The use of synchrotron sources for infrared (IR) spectromicroscopy provides greatly increased brightness that enables high-quality IR measurements at diffraction-limited spatial resolutions. This capability permits synchrotron-based IR spectromicroscopy to be applied to biological applications at spatial resolutions on the order of the size of a single mammalian cell. The question then arises, "Does the intense synchrotron beam harm biological samples?" Mid-IR photons are too low in energy to break bonds directly; however, they could cause damage to biological molecules due to heating. In this work, we present measurements that show negligible sample heating effects from a diffraction-limited synchrotron IR source. The sample used is fully hydrated lipid bilayers composed of dipalmitoylphosphatidylcholine (DPPC), which undergoes a phase transition from a gel into a liquid-crystalline state at about 315 K during heating. Several IR-active vibrational modes clearly shift in frequency when the sample passes through the phase transition. We calibrate and then use these shifting vibrational modes as an in situ temperature sensor.
Michael C. Martin, Nelly M. Tsvetkova, John H. Crowe, and Wayne R. McKinney, "Negligible Sample Heating from Synchrotron Infrared Beam," Appl. Spectrosc. 55, 111-113 (2001)