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Optical photothermal infrared (O-PTIR) spectroscopy is a method used in the Ault lab for single particle analysis. The mIRage infrared + Raman microscope (Photothermal Spectroscopy Corp.) consists of a visible objective (4×, 0.13 numerical aperture, 17.3 mm working distance, Nikon Plan Fluor), a Cassegrain reflective objective for simultaneous use of IR and visible lasers (40×, 0.78 numerical aperture, 8.3 mm working distance, 55 μm × 42 μm field of view), and the same QCL laser (880 to 1950 cm–1) and OPO laser (2700–3600 cm–1). This instrument was recently developed and used to characterize submicron aerosol particles without contacting the sample
The recent development of O-PTIR enables analysis of particles with the same spatial resolution as that of Raman microspectroscopy since the probe laser is the same (532 nm). PTIR and Raman spectra can also be collected simultaneously from the same particle. This provides information based on both the dipole moment (IR spectroscopy) and polarizability (Raman spectroscopy). An added benefit of O-PTIR is that it is not influenced by fluorescence, a common challenge for Raman spectroscopy of complex organic species using a 532 nm laser.
Fankhauser, A. M.P; Lei, Z..G; Daley, K. R.P; Xiao, Y.G; Zhang, Z.; Gold, A. Ault, B. S.; Surratt, J. D.; Ault, A. P.* Acidity-Dependent Atmospheric Organosulfate Structures and Spectra: Exploration of Protonation State Effects via Raman and Infrared Spectroscopy and Density Functional Theory, Journal of Physical Chemistry – A, 2022, 126, 35, 5974–5984.
Mirrielees, J. A.; Kirpes, R. M.; Haas, S.; Rauschenberg, C. D.; Matrai, P. A.; Remenapp, A.; Boschi, V.; Grannas, A. M.; Pratt, K. A.*; Ault, A. P.* Probing individual particles generated at the freshwater-seawater interface through combined Raman, photothermal infrared (PTIR), and x-ray spectroscopic characterization, ACS Measurement Au, 2022, 10.1021/acsmeasuresciau.2c00041.
The figure to the right compares the experimental spectra of 2-MTS standard by ATR-FTIR (green), O-PTIR (blue), and AFM-PTIR (pink).
Fankhauser, A. M.P; Lei, Z..G; Daley, K. R.P; Xiao, Y.G; Zhang, Z.; Gold, A. Ault, B. S.; Surratt, J. D.; Ault, A. P.* Acidity-Dependent Atmospheric Organosulfate Structures and Spectra: Exploration of Protonation State Effects via Raman and Infrared Spectroscopy and Density Functional Theory, Journal of Physical Chemistry – A, 2022, 126, 35, 5974–5984.
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