1 Nov 2021

Introduction to ATR-FTIR spectroscopy, part 1: the basics

This is the first in a series of short technical notes written by our Applications Scientist, Andrew Davies, on the subject of Attenuated Total Reflectance, or ATR. In this first part, we’ll learn the basics of ATR spectroscopy. In the second, we’ll learn how to calculate the effective depth of penetration for ATR, and in the third, we’ll discuss some of the practical implications of different ATR puck choices for the Quest ATR accessory.
 

Introduction

ATR has become the dominant method for the collection of an FTIR spectrum of solid materials. It has little to no sample preparation and is effective at both qualitative and quantitative measurements. Compared to the older transmission methods (such as making KBr pellets) important differences emerge. An ATR spectrum is not directly comparable to a transmission spectrum, although software algorithms are available in most commercial spectrometers capable of converting an ATR spectrum into a transmission-like spectrum should the analyst wish to compare them.

This note outlines the basics of the ATR method, providing the analyst with the information they require to gain the most out of the technique. 

How does ATR work?


The basic theory of ATR is straightforward. An IR beam is reflected off the surface of a crystal from the underside and interacts with a sample of lower refractive index placed onto the surface of the crystal. Recording the intensity of the light reflected before and after placing a sample on the crystal surface produces the characteristic infrared spectrum of your sample. The basic setup is shown in Figure 1.

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Figure 1: Graphical representation of a single bounce ATR


In order to ensure that the beam of light is reflected rather than transmitted through the surface of the ATR crystal the angle of incidence must be above the critical angle, which varies depending on the refractive index of the crystal material. The Specac QuestTM has a fixed angle of incidence of 45° which is above the critical angle for all the ATR crystals offered with it, including diamond, zinc selenide, and germanium.

Penetration Depth


It is important to state that the IR beam does not exit the surface of the crystal at the crystal/sample interface. Instead, the electric component of the light propagates into the rarer sample medium. This is known as called an evanescent wave, and its intensity decreases exponentially as it propagates into the sample. As a result, the penetration depth of ATR spectroscopy is very short, the exact depth depending on several factors including the sample type and the wavelength of the light. 

Higher frequencies have a smaller penetration depth, so one difference between a transmission and ATR spectrum is that the relative peak intensities at higher wavenumbers are reduced relative to the lower frequency end of the spectrum as illustrated in Figure 2. 

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Figure 2: Comparison of an ATR and transmission spectra of paracetamol. The two spectra have been scaled such that the peak at 1230 cm-1 (indicated by the red arrow) is of equivalent scaling between transmission (left y axis) and ATR (right y axis).

Crystal Choice


A range of materials are available to choose from for your ATR crystal. The best one to choose depends on your application. ZnSe is suitable for day-to-day applications, however care should be taken to avoid harder samples and point loads as these can cause the crystal to shatter. It is also critical to ensure the crystal is not exposed to acidic or strongly basic samples as these can cause the formation of toxic fumes. Ge is suitable for materials with a high refractive index and for surface studies owing to its smaller penetration depth.
 
Diamond is the real workhorse of ATR spectroscopy. It is extremely hard wearing and is virtually indestructible. Other commercial ATR accessories often employ a laminated diamond, which can delaminate over repeated use. To prevent this the QuestTM and Golden Gate both use a monolithic diamond crystal ensuring your ATR can last for years. Two different options are available – a standard puck with anti-reflective coating to improve the signal-to-noise ratio in the mid-IR, and an uncoated extended range diamond that extends into the far-IR.
 
For more information on this topic please contact sales@specac.co.uk, who will forward your enquiry to our technical team.