FTIR is concerned with the vibration of molecules. Each functional group has its own discrete vibrational energy which can be used to identify a molecule through the combination of all of the functional groups. This makes FTIR microscopy ideal for sample ID, multilayer film characterization, and particle analysis.
FTIR is concerned with the vibration of molecules. Each functional group has its own discrete vibrational energy which can be used to identify a molecule through the combination of all of the functional groups. This makes IR Microscopy ideal for sample ID, multilayer film characterization, and particle analysis. More can be found here on the fundamentals of FTIR Spectroscopy.
Reflection Measurements of FTIR Microscopy
Reflection measurement is performed with a Cassegrain objective, which is different from a regular microscope objective. The Cassegrain objective (Figure 1) uses a primary and secondary mirror to send the IR beam onto the sample at 35o. IR reflective materials work best with this objective, while dark samples do not work as well.
Transmission Measurements of FTIR Microscopy
Transmission analysis of samples is probably the most common and most universal method of analyzing solids, liquids, and gases. Transmission measurements provide the highest sensitivity and best detection of all infrared sampling techniques. For transmission, two Cassegrain objectives are employed: focusing and condenser (Figure 2). The focusing works the same as the one for reflection. Light passes through the sample at the focal point and then hits the condenser which collimates the beam into the detector. Samples must be thin (less than 50 μm) and need to be in either a KBr pellet or diamond anvil cell. The focal planes of the Cassegrains are matched. Fibers, laminates, and thin films are often measured in transmittance mode.
ATR Measurements with FTIR Microscopy
Attenuated total reflectance (ATR) uses special crystal materials in contact with a sample to get chemical information. The ATR (Figure 3) consists of focusing mirrors to hit the crystal at a 45o angle. The light then passes into the sample and reflected back into the spectrometer. To protect the objective, a pressure plate is required. Over pressure can damage some crystals, with the exception of diamond objectives. Diamond, ZnSe, and ZnS can be configured such that the sample can be seen when in contact with the sample.
Coatings on ‘shiny’ substrates are excellent candidates for infrared reflection-absorption studies. As the coating gets thinner, incidence and collection angles can be varied from 45-75° until the ‘grazing’ angle is reached, generally considered to be 85° (Figure 4). The term ‘reflection absorption’ describes the progress of the incident beam as it passes through the coating, reflects from the substrate, and passes through the coating again before reaching the detector. As the incident and collection angles approach the grazing angle, the incident beam strikes the coating at shallower angles and the path length through the sample gets longer, enhancing the absorption intensity. Grazing angle reflection is used to examine the thinnest of surface coatings by ‘grazing’ the sample at a very shallow angle, the resulting longer sample path length providing greater sensitivity. Infrared reflection-absorption spectroscopy (IRRAS) is often used to study monolayer coatings on metals and other substrates. Limited quantitation can be made when evaluating the composition and thickness of the coatings.
The many detectors that can be used in an FTIR microscope cover a wide wavelength range from the visible – (silicon photodiodes), NIR – (InSb or InGaAs), mid-IR – (TGS or MCT), far-IR – (Si bolometer). The IRT Series has a choice of standard detectors with the option of a second detector. The simplest detector, a Peltier cooled DLaTGS detector is used in the mid-IR region with good sensitivity, however, with much greater sensitivity an LN2 cooled mid-band MCT detector is better for measuring smaller microscopic areas. The optional second detector can be installed with a choice of either a fixed detector or a cassette system for interchangeable detectors, both can be selected from a wide range of options. To increase speed, especially for imaging or dynamic measurement a 16 element linear array detector (MCT or InSb) dramatically improves throughput (Figure 5).
IQ Mapping™ is a unique feature that allows the measurement area to be moved in a stationery Cassegrain to build up an image map without moving the stage. IQ Mapping™ can be used with transmission, reflection and ATR measurements. It is exceptionally useful for ATR measurement as an area in contact with the crystal can be mapped without lifting and repositioning the objective prism, which normally causes damage to the surface during the process. IQ Mapping™ also provides imaging for soft samples, like gels or even viscous liquids. The ATR prism will agitate the sample if it is lifted and repositioned, but using the Clear View SS type ATR objective allows the sample to be viewed and measured without disturbing the sample once it is in position.
Fluorescence Observation can be used to identify fluorescent samples that cannot be seen with visible light (Figure 6.). Two options are available with different wavelength ranges. The wavelength range is selected with a filter.
Differential Interference Contrast (DIC) Observation
Differential Interference Contrast (DIC) Observation uses two orthogonally polarized light beams and a Nomarski-modified Wollaston prism to enhance the observation of images with low contrast. DIC uses the phase difference in light to stereographically view very small step differences in the submicron order. Nomarski prisms are used to create bright and dark contrast from the differences in the two orthogonally polarized light beams directly reflected at the sample’s surface. This technique can be applied equally to low-contrast biological and non-biological samples that have small unevenness in the surface. In the example below, a printed circuit board was measured using standard observation and DIC observation; uneven points and scratches on the sample were more clearly observed using DIC observation.
Visible Polarization Contrast (VPC) Observation
Visible Polarization Contrast (VPO) observation exploits the differences in anisotropic properties to enhance the observation of materials with low contrast. VPC uses two polarized elements located in the optical path on each side of the sample being observed. It is particularly useful for samples such as biomolecules and biostructures, minerals, ceramics, mineral fibers, extended polymers, liquid crystals, etc. In the example below, a stretched vinyl sample was measured using standard observation and VPO observation; the multiple layers of the stretched area, which has very little contrast under simple unpolarized white light, can be seen when viewed with polarized light. Polarization is extremely useful for observing samples with oriented structures.
FTIR Microscopy Webinar
This introductory webinar covers FTIR imaging. After a quick recap of FTIR theory and instrumentation, sample preparation, techniques, and objectives are discussed. Additionally, hardware such as sample stages, detectors, and complimentary microscopy techniques are reviewed.
PowerPoint slides can be downloaded here.
This technique can be used for chemical or molecular analysis encompassing depth profiling and mapping of samples with spatial resolution as little as 1 μm.
An FTIR ATR method may be a suitable alternative and offers advantages such as minimal sample preparation, non-destructive measurement, and easy handling.