Ep 14 — Introduction to Sampling Techniques: ATR (Attenuated Total Reflectance) — The Most Common Modern Method

Series: Encyclopedia of Infrared Spectroscopy: From Principles to Practice
Chapter: Part 2 · Beginner Level — Entering the Laboratory
Target Audience: Undergraduate and graduate students, technicians new to the lab
Prerequisites: Ep 13 (Transmission Method)
Reading Time: Approximately 38 minutes

Introduction: A 'Peeping' Invention

In 1959, physical chemist Johannes Fahrenfort at the Royal Dutch Shell laboratory discovered a peculiar phenomenon while studying interfacial phenomena: when light travels from a high-refractive-index medium to a low-refractive-index medium, and the incident angle exceeds a certain 'critical angle,' total internal reflection occurs—but the reflection is not 'perfectly clean'; a portion of the energy 'smuggles' into the low-refractive-index medium as an evanescent wave, where it is absorbed by molecules [1][2].

Fahrenfort keenly realized: isn't this an excellent infrared sampling method? Simply place the sample on the crystal surface, allow the infrared light to undergo total internal reflection within the crystal, and the evanescent wave will 'peep' at the molecular information of the sample—no sample preparation, no pellet pressing, no liquid cell needed [1][2].

In 1961, Fahrenfort published a paper in Spectrochimica Acta formally proposing the ATR (Attenuated Total Reflectance) spectroscopic method [1]. Subsequently, Paul Wilks in New York and Harrick Scientific commercialized ATR, gradually bringing it into laboratories [2].

However, early ATR had a fatal flaw: the crystal materials were too fragile. Materials like KBr-5 and ZnSe were soft, easily scratched, and not corrosion-resistant. It wasn't until the late 1990s that the advent of diamond ATR completely changed the situation—diamond has the highest hardness, chemical inertness, and a wide spectral range, making ATR truly a 'place-and-measure' universal tool [3][4].

Today, over 90% of newly sold FTIR instruments worldwide are equipped with ATR accessories as standard [3][4]. From pharmaceutical QC to on-site forensic analysis, from food adulteration to plastic identification, ATR is almost ubiquitous. In this episode, we will delve into the principles, techniques, and practice of this 'standard modern infrared accessory.'

1. Physical Principles of ATR

1.1 Refraction, Reflection, and Total Internal Reflection

To understand ATR, let us first review the phenomena of refraction and reflection of light [5][6]:

Snell's Law (law of refraction): when light passes from medium 1 (refractive index $n_1$) to medium 2 (refractive index $n_2$):

$$n_1 \sin\theta_1 = n_2 \sin\theta_2$$

where $\theta_1$ is the angle of incidence and $\theta_2$ is the angle of refraction [5][6].

Two scenarios [5][6]:

Critical Angle: in internal reflection, the corresponding angle of incidence when $\theta_2 = 90°$ (the refracted ray propagates along the interface) [5][6]:

$$\theta_c = \sin^{-1}\left(\frac{n_2}{n_1}\right)$$

Total Internal Reflection (TIR): when the angle of incidence $\theta > \theta_c$, the light no longer enters medium 2 but is completely reflected back into medium 1 [5][6].

                Medium 1 (high refractive index n₁, ATR crystal)
                          ↘  θ₁
        Incident light ────────────→ ╲│
                             ────── Interface
        Reflected light ←───────────╱│  θ₂
                          ↗  
                Medium 2 (low refractive index n₂, sample)

   When θ₁ > θc → Total Internal Reflection (TIR) → Basis of ATR

📷 Figure 1: Schematic of total internal reflection
Source: Harrick Scientific Application Note [2]
https://harricksci.com/applic…

1.2 Evanescent Wave

Total internal reflection does not mean 'no light enters medium 2.' The rigorous solution of Maxwell's equations shows that the electromagnetic field penetrates the interface into medium 2, but its intensity decays exponentially with depth—this is the evanescent wave [2][5][6][7].

The electric field intensity of the evanescent wave decays with depth $z$ as [2][6][7]:

$$E(z) = E_0 \cdot \exp\left(-\frac{z}{d_p}\right)$$

where $d_p$ is the penetration depth, defined as the depth at which the electric field intensity decays to 1/e (about 37%) of its surface value [2][6][7].

1.3 Penetration Depth Formula

The classic literature from Harrick Scientific gives the formula for penetration depth [2][6][7]:

$$d_p = \frac{\lambda}{2\pi n_1 \sqrt{\sin^2\theta - \left(\frac{n_2}{n_1}\right)^2}}$$

where:

• $\lambda$: wavelength of infrared light (µm)
• $n_1$: refractive index of the ATR crystal
• $n_2$: refractive index of the sample
• $\theta$: angle of incidence

Typical values [2][6][7]:

• Mid-infrared region (4000–400 cm⁻¹, i.e., 2.5–25 µm)
• Diamond ATR ($n_1$ = 2.4), sample $n_2$ ≈ 1.5, angle of incidence 45°
• Penetration depth $d_p$ ≈ 0.5–5 µm (increases linearly with wavelength)

"The evanescent wave typically penetrates less than 5 µm into the sample."
—— Shimadzu ATR teaching note [6]

💡 Key insight: The penetration depth is proportional to wavelength, meaning that peaks in the long-wavelength (low wavenumber) region are relatively stronger than those in the short-wavelength (high wavenumber) region — this is the root cause of the morphological differences between ATR and transmission spectra (see Section 6 for details).

1.4 Signal Generation in ATR

When the sample absorbs infrared light at specific frequencies within the evanescent wave range [2][7]:

1. The evanescent wave energy is absorbed by sample molecules (vibrational transitions)
2. The intensity of the totally internally reflected light attenuates at that frequency
3. After multiple reflections, the attenuation of the exiting light accumulates, forming a detectable absorption spectrum

This is the origin of 'attenuated total reflectance'—the intensity of the totally reflected light is attenuated by sample absorption [2][7].

      ATR crystal
    ┌─────────────┐
    │  ╲    ╲    ╲│  ← multiple total internal reflections
    │   ╲    ╲    │
    │    ╲    ╲   │  ← evanescent wave at each reflection
    └────┴────┴───┘      penetrating sample surface
       ↑    ↑    ↑
      Sample placed on crystal bottom surface

2. ATR Crystal Materials

2.1 Why Are High Refractive Index Crystals Needed?

From the critical angle formula $\theta_c = \sin^{-1}(n_2/n_1)$, it can be seen [2][5][6]:

• The larger $n_1$ (crystal refractive index) → the smaller the critical angle → the easier to satisfy total internal reflection conditions
• Most organic compounds have $n_2$ ≈ 1.4–1.6
• Therefore, ATR crystals require $n_1$ ≥ 2.0 to operate reliably [2][6]

2.2 Comparison of Four Major ATR Crystals

The following table compares the four most commonly used crystal materials in modern ATR instruments [3][4][8][9]:

Table 1: Comparison of mainstream ATR crystal materials (data sources: Bruker [3]; Specac [4]; Harrick [8]; art photonics [9])

2.3 Diamond ATR: The "Standard" in Modern Labs

Why has diamond become the first choice for modern ATR? Bruker's technical documentation provides a clear answer [3]:

"Diamond is a great universal material as it works well for almost any sample. Diamond is extremely hard and chemically inert, so it is very resistant to chemical and physical damage."
—— Bruker ATR Technical Note [3]

Five key advantages of diamond ATR [3][4][8]:

① Extreme hardness: Mohs hardness 10, Knoop hardness 10000, the hardest among all crystal materials. It can withstand the pressure of high-pressure accessories, suitable for hard powders, minerals, ceramics, and other difficult-to-measure samples [3][8].

② Chemical inertness: Resistant to all acids and bases (pH 1–14), does not react with any common chemical reagents [3][4].

③ Wide spectral range: Transmission range 40000–200 cm⁻¹, covering mid-IR and far-IR [4].

④ Moderate refractive index: $n_1$ = 2.4, sufficient index difference with most organics ($n_2$ ≈ 1.5), critical angle approximately 39°, leaving margin [4][8].

⑤ Non-toxic and safe: Contains no heavy metals (unlike KRS-5 containing thallium, ZnSe containing selenium) [4].

Limitations of diamond ATR [3][4][8]:

• High cost: Single crystal diamond ATR crystal costs 5–10 times that of ZnSe.
• 2700–1800 cm⁻¹ absorption band: Diamond has its own absorption in this region (C-C vibration overtones), which can interfere with sample signals [3][4].
• Unsuitable for high refractive index samples: Such as black carbon-filled rubber ($n_2$ close to 2.0), requiring a Ge crystal instead [3][8].

⚠️ Note: Diamond ATR exhibits an absorption band in the 2700–1800 cm⁻¹ region due to overtone vibrations of the diamond lattice. If target peaks fall in this region (e.g., C≡N at 2250 cm⁻¹, C≡C at 2100 cm⁻¹, Si-H at 2200 cm⁻¹), choose ZnSe or Ge crystals [3][4].

2.4 ZnSe: Cost-Effective Choice

ZnSe (zinc selenide) is the workhorse that popularized ATR—inexpensive with balanced performance [3][4][8]:

• Advantages: $n_1$ = 2.4 (same as diamond), transmission range 7800–550 cm⁻¹, no mid-IR intrinsic absorption, strongest signals [4].
• Disadvantages: Low hardness (Knoop 137), not resistant to strong acids/bases (pH limited to 5–9), prone to scratching [4][8].

Suitable for: Routine liquids, soft solids, teaching labs [4][8].

Avoid: Hard powders (can scratch the crystal), strong acid/base samples, samples containing ammonia/amines (can form complexes that corrode ZnSe) [4][8].

2.5 Ge: Surface Analysis Specialist

Ge (germanium) has the highest refractive index ($n_1$ = 4.0), offering two unique advantages [3][4][8]:

① Very shallow penetration depth: Approximately 0.2–0.5 μm (5–10 times shallower than diamond), ideal for surface analysis (e.g., coatings, surface modifications) [3][8].

② Suitable for high refractive index samples: Such as black rubber, polymers containing carbon fillers ($n_2$ close to 2.0) [3].

Limitations [4][8]:

• Brittle (Knoop 580, but poor toughness, prone to breakage).
• Not resistant to oxidizing acids (HNO₃, H₂SO₄+H₂O₂).
• Narrow transmission range (5500–600 cm⁻¹).
• Refractive index varies significantly with temperature (requires temperature control).

2.6 Si: Far Infrared and Special Applications

Si (silicon) characteristics [4][8][9]:

• Refractive index $n_1$ = 3.5 (between diamond and Ge).
• High hardness (Knoop 1150), second only to diamond.
• Special transmission range: 8000–1350 cm⁻¹ and 500–33 cm⁻¹ (the intermediate 1350–500 cm⁻¹ region has strong phonon band absorption) [4].
• Main uses: Far IR measurements (inorganics, coordination compounds, metal-organic compounds) [4][9].

2.7 Crystal Selection Decision

Specac application scientist Andrew Davies provides selection principles [4]:

"When choosing a crystal, it is important to consider chemical compatibility, the required spectral range, and the refractive index of your crystal (nc) and sample (ns)."
—— Specac ATR Technical Article [4]

Selection workflow [3][4][8]:

                  Unknown sample
                         │
              ┌──────────┼──────────┐
              │          │          │
         Routine     High RI     Far IR
         (most)      (black rubber, etc.) (< 400 cm⁻¹)
              │          │          │
         ┌────┴────┐     Ge         Si
         │         │
       Hard/corrosive  Soft/routine
         │         │
       Diamond     ZnSe
      (universal) (economical)

Figure 2: Decision tree for ATR crystal material selection (based on Specac [4] and Bruker [3])

3. Single Bounce vs Multi-Bounce ATR

3.1 Single Bounce ATR

Structure: Infrared light undergoes only one total internal reflection inside the crystal before exiting to the detector [6][8][10].

    Incident → ╲    ╱ → Exit
              ╲  ╱
               ╲╱  ← Single bounce point
              ─────
               Sample

Characteristics [6][8][10]:

• Weaker signal (only one reflection absorption).
• Small sample volume (small reflection point area, approximately 1–2 mm²).
• Ideal for limited sample amounts (e.g., forensic trace evidence, small drug particles).
• Modern diamond ATR designs are mostly single bounce.

3.2 Multi-Bounce ATR

Structure: Infrared light undergoes multiple total internal reflections (typically 5–15) inside the crystal, with each reflection contributing to absorption [6][8][10].

Incident light → ╲╲╲╲╲╲╲╲ → Emitted light
─────────
Sample
(multiple reflections, signal accumulation)
```

Effective path length formula [6]:

$$\text{EPL} = N \cdot d_p$$

where N is the number of reflections, and $d_p$ is the penetration depth per reflection. For Shimadzu's HATR cell (horizontal ATR), N = 10 [6].

Features [6][8][10]:

• Strong signal (multiple reflections accumulate absorption)
• Large sample volume (large crystal surface area, approx. 80×10 mm)
• Suitable for liquids, pastes, solutions and other cases requiring large sample area
• Classic designs: Horizontal ATR (HATR), Circular ATR (CIRCLE ATR)

3.3 Comparison of the Two Types

Table 2: Single reflection vs. Multiple reflection ATR comparison (data sources: Shimadzu [6]; Harrick [8])

IV. Sample Contact and Pressure Mechanism

4.1 Why is Contact So Critical?

The evanescent wave in ATR penetrates only 0.5–5 μm into the sample surface. Therefore, close contact between the sample and the crystal is key to successful ATR [3][6][8].

Shimadzu's teaching notes emphasize [6]:

"Good contact between the sample and the crystal must always be maintained since the evanescent wave only penetrates less than 5 µm into the sample."

If contact is poor, the evanescent wave will "pass through" the air gap ($n_{air}$ = 1.0, which differs greatly from the crystal $n_1$ = 2.4; critical angle approx. 25°), effectively "seeing" air instead of the sample—resulting in very weak spectral signals [6][8].

4.2 Pressure Mechanism

To ensure good contact, modern ATR accessories are equipped with pressure mechanisms [3][8][10]:

① Spring-loaded pressure (most common)

• Hand-tightened screw + spring
• Pressure approx. 10–20 N
• Suitable for most solid samples
• Inexpensive, simple design [3][8]

② Pneumatic pressure (high-end models)

• Compressed air drives a piston
• Adjustable pressure (10–100 N)
• Good pressure repeatability, suitable for quantitative analysis
• Used in Bruker Platinum ATR, Thermo iD7, etc. [3][10]

③ High-pressure accessories (special applications)

• Pressures up to 100–500 N
• Used for hard samples (minerals, ceramics, metal coatings)
• Only diamond crystals can withstand [3][8]

4.3 Contact Techniques for Different Samples

Solid powders [3][6][8]:

• Take 1–5 mg powder to cover the crystal center
• Apply pressure slowly to avoid powder scattering
• Pressure flattens the powder to fit the crystal surface
• Common issue: large particles → poor contact; should be ground first

Bulk solids (e.g., plastics, rubber, coatings) [3][8]:

• Cut a flat small piece (approx. 5×5 mm)
• Place the flat side down on the crystal
• Apply pressure to bring the surface into contact
• Common issue: rough surface → poor contact; should be sanded smooth

Liquids [3][6]:

• Drop 1–2 drops directly onto the crystal
• Liquid spreads naturally, no pressure needed
• Advantage: Best contact for liquids, usually highest SNR
• Note: Volatile liquids require fast measurement or a cover

Pastes/Gels [3][8]:

• Apply a small amount onto the crystal and spread
• Apply light pressure for even coverage
• Suitable for food, cosmetics, biological samples

💡 Pro tip: For samples that are difficult to bring into contact (e.g., fibers, film edges), place a drop of solvent (e.g., ethanol) on the crystal to let the sample "stick" to the crystal, then measure after the solvent evaporates [8].

4.4 Cleaning and Maintenance

Cleaning procedure after ATR measurements [3][8]:

1. Use cotton swab or lint-free tissue dipped in ethanol (or isopropanol)
2. Gently wipe the crystal surface
3. Repeat 2–3 times to ensure no sample residue remains
4. Confirm that the crystal is clean and dry before collecting background

Precautions [3][8]:

• Avoid scratching the crystal with hard objects (especially ZnSe, Ge)
• Avoid strong acids or bases for cleaning (corrode ZnSe)
• Diamond crystals can be cleaned with any common solvent
• When not in use for long periods, protect the crystal with a dust cover

V. Advantages and Limitations of ATR Applications

5.1 Core Advantages of ATR

Bruker summarizes five main reasons why ATR has become the mainstream technique [3]:

① No sample preparation

• Solids, liquids, powders, pastes can be measured directly
• Eliminates tedious steps like KBr pellet pressing and liquid cell filling
• Measurement time reduced from 10–30 minutes to 1–2 minutes [3]

② Non-destructive

• Samples can be recovered after measurement
• Suitable for precious samples, forensic evidence
• Can be used for online monitoring [3]

③ Wide range of applicable samples

• "The sample can be virtually any material, size, or shape" [3]
• Almost all solids and liquids can be measured

④ Good reproducibility

• Path length is determined by crystal parameters, not operator-dependent
• Quantitative analysis reproducibility superior to KBr pellet method [3][6]

⑤ Easy cleaning

• Simply wipe the crystal after measurement
• No disassembly or cleaning required (unlike liquid cells)

5.2 Limitations of ATR

① Surface selectivity [3][8]

• The evanescent wave penetrates only 0.5–5 μm
• Measures the surface, not representative of the bulk
• For samples with different surface and bulk compositions (e.g., coatings, surface-modified materials), interpretation requires caution

② Penetration depth varies with wavelength [6][8]

• Deeper penetration and stronger signal at longer wavelengths (low wavenumber)
• This causes ATR spectra to differ in shape from transmission spectra (see Section VI for details)
• Requires ATR correction algorithm

③ Effect of sample refractive index [4][8]

• High-refractive-index samples ($n_2$ > 1.7) may approach the critical angle, causing spectral distortion
• Typical cases: carbon black-filled rubber, dark-colored samples
• Solution: Use Ge crystal ($n_1$ = 4.0)

④ "Distortion" of strongly absorbing samples [4][8]

• Anomalous refractive index variation near strong absorption bands (anomalous dispersion)
• Leads to peak asymmetry and shifts in ATR spectra
• Solution: ATR correction algorithm

⑤ Not suitable for gases [3]

• Refractive index of gases is near 1.0, very different from the crystal
• Additionally, gases cannot make "contact" with the crystal surface
• Gas measurements still require long-path gas cells (Ep 13)

5.3 Applications of ATR in Various Industries

Table 3: Typical applications of ATR in various industries

🔗 Extension: Functional group information obtained by ATR can be cross-referenced with the ftir.fun functional group database. For example, if the C=O peak increases after polymer aging, visit ftir.fun carbonyl functional group page to check the frequency range.

6. ATR Spectrum vs. Transmission Spectrum

6.1 Origin of Morphological Differences

Although ATR and transmission spectra have similar peak positions, there are systematic differences in peak intensity distribution [6][8][14]:

Root cause: The penetration depth $d_p$ is proportional to wavelength $\lambda$ [6][8]:

$$d_p \propto \lambda$$

• Long wavelength (low wavenumber): deeper penetration → more absorption → peak relatively stronger
• Short wavelength (high wavenumber): shallower penetration → less absorption → peak relatively weaker

Comparison of ATR and transmission spectra for the same polystyrene sample [6][14]:

Table 4: Intensity comparison of ATR and transmission spectra in different regions

6.2 Peak Position Shift

In addition to intensity differences, ATR spectra also exhibit peak position shifts and peak shape distortions near strong absorption bands [4][8][14]:

Cause: Anomalous dispersion [4][8]

• Near absorption peaks, the sample refractive index $n_2$ changes sharply
• This alters the critical angle and penetration depth
• Result: peak position shifts to lower wavenumber by a few cm⁻¹, peak shape becomes asymmetric

Typical manifestations [4][8]:

• Strong peak shifts are more pronounced than weak peaks
• High refractive index samples exhibit larger shifts
• The C=O peak in ATR spectra may be 2–5 cm⁻¹ lower than in transmission spectra

6.3 ATR Correction Algorithm

To enable matching ATR spectra with transmission spectral libraries, modern FTIR software includes ATR correction algorithms [6][8][14]:

Correction principle [6][14]:

1. Measure the ATR spectrum $A_{ATR}(\nu)$
2. Multiply by a wavenumber correction factor (proportional to wavenumber $\nu$):

$$A_{corrected}(\nu) = A_{ATR}(\nu) \cdot \nu \cdot \frac{\nu_{ref}}{\nu}$$

1. Correct peak position shifts (based on refractive index model)
2. Output a "pseudo-transmission" spectrum that can match standard libraries

Caveats [8][14]:

• ATR correction is an approximate algorithm and cannot completely eliminate all differences
• Distortion of strong absorption peaks is difficult to fully correct
• For precise peak position analysis (e.g., polymorph identification), it is recommended to use raw ATR spectra matched against self-built ATR libraries

💡 Practical advice:

• Qualitative identification: Use ATR-corrected spectra to match against transmission libraries
• Polymorph/precise peak position analysis: Use raw ATR spectra matched against self-built ATR libraries
• Quantitative analysis: Use raw ATR spectra with fixed measurement conditions (pressure, crystal, temperature) [8][14]

6.4 Example Comparison: Ethyl Acetate ATR vs. Transmission

Comparison of two measurement results for the same ethyl acetate sample [14]:

Table 5: Ethyl acetate ATR vs. transmission intensity comparison (data source: Thermo Scientific application note [14])

It is clear that the lower the wavenumber, the higher the relative ATR intensity, which is fully consistent with the penetration depth formula $d_p \propto \lambda$.

🔗 Extension: Detailed peak position data for ester groups can be found at ftir.fun ester functional group page.

7. Modern ATR Instrument Integration

7.1 Major Manufacturers' ATR Products

Bruker [3]:

• Platinum ATR: Diamond single reflection, pneumatic pressure, integrated in ALPHA II and TENSOR series
• Lumos: Fully automated micro-ATR for small samples
• HTS-XT: High-throughput ATR with 96-well plate

Thermo Fisher [10]:

• Smart iTR: Diamond/ZnSe/Ge interchangeable crystals, for Nicolet iS50
• iD5 ATR: Compact diamond ATR, for Nicolet iS5
• iD7 ATR: Pneumatic pressure, higher performance

Shimadzu [6]:

• QATR-S: Diamond single reflection, for IRAffinity-1 and IRTracer-100
• MIRacle ATR (manufactured by PIKE): ZnSe/Ge/Diamond options

PerkinElmer:

• Universal ATR: Diamond single reflection, for Spectrum series

7.2 Integration and Intelligence

Trends in modern ATR accessories [3][10]:

• Plug-and-play: Accessory recognition technology, automatic parameter configuration upon installation
• Automatic pressure: Pneumatic or electric pressure for reproducibility
• Visualization: Built-in camera to observe sample contact
• Software integration: One-click background and sample acquisition, automatic ATR correction

💡 Modern operation workflow (using Bruker Platinum ATR as an example) [3]:

1. Open software, select "ATR Measurement" mode
2. Collect background (crystal clean)
3. Place sample on the crystal
4. Press the pressure button (pneumatic automatic pressurization)
5. Click "Collect Sample"
6. Software automatically processes and displays the spectrum
7. Clean the crystal

Total time: 1–2 minutes!

8. Comprehensive Comparison of ATR and Transmission Methods

8.1 Comprehensive Comparison Table

Table 6: Comprehensive comparison of ATR vs. transmission methods

8.2 Selection Guide: When to Use ATR vs. Transmission?

Scenarios favoring ATR [3][4][6]:

• Routine rapid identification (preliminary screening of unknown samples)
• Aqueous samples (aqueous solutions, biological fluids)
• Difficult-to-grind samples (rubber, plastics, fibers)
• Surface analysis (coatings, modifications)
• Precious/non-destructible samples (artifacts, forensic evidence)
• High-throughput screening

Scenarios favoring transmission [1][6][15]:

• Pharmacopoeial identification (ChP, USP standard methods)

• Precise quantitative analysis (requires controllable path length)

• Gas samples
• Accurate matching with standard spectral libraries (historical data mostly transmission)
• Far-infrared measurement (requires CsI windows)
• Teaching (learning basic principles)

🔗 Extension: USP General Chapter <1854> (Mid-IR Spectroscopy) allows the use of ATR, but method equivalence must be validated [11].

9. Advanced Applications of ATR

9.1 In Situ Reaction Monitoring

ATR can be used for in situ monitoring of chemical reactions [3][13]:

• Dropping the reaction solution onto the ATR crystal
• Real-time spectral acquisition (one spectrum every 10–30 seconds)
• Monitoring reactant consumption and product formation
• Applications: Polymerization, esterification, drug degradation

Case Study: Polyurethane synthesis monitoring [3]

• Isocyanate (NCO, 2270 cm⁻¹) peak gradually disappears
• Polyurethane Amide I (1700 cm⁻¹) peak gradually increases
• Calculate conversion rate over time

9.2 ATR-FTIR Imaging

Combined with a focal plane array (FPA) detector, ATR can be extended to chemical imaging [13]:

• Spatial resolution: approximately 1–5 μm (limited by evanescent wave wavelength)
• Acquires spectra from thousands of pixels simultaneously
• Applications: Tablet dissolution, polymer blend distribution, biological tissue imaging

📷 Figure 3: ATR-FTIR imaging principle and visualization of drug dissolution process
Source: Kazarian Group, Imperial College London [13]
https://pmc.ncbi.nlm.nih.gov/…

9.3 Variable Angle ATR

By changing the incident angle, the penetration depth can be adjusted, enabling depth profile analysis [2][8]:

• Larger incident angle → Shallower penetration
• Smaller incident angle (close to critical angle) → Deeper penetration
• Applications: Coating thickness analysis, surface gradient studies

9.4 Polarized ATR

Using polarized light (s-polarization vs p-polarization), one can study molecular orientation [2][8]:

• The evanescent wave electric field of p-polarized light has a perpendicular component, sensitive to vibrations perpendicular to the surface
• The evanescent wave electric field of s-polarized light has only a parallel component
• Applications: Liquid crystal alignment, polymer stretch orientation, protein secondary structure

10. Historical Review and Future Outlook

10.1 Brief History of ATR Development

Table 7: Brief History of ATR Development

10.2 Future Trends

① Portability [3]

• Handheld ATR-FTIR (e.g., Bruker ALPHA II)
• On-site rapid inspection: food safety, forensics, environment
• Battery powered, built-in spectral library

② Automation and High Throughput [3]

• Autosampler (96-well plate)
• Unattended batch measurement
• Pharmaceutical QC high-throughput screening

③ Imaging and Multidimensional Analysis [13]

• FPA-ATR high spatial resolution imaging
• In situ kinetic imaging
• Multimodal fusion with Raman, XPS

④ Intelligent Data Processing [3]

• Machine learning-assisted identification
• Automatic spectral interpretation
• Cloud spectral library and remote analysis

Summary of This Chapter

Review Questions

1. Using a diamond ATR ($n_1$ = 2.4, incident angle 45°) to measure a polystyrene sample ($n_2$ = 1.59), calculate the critical angle and the penetration depth at 1000 cm⁻¹. (Hint: $\lambda$ = 10 μm)
2. The same sample is measured by ATR and transmission. The ATR spectrum shows a much stronger peak at 720 cm⁻¹ than at 3000 cm⁻¹, while in transmission they are similar. Explain why.
3. You need to measure the infrared spectrum of a black carbon-filled rubber sample. With diamond ATR, the spectrum is severely distorted. Which crystal should you use instead? Why?
4. Pharmacopoeia specifies KBr pellet method for API identification, but ATR is more convenient for daily lab use. What method validation is required to replace KBr pellet with ATR?
5. When measuring a sample containing water, the O-H peak of water (3400 cm⁻¹) is much weaker than the C=O peak of the sample (1700 cm⁻¹) in ATR, but similar in transmission. Why?

References

[1] Fahrenfort J. "Attenuated Total Reflection: A New Principle for the Production of Useful Infra-Red Reflection Spectra of Organic Compounds." Spectrochimica Acta, 1961, 17(7): 698–709. DOI:10.1016/0375-9397(61)80056-4.

[2] Milosevic M. "Internal Reflection and ATR Spectroscopy." Applied Spectroscopy Reviews, 2004, 39(3): 365–384. DOI:10.1081/ASR-200030195.
https://mmrc.caltech.edu/FTIR…

[3] Bruker. "Attenuated Total Reflectance (ATR)." FT-IR Technology Overview.

https://www.bruker.com/en/pro…

[4] Davies A. "Choosing the Right ATR Crystal." Specac Theory Articles, 2018.
https://specac.com/theory-art…

[5] Hecht E. Optics. 5th ed. Pearson, 2017. Chapter 4. ISBN: 978-0-13-397722-6.

[6] Shimadzu. "Determination of Sample Penetration Depth and Effective Pathlength during FTIR-ATR Measurements." Teaching Note 220-93223-01.
https://www.ssi.shimadzu.com/…

[7] Harrick Scientific. "Introduction to Attenuated Total Internal Reflectance (ATR)." Technical Documentation.
https://harricksci.com/applic…

[8] Harrick Scientific. "Internal Reflection and ATR Spectroscopy." Application Notes.
https://harricksci.com/applic…

[9] art photonics. "ATR Crystal Choice for Fiber-based Process Spectroscopy." Technical Note #001, 2024.
https://artphotonics.com/wp-c…

[10] Thermo Fisher Scientific. "Smart iTR Attenuated Total Reflectance (ATR) Sampling Accessory." Product Documentation.
https://www.thermofisher.com/…

[11] USP General Chapter <1854>. "Mid-Infrared Spectroscopy." United States Pharmacopeia.

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[13] Kazarian S G, Ewing A V. "Applications of FTIR Spectroscopic Imaging to Tablet Dissolution and Drug Release." Expert Opinion on Drug Delivery, 2013, 10(9): 1207–1221. DOI:10.1517/17425247.2013.802832.

[14] Thermo Fisher Scientific. "ATR Correction Algorithm." Nicolet FTIR Application Note AN-011.
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[15] Chinese Pharmacopoeia Commission. Pharmacopoeia of the People's Republic of China 2020 Edition, Part IV, General Rule 0402 Infrared Spectrophotometry. China Medical Science Press.

[16] Delbeck S, Heise H M. "FT-IR versus EC-QCL Spectroscopy for Biopharmaceutical Quality Assessment." Analytical and Bioanalytical Chemistry, 2020, 412(19): 4647–4658. DOI:10.1007/s00216-020-02718-1.
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Next Episode Preview: Ep 15 — ATR vs Transmission: When to Use Which?
We will compare ATR and transmission methods on different sample types (powders, liquids, polymers, gases) through practical cases, establish a systematic decision process for method selection, and provide comparative analysis of two spectra of the same sample.

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