Microspectroscopy
than 20 μm in thickness, which can be time-consuming, dif- ficult, or impossible. Another common measurement mode in FT-IR is micro-
attenuated total reflection (ATR), which minimizes the amount of sample preparation required and enhances spatial resolu- tion through the use of high refractive index internal reflec- tion element (IRE) materials, typically Ge, ZnSe, or diamond (Figure 4). With this method of analysis, the IRE of the ATR accessory or microscope objective must be in intimate contact with the sample. Te sample stage is then adjusted to apply enough pressure to maximize the contact of the IRE with the sample surface. Applying too much pressure may cause dam- age to both the sample and/or the IRE, so care must be used. When comparing ATR and transmission spectra from a sam- ple, there are spectral differences, including changes in peak intensities and peak positions. Ideally, one would like to simply reflect an IR beam off the
Figure 1: Spatial resolution and relative chemical specificity of correlative methods.
where λ is the wavelength of infrared light used, n is the index of refraction of the surrounding media (1 for air), and NA is the effective numerical aperture of the microscope objective used. Figure 2 illustrates the loss of resolution when using long IR wavelengths. When using short wavelengths of visible light (leſt), features are clearly resolved, but when using IR wave- lengths (right), resolution is lost. Figure 3 illustrates the wavelength dependence of spatial
resolution for two objective numerical apertures used in com- mercially available FT-IR and direct quantum cascade laser (QCL) based systems. Tis contrasts with spatial resolution for the O-PTIR system, which has a theoretical spatial resolution of ∼416 nm and is independent of IR wavelength. Tis is due to the spatial resolution of O-PTIR being defined by the wavelength of the probe laser (532 nm), whereas with FT-IR/QCL systems the spatial resolution is reduced by up to 30× and varies over the range of IR wavelengths. Sample preparation
prior to IR analysis can be complex and requires expertise. Te best quality and the most accurate FT-IR data are collected in trans- mission mode. Tis requires a sample thin enough for the IR light to pass through without band saturation. Tis oſten requires section- ing of the sample to less
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top surface of a sample to get a spectrum. Unfortunately, most samples reflect weakly in the mid-IR range and produce noisy spectra with dispersive artifacts. Tis makes comparison and ratioing of peaks problematic when compared to transmis- sion data. Figure 5 illustrates how the IR spectrum of the same material measured in transmission can vary depending on the size of the particle being measured [1]. O-PTIR spectroscopy is a patented technology used
exclusively in the commercial mIRage infrared microscope. O-PTIR overcomes all of the problems mentioned above that have hindered the expansion of FT-IR applications. Te approach is based on a pump-probe architecture that couples broadly tunable pulsed IR laser sources (the pump) capable of covering the standard IR spectral range, with a short- wavelength visible laser (the probe) to provide submicron IR spectroscopy. In addition to achieving submicron IR spatial resolution, the mIRage system typically operates in reflection mode, does not require sample contact, and produces artifact- free IR spectra, such as those shown in Figure 6 for three com- mon polymers. Te unique architecture of this pump-probe system takes advantage of the fact that the probe laser (532 nm or 785 nm) can simultaneously act as a Raman excitation
Figure 2: Images of a 1951 US Air Force standard-resolution target obtained with a visible camera (left) and infrared focal plane array camera (right). Each image size is 700 μm × 700 μm.
www.microscopy-today.com • 2020 May
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