attenuated total reflectance
/ uh-TEN-yoo-ay-tid TOH-tul rih-FLEK-tents /
Attenuated total reflectance is a way to take an infrared spectrum just by pressing a sample against a crystal — no grinding, no dilution, no thin films to prepare. Picture light skipping along the inside of a clear crystal, bouncing off its surface again and again like a stone skimming water. At each bounce a whisper of light reaches just past the surface and can be nibbled by whatever is touching it.
More precisely, infrared light is sent into a crystal of high refractive index at an angle steep enough that it totally reflects internally. At each reflection a faint evanescent wave pokes a fraction of a micrometer beyond the crystal; a sample in close contact absorbs at its characteristic frequencies, and the reflected beam comes out slightly weakened — attenuated — there.
It matters because ATR turns infrared spectroscopy into a touch-and-read technique: clamp a powder, liquid, gel, or rubbery solid onto the crystal and scan, with almost no preparation. The honest caveat is that the light only probes a thin surface layer, so the spectrum reflects what is touching the crystal, and good contact is essential for a reliable result.
To check a paint chip, an investigator simply presses it onto the diamond ATR crystal of an FTIR and scans. No solvent or pellet is needed; the surface in contact with the crystal gives a clean infrared spectrum in seconds.
Pressing a sample onto the crystal gives an infrared spectrum with no preparation.
ATR is a sampling accessory for infrared spectroscopy, usually bolted onto an FTIR, not a separate technique. Because it probes only a shallow surface layer, an inhomogeneous sample may give a spectrum that differs from its bulk.