A quick search on the internet using “universal sampling accessory for FT-IR” will return a number of hits, mostly from instrument vendors, indicating that an attenuated total reflection (ATR) accessory will fit the bill. Coupling this accessory with a diamond ATR element will provide a system that will allow the user to analyze any condensed phase sample. But is that true?

To determine if that statement is true, we need to look at a number of factors. In the early days of infrared (IR) spectroscopy with dispersive instruments, the beam geometry and the way the instrument collected data made it difficult to obtain spectra from any configuration except transmission. This meant that the scientists who collected ATR data were experts in the field and understood all the ramifications of each technique.^1-3

With the advent of Fourier transform infrared (FT-IR) instrumentation, the energy and beam geometry constraints disappeared, and the ATR sampling approach became very commonplace. In Applied Spectroscopy in 13 years between 1960 and 1973, there were about 40 references published discussing applications and aspects of ATR. However, between 1973 and 1993, after the introduction and the growth of FT-IR spectroscopy, the number of references in Applied Spectroscopy was over 300, and the ATR technique became more accessible to many more analysts in the laboratory. With the introduction of a diamond ATR element by Specac in the early 1990s, applications took off with almost 1000 articles over the next 20 years, and these statistics were from the academic community, not the practicing analyst in a laboratory.

However, the growth of the technique and the ease of use came at a price. The fundamental principles that impacted the applications, which have been well understood by the researchers was lost.^4,5 Since ignoring the fundamental physics of the technique results in a distorted and useless spectrum, it is well worth it to revisit the basics.

What are the basics?

Complex Refractive Index, anomalous dispersion and the critical angle

The complex refractive index describes how light propagates through a medium. Simply put:

N(λ) = n(λ) + ik(λ) [1]

N is the complex refractive index that has a real portion, that describes how the light slows down and bends when it enters another medium, and the imaginary portion, k, which is the absorptivity coefficient describing how the light is attenuated through the sample.

We know from simple spectroscopy that k varies with wavelength and produces the absorption spectrum and also varies with wavelength but show little change unless it encounters a region of absorbance. When that happens, there is a large bisignate change in the refractive index. This change is classified as anomalous dispersion.^5,6 Figure 1 illustrates the changes in both quantities over the same wavelength range.

These 2 quantities come into play in spectral sampling depending on the characteristics of the sample.

Optical Sampling Geometries

In a transmission measurement, as shown in Figure 2, the light hits the sample perpendicular to the sample surface and passes through it to be detected. The spectrum that results from this sampling geometry has the least amount of distortion.

When the sampling is in the reflection geometry, as shown in Figure 3, then the spectrum is distorted by the effect of anomalous dispersion.

ATR sampling is a multifaceted optical technique, which involves the optical geometry of the system and the effect of the complex refractive index on the absorption of light. ATR uses an optically transparent (to mid-infrared radiation) material called the ATR element in contact with the sample. The ATR element has a refractive index that is higher than the sample of interest and at the interface between the sample and the ATR element, the light is reflected into the ATR element. But a very small portion of the light does propagate into the sample is absorbed and produces an ATR spectrum.

The reflection behavior is described by Snell’s law.⁵

η₁ sin θ₁ = η₂ sin θ₂ [2]

where η₁ and η₂ are the respective refractive indices of the sample and the ATR element. The angles θ₁ and θ₂ are the angle of incidence and angle of refraction, respectively. Note, that for the combination of ATR element and sample, there is an angle θ, the critical angle, where total internal reflection occurs.

If the critical angle is violated, there is no reflection behavior, the evanescent wave does not propagate into the sample properly, and the spectrum starts to become a composite of the two components of the complex refractive index.

The impact of the wavelength dependence of the absorptivity coefficient and the real refractive index is important because distortions in ATR experiments result if the physics of the situation is ignored.First, the refractive index of the ATR element must be higher than the sample. Second, the angle of incidence of the light from the ATR element into the sample must be greater than the critical angle. If either of these are disregarded, the ATR analysis will produce unusable spectra that are distorted in band shape, band position and intensity. The result is a spectrum that is not useable for qualitative or quantitative analysis.

Many publications exist on the theory of the ATR experiment ^{4, 5, 7, 8} and the authors would encourage the reader to examine them, but these theoretical references neglect to provide examples of the practical distortions and some “rules” to decide which ATR element and what angle of incidence is the best approach. One reference that does provide some context recently appeared in Applied Spectroscopy Practica.⁹

A Pictorial View of the Problem

Figures 1, 2, and 3 allow us to visualize the behavior of the spectrum from each component of the complex refractive index. A close look at the spectrum of the same sample shows the bisignate behavior of the strong bands in the reflection spectrum.

Because the diamond ATR is most commonly used as the “universal accessory” we used an in-compartment ATR accessory (the GladiATR from Pike Technologies, Madison, Wisconsin). This accessory has a fixed angle of incidence of 45°.

Isopropanol

The first sample we will look at is isopropanol. The refractive index of isopropanol is 1.35, so with a 45° diamond ATR, the critical angle is calculated as 34°, which is well below the angle of the accessory. A comparison of a capillary film of isopropanol with the diamond ATR spectrum is shown in Figure 4. The top trace is in transmission as a capillary film, and the bottom is the ATR spectrum. The strength of the CH stretches from 2800 to 3000 cm^-1 are different in the two spectra because of the wavelength dependent depth of penetration, but no other distortions show up in the spectra.

Dibenzoyl methane

1,3-Diphenyl-1,3-propanedione, commonly known as Dibenzoyl methane (DBM), is a UV stabilizer used in polymers. The refractive index of this material is 1.6. Calculating the critical angle, we come up with 42.5°, which is very close to the angle of incidence in the ATR element. When we examine the spectrum shown in Figure 5, we can clearly see that the ATR spectrum (on the bottom) is starting to show distortions compared to the transmission spectrum of a cast film on the top. Because the ATR element is a diamond, the most obvious distortion is in the diamond phonon band.

This spectrum may not be totally useless, but using a germanium ATR element would have provided a much better spectrum since the critical angle for the germanium ATR element is 24°.

Phosphorous Pentoxide

Phosphorous pentoxide is a powerful dehydrating agent typically used in chemical synthesis. The refractive index of this material is 2.0, and when one calculates the critical angle for the diamond ATR accessory, we find that it is 57.8°. This is obviously greater than the geometry of the accessory, and when we collect the spectrum of that material in the diamond ATR accessory, what results is a grossly distorted spectrum as shown in Figure 6.

Here, the spectrum is so distorted that it is useless. The diamond phonon band is one of the dominant features in this spectrum, and the characteristic absorbance bands of the compound shown in the thumbnail are unrecognizable.

Solving the problem

As the information above shows, there is no “universal” sampling accessory, but the accessories can have wider applicability with the appropriate selection of ATR elements. Table 1 shows the most common ATR elements. As one can see from the data, zinc selenide will have the same critical angle problems as diamond with a higher refractive index sample, but germanium, with its much higher refractive index, can address a wider range of samples.

In Table 2, we can compare the critical angle for the three samples of interest in an accessory with a 45° angle of incidence.

In all three cases, the germanium would have provided a useable spectrum. Germanium has some disadvantages though. It is very brittle and temperature-sensitive, and the long wavelength cut-off (850 cm^-1) hides characteristic bands in the fingerprint region. In Figure 7, we see the spectrum of P₂O₅ using a germanium ATR element where we now see the undistorted characteristic absorbances.

As a rule of thumb to help determine if a spectrum will be useable examining the difference in refractive index of the sample and the ATR element will provide an estimate of the usability of the spectrum, as shown in Table 3.

Conclusion

In FT-IR spectroscopy, a universal sampling accessory does not exist. ATR is effective for many organic samples, but there are exceptions that are important from an industrial standpoint. Having the ability to choose between a diamond or germanium broadens the applicability of a single reflection ATR considerably, but there is nothing that will work for all samples.

References

(1) Polchlopek, S. E. Attenuated Total Reflectance Effects with Different Prisms. Appl. Spectrosc. 1963, 17 (5), 112-114. DOI: 10.1366/000370263789620962.

(2) Hirschfeld, T. High-Sensitivity Attenuated Total-Reflection Spectroscopy. Appl. Spectrosc. 1966, 20 (5), 336-338. DOI: 10.1366/000370266774385822.

(3) Sherman, B. Infrared Spectroscopy by Attenuated Total Reflection. Applied Spectroscopy 1964, 18 (1), 7-9. DOI: 10.1366/000370264789620763.

(4) Milosevic, M. Internal Reflection and ATR Spectroscopy; John Wiley & Sons, Inc., 2012. DOI: 10.1002/9781118309742.fmatter.

(5) Harrick, N. J. Internal Reflection Spectroscopy; Wiley Interscience, 1967.

(6) Born, M.; Wolf, E. Principles of Optics: Electromagnetic Theory of Propagation, Interference and Diffraction of Light; Pergamon Press, 1964.

(7) Ramer, G.; Lendl, B. Attenuated Total Reflection Fourier Transform Infrared Spectroscopy. In Encyclopedia of Analytical Chemistry: Applications, Theory and Instrumentation, 2006.

(8) Mirabella, F. M. Principles, Theory and Practice of Internal Reflection Spectroscopy. In Handbook of Vibrational Spectroscopy, 2001.

(9) Miseo, E. V.; Larkin, P. J. Fourier Transform Infrared Spectroscopy (FT-IR) Diamond Attenuated Total Reflection (ATR) Measurements: The Good, the Bad, and the (Really) Ugly. Appl. Spectrosc. Practica 2025, 3 (2), 27551857251336262. DOI: 10.1177/27551857251336262.