Seeing the Full Picture

Fourier transform infrared (FTIR) spectroscopic imaging emerged two decades ago as a promising method for analyzing a range of different materials (1). Since then, it has been perceived mainly as a technique for obtaining chemical images of heterogeneous materials and samples, such as polymer blends or biological tissues. A chemical snapshot generated using FTIR imaging by recording many of IR spectra simultaneously is somewhat similar to a photograph from a conventional camera, the main difference being that it reveals features invisible to human eye – the images are based on chemical rather than visual differences. Thus, a chemical image is truly worth a thousand spectra!

However, spectroscopic imaging has even greater potential for chemical imaging of processes and systems that change with time. Surprisingly, this very exciting capability of spectroscopic imaging is underused. Modern FTIR spectroscopic imaging (using fast array detectors) offers itself as an experimental tool for studying dynamic systems. Indeed, a modern analytical chemist will often need to apply techniques in situ, online or inline and ultimately for industrially relevant processes. FTIR spectroscopic imaging usually can be applied in conjunction with different accessories – a microscope being one of the most familiar.

From the introduction of the technique, it was proposed that FTIR spectroscopic imaging may also work for macroscale studies (1) – one of the first examples being pharmaceutical tablet dissolution (2). Nevertheless, the combination of an FTIR spectrometer with a microscope became a commercially available system, while macro FTIR imaging of larger samples required the development of a macro compartment for accommodating accessories that would be suitable for dynamic experiments. In our laboratory, a number of such accessories and approaches have been developed for FTIR imaging experiments with systems that change over time. Importantly, using a range of spectrometers from different companies, we have been able to show that the combination of a spectrometer and macro compartment without a microscope provides a very reliable platform for the chemical imaging of dynamic systems (3).

However, the potential of studying dynamic systems in transmission mode was limited by the need for very thin samples. An attenuated total reflection (ATR) approach for macro FTIR imaging provides an alternative method that is particularly useful for the analysis of aqueous systems, as the strong infrared absorption of water does not present an obstacle.  ATR-FTIR spectroscopy and imaging are often falsely perceived as surface techniques. In fact, when using ATR-FTIR spectroscopy one measures a layer of the sample, which is typically a few micrometers thick. Depending on the optics used and spectral region of interest studied, the probing depth of ATR in mid-infrared range is comparable to that used in transmission infrared spectroscopy (which measures the absorption of IR light through thin samples with a thickness of 5–20 µm but usually requires microtoming of thick samples). The advantage of the ATR approach is that it provides the possibility of obtaining depth profiles within the measured layer of the sample by changing the angle of incidence or analyzing spectral bands at different wavelengths.

Macro ATR-FTIR spectroscopic imaging approaches using inverted prisms have been established for studies of tablet compaction and dissolution, diffusion of drug molecules into tissue, polymer mixing and polymer phase separation, dynamics of emulsion systems,  systems under flow, mixing and reactions in microfluidics, protein aggregation and crystallization.  The use of spectroscopic imaging to monitor the behavior of many samples simultaneously could be very valuable in high-throughput analysis, for example screening samples of different composition under identical conditions and controlled environment.

Seeing the actual process of tablet dissolution and drug release using FTIR imaging provides an approach for tablet optimization that is being employed in the pharmaceutical industry.  Another example of a successful industrial application is the spectroscopic imaging of film formation in latex dispersions, which is related to important processes, such as paint drying.  These examples should convince analytical chemists to adopt this chemical imaging methodology, not just for obtaining chemical images of static systems but also for use in understanding chemical processes.

The applications of macro ATR-FTIR imaging exist in other areas, such as the biomedical field, where it can analyze transdermal drug delivery and other processes. Even if ATR-FTIR imaging is yet to be exploited thoroughly in the medical field, it might well have very good potential for guiding applications of conventional ATR spectroscopy in medical diagnostics.

FTIR spectroscopic imaging could evolve into a powerful tool for routine online analysis by connecting flow cells to process lines that can be placed in the  sample compartment of the imaging systems. It could also be used for inline analysis when suitable fiber-optics are available, such as bundling thousands of mid-infrared optical fibers with a focusing device on an array detector. With such advances, chemical imaging for process analysis will become even more useful and widespread – and then the full picture can be realized.