Buyer's Guide: FTIR Microscopes

Fourier-transform infrared (FTIR) microscopy is a powerful technique that combines the chemical identification abilities of FTIR spectroscopy with the spatial resolution of optical microscopy. Using an FTIR microscope, one can both visualize and obtain the IR spectra of microscopic sample regions, making it an excellent technique for materials research, failure analysis and quality control applications, including pharmaceutical QC. There are a range of systems available to enable FTIR microscopy in the lab, and several factors to consider when selecting an optimal system for your application. Here are some questions to keep in mind when shopping for an FTIR microscope:

1. Is a fully integrated system or connected system better for my lab?

Some FTIR microscopes come as standalone systems combining a spectrometer and microscope into one instrument, while others involve interfacing a microscope externally with an FTIR spectrometer. Considering whether to purchase a fully integrated FTIR microscope versus a connected system is a good first step for narrowing down your options, especially if you already have an FTIR spectroscopy system set up in your lab. 

Standalone systems offer a compact footprint and often maximize simplicity and efficiency by combining the necessary components into one streamlined, automated machine. In addition to saving bench space, these instruments can be beneficial and cost-effective if you are looking for a dedicated system solely for FTIR microscopy and imaging, in cases where traditional FTIR spectroscopy is not a priority or you wish to have your FTIR spectrometer available for other experiments. Downsides to full integration may include high upfront cost when compared to adding an FTIR microscope to an existing FTIR spectrometer, and limited upgrade and customization options.

Adding an FTIR microscope to the FTIR spectroscopy system you already have at your lab can be a less expensive option and many spectrometer manufacturers offer the ability to seamlessly interface the two instruments, along with opportunities to automate and customize the connected system. A common configuration is placing the spectrometer and microscope side-by-side, which takes up more bench space than a compact, integrated model. However, there are also microscopy modules that can be incorporated inside the sample compartment of the spectrometer. 

Products to Consider:

• LUMOS FTIR Microscope from Bruker Optics

• IRT-1000 Series FTIR Microscope from JASCO

• Nicolet iN10 IR Microscope from Thermo Fisher Scientific

2. Which type of detector best suits my application?

The two most common detector materials used in FTIR microscopes are deuterated lanthanum α-alanine doped triglycine sulphate (DLaTGS) and mercury cadmium telluride (MCT). DLaTGS detectors are not as sensitive as MCT detectors, but can be advantageous when measuring areas larger than 50 µm as they are less expensive, easy to use and do not require any external cooling.¹ 

MCT detectors are preferred for more sensitive measurements, with liquid nitrogen cooled MCTs (LN-MCTs) offering the highest resolution for sample regions down to less than 10 µm. The tradeoff of this superior sensitivity is the need to maintain the LN2 supply and allow time for the detector to cool down. These sensors are also vulnerable to suffering diminished performance from saturation or temperature changes. 

Not all applications necessitate pinpointing regions this small, and thermoelectrically-cooled MCTs (TE-MCTs) provide an excellent balance between ease-of-use and high resolution for measurements smaller than 50 µm but larger than 10 µm. While more expensive than DLaTGS detectors, TE-MCTs can provide the sensitivity necessary to resolve details that may be missed using a less powerful detector. 

In addition to detector material and cooling methods, one should also consider the specific wavelength range of each detector, the ability to expand the wavelength range by incorporating multiple detectors, and the flexibility to change or upgrade detectors, for example, to incorporate an LN-cooled detector in the future. 

Products to Consider:

• 610-IR FT-IR Microscope from Agilent Technologies

• Survey IR FTIR Microscope from Shimadzu

• IRT-5200 FTIR Microscope from JASCO

3. What are my options for chemical imaging?

FTIR chemical imaging is an extension of FTIR microscopy enabling comprehensive molecular information about a microscopic sample region to be captured, localized and visualized within a single image. One chemical image captured by an FTIR microscope can contain thousands of IR spectra, with each pixel containing the entire IR spectrum of the region it contains.² Depending on the resolution of the FTIR mode used (ex. transmission, reflection, attenuated total reflectance/ATR), each pixel can represent an area as small as ~1 µm.³ This technique is especially valuable for locating and identifying details such as flaws and contaminants that are difficult to spot using traditional optical microscopy and imaging, as “false color” maps of the sample can be generated using the spectral data. 

Some FTIR microscopes are specifically designed and optimized for chemical imaging, which requires specialized detector configurations for maximum resolution and capture speed. While chemical imaging can be achieved by using a single element/single point detector to scan and capture individual pixels, this process is time-consuming, tedious and impractical for applications that require a higher throughput. This is why most chemical imaging FTIR microscopes use either a line array or focal plane array (FPA) detector setup to capture multiple pixels simultaneously, significantly speeding up the process. 

Line array detectors capture one row of pixels at a time and scan across the sample to obtain a complete image. For example, a 1x16 linear array can capture a 16x16 px area in 16 captures vs. 256 captures with single point scanning. While linear array detectors provide much greater speed than single element detectors, FPA detectors are the gold standard for high-throughput chemical imaging, as only a single measurement is needed to capture localized spectral information from an entire 2D area – up to more than 10,000 spectra in a single scan. FPA dimensions can range from 16x16 px to 128x128 px, and these arrays can further be used to scan multiple “tiles” across a sample to capture a much larger area in a matter of minutes. 

Systems with large, high-resolution FPAs, along with the automation abilities needed for fast and precise scanning, are more costly than simpler systems that may be capable of, but not purpose-built, for chemical imaging. Therefore, you must consider if and how chemical imaging may benefit your work, and what your throughput requirements are for obtaining chemical images. 

Products to Consider:

• HYPERION Series Microscopes from Bruker Optics

• IRT-7200 FTIR Microscope from JASCO

• 620-IR FT-IR Spectrochemical Imaging Microscope from Agilent Technologies

References

1. "Guide to FT-IR Microscopy," Bruker. https://www.bruker.com/en/products-and-solutions/infrared-and-raman/ft-ir-microscopes/what-is-ft-ir-microscopy.html

2. "Guide to FT-IR Imaging," Bruker. https://www.bruker.com/en/products-and-solutions/infrared-and-raman/ft-ir-microscopes/what-is-ft-ir-imaging.html 

3. "FTIR chemical imaging using focal plane array-based systems," Technical Overview, Agilent Technologies (2011). https://www.agilent.com/cs/library/technicaloverviews/Public/si-2645.pdf