Analysis of the State of the Art: Infrared Spectroscopy

In honor of Spectroscopy's celebration of 30 years covering the latest developments in materials analysis, we asked a panel of experts to assess the current state of the art of infrared (IR) spectroscopy and to try to predict how the technology will develop in the future.

Infrared (IR) spectroscopy is a valuable technique for identifying the structures of molecules because of the characteristic absorption bands associated with the various functional groups. We asked a panel of IR experts about notable recent advances in the technique, challenges faced by spectroscopists in terms of limitations of the technique and understanding how it works, and possible future developments in instrumentation and new applications.

This article is part of a special group of six articles covering the state of the art of key techniques, also including Raman spectroscopy, near-infrared (NIR) spectroscopy, inductively coupled plasma–mass spectrometry (ICP-MS), laser-induced breakdown spectroscopy (LIBS), and X-ray fluorescence (XRF) spectroscopy.

Recent Advances in IR

Although IR spectroscopy has long been a workhorse technique in analytical laboratories, many improvements have been made in the past few years. Our panel members shared their opinions about a number of exciting recent developments.

According to Rohit Bhargava, who is a professor and the Bliss Faculty Scholar at the University of Illinois at Urbana-Champaign, the single most important development has been the opening up of the design space in IR instrumentation. A decade ago, there was very little differentiation among commercial offerings. Now that has changed. "New sources and improved computing, detectors, and advances in theoretical understanding have led to new designs and expanded choices," he said.

Timothy McIntyre, Manager of Analytical ICD at Tate & Lyle, agreed with Bhargava about the importance of computing. "An area that adds tremendous speed and efficiency to the implementation of IR spectroscopy has been software that easily integrates functions such as sample presentation and control, quantitative analysis, classification, and identification through database searching and data handling and communication," he said.

Peter Griffiths, who is an emeritus professor of chemistry at the University of Idaho and the principal of Griffiths Consulting LLC, identified two developments as being the most important recent instrumental advances: nanospectrometers and quantum cascade lasers. "If I had to choose between them, I'd probably opt for nanospectroscopy," he added.

Daniel Schroeder, a senior research specialist at 3M, also identified multiple areas of recent advancements, including sampling accessories, mobile instruments, and hyphenated instruments. "Attenuated total reflection (ATR) crystals have been used for many years, but they have been incorporated in a variety of sampling devices more recently, making FT-IR ATR ubiquitous in both laboratory and process environments," he remarked.

"An exciting advance in IR spectroscopy has been the development of coherent 2D IR spectroscopy," said Martin Zanni, who is the Meloche-Bascom Professor of Chemistry at the University of Wisconsin-Madison. "Analogous to the invention of 2D nuclear magnetic resonance (NMR) spectroscopy in the 1970s, it is now possible to create IR pulse sequences to measure 2D IR spectra with diagonal peaks and cross peaks," he explained. "Like 2D transformed NMR, 2D IR is now transforming mid-IR spectroscopy."

Key Challenges

IR spectroscopists face a variety of challenges in terms of limitations of the technique, limits to understanding how it works, and difficulties in using it for specific application types.

"Ease of sample presentation versus wavelength range is always a concern for many of our challenging problems," noted McIntyre. "Identifying mixtures of like compounds in a particular solvent or matrix can be a time-consuming process."

Schroeder's work involves interfacing with chemists, process engineers, and operators in support of material manufacturing. As a result, deploying complex analysis methods in ways that make it easy for end users to practice IR in their daily work is a key challenge. "Automation of data collection, interpretation, reporting, and archiving is key to making IR available as a tool across a wide range of job functions," he said.

The importance of correctly interpreting IR spectra was mentioned by several of our panelists. "Too many people rely on computers to interpret spectra," said Griffiths. "And even though noble attempts have been made by several organizations, identification of the components of mixtures still presents a major problem." Zanni added that a great challenge is to improve the structural identification to a higher level so that when crystallography and NMR spectroscopy cannot be applied, we can still obtain enough detailed information. Bhargava summed up this significant theme. "The codependence of spectra and structure and understanding of their decoupling is likely to affect numerous applications, design of instruments, and software for many years to come," he said. "Harnessing this convergence is the major challenge at present for IR spectroscopy [1]."

Griffiths brought up another important challenge for IR spectroscopy, related to an emerging application. "Possibly the most important challenge is the use of IR imaging spectroscopy and nanospectroscopy for medical diagnosis," he said. "Progress is being made but 'the big breakthrough' that will result in instruments being placed in most hospitals nationwide-as happened with magnetic resonance imaging-has yet to be made." Bhargava echoed this sentiment. "One of the biggest potential markets for IR spectroscopy is in the imaging of biomedical samples, which is largely limited by our ability to record sufficient data in a reasonable time," he said.

Instrument Features Needing Improvement

Our panelists noted a number of areas in which IR instruments could be improved.

"Anything that can be done to improve the signal-to-noise ratio in IR-based measurement techniques will help drive the use of IR into new areas of application," said Schroeder. He mentioned that detectors with lower noise would support higher sample throughput by requiring fewer scans per sample.

In McIntyre's view, a combination of improved photometric accuracy for instrument-to-instrument quantitative agreement and improved wavelength resolution at an affordable price are features that need attention. Griffiths supported this comment about the cost of performance. "Measurement at high resolution is no longer a problem, provided you have enough money to buy the right equipment," he said.

"The biggest topic for which improvements are desirable is sensitivity," Griffiths said. "Quantum cascade lasers (QCLs) give significantly improved sensitivity but they have a limited spectral range and are very expensive." He also mentioned that mid-IR needs improvement: "The development of a mid-IR detector that is as sensitive as a mercury–cadmium–telluride detector and has the linearity of deuterated triglycine sulfate (DTGS) detectors would be great!" he said.

Where Will Progress Be Made?

So how will the current limitations of instrument performance be addressed? Several common threads ran through the panel responses.

"Since computers will continue to become increasingly powerful, the computation of spectra of complex molecules should continue to improve over the next 15 years," noted Griffiths. Bhargava provided a similar response. "Theory-based algorithms to analyze data will become more commonplace and, indeed, indispensable," he predicted. Schroeder also commented on data analysis. "New ways of processing and visualizing spectroscopy data will be even more important in driving growth and progress in spectroscopy-based products and applications," he said.

Zanni mentioned the importance of laser sources. "We need a cheaper laser source for generating high intensity mid-IR pulses," he said. "I predict that there will be a sea change in the next few years because there is currently widespread interest in new mid-IR laser sources." Griffiths noted this possible development as well. "The development of QCLs with increased spectral range and reduced cost is already taking place, and we should see further improvements in the next 5 years," he said.

Less-Capable Instruments

Although there is a drive to push performance in terms of detection limits, signal-to-noise ratios, and so forth, there is also a move in the opposite direction, toward less-capable instruments that can satisfy the needs of specific applications. "With the opening of the design space and many new components available, there will surely be a proliferation of fit-for-purpose instrumentation," said Bhargava. "A very successful example is the work of Bernhard Lendl and colleagues using new QCL technology."

Griffiths has been aware of the advantages of such instruments for a long time. "About 15 years ago, I gave talk titled something like 'Resolution-How Low Can You Go?' in which I made the case for low-resolution measurements," he said. "I haven't changed my mind since." Low-resolution measurements can be powerful, he noted, because of the increased signal-to-noise ratio and decreased measurement time compared to those associated with higher-resolution measurements.

Schroeder gave an example of a need for an IR device with limited capability. "Transmission IR sensors for low-volume flow liquid IR analysis of fluids across the 6000–2000 cm^-1 spectral region would be useful for flow reaction studies," he said. "Such devices could use 8 cm^-1 or lower resolution detection and still obtain good quantitative data." According to Schroeder, using these sensors with 0.1–1.0 mm pathlength transmission flow cells would provide enhanced sensitivity for detection of O-H, N-H, and C-H functional groups compared with that obtained by ATR throughout that spectral region.

McIntyre had a different perspective. In his experience, general-purpose analytical instruments that can be used for many applications is more valuable. "There is a minimum level of instrument performance we look for before making an instrument purchase, and going with less-capable instruments will eventually limit the applicability," he said.

Portable Instruments

Portable or handheld instrumentation has become increasingly prevalent over the past few years. We asked our panel to comment on this trend with respect to IR spectroscopy.

"Most handheld instruments don't operate at a resolution that is much better than 4 cm^-1," said Griffiths. "If higher resolution is needed, someone will develop an appropriate spectrometer."

Schroeder envisions a new mobile IR instrument in which the sensor collects spectral data and the device then transmits the data to a remote spectral processing system for more mathematically intense computations. The advantages of such a system would be reduced cost and streamlined implementation and maintenance by requiring that methods be loaded only once, onto the single centralized processing system. Such a setup could also overcome a shortcoming of many portable instruments-their lack of quantitative analysis software, which limits their use to simple ID verification applications. In his scenario, the more expensive electronics and software (chemometric analysis, neural network, process control, statistical analysis, and so forth) for data analysis could reside in the centralized data processing system. "The portable IR instruments would only need to collect the spectra, and pass those spectra to the centralized processing unit for analysis," he explained.

Bhargava had a positive forecast for portable IR in industrial settings. "Process deployment of IR spectroscopy will be an important growth market with industry relying on spectrometers to be at the core of integrated sensors," he predicted.

Hyphenation

Combinations of tools-sometimes called hyphenation-are becoming common ways to change the analytical paradigm. "The combination of IR spectroscopy and microscopy is a natural convergence that will dominate all other hyphenation approaches," predicted Bhargava. McIntyre agreed about the importance of this hyphenation. "The combination of microscopy and IR is probably the main combination tool that we would readily utilize," he said.

Griffiths pointed specifically to the need to hyphenate IR with chromatography. "Now that mass spectrometers have been interfaced to many types of chromatographs (for example, with gas chromatography, high performance liquid chromatography, and capillary electrophoresis) with far higher sensitivity than any IR spectrometer, the main reason for linking chromatographs to IR spectrometers is to obtain information that is not obtainable by mass spectrometry-that is, differentiating between structural isomers," he said. He doesn't see such hyphenation moving into a portable instrument, however. "Since the number of applications of these hyphenated instruments is generally limited by their sensitivity, and because you can't pass as many photons through a small instrument, I don't forecast an awakening of interest in hyphenated, small FT-IR spectrometers just because of their size," he concluded.

Important Applications

IR spectroscopy is used for a wide variety of applications with many challenges, and our panel responses about the most important applications for IR reflected that diversity.

"Biomedical applications continue to drive IR spectroscopy and imaging with their tantalizing prospects of exceptional potential," said Bhargava. The primary challenge, he noted, is to integrate physics-based and computational analysis–based protocols to extract data from rapidly acquired IR data. "There has been very exciting progress in theory, algorithms, fast instrumentation and computing-all four areas will have to advance, in a synergistic manner, for true progress," he said.

Griffiths agreed with Bhargava that the most important potential applications of IR spectroscopy are in the biomedical field, specifically mentioning the use of imaging spectrometers and nanospectrometers for medical diagnosis. One of the biggest challenges, he noted, is how to compensate for the absorption of water in tissue samples. "For example, can measurements be made immediately after obtaining a biopsy tissue, or does the tissue have to be freeze dried, a procedure that can take several hours, before the necessary information can be obtained?" he asked. "Progress in this area has been incremental and we are still awaiting the big breakthrough." He mentioned the role of chemometrics in this application and, also like Bhargava, the need for faster and more powerful algorithms.

Griffiths discussed another potential stumbling block for the development of imaging spectrometers in the United States: the International Traffic in Arms Regulations. "The detector arrays that are or could be used for mid-infrared imaging are now listed on the US Military Critical Technologies list," he said. "Such listing limits sales and destroys the driver that any instrument company would have to invest in new technology for hyperspectral imaging measurements [2]."

For McIntyre, agricultural and food applications are important in his company's business, and IR spectroscopy is one of many techniques they use. "I don't view the use of this technique as competitive; it is complementary to the data collection and analysis process," he said.

Zanni identified yet another application as being a potential growth area for IR spectroscopy: solar cell research. "It is largely unknown how even the most promising molecules for dye-sensitized and other types of photovoltaics bind to their semiconductor substrates," he pointed out. "IR spectroscopy can give better information than obtained so far on the anchoring of the sensitizers, their conformations, and the structures most important for efficiency."

Acknowledgments

I would like to thank Spectroscopy Editorial Advisory Board members Michael Bradley and John Monti for their invaluable assistance in selecting panel members and formulating survey questions.

Reference

(1) R. Bhargava, Appl. Spectrosc. 66(10), 1091–1120 (2012). doi: 10.1366/12-06801

(2) P.R. Griffiths and E.V. Miseo, Chapter 1 in Infrared and Raman Spectroscopic Imaging, 2nd Edition, R. Salzer and H.W. Siesler, Eds. (Wiley-VCH, Weinheim, Germany, 2014).

Steve Brown is the technical editor of Spectroscopy magazine.