In a nutshell: What is ATR infrared spectroscopy?
Infrared spectroscopy uses infrared light (heat radiation) to identify a large number of chemical substances like fluids, solids or gases. ATR stands for attenuated total reflection and is the method of choice for almost all routine analyses of fluids and solids.
In ATR infrared spectroscopy, an infrared light is pointed from below on a crystal, an infrared transparent prism. The light is almost totally reflected on the top surface, but a small part of the energy gets absorbed by the interaction with the molecules in the sample.
This loss of energy is measured and results in a spectrum which contains information about the sample.