Photothermal infrared spectroscopy (PTIR) used on the Mirage IR microscope is a result of over a decade of expertise in photothermal physics that Anasys Instruments and its collaborators have built up since starting research on their AFM-based nanoscale IR spectroscopy platform.
Sub-micron IR spectroscopy
PTIR overcomes the IR diffraction limit by combining a mid-IR pulsed, tunable laser that heats the sample. When the IR laser is at a wavelength that excites a molecular vibration in the sample, absorption occurs, thereby creating photothermal effects including photothermal expansion. A visible probe laser, focused to 0.5 µm spot size, measures the photothermal response via the scattered light, as shown in the PTIR technique illustration.
The component of the reflected visible laser signal that is modulated at the IR pump laser repetition rate is directly proportional to the absorption coefficient of the sample at that wavenumber. The IR pump laser can be tuned through the entire fingerprint region in one second or less, to obtain an IR spectrum.
Spectra 0.5um spacing showing different polymer materials in a multilayer film
Transmission FTIR quality in reflection mode
Due to its unique operating principle, PTIR can be used in both transmission and reflection mode. However its principle operating mode is in a reflection mode which eliminates several longstanding limitations for IR microscopy leading to substantial benefits for the IR community, including minimizing sample preparation and enabling sub-micron spectroscopy. PTIR has consistently shown transmission quality spectra in reflection mode across a wide range of sample types. The sub-micron resolution is demonstrated in Figure 1 showing reflection mode spectra on a multi-layer packaging film measured 0.5µm apart with highly differentiated chemical fingerprints indicating different materials.
Correlates to bulk FTIR databases
PTIR measurements of common polymeric materials have shown excellent correlation between PTIR and bulk FTIR spectra. Figure 2 shows excellent correlation for polystyrene (PS), polyethylene terephthalate (PET) and polymethyl methacrylate (PMMA) with high correlation to spectra from the Know-it-all® database.
PTIR measurements shown in Figure 2 were made on samples that were over 20-µm thick in reflection mode, yet the strongest bands show no evidence of saturation. This is because the reflected signal is sampling only the top couple microns of the sample, making the depth of penetration comparable to what is achieved using ATR accessories, but without the optical band-shape distortions present in many ATR spectra.
Three different spectra from mirage searched against the database with high matches for PS (Top) PET (middle) and PMMA (bottom)
High resolution single wavelength imaging
Due to the wavelength tunability and high spectral resolution of Mirage, sub-micron chemical images can be created at specific wavelengths providing more accurate chemical composition of the surface at discrete wavenumbers.
Ease of use and minimizing sample preparation
PTIR is an optical, non-contact based approach, hence is fast and easy to use, while maintaining transmission quality spectra. Because of its high quality spectra in reflection mode it enables IR measurement on thick samples and eliminates the need for thin samples in many sample types. This leads to dramatically easier sample preparation, improved ease of use and faster turnaround times.
Next generation infrared spectroscopy
PTIR eliminates several longstanding limitations for IR microscopy enabling sub-micron IR spectroscopy and minimizing sample preparation, PTIR is a unique technique that provides a huge step forward for the IR spectroscopy community.
High resolution, single wavelength chemical imaging with 0.5µm spatial resolution
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