FTIR Sample Preparation for the 21st Century: ATR

The holy trinity of FTIR sample analyses is speed, accuracy, and cost. Ideally an analysis will be carried out quickly, accurately, and as fast as possible. Sample preparation has long been the Achilles' heal of FTIR, too frequently involving long, tedious, manual operations. For example, even in the hands of a skilled analyst it can sometimes take over 1 hour to prepare a KBr pellet. This is unacceptable in a 21st century lab where time is money and speed is of the essence. Fortunately, there exists a sample preparation technique that is up to the challenge of giving us accurate, fast, and inexpensive analyses, it is called Attenuated Total Reflectance (ATR). This blog post will be the first in a series exploring how ATR works, what applications it is suited for, and why it is such an advantageous technique.

In the ATR technique the infrared beam is brought to a focus on the face of a prism-shaped crystal made of a material that is infrared transparent and has a high refractive index. Common examples of ATR crystals include diamond, Zinc Selenide (ZnSe), and Germanium. The infrared light is refracted by the crystal and travels towards the top surface of the crystal as illustrated above. The magic ocurrs when the infrared beam reaches the top surface of the crystal. Because of a phenonenon too complicated to explain, here a small portion of the infrared beam sticks up above the surface of the crystal. I call this region of space a "hot spot" but it is more properly called the "evanescent wave". Infrared spectra are obtained by bringing samples into contact with the evanescent wave so they can absorb some of the infrared radiation. More to follow...

For more on the topic of ATR you can consult my book Fundamentals of FTIR, available for purchase here: FTIR Books , or take my Hands-On FTIR Sample Preparation Course as outlined here: FTIR Sample Prep. Course Outline.