needs, including: application flexibility or difficulty, sampling location, sampling speed, or equipment ruggedness. To meet those requirements with the best advantage, BUCHI NIR products are engineered using one of two technologies: Fourier-transform (FT) and dispersive NIR.

FT-NIR

An FT-NIR, or Fourier Transform NIR, uses an interferometer to modulate the NIR signal and a computer to obtain the spectra of materials.

The simplest form of an interferometer is the classical Michelson, which consists of a beamsplitter and two mutually perpendicular plane mirrors–one which remains static and one which moves. The beamsplitter reflects part of the NIR energy from the source (typically a halogen bulb) onto each mirror. The reflected beams are recombined at the beamsplitter and directed out. When the distance between the beamsplitter and the two mirrors is equidistant, the two beams interfere constructively. The interference between the beams with an optical path difference (i.e. as the moving mirror is displaced) creates an interferogram. The mathematical treatment called Fourier transform is used to convert the interferogram to a spectrum.

Advantages of FT-NIR polarization inteferometer:

  • Simultaneous measurement of all wavenumbers and greater light throughput, giving an improved signal-to-noise ratio
  • High wavelength resolution, leading to good data transferability (i.e. calibration transfer) and improved selectivity for spectral features with narrow bandwidth
  • Single-beam interferometer without typical double-beam divergence for mechanically and temperature stable beam alignment*
  • More robust design than standard Michelson interferometer*

Because there are fewer optical components to attenuate radiation, more power reaches the detector, providing the benefit of better signal-to-noise ratio. Because the resolution at discrete wavelengths is much better, elements which interact within very narrow bands can be detected (i.e. better selectivity).

* The additional benefits of insensitivity to vibration and robustness are realized with the BUCHI polization interferometer used in both the N-500 and NIRMaster products. This benefit in rough environments is attributed to the difference of spatial movements and the optical path shifts for the two light beams in a birefringent crystal of variable thickness. Whereas in a Michelson type interferometer mechanical distortions directly affect the interference, such effects are reduced by a factor of 10 to 20 in a crystal interferometer.  

In addition to displaying the performance benefits of FT-technology, the BUCHI NIRFlex-N500 product is a good fit for an R&D setting or lab where the sample types are varied (e.g. liquids and solids). The N-500 has hot-swappable sampling modules (e.g. fiber optics, solids and liquid measurement cells) and sampling add-ons (e.g. petri dish holders and 6-position vial carousels) to optimize method performance across a variety of applications.

The BUCHI NIRMaster blends the performance of FT-technology with the more rugged demands of at-line installation points. The NIRMaster is the world’s first dedicated FT-NIR standalone spectrometer available in food-grade PMMA or stainless steel. The accuracy and reproducible performance of polarization FT technology plus the hygienic design with certified dust protection (IP64 or IP65) meets the easy cleaning processes requested for at-line machinery.

As mentioned previously, the optical bench for the N-500 and NIRMaster are the same. While the N-500 isn’t suitable for at-line work due to the risk of ingress, both products have a similar robustness to mechanical disturbances.

However, when an application doesn’t require the level of spectroscopic performance delivered by FT to deliver acceptable property measurement accuracy, or when ultra-fast measurement speed or online measurements are required, then users should look to dispersive NIR technology.

Dispersive NIR

In a dispersive NIR system, light is constantly emitted from an NIR source (typically a tungsten halogen lamp) onto a sample. The diffusely reflected light from the sample is directed to a dispersive element (e.g. stationary grating) and the resulting spatially distributed monochromatic light is detected. In lieu of a moving monochromator and a single detector, a diode array may be used. A diode array usually consists of 256 diodes, each of which collects the intensity of a certain wavelength range depending on its spatial position.  These individual diode signals are commonly referred to as pixels. Division of measured intensity (I) by intensity of a white reference spectrum (I0) as well as conversion of pixels to a wavelength scale results in a so-called spectrum, I/I0 plotted against nm or cm-1.

advantages of dispersive technology

  • Measurement speed
  • Ruggedness

Utilizing diode arrays gives rise to detecting a specified wavelength range in milliseconds. Averaging up to 200 spectra per second improves signal-to-noise ratio and allows detection of fast moving products in pipes or on conveyor belts. Because diode array based process analyzers do not contain any moving parts, they are robust by design, making them suitable for rough industrial conditions such as vibrations, extreme temperatures or humidity.

Example non-contact sensing of powder moving along a conveyor belt using NIR-Online.

In a process environment diode array process analyzers are desirable over scanning or interferometer based technologies. Rotating wavelength filter wheels or Fourier Transform (FT) spectrometers with moving parts require longer measuring times and the quality of measured data relies on stable measurement conditions.

The BUCHI NIR portfolio offers two products using diode array technology: the NIR-Online process analyzer and the standalone at-line ProxiMate NIR.

The ProxiMate was born out of a call for a lower-cost and higher IP-rated at-line solution for food and feed applications. The IP-69 designation of the ProxiMate indicates complete protection against dust particle up to 4 microns in size and protection from water projected from a powerful nozzle in any direction. The instrument can be washed down in exactly the same way as other plant machinery.

Like the NIR-Online, the ProxiMate delivers fast measurements, enabling greater throughput and less downtime in the lab or on the plant floor. Because the optical bench of the NIR-Online and ProxiMate is identical, data and calibration transfer could also provide faster method transfer from at-line to on-line applications or vice-versa.

Making your selection

Here are some additional factors to consider when it comes to finding that “perfect fit:”

Spectroscopic performance – as dictated by the application. Some applications require higher spectral resolution FT to observe small peak shifts or narrow bandwidth features, while the desired accuracy for other measurements can be easily obtained with a dispersive diode array system.

Sampling considerations (i.e. type(s), frequency, volume, location). If an R&D lab needs to optimize sampling for liquids, solids and slurries, then having the flexibility of a laboratory instrument, plus the benefits of optimal spectral resolution, would lead the user to an FT-NIR like the NIRFlex N-500. When sampling volume and/or frequency requirements are moderate, sample variation is low (i.e. you don’t need to optimize measurement conditions for drastically different materials like slurries, clear liquids and powdered solids) and measurements are more practically done on the plant floor, then an ingress protected NIR solution is advantageous. When sampling points are potentially hazardous or impractical, or when an ideal sampling frequency is very high or continuous, then an in- or online NIR product that is integrated into the processing equipment is clearly the better fit.

Cost and payback. In general, the cost of a diode array instrument is less than an FT-NIR product. Payback can be determined based on the frequency or cost of wet chemistry methods, the reduced sampling time of the NIR method over alternative methods, or mitigating the risk of a failed or recalled batch or accepting out of spec raw materials. Moreover, if an application exists as an out-of-the-box solution (e.g. a “pre-calibration” for moisture, protein and fat in ground meat) then the cost of method development will decrease sharply.

Certifications & standards. Consider whether the installation points or application requires food-grade materials, explosion-proof design, GMP or non-GMP software before making a decision. For example, the BUCHI NIRMaster, ProxiMate and NIROnline products are considered safe in food environments. The BUCHI NIRFlex N-500 and NIRWare software package offers 21 CFR Part 11 compliance and IQ/OQ, whereas the other products do not. The BUCHI NIR-Online® ExProof process analyzer is constantly over-pressurized, eliminating the risk of flammable gas penetration.

Summary

Just as a mountain bike race would best handled on a mountain bike… there are well-suited NIR products to handle specific application and workplace requirements. Need some more help getting matched up? Contact us.

Supporting References

Skoog, D. A., F. J. Holler, and T. A. Nieman. 1998. Principles of Instrumental Analysis, 5th ed. Chicago, Ill.: Harcourt Brace College Publishers

Burns, D.A., Ciurczak, E.W. 2007. Handbook of Near-Infrared Analysis, 3rd ed. CRC Press.

BUCHI Product Pages

NIRSolutions Product Page

NIR-Online Product Page

Things are really getting cheesy at BUCHI.

When prodded, I suppose many at BUCHI would agree that some of the cheesiest members of the team belong to the NIR group.

Maybe that’s something to be proud of!

More cheese, please!

Cheese is delicious, after all. With a global market of around $100 Billion USD, I think there is a general agreement on the matter.

There are many ways to consume cheese, of course. Processed cheese products like fondue have their place on the cheese continuum, particular around special events like graduation parties and weddings that seem to dominate our summer calendars.

Manufacturing processed cheese products like fondue is a complex task, with special emphasis on key quality parameters like total solids and fat content.

These products are typically produced by blending one or more shredded natural cheeses with additional ingredients, such as: emulsifying salts, condiments, flavors and other goodies. This mixture is then heated and sheared until a homogeneous molten mass is obtained which can be poured into heart- or graduation-capped shaped molds or other shapes for future devouring.

In order to consistently deliver the same great-tasting product, real-time control of key quality attributes is a must. Monitoring total solids content, fat, salt, pH, homogeneity and more during a blend can allow for real-time process adjustments to meet all of the quality targets and avoid having to rework a batch. The end result: tasty cheese products (read: make money) and improved manufacturing efficiency (read: save money).

While traditional laboratory methods exist for determining the quality parameters, issues with representative sampling and the method collection times are rate-limiting.

Want to learn more? Contact Us and request Short Note #357, or ask to speak with an Applications Specialist to learn more about how you can implement NIR for better process control in cheese manufacturing.

You can also visit our NIR-Online Solutions page to learn more.

If cheese quality is on your mind, but an on-line solution isn’t a good fit, BUCHI also has milk & dairy solutions for our off-line and at-line NIR. You can use our NIR Applications Finder on-line tool to configure the perfect NIR solution for you. Choose your industry and products, then get a full listing of available pre-calibrated applications, plus a quote.

Among our pre-calibrated parameters, you’ll find: dry matter, moisture, fat, protein, lactose, fatty acids, total sugar, ash and more.

Use our new NIR Application Finder to get the latest in Pre-Calibrated Solutions for your industry.

Interested in an application that isn’t listed? Fear not! We have a team of Application Scientists at the ready. Reach them using our Application Support Request Form .

If you’re looking for the full gamut of our published cheese & dairy applications, check out the BUCHI Application Finder. You’ll find methods related to:

  • Extraction
  • Spray Drying
  • Kjeldahl for protein determination
  • Near-infrared spectroscopy

Stay cheesy, my friends!