Recent Developments in Analytical Science - Infrared Spectroscopy

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  • Published: Jul 18, 2016
  • Channels: Proteomics & Genomics / HPLC / Ion Chromatography / Electrophoresis / Gas Chromatography / Atomic / Base Peak / NMR Knowledge Base / Infrared Spectroscopy / Proteomics / Raman / MRI Spectroscopy / X-ray Spectrometry

Infrared

Infrared spectroscopy is a longstanding technique that continues to be popular among scientists, exemplified by a recent report that predicts the market for instruments will maintain growth up to 2020 when it will be worth USD 1.25 billion. The three main sub-categories of conventional IR spectroscopy relate to the irradiating wavelength - near-IR, mid-IR and far-IR - and they have been successfully deployed in many fields for decades. Developments such as Fourier transform IR spectroscopy and the introduction of hyphenated IR techniques have driven it forward.

FTIR spectroscopy affords spectra with improved resolution, signal-to-noise ratios and accuracy. This is achieved by measuring the IR frequencies simultaneously rather than individually using an interferometer, then deconvoluting the subsequent total signal into individual frequencies by Fourier transformation. Despite being a well-used technique, the majority of FTIR applications use a modified technique called attenuated total reflection FTIR in which the incident IR beam is directed through an optically dense crystal produce an interferogram that is converted to spectra. There are some drawbacks, such as the necessity for cleaning the sampling area between experiments but the technique gives sharper, more reproducible spectra in seconds for solids and liquids to give spectral databases of better quality for more accurate compound identifications.

There are benchtop, portable and handheld versions of IR, FTIR and ATR-FTIR instruments supporting analyses in the lab and in the field. Some of these are directed at particular markets and carry specific in-built spectral libraries and search software for areas such as biodiesel, milk adulterants, explosives and hazardous materials. Point-of-care applications are also on the rise, with instruments intended for the diagnosis of specific medical conditions such as chronic obstructive pulmonary disorder.27 This type of instrument allows clinical personnel to obtain rapid diagnoses within minutes, reducing the waiting time for treatment and improving patient care.

Another major development is the introduction of 2D IR spectroscopy in which, ironically, three laser pulses are employed. Two closely timed pulses are used to excite the vibrational modes of the molecules in the sample and a third laser generates the IR signal. Due to the rapid time scale, the dynamics of systems such as protein conformations28 and the structural changes occurring in catalysts29 can be examined. The procedure can separate overlapping peaks in complex IR spectra by displaying them in two dimensions: the excitation versus the detection frequencies

GC and LC systems can be connected to the IR spectrometer, the most popular using FTIR spectroscopy since it works on a shorter timescale that is more compatible high-speed GC elution times. GC-FTIR, LC-FTIR, GPC-FTIR and multidetector systems such as GC-IR-MS and thermogravimetry-IR-GC/MS are available. They have been used in a multitude of applications for analysing drugs of abuse, pharmaceuticals, environmental pollutants, natural products, polymers, foodstuffs and petrochemicals.

In recent years, both IR and Raman spectroscopy have been at the forefront of a different type of combination technology that is carrying it into the realm of imaging techniques. By linking them with optical microscopy, FT Raman microscopy and FTIR microscopy have become established for examining the surfaces of semiconductors, coatings, paints and polymers. They have the ability to map the distributions of compounds within surfaces which are displayed as false colour maps. Advances like automatic focussing and tracking are making them more user friendly.

In the clinical world, a new publication has demonstrated the potential of FTIR microscopy for producing 3D chemical images of tissue.30 Moreover, it is achieved in a quantitative manner due to novel data manipulation techniques by a process dubbed spectromics: “the ability to exploit any spectral information (or set of spectral information using any kind of calculation procedure) for metadata construction (from chemical to molecular, biological, and anatomical tissue contents).” This major development brings a fully automated IR microscopy suite for 3D digital histology one step closer but there are a number of obstacles to overcome, such as full automation of tissue slicing, data treatment and 3D reconstructions.

One of the next steps could be the use of Raman and ATR-FTIR microscopy for the analysis of single cells in vivo, the former being less sensitive than FTIR microscopy but providing higher resolution. Linking ATR-IR spectroscopy with atomic force microscopy has also been mooted. This combination would open up the time-resolved analysis of in vitro systems such as proteins interacting with ligands. This will be aided by the new generation of lasers which permit analysis on the nanoscale.31

Hyperspectral imaging

The combination of spectral and spatial information is a powerful one which is finding applications in many field including process analysis as well as out in the field, flown on satellites, planes and unmanned aerial vehicles (UAVs) or “drones” for defense as well as agricultural applications. The size of the datasets generated from such experiments pose challenges in storage, transfer and processing. Just as the advent of faster and cheaper computers enabled chemometric techniques to be applied in a real or near-real timeframe, so further advances in computing and in data handling algorithms will enable hyperspectral imaging to expand.

NIR spectroscopy

NIR spectroscopy is well-established in many industries, especially food and agriculture where it was first employed. In recent years, its applications have exploded and you will find studies in medicine (functional NIR spectroscopy in brain studies, wound care), environment, wildlife and ecology and in plant breeding studies. Applications such as the detection of melamine in milk powder and animal protein in animal feed have huge potential, but perhaps it is miniaturisation that may see the biggest growth in NIR spectroscopy. Keyfob-size devices for the consumer market are about to appear, although there is much debate about their genuine benefit, but smaller and cheaper analyzers will bring huge benefits to farmers and workers in fields including minerals and forest products..

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