Microspectroscopy


commercially available [15]. Tere are multiple mIRage installations around the world at industrial companies, government research facilities, and major academic institutions for multiple reasons, including improved spatial resolution, and non-con- tact data acquisition that does not require the use of an atomic force microscope (AFM) and uses similar optical methods to FT-IR and Raman microscopy. Te O-PTIR technique has previously


Figure 3: Theoretical spatial resolution comparison of FT-IR, QCL, and O-PTIR microscopes as a func- tion of wavenumber. At 1000 cm−1


, the theoretical spatial resolutions of the FT-IR, QCL IR, and O-PTIR


microscopes are 12.2, 8.7, and 0.416 μm, respectively, assuming a 532 nm probe laser for the O-PTIR microscope.


source, thus providing simultaneous IR and Raman spectro- scopic analysis of the same sample location and at the same spatial resolution. Such a combination provides complemen- tary IR and Raman spectral information that can help solve a chemical problem or provide a confirmatory analysis of an unknown contaminant. Tis ease of sample preparation allows the analysis of


manufactured products like extruded or electrospun fibers to be measured without changing the mechanical properties induced by cutting, melting, or pressing into thin films, thus allowing the study of these materials in their native metrologi- cal forms.


Innovation Te O-PTIR tech-


nique provides infrared spectroscopy and chemical imaging with


submicron


spatial resolution that is up to 30×better than the con- ventional mid-IR diffraction limit, does not require con- tact, does not require thin sections to minimize sat- uration, and provides wave- length-independent spatial resolution. Te O-PTIR technique has been devel- oped by researchers at the Naval Research Laboratory [2] and other groups [3–14] and has recently become


been used to study chemicals [2], liquid crystals [3], polymer particles and films [5,7–9], biological tissues [4], cells [7,10,12] and living organisms [7], and pharmaceu- ticals [14]. Te O-PTIR technique provides an elegant non-contact way to chemically identify contaminants with higher spatial resolution than conventional IR micro- scopes, while maintaining the advan- tages of an optical microscope-based platform. O-PTIR has been referred to by several names and acronyms including photothermal IR imaging spectroscopy (PT-IRIS), mid-IR photothermal (MIP), mid-IR photothermal microscopy, and IR photothermal heterodyne imaging (IR-


PHI). We prefer O-PTIR because it is both easy to pronounce and suggestive of optical IR, which in turn alludes to two of the key advantages: (1) better spatial resolution is achieved through a (visible) optical probe beam that can be focused much smaller than the IR beam; and (2) the technique is based on optical microscopy. O-PTIR builds on prior work in vis- ible light photothermal imaging and spectroscopy [16–19] and photothermal infrared standoff detection techniques [20–22]. A schematic diagram of the O-PTIR technique is illus-


trated in Figure 7. O-PTIR uses a visible probe beam to detect the photothermal response of IR-absorbing regions of a


Figure 4: Example of an ATR microscope objective. Such measurements require sample contact, can affect native ori- entations, and are subject to cross-contamination, as shown on the right. Image size is 1.53 mm × 1.15 mm. μATR can be extremely useful, but pressure needed for intimate contact can damage the sample or μATR. It may cost several thousand dollars to repair a μATR.


2020 May • www.microscopy-today.com 29


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