Modern trends in infrared spectroscopy

The utility of infrared (IR) spectroscopy in the analytical laboratory has been recognised for decades However, it is only over the past few years that developments in instrumentation and data processing have enabled the full potential of this technique to be realised Dr Simon Nunn reports on how this technique is now meeting the demands of modern quality control

Every chemist would like to be able to see her sample on a molecular scale Unfortunately, this is not practical due to a number of physical limitations, not least the resolving power of an eyeball Infrared (IR) spectroscopy is one technique that can be used to obtain information at the molecular level Students of chemistry will be familiar with the idea that an IR spectrum is a characteristic fingerprint of a material

A molecule can be thought of as a collection of atomic balls connected by spring-like bonds These bonds vibrate at a frequency that depends on the strength of the spring and the masses of the atomic balls Organic compounds, and some inorganic materials, possess characteristic vibrations in the mid-IR region of the electromagnetic spectrum The IR spectrum of a material is therefore diagnostic of its chemical composition In the analytical laboratory, the technique can be used to identify a substance or to determine the concentration of a component within that material

Despite the wealth of chemical information contained in an IR spectrum, difficulties associated with obtaining a quality spectrum from a sample and then extracting useful data from that spectrum have limited its application in the quality control laboratory These problems have been exacerbated by moves within the chemical industry to restructure quality control (QC) operations QC testing is generally moving from the laboratory closer to the production process, with an associated loss of skilled, laboratory-based, spectroscopists Recent developments in instrumentation and software have enabled IR spectroscopy to meet the demands of the modern QC environment

The heart of an infrared spectroscopy system is the spectrometer itself Early spectrometers used a prism, and later diffraction gratings, to split the light from the source into its constituent frequencies These dispersive instruments have been superseded by Fourier transform (FT) spectrometers Here the diffraction grating is replaced by an interferometer The advantages of using interferometers rather than gratings have been recognised since the 1960s However, the fast FT calculation, which is required to convert the output of an interferometer into a usable spectrum, requires significant computation The use of FTIR spectrometers did not become commonplace until the advent of fast, inexpensive computers in the 1980s A PC can perform the necessary calculations so quickly that the analyst is seldom aware that any processing is being done A modern FTIR designed for QC applications is compact (about the size of a desktop PC), robust and has a performance that outstrips that of a ten-year-old system costing three times the price Spectrometers are available with dynamic optical alignment and sophisticated on-board diagnostics that inform the user of the state of the instrument A detailed knowledge of the workings of the instrument is therefore not required to ensure optimum performance

An infrared spectrum is generally obtained from a few milligrams of material Until recently there have been comparatively few methods in common use for ensuring that the optimum amount of material is presented to the instrument Liquids were typically squashed between two salt plates such that a film of a few microns was achieved Solids were ground and mixed with potassium bromide The mixture was then subjected to high pressure to form a fragile but self-supporting disc A high degree of user skill is required to reliably obtain quality IR spectra using either of these techniques There is now a large number of sampling accessories available that are designed to ensure that the optimum amount of sample is presented to the instrument with the minimum of preparation

Many of these accessories are based on the attenuated total reflection (ATR) principal In ATR, the spectrometer beam travels through an IR-transparent prism The beam is totally reflected at the crystal surface The IR spectrum of a sample is obtained by placing the material against the crystal surface Careful control of the amount of sample placed on the accessory is not required as the instrument only senses the portion of the material that is in direct contact with the prism A variety of crystal materials is available, including diamond The mechanical and chemical properties of diamond are in marked contrast to the fragile materials, such as potassium bromide, which are traditionally associated with IR Spectra can now easily be obtained from samples that were once considered difficult, including viscous liquids, aqueous solutions, corrosive and abrasive materials

IR radiation can be focused, like visible light This opens up the possibility of combining infrared spectroscopy with microscopy, enabling IR spectra to be obtained from samples down to 10 microns diameter Infrared microscopy is routinely used in forensic science and contamination analysis Automation of IR microscopy systems enables chemical maps to be produced The construction of a polymer laminate or the distribution of a component within a blend can be elucidated using this technology

So we now have high-performance spectrometers equipped with all manner of cunning accessories which allow quality spectra to be obtained with ease from a vast range of samples, but what about the spectra themselves? The output of all this sophisticated technology is a wiggly line Clearly some work needs to be done to transform this obtuse trace into meaningful information This interpretation is the domain of the spectroscopist However, with the rationalisation of corporate analytical facilities, many companies find themselves without such a resource Fortunately, recent advances made in spectroscopic software have enabled relatively unskilled operators to obtain meaningful information from their instrumentation

Perhaps the largest single use of FTIR is in the identification of materials The spectrum of an unknown is matched against a database Early databases were paper-based Matching an unknown spectrum against a paper database is time-consuming and can be inaccurate, as there can be no quantitative measure of the quality of fit of a astare-and-compare' analysis Furthermore, when matching against a paper library, who knows where to begin, and who could comprehensively search through a database containing more than a few dozen spectra? Now, of course, spectral databases are stored digitally Users can create their own spectral libraries or chose from a selection of commercially-available databases

The searching of IR libraries is a powerful technique for the identification of unknowns but can be abused For example, if a plant receives deliveries of soybean oil, it may wish to confirm the identity of the material in the tanker by FTIR spectroscopy The IR spectrum will give a very good match to a library spectrum of soybean oil However, the spectrum of, say, peanut oil will very similar In fact there may be so few differences between the spectra of the two materials that a astare and compare' analysis or a computerised library search cannot distinguish them Therefore, the identity of the soybean oil has not been confirmed

Fortunately, there are data analysis techniques that facilitate such confirmation Discriminant analysis and comparison libraries are just two techniques that study not only the similarities but also the small differences between spectra Such techniques can readily distinguish between the IR spectra of very similar materials, such as natural oils, and are therefore widely used in the QC environment for the checking of incoming materials

IR spectroscopy can also be used to determine the concentration of a component in a mixture Early attempts at quantitative analysis were fraught with difficulty, due in no small part to the problems associated with presenting a sample reproducibly to the instrument However, the advances in accessory technology, described above, have removed many of the sources of experimental error that have traditionally hampered the development of quantitative FTIR methods Furthermore, sophisticated statistical data analysis packages now form an integral part of FTIR software suites

The developments in FTIR instrumentation, accessories and software have combined to produce a uniquely powerful analytical technique Reliable results are obtained in seconds with a minimum of operator skill FTIR is well placed to meet the needs of the industry as QC operations move ever closer to the production line

Dr Simon Nunn is with Nicolet Instruments Ltd, Nicolet House, Budbrooke Road, Warwick, CV34 5XH Tel: +44 1926 417700 Fax: +44 1926 417701 wwwnicoletcouk

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