NIR: a Spring-y subject

Winter felt brutal and eternal, as it always does for someone who doesn’t ski or care for hot chocolate, I suppose. What a relief it is to see signs of Spring emerging from my brownish-colored yard and hear birds chirping outside once again.

Did you know NIR is quite Spring-y as well. This blog will explore some spring-themed theory.

beautiful bird bloom blossom
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At temperatures above absolute zero (i.e. even in the dead of a Northeast USA snowmageddon), all of the atoms in a molecule are in continuous vibration with respect to each other.

The behavior of molecular vibration is analogous to a mechanical model in which two masses connected to the ends of a spring! A disturbance of one of these masses along the axis of the spring results in a vibration called the simple harmonic oscillation.

close up of metal
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Vibration, or the displacement of an atom relative to its equilibrium position, produces potential energy proportional to the work required to displace the mass.  This energy is at its maximum when the spring is stretched or compressed to its maximum amplitude. It is at its minimum (i.e., zero) at the equilibrium position.

The fundamental requirement for infrared (Far-IR, mid-IR, near-IR) activity, leading to the absorption of infrared radiation, is that the energy of incident radiation matches the vibrational energy levels exactly, and that the vibration itself causes a change in dipole moment.  The frequency of radiation that will bring about this change can be calculated by Hooke’s Law:

Hookes Law

where c is the speed of light (3×103 cm/s), f is the force constant of the bond (dyne/cm), and Mx and My are the masses of  the atom x and atom y involved in the bond, respectively.  The force constant is positively correlated to properties such as bond order or bond strength (i.e. the “springiness” of the bond).  In accordance with the Boltzmann distribution, frequencies which correspond to fundamental transitions between the ground state and first vibrational level (n = 1) dominate the vibrational absorption spectrum.  Because the majority of absorption bands of chemical compounds correspond to fundamental vibrations at infrared frequencies, it is a common tool for structural elucidation.

How does thee vibrate? Let me count the ways.

The number of possible or theoretical fundamental vibrations is determined by the total degrees of freedom of the molecule.  Each atom requires three degrees of freedom in order to describe its position relative to other atoms in the molecule.  Therefore, a molecule of N atoms has 3N degrees of freedom.  For nonlinear molecules, six degrees of freedom are used to describe translation and rotation; the remaining 3N – 6 degrees of freedom are vibrational degrees of freedom (i.e., fundamental vibrational modes).  For linear molecules, only two degrees of freedom are required to describe rotation, resulting in 3N-5 normal modes.

Disclaimer: Sometimes 1+1 doesn’t equal 2

The number of theoretical bands will not necessarily equate to the number observed experimentally.  The number of theoretical bands observed may be reduced by: lack of a change in the molecule’s dipole as it vibrates or rotates, fundamental frequencies that fall outside of the infrared region or are too weak to be observed, vibrations that coalesce, or the occurrence of a degenerate band from several absorptions of the same frequency in highly symmetrical molecules.  On the other hand, vibrations at integer-multiples of a given frequency and combination tones will increase the actual number of bands observed.  It is from the combination and overtones transitions that NIR spectra arise.

The NIR region of the electromagnetic spectrum covers the range of approximately 14,000 to 4,000 cm-1, or about 700 to 2,500 nm.   The most prominent absorption bands occurring in the NIR region include overtones and combinations of fundamental vibrations of the IR-active –CH, –OH, -CO, –NH and –SH functional groups present in most pharmaceutical drug molecules.  Due to the relatively weak molar absorptivities of the transitions responsible for the peaks observed, sample dilution is not required.  This characteristic also provides for relatively deep sample penetration up to several millimeters thick, especially at shorter wavelengths (e.g., 700-1500 nm).  Even though NIR spectroscopy is characterized by spectra which are typically broad, overlapping and of low intensity relative to the fundamental mid-IR absorption bands, it has some practical advantages.  The richness and utility of NIR spectra is a consequence of anharmonic oscillation.

Anharmonicity: where the simple rules start to break down and things start to get interesting

Bonds which share a common atom seldom behave as independent oscillators.  As the interatomic distance separating two atoms decreases, coulombic repulsion between the nuclei results in an additional force which acts in the same direction as the force restoring the system toward equilibrium.  Thus, the potential energy of the system increases more rapidly than predicted by the harmonic oscillator.  On the other hand, as the interatomic distance approaches that at which dissociation of the atoms takes place, a decrease in the restoring force and potential energy of the system occurs.  The intramolecular interactions produce non-symmetric vibrations about the equilibrium position.  The anharmonicity results in non-equivalent energy changes between vibrational states, where ΔE becomes smaller at higher quantum numbers.   Moreover, the selection rule is not rigorously followed (because rules are made to be broken), thus allowing the overtones responsible for much of the NIR spectra, where Δn = ±2, ±3 and ±4 represent the first, second and third overtones, respectively.

The degree of anharmonicity determines the extent of the displacement from an integer multiple of the fundamental frequency, as well as the intensity of the overtones.   Vibrations stemming from intramolecular hydrogen-bonding vibrations have the highest anharmonicity constants, leading to their prevalence and high intensity in the NIR region. That’s why NIR is so great at measuring low levels of water in samples!

More interesting stuff in the spectra:

NIR spectra are further enriched when vibrational modes interact to give absorptions at frequencies that are the approximate sums or differences of their fundamental frequencies.  These combination bands, which generally occur between 1900 and 2400 nm, are a consequence of energy absorption by two bonds rather than one, allowing the photon to excite two vibrational modes simultaneously.  As with overtones, the intensities of combination bands are weaker than their fundamental frequencies.

A special type of interaction called Fermi resonance occurs as a consequence of accidental degeneracy of different vibrational modes have the same symmetry and approximately the same frequency as a fundamental vibration.  This results in two relatively strong absorbance bands which are displaced at slightly higher and lower frequencies than expected, respectively.

Darling and Dennison resonance affects vibrations which have identical symmetry species and similar energies, leading to several pairs of absorption bands.

Coupling between oscillators results in slight to moderate shifts in the absorption frequency of the molecules involved.  In general, coupling requires that vibrations be of the same symmetry species and a common atom or bond between the two vibrations or vibrating groups, respectively.  The interaction is greatest when the coupled groups have nearly equivalent energies; little to no interaction is observed by groups separated by two or more bonds.  Despite the fact that coupling leads to uncertainties in functional group identification, it is this phenomenon that provides the unique features of a spectrum enabling compound identification.

 

For more reading on this spring-y subject:

Skoog D.A. and Leary J.J. Principles of Instrumental Analysis. 4th Edition. 1992.

Silverstein R.M, Spectrometric Identification of Organic Compounds, 5th Edition. 1991.

D. Burns, and E. Ciurczak,Handbook of Near-Infrared Analysis 2nd Edition, Marcel-Dekker, Inc. New York, 2001.

E. Ciurczak and J. Drennen, Near-Infrared Spectroscopy in Pharmaceutical and Medical Applications, Marcel-Dekker, Inc. New York, 2002.

L. Weyer and S.-C. Lo, “Spectra-Structure Correlations in the Near-infrared,” In Handbook of Vibrational Spectroscopy, Volume 3, Wiley, U.K., 2002.

 

 

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Evolution of BUCHI NIR

February 12 was Darwin Day. Darwin, of course, was known for his theories of evolution. While his theories continue to be hotly debated 137 years after his passing, none could argue that products and companies must evolve in some capacity to keep pace with, and meet the demands of customers-at-large.

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Borrowing the words of Spanish soccer player Gerard Pique, “Evolution is all about looking forward.” The BUCHI NIR portfolio shows evidence of evolution in that sense. 

The BUCHI NIR “big bang” occurred in 1999 with the acquisition of Buhler NIR, with incremental steps leading up to the dramatic upgrade to the NIRFlex series, and so far culminating with the N-500 FT-NIR product.

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The NIRFlex bench-top instrumentation was designed to meet consumer demands in both R&D and routine testing labs for true flexibility and high performance. Hot-swap modules accommodate nearly any sample type (i.e. solid, liquid, semi-solid or slurry), while the Fourier-transform (FT) technology provides exceptional precision. A novel single-beam FT design further propels instrument performance, stability and robustness, while hardware and software components continue to meet even the demands of the  pharmaceutical regulatory agencies.

NIRMaster_NIRMaster_ProNext, the NIRMaster was added to the portfolio specifically to serve the demands of the food and feed industries, bringing with it IP-54 and IP-65 ingress protection and features designed to ensure food safety. This revolutionary design was the first to marry the accuracy of FT-NIR with a robust and hygienic standalone at-line design suited for the production floor.

NIR-Online

In 2005, the BUCHI portfolio expanded with the acquisition of German on-line sensor manufacturer NIR-Online, just a modest train ride away from company headquarters in Flawil, Switzerland. With this acquisition, BUCHI was finally able to offer a true in- and on-line NIR solution for process control. Since its introduction, the NIR-Online product has further evolved, expanding capabilities to include multiplexing, and continuing to meet requirements for workplace safety, including explosion-proof options for hazardous environments.

In late 2018, the BUCHI NIR portfolio saw its next evolution: the ProxiMate. This at-line workhorse took the speed and agility of the NIR-Online solution at-line, creating an affordable option for the typical food and feed industry customer without compromise in quality or performance.

ProxiMate_masterProxiMate offers 3 main benefits for food and feed industry users: applicative fit, extreme robustness and simple operation.

  • BUCHI application chemists have developed many ready-to-use pre-calibrations typically required by the food and feed industries, enabling accurate results with minimal effort and by users of any ability level

Of course, the idea of the “survival of the fittest” comes to mind on Darwin Day. It seems that the more recent of our evolutionary steps in the NIR product portfolio has been hyper-focused on robustness–survival in the dirtiest, harshest, or most hazardous environments. Learn more about ProxiMate and join us on this Extreme Journey along the NIR evolutionary path!

Near vs. Mid-IR: pick your poison

Is there a simple answer?

Of course not! When it comes to the debate regarding which infrared spectroscopy reigns superior, near-infrared (NIR) or mid-infrared (IR), the answer should be a reflect the merits of the technology in light of the application of interest. It’s like asking whether a knife is better than a spoon. Well, are you trying to cut an apple or eat ice cream? You see my point.

Basic theory

Put simply, infrared spectroscopy is the study of the interaction of infrared light with matter, where infrared light is characterized by wavenumber range spanning from 12,800 to 10 cm^-1 (or wavelengths of 0.78 to 1000 micron). Mid-IR is typically defined as light between 4000 and 400 cm^-1, and NIR as light between 10,000 and 4,000 cm^-1, give or take. Mid- and near-IR are included under the umbrella of molecular spectroscopy.

Imagine your sample at the molecular level, with carbon, hydrogen, oxygen and nitrogen atoms coordinated by chemical bonds in such a way as to produce the water, fat and protein content in that sample. The relative positions of the atoms in the molecules of your sample are not fixed; they fluctuate continuously as a consequence of a multitude of different types of vibrations (i.e. stretching and bending) and rotations about the bonds in the molecule. Check out this page for some nice illustrations and more in-depth theory. When the frequency of a specific vibration is equal to the frequency of the IR radiation directed on the molecule (*and the molecule undergoes a net change in dipole moment as a consequence of the vibrational or rotational motion), the molecule absorbs the radiation. A plot of the measured infrared radiation intensity versus wavenumber is known as an infrared spectrum.

Consider the difference in the wavenumber range (and hence, energy) of mid- and near-IR radiation. The higher-energy mid-IR is exciting fundamental vibrations; that is, when energy is absorbed by the molecule in its ground state to the first vibrational state. NIR spectroscopy is comprised of combination bands of overtones of those fundamental vibrations.  The latter are of much lower intensity than their fundamental analogs, owing to their lower transition probabilities. This can be an advantage OR disadvantage – depending on what you’re trying to do (keep reading!).

The bonds defining functional groups (structural fragments within the molecule, like C=O, N-H or C-H), tend to absorb IR radiation at predictable wavenumber ranges, regardless of the rest of the molecule’s structure. Organic functional groups have characteristic and well-delineated absorption bands in the mid-IR, lending the technique to structural elucidation and compound identification, especially when paired with other analytical methods like NMR. While the broad peaks and overlapping of the overtone and combination bands strongly decrease the specificity of NIR spectroscopy for spectral interpretation, low absorptivity and efficient light scattering by NIR radiation can be advantageously exploited. In other words, because the absorption intensity is low, NIR samples do not need to be diluted (as with mid-IR) to avoid saturation at the detector; sample thickness interrogated by NIR light can be extended from millimeters up to centimeters, depending on the sample composition. This large sampling volume is valuable for quantitative analysis of samples with some degree of heterogeneity.

Let’s now consider a common application where both mid-IR (FT-IR) and NIR methods are commonly employed: raw material identification.

Mid-IR Advantages

  • Characteristic and well-delineated absorption bands  for organic species in the mid-IR lend the technique to structural elucidation and compound identification; detailed tables of characteristic group frequencies facilitate structural elucidation efforts

Mid-IR Disadvantages

  • The need for sample dilution (e.g. KBr pellets, salt plates) is common, requiring extra time for material evaluation, as well as effective “destruction” of the sample (i.e. the sample cannot be used beyond the mid-IR measurement)
  • The small sampling volume of mid-IR when using attenuated total reflectance (ATR) is small, thus limiting method repeatability for less homogeneous samples

NIR Advantages

  • NIR spectra are impacted by both chemical and physical attributes of the sample; therefore, NIR can be used to discriminate between grades of the same chemical substance
  • NIR radiation achieves more sample penetration; increased sampling volume may increase sensitivity to contaminants
  • No sample preparation  (i.e. no pellets or salt plates), nor purge gas is required, thus reducing the sampling efforts and costs
  • Spectra are collected in seconds (typically 4 to 30s)

NIR Disadvantages

  • Some functional groups having both fundamental and first order (or higher) overtones in the mid-IR region will not appear in the NIR region, potentially limiting the discriminatory power of NIR for certain sample sets
  • Due to the more complex (i.e. broad and overlapping) signal of NIR spectroscopy, chemometric procedures are required for qualitative discrimination.  The superposition of bands However, software capable of handling these procedures is widely available and quite capable when paired with solid experimental design

Conclusion

What’s the moral of the story? If you have a label on a bag of white powder and you want to quickly see if that label is correct, then NIR is likely to be the right choice for you. You’ll complete your analysis quicker and be able to retain or use the NIR sample as you see fit. However, if you are synthesizing compounds in the lab and you want to know what you brewed up, mid-IR is the clear choice.

Chemical Industry QC with NIR

 

Read this post, or watch the webinar instead!

Quality control for many labs involves a heavy dose of wet chemistry methods, things like titration and separation techniques that take skill, time and (even more) chemicals to execute. Luckily, some of these traditional testing methods can be replaced by simple, fast and safe NIR spectroscopy.

While this blog title indicates applicability to the Chemical Industry, “chemical” is one broad umbrella. There are myriad products and processes that fall under the chemicals category, from natural products like wood and pulp to personal care products to standard bulk chemicals. Reaching all of these audiences with one blog post seemed a little daunting until we broke it down to some common key themes for implementation of NIR for the chemical (or any!) industry:

  • Raw material qualification
  • Intermediate/in-process testing
  • Finished product testing

Of course, the typical applications that might fall into any one of these categories will differ based on the products being produced. Some of the more common applications include:

  • Material identification
  • %-Moisture or %-solvent quantification
  • Reaction extent or %-polymerization
  • Hydroxyl and acid number determination

As with many other industries, the raw materials used for production of chemical products are often non-discrete, sourced from various parts of the galaxy, and labeled–sometimes correctly, sometimes not.  If you follow product recalls, you’ll find that millions of dollars have been lost due to mislabeled containers being poured into mixers, placed on trucks for distribution to other producers, or stocked on store shelves.

NIR is one quick tool used for identity testing of routinely received materials. There is potential to differentiate isomers, crystalline forms, chemical analogs, fatty acids, and even contaminated materials. Because identity testing with NIR takes seconds and can be done in the warehouse, more frequent testing can be accomplished without backlogging the QC guys and gals.

On the quantitative side, there is plenty to measure keeping in mind the inherent sensitivity of NIR to particular molecular bonds, including O-H, C-H, N-H and C-O bonds. So, if those bonds are changing in type or in number, NIR could be a great fit. This is the case in the typical chemical application of determining hydroxyl number, where we observe a decrease in NIR signal attributed to O-H bonds as those O-H end groups are consumed during polymerization. In fact, determining hydroxyl number of polyols by NIR is a standard practice per ASTM and ISO.

BUCHI Market Manager and former BUCHI NIR Applications Specialist Ryanne Palermo produced a short webinar on these topics, including a fiery example of tracking nitrogen substitution in nitrocellulose. Tune into the webinar by clicking here.

Find more free, streaming content on our BUCHI Webinar On-Demand page, including information about preparative chromatography, laboratory and industrial evaporation, drying, encapsulation and more.

 

Be a Champion of Final Goods Inspection

Max won’t let a pile of untested final goods (or third wheel) stand between him and a coffee date with his lady love. Check out the newest and last installment of the Food Quality Champion Series animated videos, then download the Guidebook and become a Final Goods Inspection Champion, yourself!

The Final Goods Inspection Guidebook is ripe with information to understand or expedite quality control operations in the food and feed industry. Topics include:

  • Regulations impacting final product quality control
  • Representative sampling & sample preparation
  • Tips for optimizing Kjeldahl workflow for protein determination
  • Tips for optimizing extraction and hydrolysis workflow for fat determination
  • Tips for optimizing NIR methods for proximate determination in food and feed products

Download the guidebook for helpful insights, then start a conversation with your local BUCHI Application Specialists to see how you can be a Champion!

 

 

Champion saves the day: Volume 2 Production

In-process and at-line NIR for production

Beat the costs in production! Download Volume 2 of the Champions’ Guidebook and find out how to save money while monitoring production lines.

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Our determined (and love-struck) food champion, Max, is back at it. Check out the newest animated video to see how NIR can avoid costly production errors (and increase profitability) after googly-eyed Max’s big goof-up.

One of the greatest assets of on-line and at-line NIR is having a second set of (focused) “eyes” on production operations. The NIR can be trained to measure critical material properties for in-process or finished products, or even do simple identification procedures to confirm questions like: is Product A is actually being produced?

Max may be a little distracted at times, but NIR can still make him a champion!

Be a Food Analysis Champion!

Save time with efficient incoming goods inspection

New BUCHI campaign delivers 3 e-booklets to create Food Analysis Champions!

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Every day, food producers undergo myriad processes and procedures designed to achieve a quality product and (hopefully) a profitable business.

The loading docks and warehouse serve as initial points of contact for ingredients and foodstuffs that will become integrated into delicious (and sometimes nutritious) food products. It is the obligation of the producer to ensure that they are obtaining the highest quality and correctly priced goods prior to feeding those ingredients into the production process.

Our first booklet provides insight into challenges and opportunities related to incoming goods inspection, including:

  • Typical slow-downs in incoming goods receiving
  • Tips to meeting incoming goods inspection requirements efficiently
  • Benefits of using fast, non-destructive NIR analysis for testing incoming goods
  • Improving time-to-result for classical reference methods (i.e. extraction and Kjeldahl)
  • Sample NIR and classical testing applications to help you save time!

Download this complimentary resource, and stay tuned for future additions to the series, including: production and finished goods control!

For some nice (and enlightening) lunch break entertainment, watch our Food Analysis Champion, Max, save the day when production is halted due to QC backlog in the BUCHI animated video short series for “Beat the Clock.”