Ep 15 — ATR vs Transmission: When to Use Which?

Series: Encyclopedia of Infrared Spectroscopy: From Principles to Practice
Chapter: Part 2 · Beginner Level — Entering the Lab
Audience: High school/undergraduate/graduate students, lab technicians new to the field
Prerequisites: Ep 13 (Transmission), Ep 14 (ATR)
Reading time: ~35 minutes

Introduction: The "Classic Soul-Searching Question" in the Lab

The question most frequently asked by beginners to FTIR trainers is not "Which functional group does this peak correspond to?" but rather a more basic one:

"Instructor, for this sample... should I use ATR or KBr pellet?"

This question seems simple but puts many novices in a dilemma during their first independent experiment. On one hand, there is modern ATR where you "just put it on and measure" — a spectrum in 1 minute; on the other hand, there is the "classic ancient method" of KBr pellet transmission — controlled pathlength, standard spectra, pharmacopoeia-compliant. Each method has its own temperament; choosing incorrectly not only wastes time but may also yield a misleading spectrum.

Moreover, the IR community circulates many "rules of thumb" — "ATR cannot do quantification," "KBr must be dried overnight," "aqueous solutions can only use ATR" — among which truth and falsehood are mixed, some even being outdated experiences from the 1990s that are no longer fully applicable today [1][2].

This episode systematically compares ATR and transmission methods across dimensions such as principle, sample preparation, spectral morphology, quantitative capability, and regulatory compliance. It then provides selection strategies for four typical sample types: powders, liquids, polymers, and gases, culminating in a decision flowchart. After reading this episode, you will be able to make a reasonable judgment for a new sample within 10 seconds.

💡 Positioning: Ep 13 and Ep 14 introduced the principles of transmission and ATR, respectively. This episode is their "comprehensive comparison," equivalent to a practical decision framework for Ep 13 + Ep 14.

1. Review: Physical Essence of the Two Methods

To make a sound choice, one must first understand the differences in their physical essence. Infrared spectroscopy essentially measures the absorption of IR light by the sample, with "pathlength" being the key variable.

1.1 Transmission Method: Light "Passes Through" the Sample

In transmission, IR light passes directly through the sample (thin film, KBr pellet, liquid cell), and the detected light intensity follows the Beer-Lambert law:

$$A(\nu) = \varepsilon(\nu) \cdot b \cdot c$$

where $A(\nu)$ is absorbance at wavenumber $\nu$, $\varepsilon(\nu)$ is molar absorptivity, $b$ is pathlength (cm), and $c$ is concentration (mol/L) [1].

Key characteristic: Pathlength $b$ is controllable and wavelength-independent — this is the physical basis for quantification using transmission. Transmission is the "standard method" of IR spectroscopy; almost all historical spectral libraries (e.g., Sadtler, Aldrich) were built using transmission [1][3].

1.2 ATR: Light "Peeps" at the Sample Surface

In ATR, IR light undergoes total internal reflection inside a high-refractive-index crystal, and the evanescent wave penetrates the sample surface to a depth of:

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

The effective pathlength $b_{\text{eff}} = N \cdot d_p$, where $N$ is the number of reflections [2][4].

Key characteristic: Pathlength $b_{\text{eff}}$ varies with wavelength ($d_p \propto \lambda$), meaning ATR spectra differ systematically from transmission spectra. Specifically, the low-wavenumber (long-wavelength) region shows relatively enhanced signals and strong absorption bands shift in position [2][4].

Transmission           ATR

   Sample ┃┃┃┃┃┃┃          ATR crystal     ┃        
        optical path b           \  \  \      Sample attached
  ────────►              ─────────    surface 0.5–5 μm
        fixed pathlength           (evanescent penetration depth varies with wavelength)

📷 Fig. 1: Schematic comparison of optical paths: transmission vs. ATR
Source: Author's drawing, based on [1][2]

1.3 Core Difference in One Sentence

Transmission measures bulk information of the sample with precisely controllable pathlength; ATR measures surface information with wavelength-dependent pathlength [1][2].

Once this sentence is understood, all subsequent differences can be derived.

2. Advantages and Limitations of ATR

2.1 Five Major Advantages of ATR

① No sample preparation [2][4][5]

This is the most straightforward advantage of ATR. Solids, liquids, powders, pastes, and films can almost always be "placed and measured." No KBr pellet pressing, no liquid cell filling, no film coating. Measurement time is reduced from 10–30 minutes for transmission to 1–2 minutes [2][5].

② Non-destructive [2][5]

Samples can be recovered unchanged after measurement. This is especially important for precious samples (forensic evidence, artifacts, archaeological samples) — after one measurement, the sample can still be used for subsequent DNA, isotope, or morphological analysis.

③ Surface selectivity [2][4]

The evanescent wave penetrates only 0.5–5 μm, meaning ATR is highly sensitive to the sample's surface information. While this can be a "limitation" (not representative of the bulk), it is an advantage for studying surface coatings, surface modifications, surface contamination, etc. — directly revealing surface chemical information [4][6].

④ Suitable for aqueous samples [2][5]

Transmission measurement of aqueous solutions requires special liquid cells (CaF₂, ZnSe windows, very short pathlength 6–10 μm), and the strong absorption of water can mask sample signals. Due to its short pathlength (μm level), ATR still shows water bands but they are relatively manageable, making it especially suitable for biological fluids, foods, and aqueous solutions [5][7].

⑤ Simple operation and good reproducibility [2][4]

ATR operation requires low skill from the experimenter; results depend mainly on crystal parameters and pressure, and reproducibility is better than KBr pellets (which are affected by grinding, mixing, and pellet quality).

2.2 Four Major Limitations of ATR

① Penetration depth varies with wavelength [2][4]

This is the most fundamental limitation of ATR. $d_p \propto \lambda$ causes systematic differences in spectral morphology compared to transmission:

Table 1: Intensity comparison between ATR and transmission for the same polystyrene sample (data from [2][8])

② Risk of misleading due to surface selectivity [4][6]

ATR only sees the surface 0.5–5 μm. If the sample surface differs compositionally from the bulk (e.g., coating, contamination, oxidation), ATR measures the surface composition, not the overall composition. For identification purposes where "representative of the whole" is needed, this can be misleading.

For example: A plastic block with an oily surface will show strong oil signals in ATR (surface), while transmission would measure the plastic bulk (light passes through the entire sample).

③ Refractive index matching issue [4][6]

ATR requires the sample's refractive index $n_2$ to be significantly less than the crystal's $n_1$. For high-refractive-index samples ($n_2$ > 1.7), such as carbon-black-filled rubber, dark minerals, or semiconductor materials, diamond ATR can produce spectral distortions [4]. A solution is to use a Ge crystal ($n_1$ = 4.0) [4][6].

④ Unsuitable for gases [2]

Gases have refractive indices close to 1.0 and cannot be made to "contact" the crystal surface well; ATR is practically incapable of measuring gases. Gas measurement must use transmission with long-pathlength gas cells (10 cm or longer) [1].

3. Advantages and Limitations of Transmission

3.1 Five Major Advantages of Transmission

① Precisely controllable pathlength [1][3]

The pathlength of a liquid cell is precisely determined by the spacer thickness (available in specifications such as 0.025, 0.05, 0.1, 0.5, 1 mm). The effective pathlength of a KBr pellet is determined by sample concentration and pellet thickness, theoretically also controllable. This "controllable pathlength" is the foundation for quantification [1][3].

② Standard spectral morphology [1][3]

Transmission spectra follow the Beer-Lambert law, with peak intensity linearly related to concentration and independent of wavelength. This "standard morphology" allows transmission spectra to be directly matched against commercial spectral libraries (Sadtler, Aldrich, Hummel) [3][9].

③ Gold standard for quantitative analysis [1][3]

Because the optical path is controllable and the spectra are standardized, transmission is the "gold standard" for infrared quantitative analysis. Almost all infrared quantitative methods in pharmacopoeias and national standards are based on transmission [10][11].

④ Suitable for gas samples [1]

Gas cells (10 cm, 20 cm, multiple-reflection long-path 10 m) are the only choice for measuring gases, with path lengths up to 10 m or more, providing extremely high sensitivity [1].

⑤ Far-infrared measurement [3]

The far-infrared region (< 400 cm⁻¹) requires CsI or polyethylene windows, which can only be used in transmission mode [3].

3.2 Four major limitations of transmission

① Tedious sample preparation [1][3]

Different sample types require different preparation methods: solids require KBr pellet (grinding, mixing, pressing 10–20 minutes); liquids require liquid cells (filling, cleaning, window maintenance); films require coating or casting. Each step is a source of error [1][3].

② KBr moisture absorption [1][3][12]

KBr is highly hygroscopic. Insufficiently dried KBr will show strong water absorption at 3400 cm⁻¹ and 1640 cm⁻¹, interfering with sample signals [12]. Even after drying, it is difficult to avoid in environments with humidity > 50%. This is one of the biggest pain points of the KBr pellet method [1][12].

③ Sample destruction [1][3]

KBr pellet requires grinding the sample, and the sample cannot be recovered after mixing with KBr. The liquid cell method requires dissolving the sample, which may alter its state.

④ High operating skill requirement [1][3]

The quality of KBr pellets (transparency, uniformity) directly affects spectral quality. Beginners often encounter issues like "opaque pellet", "uneven sample distribution", or "pellet cracking". Extensive practice is needed to consistently produce high-quality pellets.

⑤ Difficulty with aqueous samples [1][12]

KBr is soluble in water; aqueous solutions cannot be directly pressed into pellets. Special liquid cells (CaF₂, ZnSe) are required, and the strong water absorption peaks mask sample signals [12].

IV. Comprehensive comparison: one table clarifies the differences

The following table systematically compares ATR and transmission across 12 dimensions [1][2][3][4][5]:

Table 2: Comprehensive comparison of ATR vs Transmission (compiled from [1][2][3][4][5])

V. Selection by sample type: four major scenarios

5.1 Powder samples

Scenario: Identify unknown white powder; determine drug raw materials; analyze mineral powders.

First choice: ATR (90% of cases) [2][5]

Rationale:

• No KBr pellet required, spectrum obtained in 1 minute
• Non-destructive, sample can be recovered
• Good contact yields high-quality spectra
• Even coarse particles can be measured after slight grinding

Exceptions: transmission required [1][3]

• Pharmacopoeial identification (ChP, USP <197> requires KBr pellet) [10][11]
• Quantitative analysis (e.g., assay)
• Precise matching with historical transmission spectral libraries

Operational tips [2][5]

• ATR: Place 1–5 mg powder on crystal center, slowly tighten pressure screw
• Transmission: Grind to < 2 μm, mix with KBr at 1:100, press into transparent pellet

⚠️ Common error: Coarse powder particles (> 10 μm) result in poor ATR contact, severe scattering, and tilted baseline. Grind in an agate mortar first [5].

5.2 Liquid samples

Scenario: Pure liquids (ethanol, ethyl acetate); solutions (polymer solutions, reaction mixtures); aqueous samples (serum, food).

First choice: choose based on sample nature

Pure liquids/organic solutions → ATR [2][5]

Rationale:

• Drop 1–2 drops on crystal
• Best liquid contact, highest SNR
• No cell filling and cleaning
• Volatile liquids can be measured quickly

Aqueous solutions/biological fluids → ATR (preferred) or short-path transmission cell [5][7][12]

Rationale:

• ATR: water peaks are relatively controllable (μm-scale path length)
• Transmission: requires CaF₂/ZnSe cells, path length 6–10 μm, water peaks still strong but subtractable

Exceptions: transmission required [1][3]

• Very dilute solutions (e.g., 0.1% solute) → need long-path cell (0.5–1 mm) for signal enhancement
• Quantitative analysis (precise path length)
• Matching with historical spectral libraries

Operational tips [2][5]

• ATR: Drop 1–2 drops of liquid, cover to prevent evaporation
• Transmission: Choose appropriate path length cell (pure organic liquid 0.025–0.1 mm; aqueous solution 6–10 μm)

5.3 Polymer samples

Scenario: Plastic identification; rubber analysis; film quality; polymer aging.

First choice: depends on sample form

Bulk plastics/rubber/films → ATR [2][4][5]

Rationale:

• No sample preparation, place directly
• Surface information (suitable for surface aging, coating analysis)
• Fast measurement

Exceptions: transmission required [1][3]

• Films with thickness < 50 μm → direct transmission measurement (appropriate path length)
• Polymer solutions → cast film then transmission
• Precise matching with Hummel polymer library (library uses transmission spectra) [9]

Special consideration: Black rubber (with carbon black filler) [4]

• Carbon black has high refractive index (n₂ close to 2.0), distorts diamond ATR spectra
• Use Ge ATR crystal (n₁ = 4.0)
• Or use transmission method (ultrathin section)

Operational tips [2][4]

• ATR: Cut a small flat piece (5×5 mm), place face down, apply pressure for contact
• Transmission: Clamp film directly; thick samples need ultrathin sections (1–10 μm)

💡 Professional tip: For polymer films with thickness 20–100 μm, transmission often gives the cleanest spectra. For example, to measure polyethylene food bags, simply clamp the film in the sample holder for transmission measurement.

5.4 Gas samples

Scenario: Ambient air monitoring; reaction gases; breath analysis.

Only choice: transmission long-path gas cell [1][3]

Rationale:

• Gas concentrations are low (ppm–ppb), requiring long path lengths (10 cm–10 m)
• Gas refractive index ≈ 1.0, ATR completely unsuitable
• Multipass gas cells (White cell, Herriott cell) achieve 10–100 m path lengths

Operational tips [1]

• Evacuate then fill with sample gas
• Choose appropriate path length (lower concentration requires longer path)
• Note the effect of gas pressure on peak width (peaks broaden at high pressure)

📷 Figure 2: Schematic of a multipass long-path gas cell
Source: Thorlabs application note
https://www.thorlabs.com/newg…

VI. Example comparison: ATR vs transmission spectra of polystyrene

Polystyrene is the "standard sample" for infrared teaching, used for calibration on almost every instrument [2][8]. Below, using polystyrene as an example, we compare spectral differences between the two methods for the same sample.

6.1 Comparison of major peak positions and intensities

| 2925 | Alkyl -CH₂- asymmetric | 0.55 | 0.35 | 0.64 |
| 2850 | Alkyl -CH₂- symmetric | 0.40 | 0.27 | 0.68 |
| 1942–1740 | Aromatic ring combination band | 0.05–0.15 | 0.05–0.18 | ~1.0 |
| 1601 | Aromatic C=C stretch | 0.45 | 0.40 | 0.89 |
| 1493 | Aromatic C=C stretch | 0.50 | 0.50 | 1.00 |
| 1452 | CH₂ scissor | 0.40 | 0.45 | 1.13 |
| 1028 | Aromatic CH in-plane | 0.30 | 0.42 | 1.40 |
| 757 | Aromatic CH out-of-plane (adjacent 5H) | 0.55 | 1.10 | 2.00 |
| 698 | Aromatic CH out-of-plane (out-of-plane ring deformation) | 0.80 | 1.75 | 2.19 |

Table 3: Polystyrene ATR vs transmission intensity comparison (Typical data from [2][8], actual intensities may vary slightly with sample thickness and instrument parameters)

🔗 Extension: For detailed peak positions of aromatic and alkyl C-H, see ftir.fun alkyl C-H functional group page.

6.2 Analysis of Differences

Clear patterns are observed from Table 3:

① High wavenumber region (>2500 cm⁻¹): ATR is weaker [2][8]

• 3082 cm⁻¹ aromatic =C-H: ATR is only 50% of transmission
• Reason: High wavenumber corresponds to short wavelength, shallow penetration depth

② Mid wavenumber region (1500–2000 cm⁻¹): Both are similar [2][8]

• 1601, 1493 cm⁻¹ aromatic C=C: ATR slightly weaker or comparable
• Reason: Moderate wavelength, insignificant optical path difference

③ Low wavenumber region (<1000 cm⁻¹): ATR significantly enhanced [2][8]

• 757, 698 cm⁻¹ aromatic CH out-of-plane: ATR is twice that of transmission
• Reason: Low wavenumber corresponds to long wavelength, large penetration depth, strong absorption

6.3 Spectral Shape Illustration

Absorbance A
   ↑
2.0│                  ▓▓              ← ATR 698 (strong)
1.5│              ▓▓  ▓▓              
1.0│              ▓▓                  ← ATR 757
   │
0.8│  ──Transmission──    ▓▓  ▓▓              ← Transmission 698 (medium)
0.5│  ▓▓  ▓▓      ▓▓  ▓▓  ▓▓         ← Transmission 757, 1493
0.3│  ▓▓  ▓▓  ▓▓ ▓▓  ▓▓  ▓▓  ▓▓     
0.1│  ▓▓  ▓▓  ▓▓ ▓▓  ▓▓  ▓▓  ▓▓     ← ATR high wavenumber weak
   └────────────────────────────→  Wavenumber ν
   3000      2000      1000      500

Figure 3: Schematic ATR vs transmission spectral shape of polystyrene (drawn based on actual spectra from [2][8])

6.4 Practical Implications

This intensity difference has two implications for practical work [2][4][8]:

① Apply ATR correction for library searching
Commercial spectral libraries (Sadtler, Aldrich) are transmission spectra. Directly matching an ATR spectrum will lower the match quality due to intensity differences. Modern FTIR software includes an "ATR Correction" function, which significantly improves the match after correction [4][8].

② Prioritize peak positions for qualitative analysis
Although intensities differ, peak positions are essentially the same (with possible shifts of 2–5 cm⁻¹ near strong absorptions). Therefore, peak assignments are not affected by the method, and qualitative analysis conclusions are consistent [2][4].

7. Decision Flowchart

Based on the above analysis, a practical method selection flowchart is provided [1][2][3][5]:

                  ┌─────────────────────┐
                  │  Unknown sample requires │
                  │  infrared measurement │
                  └──────────┬──────────┘
                             │
                  ┌──────────▼──────────┐
                  │   Is the sample a  │
                  │   gas?             │
                  └──────────┬──────────┘
                             │
              ┌──────────────┴──────────────┐
            Yes                               No
              │                              │
     ┌────────▼────────┐         ┌──────────▼──────────┐
     │ Transmission +  │        │   Is pharmacopoeia │
     │ long-path gas   │        │   compliance       │
     │ cell            │        │   required?        │
     └─────────────────┘         └──────────┬──────────┘
                                             │
                            ┌────────────────┴────────────────┐
                           Yes                                   No
                            │                                  │
                 ┌──────────▼──────────┐         ┌────────────▼────────────┐
                 │ Pharmacopoeia      │        │   Is precise pathlength │
                 │ prescribed method  │        │   quantification needed?│
                 │ (KBr pellet) or    │         └────────────┬────────────┘
                 │ ATR after method   │                      │
                 │ validation         │                      │
                 └─────────────────────┘                      │
                                              ┌──────────────┴──────────────┐
                                             Yes                               No
                                              │                              │
                                 ┌────────────▼────────────┐   ┌────────────▼────────────┐
                                 │ Transmission method     │   │   Is the sample an     │
                                 │ (liquid cell/pellet/    │   │   aqueous liquid/      │
                                 │ film)                   │   │   biological?          │
                                 │ Controllable pathlength,│   └────────────┬────────────┘
                                 │ can make calibration    │                │
                                 │ curve                   │                │
                                 └─────────────────────────┘                │
                                                                  ┌──────────┴──────────┐
                                                                 Yes                     No
                                                                  │                    │
                                                       ┌──────────▼──────────┐      │
                                                       │ ATR (suitable for   │   ┌──▼──────────┐
                                                       │ aqueous)            │   │ ATR (routine)│
                                                       └─────────────────────┘   └─────────────┘

FTIR Experiment Guide (4): How to Choose Between ATR and Transmission?

7. ATR vs Transmission: A Quick Decision Flowchart

                                  │                    │
                       ┌──────────▼──────────┐ ┌─────────▼─────────┐
                       │ ATR (preferred) or  │ │ ATR (quick test)  │
                       │ Transmission CaF₂   │ │ 1–2 min spectrum  │
                       │ short path length   │ │                    │
                       └─────────────────────┘ └────────────────────┘

Figure 4: ATR vs Transmission decision flowchart

Seven-Step Quick Reference Checklist

1. Gas? → Transmission long-path gas cell
2. Pharmacopoeia compliance? → Follow pharmacopoeia method (mostly KBr pellet)
3. Precise quantification? → Transmission (controlled path length)
4. Aqueous liquid? → ATR preferred, transmission CaF₂ secondary
5. Bulk polymer? → ATR directly
6. Dilute solution? → Transmission long-path liquid cell
7. Routine quick test? → Always ATR first

💡 Simple rule: If no special requirements (pharmacopoeia, quantification, gas), try ATR first. Most samples yield a usable spectrum in 1–2 minutes [2][5].

8. Compliance Considerations: Pharmacopoeia and Standards

Method selection is not only a scientific issue but also involves compliance [10][11].

8.1 Chinese Pharmacopoeia (ChP 2020) [10]

General Chapter 0402 Infrared Spectrophotometry: KBr pellet transmission is required in principle. If ATR is used as a substitute, method validation (specificity, accuracy, precision, robustness) is required to prove equivalence with the KBr pellet method.

8.2 United States Pharmacopoeia (USP <1854>) [11]

ATR is allowed, but requires:

• Validation of equivalence between ATR and transmission
• Clear documentation of measurement conditions (crystal material, pressure, angle of incidence)
• System suitability testing using a reference standard (e.g., polystyrene film)

8.3 Practical Recommendations

• New drug registration, release testing: Strictly follow pharmacopoeia method (KBr pellet) [10][11]
• R&D, internal QC, preliminary screening: ATR can be used, but requires SOP documentation
• Inter-laboratory comparison: All labs must use the same method (transmission recommended for better spectral library matching)

⚠️ Compliance risk: If the pharmacopoeia method specifies KBr pellet but the lab routinely uses ATR without method validation, it will be considered "not following pharmacopoeia method" during audits, posing a compliance risk [10].

9. Common Misconceptions Clarified

Misconception 1: "ATR cannot be used for quantitative analysis"

Partially correct, partially outdated [2][4][7].

Theoretically, ATR path length varies with wavelength and does not strictly follow the Beer-Lambert law. But in practice:

• Fixed measurement conditions (crystal, pressure, temperature, sample amount)
• Use relative intensities (peak height ratio or peak area ratio) at the same wavenumber
• Standard curve method can achieve ±2% relative standard deviation

ATR quantification is widely used in pharmaceuticals, food, and polymer QC, e.g., ATR-FTIR determination of milk fat content [7].

Misconception 2: "KBr must be dried overnight"

Basically correct, but there are quicker methods [1][12].

Drying KBr at 105°C for 2–4 hours is usually sufficient. More stringent: 150°C vacuum oven overnight. However, even if dried, rapid water absorption occurs when operating in environments with >50% humidity. Recommendations:

• Store dried KBr in a desiccator
• Perform pellet pressing in low-humidity conditions (<30% RH)
• Subtract background (blank KBr pellet) during measurement

Misconception 3: "Aqueous solutions can only use ATR"

Not entirely correct [5][7][12].

Aqueous solutions can use:

• ATR (preferred, simple)
• Transmission CaF₂/ZnSe short-path liquid cell (6–10 μm)
• Alternative: DRIFTS (diffuse reflectance)

The choice depends on:

• Solute concentration (dilute solutions need long path → transmission)
• Whether water peak subtraction is needed (more accurate with transmission)
• Whether quantification is needed (transmission better for quantification)

Misconception 4: "ATR spectra are completely different from transmission spectra"

Exaggeration [2][4][8].

Actual differences:

• Peak positions: basically consistent (strong peaks may shift by 2–5 cm⁻¹)
• Number of peaks: exactly the same
• Peak intensities: stronger at low wavenumbers, weaker at high wavenumbers (systematic difference)
• After ATR correction, matching score with transmission spectral libraries can exceed 0.95

Qualitative analysis conclusions are identical; differences mainly affect quantitative analysis.

10. Future Trends: Method Integration and Intelligence

10.1 Automated ATR

Modern high-end ATR (e.g., Bruker Platinum ATR, Thermo iD7) already achieve [2][5]:

• Pneumatic pressure with excellent reproducibility
• Automatic background subtraction
• Automatic ATR correction
• One-click measurement, entire process < 1 minute

10.2 Portability and On-Site Use

Portable FTIR (e.g., Bruker ALPHA II, Thermo Nicolet iS5) come standard with ATR [2][5]:

• On-site rapid testing: food safety, forensics, environment
• Battery powered, built-in spectral libraries
• Suitable for emergencies, customs, field enforcement

10.3 Cloud-Based Spectral Libraries

Cloud spectral libraries (e.g., ftir.fun) enable real-time spectrum retrieval [3][13]:

• Online search without local library
• Continuously updated, covering more substances
• Multi-user sharing, community-contributed spectra

🔗 Extension: Check standard peak positions of various functional groups at the ftir.fun functional group database to aid spectrum interpretation.

Summary of This Module

Review Questions

1. The lab receives an unknown plastic sample (white bulk). The client wants an IR spectrum to identify the plastic type within 30 minutes. Do you choose ATR or transmission? Why?

2. You need to quantify an impurity (0.5%) in a drug substance, with the impurity standard available. Is transmission mandatory? Can ATR be used? What validation is needed?

3. When measuring polystyrene by ATR, the 698 cm⁻¹ peak is much stronger than the 3082 cm⁻¹ peak, while in transmission they have similar intensities. Explain this phenomenon quantitatively (hint: $d_p \propto \lambda$, 698 cm⁻¹ corresponds to $\lambda$ = 14.3 μm, 3082 cm⁻¹ corresponds to $\lambda$ = 3.2 μm).

4. A pharmacopoeia specifies KBr pellet for identifying a drug substance, but your lab has an ATR accessory which is more convenient. Design a method validation protocol to prove equivalence between ATR and KBr pellet.

5. You need to measure the IR spectrum of an organic compound in water at 100 ppm. ATR signal is too weak (solute signal buried by water). What alternative method should you use?

References

[1] Griffiths P R, de Haseth J A. Fourier Transform Infrared Spectrometry. 2nd ed. Wiley, 2007. Chapter 11–13. ISBN: 978-0-471-19404-0.

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

[3] Coates J. "Interpretation of Infrared Spectra, A Practical Approach." In: Encyclopedia of Analytical Chemistry, Meyers R A (Ed.). John Wiley & Sons, 2006. DOI:10.1002/9780470027318.a5607.

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

[5] Bruker. "Attenuated Total Reflectance (ATR)." FT-IR Technology Overview.
https://www.bruker.com/en/pro…

[6] Mirabella F M. Internal Reflection Spectroscopy: Theory and Applications. Marcel Dekker, 1993. ISBN: 978-0824790595.

[7] 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.

[8] Thermo Fisher Scientific. "ATR Correction Algorithm." Nicolet FTIR Application Note AN-011.
https://www.thermofisher.com/…

[9] Hummel D O, Scholl F. Atlas of Polymer and Plastics Analysis. 3rd ed. Hanser, 1991. ISBN: 978-3446157035.

[10] Chinese Pharmacopoeia Commission. Pharmacopoeia of the People's Republic of China 2020 Edition, General Chapter 0402 Infrared Spectrophotometry. China Medical Science Press.

[11] USP General Chapter <1854>. "Mid-Infrared Spectroscopy." United States Pharmacopeia.
https://www.usp.org/

[12] Socrates G. Infrared and Raman Characteristic Group Frequencies. 3rd ed. Wiley, 2001. Chapter 1. ISBN: 978-0470093078.

[13] ftir.fun. "Infrared Functional Group Database."
https://ftir.fun/ir/group/alk…

Next Episode Preview: Ep 16 — Sample Preparation Practice and Common Mistakes
We will enter the laboratory and systematically explain practical techniques for solid sample preparation (KBr pellet, grinding particle size), liquid sample preparation (cell selection, aqueous solution handling), and analyze 10+ common erroneous spectra cases, providing a "mine-clearing checklist" to help you obtain high-quality spectra in one go.

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