attenuated total reflectance ( ATR )
an attenuated total reflection accessory operates by measuring the changes that occur in a totally internally reflected infrared beam when the beam comes into contact with a sample ( indicated in figure 2) an infrared beam is directed onto an optically dense crystal with a high refractive index at a certain angle this internal reflectance creates an vanescent wave that extends beyond the sueface of the crystal into the sample held in contact with the crystal this evanescent wave is like a bubble of infrared that sits on the surface of the crystal this evanescent wave protrudes only a few microns ( 0.5 -5 ) beyond the crystal surface and into the sample. consequently there must be good contact between the sample and the crystal surface