Microscopic FTIR Instrumentation

FTIR microspectroscopy is a microanalytical technique, which interfaces an FTIR spectrometer to an optical microscope. Regions of interest in the sample are spatially isolated using the microscope s apertures. It enables the IR spectrum of sampling regions down to about 10 pm resolution to be taken. Consequently, FTIR microscopy is ideal for compositional mapping and analysis of heterogeneous samples whose domain sizes are in the tens of micrometre range. 

FTIR microscopes use xl5 and xlO condenser lenses of the non-IR absorbing Cassegrainian type. They are constructed from front surface mirrors and are mounted 

Diagram of an FRA camera imaging system (x and y refer to axis) 

The IR chemical imaging system measures chemically-specific IR spectra using a mercury cadmium telluride (HgCdTe) FRA detector which provides broad frequency response (out to 18 i.m), high sensitivity (2 X 10 cm Hz 2/Watt), and an operating temperature of 40-60 K. 

Micro-spatial ( 7 pm) chemical mapping of heterogeneous complex samples. 

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The IR microscope is a valuable tool for the analysis of fibers, particulates, and inclusions. A dedicated microscope-FTIR instrument with appropriate hardware and software to perform a wide range of microscope experiments has been developed recently. 

FTIR instrumentation is mature. A typical routine mid-IR spectrometer has KBr optics, best resolution of around 1cm-1, and a room temperature DTGS detector. Noise levels below 0.1 % T peak-to-peak can be achieved in a few seconds. The sample compartment will accommodate a variety of sampling accessories such as those for ATR (attenuated total reflection) and diffuse reflection. At present, IR spectra can be obtained with fast and very fast FTIR interferometers with microscopes, in reflection and microreflection, in diffusion, at very low or very high temperatures, in dilute solutions, etc. Hyphenated IR techniques such as PyFTIR, TG-FTIR, GC-FTIR, HPLC-FTIR and SEC-FTIR (Chapter 7) can simplify many problems and streamline the selection process by doing multiple analyses with one sampling. Solvent absorbance limits flow-through IR spectroscopy cells so as to make them impractical for polymer analysis. Advanced FTIR... 

Figure 10.13—Computer controlled IR spectrometer. Many models of FTIR instruments permit the adaptation of an infrared microscope for the study of microsamples model NEXUS (11>W), reproduced by permission of Nicolet).

FTIR spectroscopy can combine with microscopy for generating FTIR spectra from microscopic volumes in materials. The instrument for FTIR microspectroscopy is simply called the FTIR microscope, which is often attached to the conventional FTIR instrument. The FTIR microscope is increasingly used for materials characterization because of its simple operation and FTIR spectra can be collected rapidly from microscopic volumes selected with the microscope. 

In principle, operation of the FTIR microspectroscopy is the same as for a conventional FTIR instrument except the spectrum is obtained from a microscopic area or intensity distribution is mapped in the sample plane. A spectrum from an area in the order of 10 x 10 /im can be obtained. Mapping or FTIR imaging at micro-level resolution can be achieved by scanning a sample using a motorized sample stage. The resolution is primarily determined by the size of the focused IR beam and precision of motorized stage. Reflectance microspectroscopy is more widely used than the transmittance mode in FTIR microscopy because minimal sample preparation is required. 

NOTE ADDED IN PROOF Since this conference, considerable progress has been made in reflection microscope attachments on FTIR Instruments(30).]... 

Infrared spectra of hybrid thin films were measured by using a Jasco Model FTIR 410 spectrophotometer. An atomic force microscope (Digital Instrument, Inc., Model DI 5000 AFM) was used to probe the surface morphology of the... 

Infrared (IR) spectroscopy, especially when measured by means of the Fourier transform method (FTIR), is another powerful technique for the physical characterization of pharmaceutical solids [17]. In the IR method, the vibrational modes of a molecule are used to deduce structural information. When studied in the solid, these same vibrations normally are affected by the nature of the structural details of the analyte, thus yielding information useful to the formulation scientist. The FTIR spectra are often used to evaluate the type of polymorphism existing in a drug substance, and they can be very useful in studies of the water contained within a hydrate species. With modem instrumentation, it is straightforward to obtain FTIR spectra of micrometer-sized particles through the use of a microscope fitted with suitable optics. 

Similarly to FTIR spectroscopy, Raman spectroscopy is a versatile technique of analyzing both organic and inorganic materials that has experienced noticeable growth in the field of art and art conservation, in parallel to the improvement of the instrumentation [38]. In particular, the introduction of fiber optic devices has made feasible the development of mobile Raman equipments, enabling nondestructive in situ analyses [39]. On the other hand, the coupling of Raman spectroscopes with optical microscopes has given rise to Raman microscopy vide infra). 

Instrumental analysis was performed with an IBM IR/32 (FTIR), Hewlett Packard 8450A UV/Vis spectrophotometer, and a Tencor Alpha Step 200 (film thickness). Photomicrographs were taken with a Reichert-Jung Polyvar-met optical microscope and electron micrographs with a Philips SEM505. 

Raman microscopes are more commonly used for materials characterization than other Raman instruments. Raman microscopes are able to examine microscopic areas of materials by focusing the laser beam down to the micrometer level without much sample preparation as long as a surface of the sample is free from contamination. This technique should be referred to as Raman microspectroscopy because Raman microscopy is not mainly used for imaging purposes, similar to FUR microspectroscopy. An important difference between Raman micro-and FUR microspectroscopies is their spatial resolution. The spatial resolution of the Raman microscope is at least one order of magnitude higher than the FTIR microscope. 

More recent spectroscopic work. Most of the recent advances in IR and Raman studies of mineralized tissue have been possible as a result of improvements in data analysis and instrumentation, particularly microscopic methods that allow spectra to be obtained from micron sized regions of tissue. For example, Paschalis et al. (1996) used FTIR microspectroscopy to study the mineral/matrix, (total carbonate)/phosphate, and CO3 Type A/Type B ratios across human osteons in 10 pm steps. The resolution was 4 cm for spectra from 20 x 20 pm areas. Similar instrumentation and data analysis have been used to study changes in mineral content and composition in biopsies of non-... 

Figure 4.24 IR microscope schematic with the detector integrated into the microscope. The microscope is usually coupled to a light port on the side of the FTIR spectrometer. The FTIR spectrometer supplies a modulated, collimated beam of light to the microscope. Courtesy of PerkinElmer Instruments, Shelton, CT (www.perkinelmer.com). (From Coates, used with permission.)...

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