Wavelength Tech Forum: NIR

This month, our Technology Forum contributors share their views on Fourier transform-infrared spectroscopy (FT-IR) and Fourier transform-near infrared spectroscopy (FT-NIR), analyzing the current state and future trends of these popular techniques. Our participants are: Richard Larsen, scientific applications manager and Wendy Gardinier, scientific product specialist at Jasco, Inc.; Francis Mirabella, principal scientist at Lyondell Chemical Co.; and Sharon Williams, infrared business manager at PerkinElmer Life and Analytical Sciences.

Fourier Transform IR is currently the most commonly used infrared spectrometry method. Can you tell us what advantages have led to its popularity?

Larsen, Gardinier: The speed, sensitivity, and repeatability of wavelength assignment and the ability to use reflective sample accessories have contributed to the popularity of FT-IR instrumentation.

Mirabella: FT-IR in the mid-IR is information-rich and contains isolated bands that are readily assignable to specific moeties. It also permits convenient calibration for quantification, and polarization permits quantification of molecular orientation. Moreover, perturbations in band wavelength position, breadth, and shape can be associated with electronic effects in the molecule, mechanical deformations, etc.

Williams: FT-IR now is the preferred technology for IR analyses due to its faster analysis times and higher sensitivity primarily due to two well documented advantage. The multiplex advantage: In FT-IR, data points are collected simultaneously, and therefore for a given data collection time any given frequency can be measured many more times, reducing the signal-to-noise ratio. The throughput advantage: The simple optical path of the interferometer means more energy gets to the sample than is possible with dispersive spectrophotometers. This means more energy reaches the detector, increasing the spectrum's potential signal-to-noise ratio.

The FDA's Process and Analytical Technology (PAT) initiative is expected to boost NIR growth. Do you believe that it will? How do you think this growth due to PAT will compare to that of Raman spectroscopy?

Larsen, Gardinier: The PAT initiative will heavily depend upon NIR analysis methods due to ease of use and sampling advantages compared to Raman spectroscopy. In addition, although NIR data does not provide easily interpreted spectra, it has superior capabilities for quantitative analysis applications.

Mirabella: NIR will grow in pharmaceuticals and agriculture, but I doubt it will surpass Raman in other applications.

Williams: Absolutely. FT-NIR is a very useful tool for the measurement of pharmaceutical samples, particularly powders. Because of this, it is a well-established technology in the pharmaceutical industry, particularly in laboratories performing raw material testing. FT-NIR growth within the PAT initiative will involve taking these analyses out of the laboratory and near-, at-, or on-line, and applying existing expertise to new applications, such as analyzing the distribution of components within a tablet or powder blend. I do not envisage that Raman spectroscopy will be adopted for PAT applications as fast for two key reasons: concerns over the safety when moving the technology out of the laboratory, and few Raman experts within the industry to provide application development.

Dispersive IR is the most popular NIR technique used in labs. Do you think FT-NIR could ever be as widely used? Why?

Larsen, Gardinier: Although NIR does not require the resolution capability of mid-IR spectroscopy, the cost of FT-NIR is expected to decline, which might make it as popular as dispersive NIR. Instruments for FT-NIR also provide more repeatable wavelength assignment, and although they are more complicated than dispersive instrumentation (e.g., filter instruments) they enjoy greater sensitivity and faster scan rates. However, developing application-specific methods for FT-NIR instruments will require additional effort by instrumentation vendors.

Mirabella: Not as FT-NIR now stands.

Williams: When NIR applications were first investigated, dispersive technology was the only technology available. Initial FT-NIR instruments provided greater speed and spectral resolution. However, these early instruments did not provide the signal-to-noise available with the existing dispersive technology. Modern FT-IR instruments available on the market have addressed this. I believe that the market does appreciate the benefits of FT-NIR technology and many customers are transitioning to this newer technology. The transition is much slower than seen in FT-IR as many established solution specific analyzers, particularly for the food and agriculture market, are still based on dispersive technology.

What application areas for FT-IR/NIR remain unexplored?

Williams: With mature technology such as FT-IR and FT-NIR spectroscopy, there are very few existing application areas that remain unexplored. However, as new materials and scientific disciplines develop and regulatory demands increase, FT-IR and FT-NIR continually are finding applications in new areas. An example of this is the measurement of the identity and quality of traditional Chinese medicines using 2D-IR spectroscopy applications, an application that has been developed due to increased pressures to regulate the quality and safety of products in this very large market.

What improvements do current FT-IR/NIR techniques need in order to expand their use?

Larsen, Gardinier: They need greater development of application-specific instruments and software methods.

Williams: Reliability, reproducibility, and robustness are going to be key factors to an instrument's success when moving into process environments. In addition, applications within the field of chemical imaging using FT-IR and FT-NIR are becoming well accepted. For wider adoption, instruments for these applications need to allow more and bigger samples to be analyzed by providing faster imaging speed while maintaining the sensitivity. Furthermore, FT-IR and FT-NIR imaging systems need to provide a user interface that allows less experienced spectroscopists to be able to use the technology and obtain meaningful information in as short a time as possible.