NIR; IR spectroscopy; MIR; molecular spectroscopy

Chapter 18

The detectives battle it out over NIR vs IR spectroscopy

🗂 Workshop overview: Lieutenant Cornlumbo and Nancy Beef clash in a battle of the wits, each trying to prove NIR or IR spectroscopy is the superior technique. In a workshop like no other, the food detectives learn the basics of NIR and IR spectroscopy, as well as the advantages and disadvantages of the methods. But will there be a clear winner? Read on to find out.

Shallot Holmes can feel that his team is close to a case-burn-out. He lies on his sofa and considers what kind of a workshop he can offer to increase their knowledge during a much-deserved case-break. He sits up suddenly, grabs a notebook, jots down his idea and calls up Lieutenant Cornlumbo and Nancy Beef.

The next morning, the detectives pile into the office yawning and murmuring grumpy greetings of good morning. They settle down on the sofa slowly and wait for Shallot Holmes to assign them their next case. The detective, full of caffeine and energy, jumps up and announces that they will be having a workshop today. But not just any workshop. Today, they will witness the battle of the spectroscopies: near-infrared NIR vs IR (mid-infrared).

On cue, Nancy Beef and Lieutenant Cornlumbo leave their seats and join Shallot Holmes to face the sofa. Nancy Beef has been given the task to represent NIR, whereas Lieutenant Cornlumbo must present the merits of IR spectroscopy. The two have spent the entire evening preparing by reading up on free resources and watching webinars on NIR. They want to achieve a different workshop style than when learning about Raman spectroscopy vs NIR. So, after presenting their arguments, the rest of the team would vote on which method they find superior. Miss Mapple, Eggcule Poirot and Shallot Holmes settle back into their seats and look on as the battle begins.

Nancy Beef and Lieutenant Cornlumbo narrow their eyes at each other but shake hands good naturedly to mark the beginning of their NIR vs IR showdown. They start off with the basic theory of each technique. Lieutenant Cornlumbo begins by giving a brief overview of infrared spectroscopy in general.

Infrared spectroscopy is the study of the interaction of infrared light with matter, with infrared light characterized by wavenumber range of 12,800 to 10 cm^-1, or wavelengths of 0.78 to 1000 micron. NIR is defined as light between 10,000 and 4,000 cm^-1, whereas mid-IR is defined as light in the wavenumber range of 4,000 and 400 cm^-1. Both mid and near-IR are included under molecular spectroscopy.

Before butting heads in the NIR vs IR battle, Nancy Beef takes her team on a little journey to refresh their memory on how infrared spectroscopy works. On a molecular level, she tells them, their samples contain water, fat, and protein build up by carbon, hydrogen, oxygen and nitrogen atoms held together by chemical bonds. When irradiated with light in the infrared range, the relative positions of the atoms in the molecules of the sample are not fixed. Instead, they fluctuate continuously due to the multitude of different types of vibrations, such as stretching and bending, and rotations about the bonds in the molecule. For the molecule to absorb radiation, two conditions must be met:

  • The energy of the IR radiation must equal the energy needed to induce the specific vibration in the molecule
  • The molecule must undergo a net change in dipole moment during the vibrational or rotational motion

If these conditions are met, a plot of the measured infrared radiation intensity versus wavenumber is produced. This plot is called an infrared spectrum.

Lieutenant Cornlumbo takes over. He tells the other investigators to consider the difference in the wavenumber range, and therefore energy, of NIR vs IR radiation. The higher-energy mid-IR is exciting fundamental vibrations, or those resulting when energy is absorbed by the molecule in its ground state to the first vibrational state.

Nancy Beef then butts in. She explains that NIR spectroscopy produces combination bands and overtones of those fundamental vibrations. These are of much lower intensity than their fundamental counterparts of mid-IR due to their lower transition probabilities.

Here, Shallot Holmes feels forced to interrupt as the exchange between his colleagues is starting to heat up. He points out that bands of lower intensities can be an advantage or a disadvantage, it all depends on your application.

Both Lieutenant Cornlumbo and Nancy Beef nod in agreement, having cooled off during the interruption. Lieutenant Cornlumbo continues his mini lecture. He points out that bonds-defining functional groups, meaning structural fragments within the molecule such as C=O, N-H or C-H, tend to absorb IR radiation at predictable wavenumber ranges, independent of the rest of the molecule’s structure. Organic functional groups have characteristic and well-delineated absorption bands in the mid-IR. Lieutenant Cornlumbo pauses, ready to deal a blow in IR’s favour in the NIR vs IR battle:

Since organic functional groups are readily distinguished with mid-IR, the technique is ideal for structural elucidation and compound identification, especially when coupled with other analytical methods such as NMR.

Nancy Beef senses Cornlumbo has gained ground and is quick to bring the attention back to NIR. She admits that broad peaks and overlapping of the overtone and combination bands strongly decrease the specificity of NIR spectroscopy for spectral interpretation. But she argues that low absorptivity and efficient light scattering by NIR radiation can be very beneficial to certain applications. She reiterates that since the absorption intensity of NIR is low, NIR samples do not need to be diluted (as with mid-IR) to avoid saturation at the detector.

This means that the thickness of the sample being analyzed by NIR can range from millimeters up to centimeters, depending on sample composition. This large sampling volume is beneficial for quantitative analysis of samples, especially heterogeneous samples. In addition, sample preparation is very fast and easy compared to Mid-IR.

Shallot Holmes, Miss Mapple and Eggcule Poirot are following closely, mesmerized by the passion of their colleagues in defending their method in NIR vs IR spectroscopy.

Shallot Holmes tries to stir Nancy Beef and Lieutenant Cornlumbo towards a meaningful conclusion. He asks them to take a closer look at a common application where both mid-IR (FT-IR) and NIR methods can be used: raw material identification.

First up is Lieutenant Cornlumbo who lists the advantages of using IR over NIR. In such an application, mid-IR offers characteristic and well-defined absorption bands for organic species in the mid-IR, enabling structural elucidation and compound identification. He adds that detailed tables of characteristic group frequencies are readily available to facilitate structural elucidation efforts. This makes mid-IR the clear choice for him in NIR vs IR.

Nancy Beef is quick on her feet and ready to point out the limitations of mid-IR. She reveals that the need for sample dilution, for example with KBr pellets or salt plates, is common with mid-IR. The dilution step costs extra time for material evaluation and the dilution destroys the same, so the diluted sample cannot be used beyond the mid-IR measurement. She is not yet finished. The second disadvantage she refers to is that since the sampling volume of mid-IR when using attenuated total reflectance (ATR) is small, the repeatability of the method for more heterogenous samples is limited.

Lieutenant Cornlumbo listens tight lipped but allows Nancy Beef to go on uninterrupted. It is now her turn to list some benefits for NIR over IR in raw material identification. She begins by telling the others that NIR spectra are influenced by both chemical and physical sample attributes and NIR can be readily used to discriminate between grades of the same chemical substance. Secondly, NIR radiation achieves deeper sample penetration, which is advantageous, as bigger sampling volume may increase sensitivity to contaminants. Thirdly, she announces that no sample preparation, no dilution, no pellets or salt plates are needed, nor purge gas required, which reduces the sampling efforts and costs of the process. Lastly, she points out that spectra are collected in 4 to 30 seconds, making NIR an extremely fast method.

She faces Lieutenant Cornlumbo and offers him the floor for his rebuttal. He nods to her respectfully and proceeds to point out the disadvantages of NIR vs IR. He says that since some functional groups have both fundamental and first order (or higher) overtones in the mid-IR region, they will not appear in the NIR region at all. This could limit the discriminatory power of NIR for certain sample sets. He proceeds to add another strike against NIR. He explains that due to the more complex (broad and overlapping) signal of NIR spectroscopy, chemometric procedures are needed for qualitative analysis.

Nancy Beef cannot contain herself anymore and counters that software capable of chemometric procedures is readily available and powerful when coupled with solid experimental design.

Before Lieutenant Cornlumbo can open his mouth to argue further, Shallot Holmes jumps in to put an end to debate. He then asks Miss Mapple and Eggcule Poirot about their opinion on the NIR vs IR matter for raw material identification.

The two whisper quietly amongst themselves for several minutes. Miss Mapple then thanks her colleagues and says she is ready to announce their verdict. The takeaway message from the debate and workshop for her and Eggcule Poirot was:

  • If you have a labeled ingredient in a sample and you want to double check if the label is correct, then NIR is the ideal choice. With this method, you can complete your analysis quicker and you can retain and use the NIR sample in downstream processes.
  • If you are synthesizing compounds in the lab and you want to know what you have just created, then mid-IR is the clear winner.

She then sits down and looks up shyly at her sparing teammates. Nancy Beef and Lieutenant Cornlumbo exchange amused glances and high five each other. They have managed to drive their point across. There is no better method between NIR vs IR, each can be considered as more or less suitable depending on the application.

The detectives, all in good spirits, continue to chatter about their workshop as the evening draws near. It is starting to pour and Eggcule Poirot waits impatiently for Miss Mapple to put on her rain pants and raincoat before they can leave. Finally outside, Eggcule Poirot opens his umbrella and begins lecturing her on how much faster and more convenient an umbrella is than rain clothes. Until a sudden gust of wind turns his umbrella upside down leaving him exposed and wet. The two of them look at each other and burst out laughing. Like NIR vs IR, umbrellas and raincoats can both be invaluable, but it all depends on the application.