Ep 13 — Introduction to Sampling Techniques: Transmission Method (KBr Pellet, Liquid Cell, Thin Film)

Series: Encyclopedia of Infrared Spectroscopy: From Principles to Practice
Section: Part 2 · Beginner Level — Entering the Laboratory
Target Audience: Undergraduate and graduate students, new laboratory technicians
Prerequisites: Ep 11 (Dispersive vs FTIR), Ep 12 (Michelson Interferometer)
Reading Time: Approximately 35 minutes

Introduction: A Good Spectrum Depends 70% on Sample Preparation

An old saying in the lab goes: "The instrument determines the lower limit, but sample preparation determines the upper limit" [1].

No matter how advanced the FTIR, if the sample is poorly prepared, the resulting spectrum will be "garbage"—baseline tilt, severe scattering, absorption saturation, water peaks covering target peaks... The root cause of these problems is almost never the instrument, but sampling.

Before the popularity of ATR (Attenuated Total Reflectance), transmission was almost the only available sampling method for infrared spectroscopy. From prism instruments in the 1940s to early FTIR in the 1980s, generations of chemists used these "traditional crafts"—KBr pellets, liquid cells, thin films—to produce beautiful spectra [1][2].

Even today, when ATR is nearly dominant, transmission remains irreplaceable [2][3]:

• Quantitative analysis needs precisely controlled path length → liquid cell
• Gas analysis needs long path length → gas cell
• Polymer films need bulk information → thin film transmission
• Pharmacopoeia identification standard methods still require KBr pellets [4]

In this episode, we will systematically learn the three classic transmission sampling techniques—KBr pellet method, liquid cell method, thin film method—and extend to gas cells. Mastering these "basic skills" will help you truly understand where every peak in an infrared spectrum comes from.

1. Basic Principles of Transmission

1.1 What is Transmission Measurement?

Transmission measurement is the most direct infrared sampling method [3][5]: the infrared beam passes through the sample, and the detector measures the intensity after transmission. The sample absorbs light at specific frequencies, so the transmitted light is weakened at corresponding frequencies, forming absorption peaks.

     IR Source → [Sample] → Detector
               Transmission Measurement

Transmittance (T) is defined as [5]:

$$T = \frac{I}{I_0}$$

where $I0$ is the incident light intensity, and $I$ is the transmitted light intensity. Absorbance $A = -\log{10}(T)$, which is proportional to sample concentration and path length (Beer-Lambert law, detailed in Ep 20) [5].

1.2 Core Requirements for Transmission

To obtain useful spectra from transmission measurements, two key conditions must be met [1][3]:

① Appropriate path length: Absorption should be neither too strong (transmittance approaches 0, spectrum "blacks out") nor too weak (poor signal-to-noise ratio). In the mid-infrared region, the suitable path length for most organic compounds is on the order of 10–50 μm [1][3]—thinner than a sheet of A4 paper!

② Low scattering: The size of sample particles or droplets must be much smaller than the measurement wavelength (mid-IR 2.5–25 μm); otherwise, Mie scattering occurs, causing baseline tilt and energy loss [1][3]. A rule of thumb is that particle size should be < 2 μm [1].

These two stringent requirements have given rise to a variety of "fancy" sampling techniques.

1.3 Why is IR So Sensitive to Path Length?

UV-Vis spectroscopy commonly uses 1 cm cuvettes, while IR uses μm-level path lengths. The reason lies in the difference in absorption cross-section [5]:

• UV-Vis electronic transitions: molar absorptivity ε ~ 10⁴–10⁵ L·mol⁻¹·cm⁻¹
• IR vibrational transitions: molar absorptivity ε ~ 10–10³ L·mol⁻¹·cm⁻¹

Although ε is smaller for IR, the peak density of organic compounds in the mid-infrared region is extremely high (there may be peaks at every cm⁻¹). If the path length is too large, peaks will overlap and saturate into "black lumps." Therefore, IR measurements must use very short path lengths [1][5].

💡 Tip: For pure liquids (e.g., water, ethanol) directly measured, the path length should be controlled at 10–100 μm; dilute solutions can use 0.1–1 mm; gases, due to low molecular density, require cm to m-level long path lengths [1][3].

2. KBr Pellet Method: The "Gold Standard" for Solid Samples

2.1 Method Overview

The KBr pellet method is a classic standard method for infrared identification of solid organic compounds, widely used since the 1950s [1][6]. Its core idea is:

Mix a small amount of sample with high-purity KBr powder, grind, press into a transparent pellet under high pressure, and place it in the light path for measurement [1][6].

Why KBr? Because KBr is almost completely transparent in the mid-infrared region (4000–400 cm⁻¹), producing no absorption peaks that interfere with the sample signal [1][6][7]. KBr is an ionic crystal with no covalent bond vibrations; it only has lattice vibrations below 250 cm⁻¹, thus "vacating" the entire mid-infrared window.

🔗 Further Reading: To understand the transmission range comparison of window materials like KBr, visit the KBr-related functional group page on ftir.fun to see O-H interference peaks from KBr moisture absorption.

2.2 Detailed Operating Steps

The standard KBr pellet procedure is as follows [1][6][8]:

Step 1: Prepare KBr Powder

• Use spectroscopic grade KBr, purity ≥ 99.9% [1][6]
• KBr must be thoroughly dried: in an oven at 130 °C for at least 24 h, or vacuum drying for 4 h [6][8]
• Store dried KBr in a desiccator (relative humidity < 30%), preferably in a glovebox [8]
• Moist KBr will show water peaks at 3400 cm⁻¹ (O-H stretch) and 1640 cm⁻¹ (H-O-H bend) [1][8]

📷 Figure 1: KBr pellet operation flowchart
Source: University of Michigan Chemistry Department Teaching Resources [6]
https://www.umich.edu/~chemen…

Step 2: Grind the Sample

• Weigh 1–2 mg of sample + 100–200 mg of KBr (mass ratio 1:100 to 1:200) [1][6][8]
• Grind thoroughly in an agate mortar in one direction for 1–2 minutes
• The purposes of grinding are [1][8]:
  • To uniformly mix the sample with KBr
  • To reduce particle size to < 2 μm (avoid scattering)
  • Insufficient grinding will cause baseline tilt and energy loss in the spectrum

"Grind the mixture thoroughly until the particle size is less than 2 μm. Insufficient grinding is the most common cause of poor spectra."
—— Shimadzu Application Note [8]

Step 3: Load the Die and Press

• Load the ground powder into a pellet die (typically 13 mm diameter)
• Spread evenly with a spatula to ensure uniform distribution
• Place the die in a hydraulic press, evacuate for 1–2 minutes (remove air) [6][8]
• Apply pressure of 7–10 tons (about 600–800 MPa), hold for 1–2 minutes [1][6][8]
• Release pressure slowly, remove the pellet

Step 4: Measure

• Place the pellet in a sample holder and position it in the beam path
• Collect background: use a pure KBr pellet or an empty beam path as background [1]
• Collect the sample spectrum
• Recommended: scan 16–32 times at 4 cm⁻¹ resolution [3]

2.3 Common Problems and Troubleshooting

The KBr pellet method has many "pitfalls"; the table below summarizes the most common issues and solutions [1][6][8][9]:

(To be continued in the next section...)

| Baseline tilt (high frequency low, low frequency high) | Scattering curve | Particles too large | Regrind |
| Absorption saturation (flat-top) | Flat-topped peak, no shape | Too much sample | Reduce sample amount (1 mg or less) |
| Poor signal-to-noise ratio | Noisy spectrum | Too little sample; insufficient scans | Increase sample amount or number of scans |
| CO₂ interference (2350 cm⁻¹) | Double peak | Atmospheric CO₂ | Purge with dry N₂; synchronize background collection |

Table 1: Common problems and troubleshooting for KBr pellet method (Data sources: Shimadzu [8]; UMich [6]; Bruker [9])

💡 Professional tip: When the pellet is "too hard" (opaque), it is often due to excessive drying of KBr, which loses the trace moisture required for forming. Breathe a puff of air (~1 second) into the KBr before pressing, which often significantly improves the result [1][8].

2.4 Applicability and limitations of the KBr pellet method

Applicable [1][6]:

• Most solid organic substances (polymer powders, pharmaceuticals, dyes, catalysts, etc.)
• Standard pharmacopoeial method for API identification (adopted by ChP, USP, EP) [4]
• Samples insoluble in common solvents

Limitations [1][6][9]:

• Sample requires grinding (may disrupt crystal form, affect polymorph analysis) [4]
• KBr hygroscopicity interferes with O-H region detection
• Not suitable for hygroscopic or sublimable samples
• Not suitable for quantitative analysis (difficult to precisely control pathlength) [1]
• Time-consuming operation requiring skill

🔗 Extension: For pharmaceutical polymorph analysis, grinding pressure during KBr pellet preparation may induce polymorphic transformation. It is recommended to use ATR method (Ep 14) or low-temperature grinding [4]. See the case study on crystal form effects on C=O frequency at ftir.fun carboxyl group page.

3. Liquid cell method: Standard method for liquid samples

3.1 Measurement principle for liquid samples

When measuring liquid samples, the pathlength is determined by the gap between two windows [1][3][10]. Key considerations [10]:

• Neat liquids: High molecular density, pathlength of 10–100 μm (capillary film or short-pathlength cell)
• Dilute solutions: Pathlength of 0.1–2 mm (medium/long-pathlength cell)
• Aqueous solutions: Strong water absorption requires pathlength < 30 μm [10]

"For neat liquids, a pathlength of 0.01–0.1 mm is typical; for solutions, 0.1–1 mm is common."
—— Purdue University Analytical Chemistry Lecture [3]

3.2 Types of liquid cells

3.2.1 Demountable cell

A demountable cell consists of a front window, spacer, rear window, and metal holder [1][10]:

   ┌──────┐  ┌──────┐  ┌──────┐  ┌──────┐
   │ Front │  │Spacer│  │ Rear │  │Holder│
   │Window │→ │(fixed│→ │Window│→ │      │
   │       │  │path) │  │       │  │      │
   └──────┘  └──────┘  └──────┘  └──────┘

• Advantages: Easy to disassemble and clean; spacers can be changed to adjust pathlength; suitable for viscous liquids and pastes [1][10]
• Disadvantages: Pathlength varies with each assembly, not suitable for quantitative analysis [1]
• Typical pathlength: 0.025–1 mm

3.2.2 Fixed cell

In a fixed cell, the pathlength is permanently sealed with silver or lead spacers during factory assembly and cannot be disassembled [1][10]:

• Advantages: Precise and constant pathlength, suitable for quantitative analysis; good sealing, can hold volatile liquids [1][10]
• Disadvantages: Difficult to clean; window damage requires full replacement; higher cost
• Typical pathlength: 0.1 mm, 0.5 mm, 1 mm

3.2.3 Capillary film method

The simplest method for liquid measurement [1][10]:

• Place a small drop of liquid between two KBr/NaCl windows
• Gently press to form a capillary film a few micrometers thick

• Directly place in the optical path for measurement

• Advantages: Very fast, no cell equipment needed; suitable for rapid identification of neat liquids [1]

• Disadvantages: Pathlength not controllable, not suitable for quantitative analysis; not suitable for volatile liquids (evaporation)

📷 Figure 2: Comparison of demountable cell, fixed cell, and capillary film method
Source: Harrick Scientific Product Technical Data [10]
https://harricksci.com/applic…

3.3 Choice of window material

The window material of the liquid cell determines the measurable spectral range, water resistance, and mechanical properties [1][7][10]. Comparison of common window materials:

Table 2: Comparison of common IR window materials (Data sources: Sigma-Aldrich [7]; Harrick [10])

Selection principles [1][10]:

• Dry organic samples → KBr or NaCl (inexpensive, good transmission)
• Aqueous samples → CaF₂, BaF₂, ZnSe (water-resistant)
• Need to measure below 400 cm⁻¹ → CsI (far-infrared)
• Avoid: KRS-5, as it contains thallium and is highly toxic; essentially obsolete in modern instruments [1]

⚠️ Safety note: KRS-5 (thallium bromoiodide) contains heavy metal thallium with cumulative toxicity. Wear gloves during handling and dispose as hazardous waste. For new instruments, ZnSe or diamond is recommended [1].

3.4 Pathlength selection

Pathlength selection directly affects spectrum quality [1][3][10]:

Table 3: Pathlength selection for different samples (Data sources: Purdue University [3]; Harrick [10])

💡 Professional tip: If unsure about the appropriate pathlength, start with a short path and gradually increase until the target peak transmittance is between 20%–60% (absorbance 0.2–0.7) [1][10].

3.5 Solvent compensation

When measuring solutions, the solvent itself also absorbs IR light, requiring background "subtraction" [1][3]:

Method 1: Dual-beam subtraction (old dispersive instruments)

• Place sample cell in sample beam, same solvent cell in reference beam
• Instrument automatically subtracts solvent absorption [1]

Method 2: Background collection method (modern FTIR)

• First collect spectrum of pure solvent as background
• Then collect spectrum of solution
• Software automatically calculates $-\log(I{\text{solution}}/I{\text{solvent}})$ to obtain pure sample spectrum [3]

Solvent selection principles [1][3]:

• Solvent absorption should not overlap with sample peaks

• Common solvents: CS₂ (transparent 4000–1350 cm⁻¹), CCl₄ (transparent 4000–1700 cm⁻¹), CHCl₃

• Avoid: water (strong absorption across spectrum), alcohols (O-H interference)

⚠️ Note: Although CS₂ and CCl₄ are "ideal" infrared solvents, they are highly toxic; operations must be performed in a fume hood [1].

IV. Film Method: A Powerful Tool for Polymer Measurement

4.1 Method Overview

The film method (Cast Film / Melt Film) is specifically for polymer samples [1][11]. The principle is to prepare a uniform film with a thickness of 10–50 μm and measure directly in transmission.

4.2 Solution Casting Method

Procedure [1][11]:

1. Weigh 0.1–0.5 g of polymer
2. Dissolve in 5–10 mL of appropriate solvent (e.g., PS in toluene, PMMA in acetone, PA in formic acid)
3. Pour the solution onto a glass plate or KBr window
4. Slowly evaporate the solvent in a fume hood (several hours to overnight)
5. Peel off the film and measure directly

Applicable to: Soluble polymers such as PS, PMMA, PVAC, PC, etc. [1][11]

Precautions [1][11]:

• The solvent must be completely removed; otherwise, solvent peaks will interfere with the spectrum
• Residual solvent can be accelerated by heating or using a vacuum oven
• Film thickness control: adjust via concentration and solution volume

4.3 Hot Pressing Method

Procedure [1][11]:

1. Place a small amount of polymer (0.1–0.2 g) between two aluminum foil sheets
2. Heat to 20–30 °C above the polymer melting point
3. Apply pressure of 1–3 MPa with a hot press, hold for 30 seconds to 1 minute
4. Cool to room temperature and peel off the film

Applicable to: Thermoplastic polymers such as PE, PP, PET, PA, etc. [1][11]

Advantages: No solvent residue; uniform film; suitable for insoluble polymers

4.4 Common Problems in Film Method

Table 4: Common problems in film method (data source: Hummel Polymer Analysis [11])

🔗 Further reading: For polymer IR characteristic peaks, see ftir.fun Alkyl C-H Functional Group Page (C-H peaks of PE and PP) and ftir.fun Ester Functional Group Page (C=O peaks of PET and PMMA).

4.5 Film Method Example: Measurement of Polyethylene (PE)

PE is insoluble in common solvents, so the hot pressing method is used [11]:

1. Take 0.15 g of PE pellets and sandwich between aluminum foil
2. Heat to 160 °C (PE melting point 130 °C, add 30 °C margin)
3. Apply 2 MPa pressure, hold for 45 seconds
4. Cool with water to room temperature
5. Peel off the film (thickness ~30 μm) and measure directly

Expected spectral features (ftir.fun Alkyl C-H Page):

• 2915 cm⁻¹: CH₂ asymmetric stretching
• 2850 cm⁻¹: CH₂ symmetric stretching
• 1470 cm⁻¹: CH₂ scissoring bending
• 720 cm⁻¹: CH₂ rocking (characteristic of long chains, n > 4)

V. Gas Cell: A Dedicated Tool for Gas Samples

5.1 Specifics of Gas Measurement

Gas samples have much lower molecular density than liquids and solids, requiring long path lengths to achieve sufficient absorption [1][12]:

$$A = \varepsilon \cdot c \cdot l$$

• Gas concentration c is typically ppm–%
• Path lengths of cm–m are needed to compensate [12]

5.2 Short Path Length Gas Cell (5–10 cm)

Structure: Glass or metal tube sealed with KBr/NaCl windows at both ends, length 5–10 cm [1][12]

Applicable to:

• High concentration gases (> 1%)
• Strongly absorbing gases (e.g., CO₂, CH₄)
• Pure gas vapor spectrum measurement

Typical applications: Measurement of organic solvent vapors, pure gas identification [1]

5.3 Long Path Length Gas Cell (Multireflection Type)

To achieve meter-level path lengths in a limited volume, multiple reflection designs are used [1][12]:

5.3.1 White Cell

Invented by John U. White in 1942 [12][13]:

• Three concave mirrors (one entrance mirror + two reflecting mirrors)
• Beam reflects multiple times between mirrors (tens of times)
• Path length can reach 1–20 m (cell volume hundreds of mL)
• Typical applications: Ambient air analysis, ppm-level gas detection [12][13]

📷 Figure 3: Schematic of White cell optical path
Source: Aero Laser GmbH technical documentation [13]
https://www.aero-laser.de/pro…

5.3.2 Herriott Cell

Proposed by D. R. Herriott and H. J. Schulte in 1964 [12][14]:

• Two concave mirrors placed opposite each other
• Beam forms an elliptical trajectory between mirrors
• Path length can reach 10–100 m (small volume)
• Typical applications: Laser spectroscopy, breath gas analysis [12][14]

5.4 Practical Points for Gas Sample Measurement

Pressure and Concentration [1][12]:

• Gas cells can be evacuated and then filled with sample gas
• Total pressure affects absorption peak width: low pressure (< 1 kPa) gives sharp peaks; atmospheric pressure broadens peaks
• Quantitative analysis requires precise control of pressure and temperature

Water Vapor Interference [1][12]:

• Atmospheric water vapor exhibits many sharp rotational structure peaks in the regions 3400–4000 and 1300–2000 cm⁻¹
• Solutions: Purge the optical path with dry N₂; simultaneously collect background

🔗 Further reading: Water vapor and CO₂ are the main interferences in atmospheric IR measurements. See ftir.fun Water Molecule Functional Group Page for the IR features of water molecules.

5.5 Gas Measurement Application Examples

Table 5: Typical gas IR measurement applications (data sources: Bruker Gas Analysis [12]; Aero Laser [13])

VI. Comprehensive Comparison and Selection Guide for Transmission Methods

6.1 Comparison of Sample Preparation Methods

| Solution-cast film | Soluble polymer | 10–50 μm | Hours | Medium | Medium | Solvent residue |
| Hot-pressed film | Thermoplastic polymer | 10–50 μm | 10 min | Medium | Medium | Requires hot press |
| Short-path gas cell | High concentration gas | 5–10 cm | 5 min | Easy | Medium | Concentration limit |
| Long-path gas cell | Trace gas | 1–100 m | 10 min | Difficult | Excellent | Water interference, optical alignment |

Table 6: Comprehensive comparison of sample preparation methods for transmission mode

6.2 Decision-making flowchart

When faced with a new sample, how to choose a preparation method? Refer to the following decision tree [1][3]:

                      Unknown sample
                         │
            ┌────────────┼────────────┐
            │            │            │
          Solid        Liquid        Gas
            │            │            │
     ┌──────┴──────┐     │            │
     │             │     │            │
  Soluble/Grindable  Insoluble/Not grindable  │            │
     │             │     │            │
  ┌──┴──┐       ATR     Aqueous?     Concentration?
  │     │       (Ep 14)  │            │
 KBr  ATR              Yes→CaF₂ cell High→Short cell
  pellet (Ep14)          No→KBr/NaCl   Low→Long cell
                           cell
     │             
  Polymer?        
     │             
  ┌──┴──┐          
Soluble  Thermoplastic       
Cast     Hot-press

Figure 4: Decision tree for transmission sample preparation method selection

6.3 Practical case: Sample preparation approach for an unknown sample

Case: The lab receives a bottle of pale yellow viscous liquid requiring infrared identification.

Steps:

1. Observe physical state: Liquid → Liquid cell method
2. Determine volatility: Viscous, not volatile → Either demountable or fixed cell
3. Check for water: Color and odor suggest possible water content → Choose CaF₂ windows
4. Test path length: First use 0.05 mm spacer for test
5. Observe result: If main peak saturates → reduce path; if SNR poor → increase path
6. Confirm solvent background: If sample is a solution, use pure solvent for background

Final plan: CaF₂ demountable liquid cell, 0.05 mm path length, background subtraction with pure solvent.

7. Future of transmission: Coexistence with ATR

7.1 Current status of transmission

Although ATR has become the mainstream for routine analysis (detailed in Ep 14), transmission remains irreplaceable as the standard method in the following areas [1][2][3]:

1. Pharmacopoeia identification: ChP, USP, EP for most APIs still specify KBr pellet method [4]
2. Quantitative analysis: Fixed liquid cells with controllable path length are the gold standard for quantification
3. Gas analysis: Long-path gas cells are the only choice for gas measurement
4. Reference spectral libraries: Commercial libraries (Sadtler, Aldrich) mostly use transmission method [2]
5. Teaching demonstrations: University lab courses still consider KBr pellet a basic skill

7.2 Transmission vs ATR: Not "one replaces the other"

Many beginners think that "after the advent of ATR, transmission is obsolete." This is a misconception [1][2]:

Table 7: Transmission vs ATR comparison

Conclusion: Transmission and ATR each have advantages; they are complementary rather than substitutes [1][2].

🔗 Next episode preview: Ep 14 — Sampling Techniques Primer: ATR (Attenuated Total Reflection). We will delve into the physical principles of ATR (total reflection, evanescent wave), crystal material selection (diamond, ZnSe, Ge, Si), differences between single vs multiple reflection ATR, and differences between ATR and transmission spectra and correction methods.

Summary of this episode

Questions

1. You receive a highly hygroscopic solid sample. After KBr pellet preparation, a broad peak appears at 3400 cm⁻¹. How to determine whether this is the sample's own O-H or KBr moisture? How to eliminate water interference?
2. To measure the infrared spectrum of 10% ethanol aqueous solution, which window material and path length should be chosen? Why?
3. Using KBr pellet method to measure a drug polymorph sample, you find that the C=O peak position after grinding does not match literature reports. What could be the reason? What alternative method should be used?
4. An environmental monitoring station needs to measure 5 ppm methane in ambient air. Which gas cell should be chosen? What path length is approximately required? (Assume detection limit for methane is about 1 ppm·m)
5. Why can polyethylene (PE) only be prepared by hot-pressing and not by solution casting? Explain from solubility perspective.

References

[1] Stuart B. Modern Infrared Spectroscopy. John Wiley & Sons, 1996. ISBN: 978-0-471-95917-0.

[2] Griffiths P R, de Haseth J A. Fourier Transform Infrared Spectrometry. 2nd ed. John Wiley & Sons, 2007. ISBN: 978-0-471-19404-0.

[3] Purdue University. "Infrared Spectroscopy: Sample Preparation." Analytical Chemistry CHM 325 Lecture Notes.
https://www.chem.purdue.edu/a…

[4] Chinese Pharmacopoeia Commission. Pharmacopoeia of the People's Republic of China 2020 Edition, Part IV, General Rule 0402 Infrared Spectrophotometry. China Medical Science Press.

[5] Skoog D A, Holler F J, Crouch S R. Principles of Instrumental Analysis. 6th ed. Thomson Brooks/Cole, 2007. Chapter 16.

[6] University of Michigan. "FTIR Sample Preparation: KBr Pellet Method." Department of Chemistry Teaching Resources.
https://www.umich.edu/~chemen…

[7] Sigma-Aldrich. "IR Sampling Materials: Window Selection Guide." Technical Article.
https://www.sigmaaldrich.com/…

[8] Shimadzu. "Preparing KBr Pellets for FTIR Analysis." Application Note FTIR-0201.
https://www.shimadzu.com/an/s…

[9] Bruker. "FTIR Sample Preparation Guide." Technical Note TN-FTIR-007.
https://www.bruker.com/en/pro…

[10] Harrick Scientific. "Liquid Sampling for IR Spectroscopy." Application Notes.
https://harricksci.com/applic…

[11] Hummel D O, Scholl F. Atlas of Polymer and Plastics Analysis. 3rd ed. Carl Hanser Verlag, 1991. ISBN: 978-3-446-15752-5.

[12] Bruker. "Gas Analysis by FTIR Spectroscopy." Application Note AN-FTIR-GAS-014.
https://www.bruker.com/en/app…

[13] Aero Laser GmbH. "White Multi-Pass Gas Cells for FTIR." Technical Documentation.
https://www.aero-laser.de/pro…

[14] Herriott D R, Schulte H J. "Folded Optical Delay Lines." Applied Optics, 1965, 4(8): 883–889. DOI:10.1364/AO.4.000883.

[15] LibreTexts. "10.8: Infrared Spectroscopy — Sample Preparation." Organic Chemistry.
https://chem.libretexts.org/B…

[16] JoVE. "Infrared Spectroscopy to Determine Functional Groups in Organic Compounds." Science Education, Analytical Chemistry.
https://www.jove.com/science-…

Next Episode Preview: Ep 14 — Introduction to Sampling Techniques: ATR (Attenuated Total Reflection) — The Most Commonly Used Modern Method
We will delve into the physics of ATR: how total reflection creates an "evanescent wave" at the sample surface, how the evanescent wave "steals" molecular information, and why diamond ATR has become the standard in modern laboratories.

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