Since the early to mid-1940’s, scientists using infrared spectroscopy have been trying to obtain spectral data from ever smaller samples. Starting with micro-KBr pellets and then beam condensers and micro-MIR (multiple internal reflectance) accessories, the ability to obtain a spectrum from an extremely small sample has been attempted on a regular basis.
The introduction of FTIR spectrometers in the mid-1960’s stimulated the interest in infrared microscopy due to the increased throughput of the interferometer and the ability to perform multiple scans to increase the signal-to-noise ratio. Infrared microscope systems were initially introduced in the late 1970’s and early 1980’s by Digilab and Spectra-Tech. The IRPlan was introduced in 1983 by Spectra-Tech as an FTIR microscope accessory that could be interfaced with instruments from various manufacturers. With the ability to collect infrared spectra from samples as small as 15 μm in either transmission or reflection mode, the infrared microscope accessory became the preferred tool for the collection of infrared spectra from sample areas of 250 μm or less.
There are numerous applications for FTIR microscopy, including polymer and fiber analysis, pharmaceutical and materials contamination, forensics, semiconductors, biochemistry, and chemical analysis applications. For almost any sample that can be scanned with a traditional infrared method, the FTIR microscope can be used to collect an infrared spectrum from a fragment of a sample. The spatial resolution of an infrared microscope is primarily determined by the diffraction limit of the infrared wavelengths used for analysis, typically 12 – 20 μm. The aperture system in the microscope is used to control the spatial dimensions of the sample beam, and this aperture cannot be smaller than the infrared wavelengths to be focused onto the sample (this is defined by the Abbe diffraction limit).
The FTIR microscope
FTIR microscopes are a compromise between the desire to visually image the sample, much like a refractive microscope using glass objectives, but with the ability to transmit and reflect infrared radiation. Since mid-infrared wavelengths do not transmit through glass, cassegrain reflection optics are used to focus the infrared source energy small enough to illuminate the microscope sample while minimizing losses of infrared energy. Infrared and visual beam paths are combined using optical components within the body of the microscope so that a visual image of the sample can be used for focusing and positioning of the sample and the infrared beam can be used for measuring the sample spectrum. Cassegrains are reflective optics with primary and secondary mirrors. A Cassegrain objective provides an extremely small focal plane with minimal distortion of the source energy.
The sample is imaged through the cassegrain objective, brought into a visual focus that matches the infrared focal point, and then the sample data are collected using the infrared optical system. Dual cassegrain objectives are required to focus and then defocus the infrared beam for transmission measurements using the rest of the microscope optical train to conduct the source energy to the sample and onto the integrated detector. An aperture is used to exclude portions of the sample outside of the measurement area. The aperture is positioned in the plane of the infrared beam, and the aperture is used to define the area for measurement of both the background and sample spectra. The combination of an FTIR microscope with an FTIR spectrometer can provide a powerful analysis method for samples that cannot generally be examined using traditional methods of analysis. Traditionally, FTIR microscopes have been large instruments interfaced using an external beam from the FTIR spectrometer. This arrangement requires additional optics, a dedicated detector, and extended bench space. While useful, these systems can be expensive compared to traditional sampling accessories that fit in the standard instrument sample compartment such as diffuse reflectance or attenuated total reflectance (ATR).
The IRT-1000 (Figure 1)is an FTIR microscope accessory that fits into the instrument sample compartment of the FT/IR-4000 or FT/IR-6000 series instruments. The IRT-1000 includes a manual stage, CCD video camera, and LCD video screen in a body which easily interchanges with other accessories and weighs no more than 15 lb. It also makes use of the standard spectrometer detector(s) for measurement. The microscope can acquire sample data using either the standard deuterated triglycine sulfate (DLaTGS) or an optional liquid N2 cooled (MCT) detector. The microscope accessory can be used to measure spectral data in the range 15,000 to 250 cm –1 , depending on the instrument configuration.
Using the IRT-1000, samples as small as 20 μm can be easily observed, and spectral data can be measured using either transmission or reflection mode. The addition of an ATR objective, such as diamond, germanium (Ge), or zinc selenide (ZnSe), allows measurement of ATR spectra from samples, minimizing the requirement for extensive sample preparation.
The IRT-1000 is integrated with the instrument power supply and Spectra Manager. For easy operation the IRT-1000 can be controlled using a front panel for the collection of either background or sample spectra, with additional functions to set the size of the sample aperture or Aperture Through Observation System (ATOS). This sample aperture system, which is illuminated by an integrated red LED, can be set to as small as 20 × 20 μm or as large as 800 × 800 μm.
Figure 2 illustrates a spectrum of a simple nylon fiber that was collected using the IRT-1000 microscope. The fiber was flattened and mounted across an open aperture on the sample stage using the standard sample holder. The sample fiber was observed using the visible system and the ATOS aperture set to exclude areas other than the fiber, the fiber was moved away from the aperture, and a background of an open area was collected. The fiber was then moved back into the aperture area, and the sample spectrum of the fiber was collected.
Figure 3 demonstrates a comparison between the MCT and DLaTGS detector performance for a polyester fiber sample. The sample was flattened and mounted over an open aperture on the standard sample holder. The ATOS aperture was set to 35 × 80 μm and the data were collected with both detectors. As illustrated in the figure, the DLaTGS detector is capable of providing similar quality data for the same sample, but does require a greater number of scans, and time, to obtain the same quality data as the MCT detector.
For samples that contain functional groups that absorb outside the range of a standard MCT detector, the DLaTGS detector can provide an advantage for these measurements given that the DLaTGS detector has a greater spectral range down to 400 cm –1 . Figure 4 illustrates a comparison between the spectra of a thin film sample of poly(tetrafluoro-ethylene) (PTFE) sample collected using the two different detectors. The DLaTGS detector was able to obtain the peaks associated with the C – F bending modes below 550 cm –1 , while the MCT detector spectral range is limited to approx. 600 cm –1 . While it is possible to specify an MCT detector with a greater spectral range, there is a sacrifice in sensitivity performance with this detector and often it is better to use a mid-band or narrow-band MCT even with the restricted spectral range.
Figure 5 demonstrates the spectra collected from a cross-section of a polymer laminate sample. The spectrum of the polypropylene layer was measured using the ATOS aperture set to 20 × 700 μm; the nylon layer was larger and the aperture was set to 50 × 700 μm for the spectrum collection. The DLaTGS detector was used to collect the spectra of the polymer layers. The spectra demonstrate the ability of the ATOS aperture to exclude the portions of the nylon layers that are on either side of the polyethylene layer in the laminate sample.
When the FTIR is configured with a CsI beamsplitter, the spectral range of the spectrometer can be extended into the far-IR region approaching 200 cm –1 . Figure 6 illustrates the spectrum of a germanium oxide crystal that has several absorption bands below 600 cm –1 . The GeO2 crystal was supported on a polyethylene sheet, the band at 725 cm –1 due to the polyethylene. The data were collected with a standard DLaTGS detector, which can obtain data from organometallic compounds, well beyond the range of even a wide-band MCT detector typically used in standard FTIR microscope systems.
To obtain reflectance spectra, the upper cassegrain is used; infrared source energy is directed through one- half of the cassegrain, and the other half used to collect the reflected energy and direct it onto the detector. The sample aperture is set as required for the sample, a background spectrum collected from a reference mirror, and the sample area collected as a sample spectrum and ratioed versus the background spectrum of the reference mirror. In Figure 7, shows a reflectance spectrum of a soda can lining; the spectrum was collected with an MCT detector with the aperture set to 100 × 100 μm. The results of a library search provided a spectral match to a two-part epoxy compound.
The use an ATR objective allows spectral measurement with minimal sample preparation. Figure 8 demonstrates a spectrum of a polymer laminate sample collected using a ZnSe ATR objective and MCT detector. The sample contact area for the ATR objective was 250 μm; the poly(ethylene terephthalate) sample was much larger than the active area of the ATR objective.
The IRT-1000 is an FTIR microscope that can be easily added to any new or existing JASCO FTIR spectrometer, it fits directly into the sample compartment and requires no complicated alignment. It can be used with either the standard DLaTGS detector or an optional liquid N2 cooled MCT detector, for the measurement of transmittance, reflectance, or ATR spectra. The IRT-1000 offers excellent functionality without compromise in performance.