Infrared microscope spatial resolution

The spatial resolution of the Raman microprobe is about an order of magnitude better than that obtainable using an infrared microscope. Measurement times, typically of a few seconds, are the same as for other Raman spectrographs. To avoid burning samples, low (5—50-mW) power lasers are employed. 

In transmission mode a spatial resolution of about 15-20 pm can be achieved with infrared microscopes [32]. This is generally sufficient to properly identify such as small impurities, inclusions, gels or single components of multilaminate foils. Similar to Raman spectroscopy, line profiles or maps over larger sample areas can be performed. 

Infrared microscopic imaging provides the significant advantages of direct spatially resolved concentration and molecular structure information for sample constituents. Raman microscopy (not further discussed in this chapter) possesses the additional benefit of confocal acquisition of this information and a 10-fold increase in spatial resolution at the expense of reduced signal-to-noise ratios compared with IR. The interested reader is urged to check the seminal studies of the Puppels group in Rotterdam,38 0 as well as our own initial efforts in this direction.41 The current section describes the initial applications of IR microspectroscopic imaging to monitor the permeation and tissue distribution of the dermal penetration enhancer, DMSO, in porcine skin as well as to track the extent of permeation of phospholipid vesicles. 

Our group was the first to report imaging with a diamond ATR accessory that provided a field of view of ca. 1 mm2 and the spatial resolution of ca. 15 pm without the use of an infrared microscope [18], The demonstration of the applicability of a diamond ATR accessory for FTIR imaging opened up a range of new opportunities in polymer research, from compaction of tablets [21-23] to studying phase separation in polymer blends subjected to supercritical fluids [24], This imaging approach was successfully utilised for the study of dissolution of tablets in aqueous solutions [25], We have also demonstrated macro... 

The optical layout for the measurement of biological samples (cells) is shown in Figure 29.3b. The sample was irradiated with co-linear IR and visible light beams. The transient fluorescence from the sample was collected from the opposite side by an objective lens. In this optical layout, the spatial resolution was determined by the objective numerical aperture (NA) and the visible fluorescence wavelength IR superresolution smaller than the diffraction limit of IR light was achieved. Here, Arabidopsis thaliana roots stained with Rhodamine-6G were used as a sample. We applied this super-resolution infrared microscope to the Arabidopsis thaliana root cells, and also report the results of time-resolved measurements. 

We have performed super-resolution infrared microscopy by combining a laser fluorescence microscope with picosecond time-resolved TFD-IR spectroscopy. In this chapter, we have demonstrated that the spatial resolution of the infrared microscope improved to more than twice the diffraction limit of IR light. It should he relatively straightforward to improve the spatial resolution to less than 1 pm by building a confocal optical system. Thus, in the near future, the spatial resolution of our infrared microscope will be improved to a sub-micron scale. 

We have also demonstrated picosecond time-resolved TFD-IR imaging of the vibration relaxation of Rhodamine-6G in Arabidopsis thaliana roots, and found an abnormally long-lived component of vibrational relaxation in a cell. This may result from a site dependence of vibrational relaxation within whole cells. These results indicate the possible utility of the two-color super-resolution infrared microscope in mapping specific IR absorptions with high spatial resolution, and the observation of dynamics in a non-uniform environment, such as a cell. By using this infrared super-resolution microscope, we will be able to visualize the structure and reaction dynamics of molecules in a wide range of non-uniform environments. 

Figure 6. Infrared-NSOM image of a blended polystyrene/polyethylacrylate film ( 1 micron thick) on Si. The image (8 /xm X 8 /xm), collected at 3125 cm b shows the domains of polystyrene (PS) embedded within the polyethylacrylate (PEA). This chemical map of the surface has 10 times higher spatial resolution than a conventional optical infrared microscope image. (Courtesy of Dr. Chris Michaels and Dr. Stephan Stranick, NIST.)...

The inventions of the scanning tunnelling microscope (STM) [16] and the atomic force microscope (AFM) [17] have allowed sub-micrometre and, at times, atomic-scale spatially-resolved imaging of surfaces. Spatially-resolved temperature measurements using optical systems are diffraction limited by the wavelength of the radiation involved, which is about 5-10 pm for infrared thermography and about 0.5 pm for visible light [18]. The spatial resolution of near-field techniques (such as AFM) is only limited by the active area of the sensor (which in the case of STM may be only a few atoms at the end of a metal wire). 

Since the angle of incidence is very large in RAIR, a large area of the substrate (typically several square centimeters) is usually examined and the spatial resolution of the technique is limited. However, several manufacturers have developed objective lenses that enable RAIR spectra to be acquired using an infrared microscope. These lenses make it possible to obtain RAIR spectra from areas as small as about 25 p,m in diameter and to use RAIR for failure analysis of adhesive bonds. 

The FT-SERS spectra obtained in the near infrared spectral range were measured with a Bruker FT-Raman system (RSF-100) connected to a Raman microscope (Bruker R590B). Excitation radiation of 1064 nm is provided by a Nd-YAG laser. A spectral solution of 4 1/cm was obtained while the spatial resolution was 100/im. ... 

An alternative approach to using interferometers for imaging is to use another source that provides spectral discrimination directly. One such source is an infrared diode laser. A laser coupled to an infrared microscope provides a means to rapidly obtain spatial resolution approaching the diffraction limit (142 ]. A multi-layer polymer composite contaminated by migration of a volatile additive from an exogenous source was imaged using this system. The distribution of this additive in the layered structure was shown to correlate with specific layers (Fig. 21). A con-... 

In principle, Raman spectroscopy is a microtechnique [161) since, for a given light flux of a laser source, the flux of Raman radiation is inversely proportional to the diameter of the laser-beam focus at the sample, i.e., an optimized Raman sample is a microsample. However, Raman microspectroscopy able to obtain spatially resolved vibrational spectra to ca. 1 pm spatial resolution and using a conventional optical microscope system has only recently been more widely appreciated. For Raman microspectroscopy both conventional [162] and FT-Raman spectrometers [ 163], [ 164] are employed, the latter being coupled by near-infrared fiber optics to the microscope. 

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