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
Photo by Pixabay on Pexels.com

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
Photo by Pixabay on Pexels.com

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.

 

 

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.

What is NIR?

More alphabet soup

Near-infrared spectroscopy. “N-I-R.”  Let’s illuminate the subject a bit, shall we?

Spectroscopy is a branch of science interested in the interaction of light with matter. Near-infrared (NIR) spectroscopy happens when the light used to do the measurements falls within a certain energy or frequency range; typically, 12000 – 4000 cm-1 (or about 700 – 2500 nm in terms of wavelength).

This idea isn’t new. The first observations of NIR light were made by Herschel in 1800, and Coblentz was considered its pioneer in the early 1900’s. However, this small but mighty portion of the electromagnetic spectrum didn’t debut commercially until the 1970’s, coinciding with advancements in PC computing power that radically simplified it’s application.

Why do people use NIR?

Everything you’ve come to love in your life: people, places, baked goods… they are all made up of molecules. Those molecules are made of atoms, and those atoms are moving and grooving (i.e. the bond lengths and bond angles aren’t static, but rather wagging and scissoring and bending and stretching). We can use NIR to measure that molecular dance party, or more technically, molecular vibrations. Those vibrational modes can tell us stuff about the sample that most QC departments like, think: sample identity or composition. 

When the molecules of a sample are hit with NIR light, the light is either absorbed or scattered. When the light is absorbed, we see a peak in our NIR spectrum. When light scatters due to the physical properties of the sample (e.g. particle size, particle morphology, bulk density), the overall slope of the spectrum is impacted. Chemical bonds that absorb NIR well are: oxygen-hydrogen (O-H), carbon-hydrogen (C-H), carbon-oxygen (C-O), nitrogen-hydrogen (N-H), and sulfur-hydrogen (S-H). While NIR isn’t the magic bullet for every analysis, we see samples that are dominated by these types of bonds in many industries, from pharmaceuticals to pet food.

The series of peaks and valleys that appear in an NIR spectrum is the summation of molecular vibrations of the sample. Consider the spectra of 5 different solutions in the figure below, where Absorbance is shown as a function of wavelength (nm). The red spectrum is pure methanol, the green spectrum is pure water. Peaks resulting from the -OH vibrations of both the alcohol and water, as well as the -CH vibration of the alcohol, are labeled.

water-alcohol

Since the energy of the -OH (and -CH) bonds of water and methanol (or donuts) differ, they produce unique spectra, as illustrated in the figure above. Combine that with the fact that spectra (of water, methanol or donuts) is repeatable, and we’re going somewhere. That means, if I scan water with my NIR analyzer 10x, I will get (more or less) the same spectrum, and that spectrum is unique from the other stuff I want to analyze (here, methanol). For those reasons, NIR would be a good tool for identification purposes.

Take another look at the figure above. The peak intensities corresponding to each molecular vibration reflect the relative composition of the molecules (water and alcohol) contributing those bonds to the solution. More specifically, you can see the peak corresponding to the water -OH vibration around 1450 nm increase as the relative proportion of water increases in solution. In the same way, the peak corresponding to the -CH combination vibration around 2250 nm increase as the proportion of methanol increases in the solution. That’s Beer’s Law working for us, where Absorbance at a given wavelength is proportional to the pathlength * molar absorptivity * concentration of the absorbing analyte. Note to the academics: if you dig into the theoretical research, you will see that Beer’s Law applies strictly to ideal solutions. Oh, and if you hadn’t heard, almost nothing is ideal in real life. However, we have mathematical ways to address non-linearity, and in the end, most methods work well. And by work well, we mean produce satisfactory standard errors of prediction. 

But before you get too excited about the infinite possibilities of NIR, let me give you some fine print. Our mortal bodies typically can’t run marathons without training first… and an NIR can’t run a qualitative or quantification application without being trained, either.

Additional resources:

For a good historical, theoretical and applications overview, see the Handbook of Near-Infrared Analysis, edited by Donald A. Burns and Emil W. Ciurczak