Ep 11 — Dispersive IR vs Fourier Transform Infrared: Why FTIR Won?

Series: Encyclopedia of Infrared Spectroscopy: From Principles to Practice
Section: Part Two · Beginner Level — Entering the Lab
Target Audience: Undergraduate and graduate students, laboratory technicians new to the lab
Prerequisites: Ep 01–10 (all content from introductory section)
Reading Time: Approximately 30 minutes

Introduction: The Revolution from 15 Minutes to 1 Second

Imagine this scenario: In the 1960s, a chemistry graduate student needed to measure an infrared spectrum. He carefully placed a KBr pellet into the sample holder, started the instrument, and then... waited 10 to 15 minutes as the recording pen slowly drew one peak after another on the chart [1][2]. If the sample absorbed moisture or the concentration was wrong, he had to start over—another 15 minutes.

Today, you place your sample on an ATR crystal, press the "acquire" button, and the complete spectrum appears on the screen in 1 second [1][2]. If higher signal-to-noise ratio is needed, scanning 32 times takes only half a minute.

This earth-shattering change stems from a technological revolution—Fourier transform infrared spectroscopy (FTIR) replacing traditional dispersive infrared spectrometers [1][3].

In this episode, we will answer three questions:

1. How does a dispersive infrared spectrometer work?
2. How does FTIR work, and how can it accomplish in 1 second what takes a dispersive instrument 15 minutes?
3. Why do almost all laboratories use FTIR today?

1. Dispersive Infrared Spectrometer: The "Workhorse" of Sequential Scanning

1.1 Basic Structure

A dispersive infrared spectrometer consists of three core components [1][4]:

Table 1: Basic components of a dispersive infrared spectrometer (Data sources: LibreTexts [1]; Bruker [4])

1.2 Working Principle—"Measuring One Frequency at a Time"

The workflow of a dispersive instrument is as follows [1][4]:

Source → Chopper (alternates sample/reference) → Monochromator (prism/grating) → Slit → Detector
                                               ↑
                                      Rotating grating to select wavelengths one by one

1. The source emits broadband infrared radiation.
2. The beam is split by a chopper into two paths—one through the sample and one through the reference (double-beam design).
3. The chopper alternately directs the sample and reference beams to the monochromator.
4. The monochromator uses a prism or diffraction grating to disperse the polychromatic light into different frequencies.
5. By rotating the grating, a specific wavelength is selected sequentially.
6. A slit allows a narrow frequency band to reach the detector.
7. The entire infrared region is scanned step by step, measuring the absorbance at each frequency point.

LibreTexts describes this directly [1]:

"The wavelengths are measured one at a time, with the slit controlling the spectral bandwidth."

"Each frequency is measured individually by the detector, so scanning the entire infrared region is very slow."

1.3 Prism vs Grating—Two Generations of Dispersive Elements

Dispersive instruments evolved from prism to grating designs [5]:

Table 2: Comparison of prism and diffraction grating (Data source: Shimadzu [5])

First-generation infrared spectrometers (1940s) used NaCl prisms, with narrow scanning range, poor reproducibility, and high sensitivity to temperature and humidity [1][6]. Second-generation (1960s) switched to diffraction gratings, offering improved performance but still suffering from fundamental drawbacks such as low sensitivity, slow scanning, and poor wavelength accuracy [1].

1.4 The Fatal Bottleneck of Dispersive Systems—The Trade-off between Resolution and Sensitivity

Dispersive instruments have an insurmountable fundamental contradiction [1][4]:

• Narrower slit → Higher resolution, but less light energy reaches the detector → Lower sensitivity
• Wider slit → Higher throughput, but adjacent wavelengths cannot be resolved → Lower resolution

This means you cannot achieve both high resolution and high sensitivity simultaneously—you always have to compromise between the two.

Thermo Fisher's technical note states [3]:

"Dispersive spectrometers require an external wavelength calibration source, since there is no internal reference."

In other words, dispersive instruments also require external calibration to define the wavelength axis, which adds uncertainty and operational complexity [3].

📷 Figure 1: Optical path diagram of a dispersive infrared spectrometer
Source: LibreTexts Analytical Chemistry [1]
https://chem.libretexts.org/C…

2. Fourier Transform Infrared Spectrometer (FTIR): Capturing the Full Spectrum in One Shot

2.1 Core Idea—Replacing the Monochromator with an Interferometer

The key change in FTIR is: replacing the prism/grating monochromator with a Michelson interferometer, eliminating the need for a slit and sequential frequency scanning [2][7].

Bruker explains it concisely [4]:

"FT-IR (Fourier transform infrared) spectrometers collect an interferogram using an interferometer, then mathematically transform it into a spectrum using Fourier transformation."

2.2 Structure of the Michelson Interferometer

The Michelson interferometer consists of the following components [2][7][8]:

                    Fixed mirror
                      ↕
   Source → Beamsplitter ←→ Moving mirror (constant velocity)
            ↓
          Detector

The two beams recombine at the beamsplitter after reflection, producing interference [2][7].

💡 Preview: The detailed principle of the Michelson interferometer will be covered in Ep 12.

2.3 From Interferogram to Spectrum—Fourier Transform

What the FTIR instrument directly collects is not a spectrum, but an interferogram [2][7].

Shimadzu's FTIR technical communication clearly states [2]:

"The data directly acquired by the FTIR instrument is in the form of an interferogram... To get a normal spectrum with the wavenumber (wavelength) along the horizontal axis requires Fourier transform by a computer. This is the major characteristic of the FTIR instrument and differentiates it from a dispersive spectrophotometer, which measures spectra directly."

Mathematical Description [2][8]:

When the moving mirror moves to produce an optical path difference δ, for monochromatic light with wavenumber σ, the intensity at the detector is:

$$I(\delta) = B(\sigma) \cos(2\pi\sigma\delta)$$

For a broadband continuous spectrum, the interferogram is the superposition of all frequencies:

$$I(\delta) = \int_0^\infty B(\sigma) \cos(2\pi\sigma\delta)\, d\sigma$$

Through Fourier transform, the spectrum is retrieved from the interferogram:

$$B(\sigma) = \int_0^\infty I(\delta) \cos(2\pi\sigma\delta)\, d\delta$$

where B(σ) is the familiar spectrum—the light intensity at each wavenumber σ [2][8].

Key Points [2][7]:

• When δ=0, all wavelengths interfere constructively, forming the center burst—a hallmark feature of the interferogram
• In the interferogram generated by the moving mirror scan, each data point contains information from all infrared frequencies
• Fourier transform must be performed by a computer to obtain the "absorbance vs. wavenumber" spectrum

Interferogram (time domain)        Fourier transform          Spectrum (frequency domain)

  │ center burst                                         │ peak
  │   /\                                                 │   /\
  │  /  \    ~~~~    ~~~~                               │  /  \    /\
  │ /    \~~    \~~    \~~                              │ /    \~~  \~~
  └────────────────────────→ δ       →         └────────────────────→ σ
  (optical path difference)                             (wavenumber cm⁻¹)

📷 Figure 2: Comparison of interferogram and Fourier-transformed spectrum
Source: Shimadzu FTIR Talk Letter vol.15 [2]
https://www.shimadzu.com/an/s…

2.4 He-Ne Laser — built-in "ruler"

The FTIR instrument contains a helium-neon (He-Ne) laser (wavelength 632.8 nm), and the laser beam travels parallel to the infrared light through the interferometer [3][9].

Roles of the laser [3][9]:

1. Precise tracking of the moving mirror position — each zero-crossing of the laser interference fringe triggers a data acquisition
2. Absolute calibration of the wavenumber axis — because the He-Ne laser wavelength is known with extreme accuracy and stability

An impressive statement from Thermo Fisher's introductory material [3]:

"These instruments are self-calibrating and never need to be calibrated by the user."

"These instruments are self-calibrating and never need to be calibrated by the user."

This means that the wavenumber accuracy of FTIR is determined by the laser wavelength — no external calibration required, and with extremely high long-term stability [3][9].

III. Three Core Advantages of FTIR

The advantages of FTIR over dispersive IR can be summarized as three "secret weapons," each named after its proposer [9][10][11].

3.1 Fellgett Advantage — Multiplex Advantage

Proposer: Peter Fellgett, 1951–1952 [9][10]

Principle [9][10]:

In a dispersive instrument, only one narrow frequency band is measured at a time. If the entire spectrum has N resolvable elements, each element can only be allocated 1/N of the total measurement time T.

In FTIR, the detector receives all frequencies simultaneously (multiplexing), so each frequency enjoys the entire measurement time T [9][10].

Dispersive instrument (example with N=5 frequencies):

Time →  ██████ ██████ ██████ ██████ ██████
        Freq1   Freq2   Freq3   Freq4   Freq5
        Each frequency only gets T/5 time

FTIR instrument (all frequencies measured simultaneously):

Time →  ██████████████████████████████
        Freq1+Freq2+Freq3+Freq4+Freq5
        Each frequency enjoys the full T time

Quantitative Conclusion [9][10]:

Assuming the noise is mainly detector noise and independent of signal (as is the case for infrared detectors), the signal-to-noise ratio of FTIR compared to dispersive is improved by approximately √N times, where N is the number of resolvable elements.

"Fellgett in 1952 showed that advantages can be obtained by doing multiplex spectroscopy; that is, examining the whole spectrum, all of the dispersed light, for all of the recording time... In the infrared spectral region, where all detectors are small point detectors limited by noise which is independent of signal level, the multiplex method can result in a decrease in recording time by a factor equal to the number of resolved elements."
—— JHU APL Technical Digest (Hochheimer) [9]

LibreTexts gives a more intuitive description [10]:

"Because the detector monitors all frequencies simultaneously, a spectrum takes approximately one second to record, as compared to 10–15 minutes when using a scanning monochromator."

Practical Effect: Over the range 4000–400 cm⁻¹ at 4 cm⁻¹ resolution, there are about 900 resolvable elements. √900 = 30, so theoretically the SNR of FTIR is about 30 times that of a dispersive instrument (Fellgett advantage alone) [9].

3.2 Jacquinot Advantage — Throughput Advantage

Proposer: Pierre Jacquinot, 1954 [9][10]

Principle [9][10]:

Dispersive instruments require slits to select a single wavelength, and slits block most of the light energy. In contrast, interferometers use a circular aperture (J-stop) without slits, greatly increasing the light throughput.

Dispersive instrument:                    FTIR instrument:

Source → [slit] → grating → [slit]      Source → [circular aperture] → interferometer → detector
        ↑blocks most light                   ↑does not block light

Quantitative conclusion [9][10]:

"Because an interferometer does not use slits and has fewer optical components from which radiation is scattered and lost, the throughput of radiation reaching the detector is 80–200 times greater than for a dispersive spectrometer."
—— LibreTexts (Harvey) [10]

That is, the light throughput of FTIR is 80–200 times higher than that of dispersive instruments [9][10].

Historical literature from JHU APL explains this [9]:

"This increase in light-gathering power comes about because interferometers can use a large circular entrance aperture while spectrometers are limited by narrow slits."

3.3 Connes Advantage——Wavelength Precision Advantage

Proposer: Pierre Connes (French physicist) [3][9]

Principle [3][9]:

As mentioned, FTIR instruments have a built-in He-Ne laser providing absolute calibration of the wavenumber axis. The wavelength precision of dispersive instruments depends on [3][9]:

• (i) External calibration standards
• (ii) Uniformity of mechanical movement of the grating and slits

Jaggi & Vij discuss this in detail in their monograph [9]:

"An FT-IR spectrometer determines frequencies by direct comparison with a visible laser output, usually a He-Ne laser. Potentially, this offers an improvement in frequency accuracy and was determined by Connes... With dispersive instruments, frequency precision and accuracy depend on (i) calibration with external standards and (ii) ability of electromechanical mechanisms to uniformly move gratings and slits. By contrast, in FT-IR spectrometers the direct laser reference has the advantage that no calibration is required."

Actual precision [10]:

• FTIR wavenumber precision: ±0.01 cm⁻¹
• Dispersive instrument wavenumber precision: depends on mechanical movement precision, easily affected by temperature drift

3.4 Combined Effect of the Three Major Advantages

Table 3: Three core advantages of FTIR

IV. SNR Comparison: How Much Better?

4.1 Combined SNR Improvement

The SNR improvement of FTIR over dispersive IR is the combined result of the Fellgett advantage and the Jacquinot advantage [9][11].

Quantitative estimation (Hochheimer, JHU APL, etc.; varies with spectral range, resolution, and noise model) [9]:

• Fellgett advantage: order of √N. As previously, with 4000–400 cm⁻¹ and 4 cm⁻¹ resolution, N≈900 → about 30 times; if a wider effective bandwidth or different resolution assumption is taken, √N can also be about several tens of times (in literature examples, it may be written as ~50 times). The two are not mutually exclusive but are based on different premises.
• Jacquinot advantage: can be up to about 10² times order (strongly dependent on aperture/slit comparison)
• Combined: Historical estimates often give an effective improvement of about 10² times; do not write a specific multiple as the factory specification of all modern instruments.

"Thus, the signal-to-noise ratio for the interferometer is about hundred times the signal-to-noise ratio for the grating spectrometer."
—— Jaggi & Vij, Chapter 9 [9]

Fahelelbom et al. in Reviews in Analytical Chemistry (2022) point out [11]:

"It is typical to obtain an SNR of 100 or higher by modern FTIR spectrometers. Non-FTIR spectrometers suffer relatively low SNRs. The reason might be attributed to the fact that the beam in such instruments needs to pass through slits, prisms, gratings, and gets reflected..."

4.2 Modern FTIR Performance Example

Take the Thermo Scientific Nicolet iS10 as an example [3]:

• Signal-to-noise ratio: up to 10,000:1 in 5 seconds
• Resolution: better than 0.4 cm⁻¹
• Scan speed: full spectrum collection in 1 second

4.3 Multiple Scan Co-adding——Further Sensitivity Enhancement

FTIR's fast scanning capability brings another advantage: multiple scans can be performed and averaged in a short time [1][3].

Since random noise accumulates as √N (N = number of scans) and the signal accumulates linearly, the SNR improves by √N with the number of scans [1]:

• 1 scan: SNR = S
• 16 scans: SNR = 4S (4x improvement)
• 64 scans: SNR = 8S (8x improvement)
• 256 scans: SNR = 16S (16x improvement)

A dispersive instrument takes 15 minutes for one scan, and 64 scans would take 16 hours—completely impractical. In contrast, FTIR can complete 64 scans in about 1 minute [1].

4.4 Important Prerequisite: Applicability of Fellgett Advantage

It must be emphasized that the Fellgett multiplex advantage holds only when the noise is primarily detector-noise limited and the noise is independent of the signal [9][11].

Infrared thermal detectors (TGS, MCT, etc.) are exactly such cases—noise is mainly determined by the detector's thermal noise, independent of the signal [9].

However, in the visible/ultraviolet region, shot noise dominates—noise is proportional to the square root of the signal intensity, and the multiplex advantage does not apply, and may even become a "multiplex disadvantage" [12]. This is one reason why dispersive instruments still retain a place in the VIS/NIR region.

5. Historical Evolution: A Century from Prisms to FTIR

5.1 Key Milestones

Table 4: Key milestones in the development of IR spectrometers (Data sources: LibreTexts [1]; Technology Networks [6]; JASCO [13])

5.2 Three Generations of IR Spectrometers

LibreTexts divides the development of IR spectrometers into three generations [1]:

"The first generation IR spectrometer was invented in late 1950s. It utilizes prism optical splitting system... The second generation IR spectrometer was introduced to the world in 1960s. It utilizes gratings as the monochrometer... The invention of the third generation IR spectrometer, Fourier transform infrared spectrometer, marked the abdication of monochrometer and the prosperity of interferometer."

Table 5: Comparison of three generations of IR spectrometers

5.3 Cooley-Tukey FFT Algorithm—A Paper That Changed History

The biggest bottleneck for FTIR commercialization was the computational speed of Fourier transforms [14][15].

Problem: Direct computation of the discrete Fourier transform (DFT) requires O(N²) complex multiplications—for a dataset of N=1000, millions of operations, which could not be performed in real time on computers of the 1960s [14].

Breakthrough: In 1965, James Cooley of IBM and John Tukey of Princeton University published a short paper of only 5 pages in Mathematics of Computation—"An Algorithm for the Machine Calculation of Complex Fourier Series" [14].

The divide-and-conquer algorithm they proposed decomposes a DFT of length N into two DFTs of length N/2 [14]:

DFT_N[k] = DFT_even[k] + W_N^k × DFT_odd[k]

Complexity breakthrough [14]:

• Naive DFT: O(N²)
• Cooley-Tukey FFT: O(N log N)
• For N=1024: from 1,048,576 to 10,240—about 100 times faster

5.4 The Legendary Story of FFT

The birth of FFT has an interesting story [14][15]:

• 1963: Meeting of the Kennedy Administration's Science Advisory Committee discusses monitoring Soviet underground nuclear tests (using seismographs to distinguish earthquakes from nuclear explosions), requiring fast Fourier analysis
• Tukey proposes the divide-and-conquer algorithm idea at the meeting
• Richard Garwin (IBM physicist) recognizes the universality of the algorithm and passes it to Cooley at IBM Watson Research Center for implementation
• Cooley completes the program implementation in about 6 weeks
• 1965 paper published

Rockmore (Dartmouth College) writes in his historical review of FFT [14]:

"This publication, as well as Garwin's fervent proselytizing, did a lot to publicize the existence of this (apparently) new fast algorithm... Tukey did not work at IBM, the patentability of the idea was doubted and the algorithm went into the public domain."

Gilbert Strang (MIT) in 1994 called FFT "the most important numerical algorithm of our time" [14].

FFT was named by IEEE Computing in Science & Engineering as one of the top ten algorithms of the 20th century [14].

Interesting discovery: Gauss had similar ideas in his private manuscripts in 1805 (for asteroid orbit interpolation), but never published [14][15]. In 1942, Danielson & Lanczos also published a similar algorithm for X-ray crystallography [14].

5.5 Significance of FFT for FTIR

Technology Networks' historical review points out [6]:

"In 1966 the development of the Cooley-Tukey algorithm provided a short-cut calculation, the 'fast Fourier transform' or FFT. This, along with the advent of the first commercial computing systems, allowed the launch of the first commercial FTIR, the FTS-14, in 1969."

FFT reduced the computation time of Fourier transforms from hours to seconds. Combined with the widespread adoption of microcomputers in the 1970s-80s, it unlocked the practical potential of FTIR design, transforming it from an expensive instrument only for advanced research into a routine laboratory device [6][13].

📷 Figure 3: 1969 Digilab FTS-14—the world's first commercial FTIR
Source: Technology Networks [6]
https://www.technologynetwork…

6. Why did FTIR win?

Based on the above analysis, FTIR comprehensively surpasses dispersive IR in the following aspects [1][3][9][10]:

Table 6: Comprehensive comparison of dispersive IR vs FTIR

Technical documentation from Thermo Fisher concludes [3]:

"Today, FT-IR is the standard for organic compound identification work in academic, analytical, QC/QA and forensics laboratories."

JASCO's historical review also confirms [13]:

"By the 1980s, dispersive infrared spectrometers were phased out for the more desirable interferometer type spectrometers as computers became more widespread."

Summary in one sentence: FTIR is not just slightly better than dispersive IR in one aspect; it achieves order-of-magnitude leaps simultaneously in speed, sensitivity, accuracy, and resolution. This overwhelming advantage caused dispersive IR to essentially exit the field of general analytical chemistry after the 1980s [1][13].

7. "Remaining strongholds" of dispersive IR

Although FTIR dominates general analysis in the mid-infrared (MIR), dispersive instruments still find use in the following areas [12][16][17]:

7.1 Visible and near-infrared region (VIS-NIR)

In the VIS/NIR region, photon energy is higher, and the main limitation is shot noise — noise is proportional to signal, and the Fellgett multiplex advantage is lost [12].

Newport technical documentation points out [12]:

"Dispersive spectrometers are typically used for VIS and NIR spectral regions rather than for the MIR region."

7.2 Process online analysis / Industrial process monitoring

Dispersive NIR instruments are more robust, allow model transfer, and are friendly to fiber optic coupling [16].

Metrohm blog states [16]:

"Dispersive instruments are a robust possibility with ideal opportunities for model transfer, high resolution, and high light throughput even for sensitive applications. Dispersive NIR is at least as good as FT-NIR."

7.3 Portable / miniaturized instruments

Dispersive designs (especially spectrographs based on detector arrays) facilitate hardware miniaturization [12].

7.4 Emerging dispersive technology — QCL

Mid-infrared dispersive spectrometers based on tunable quantum cascade lasers (QCL) have, in time-resolved chemical reaction monitoring, for the first time surpassed the sensitivity of research-grade FTIR on specific evaluation parameters, without the need for liquid nitrogen cooling [17].

🔗 Further reading: Emerging infrared light source technologies such as QCL will be introduced in the advanced section (Ep 36–45).

7.5 NDIR gas analysis

Simple filter-type non-dispersive infrared (NDIR) instruments for quantitative gas analysis are still widely used in atmospheric monitoring, automotive exhaust detection, etc. These instruments are essentially a simplified form of dispersive type.

8. Other advantages of FTIR

Besides the three core advantages, FTIR also has the following benefits [1][3][9]:

8.1 Low stray light interference

The interferometer is essentially unaffected by stray light — because all frequencies are modulated by interference, stray light (unmodulated ambient light) is not synchronously detected by the detection system [3][9].

8.2 Mechanical simplicity and reliability

FTIR has only one moving part (the moving mirror), whereas dispersive instruments have multiple precision mechanical components such as grating rotation mechanisms and slit adjustment mechanisms [1][3]. Fewer moving parts → higher reliability → lower maintenance costs.

8.3 Wide scanning range

By changing beamsplitters and detectors, FTIR can cover a very wide range from ultraviolet to far-infrared (1000 ~ 10 cm⁻¹) [3]. Dispersive instruments require changing prisms/gratings to switch wavelength ranges.

8.4 Non-destructive

FTIR measurement does not destroy the sample, and with ATR accessories, even sample preparation is unnecessary [3].

🔗 Further reading: ATR technology will be explained in detail in Ep 14. Infrared data for various functional groups can be found at ftir.fun.

9. Current status and future of FTIR instruments

9.1 Typical performance of modern FTIR

Taking commonly used laboratory FTIR instruments as an example [3][11]:

9.2 Future trends

🔗 Further reading: FTIR imaging and hyphenated techniques will be covered in intermediate and advanced sections.

Summary of this episode

Table 7: Quick reference of core knowledge points from this episode

Review Questions

1. How does the slit width of a dispersive infrared spectrometer affect resolution and sensitivity? Why is this an irreconcilable contradiction?

2. What is the raw data collected by an FTIR instrument? Why is Fourier transform needed to obtain the familiar spectrum?

3. What assumption is the mathematical derivation of Fellgett's multiplex advantage based on? Under what conditions does this advantage fail? (Hint: consider the type of noise)

4. If a dispersive infrared instrument takes 15 minutes to scan a full spectrum, how many scans are needed to achieve the same signal-to-noise ratio as FTIR (1 second/scan)? How long will it take in total?

5. Why is the Cooley-Tukey FFT algorithm crucial for the commercialization of FTIR? What bottleneck did FTIR face before the advent of FFT?

6. Why are dispersive instruments still used in the visible/near-infrared region, while they have been largely replaced by FTIR in the mid-infrared region?

7. What role does the He-Ne laser play in FTIR? Without it, how would the wavenumber accuracy of FTIR be affected?

References

[1] LibreTexts. "4.3: Instrumentation." Chemistry 219 - Heffern, Infrared Spectroscopy.
https://chem.libretexts.org/C…

[2] Shimadzu. "Fourier Transform and Apodization." FTIR Talk Letter, vol.15.
https://www.shimadzu.com/an/s…

[3] Thermo Fisher Scientific. "Introduction to FTIR." Brochure BR50555.
https://documents.thermofishe…

[4] Bruker. "FT-IR or IR Spectroscopy? What is the Difference?"
https://www.bruker.com/en/pro…

[5] Shimadzu. "Monochromators." Technical Support, Analysis Basics.
https://www.shimadzu.com.au/s…

[6] Technology Networks. "IR Spectroscopy and FTIR Spectroscopy: How an FTIR Spectrometer Works and FTIR Analysis." 2023.
https://www.technologynetwork…

[7] Rioux, F. "The Michelson Interferometer and Fourier Transform Spectroscopy." CSB|SJU.
http://www.users.csbsju.edu/~…

[8] Rundle, R. "Construction of a Michelson Interferometer for Fourier Spectroscopy." NIST Journal of Research, 1965, 69C(1).
https://nvlpubs.nist.gov/nist…

[9] Hochheimer, B.F. "Fourier Transform Spectroscopy." JHU APL Technical Digest, 1986, 7(2).
https://secwww.jhuapl.edu/tec…

[10] Harvey, D. "6.1: Overview of Spectroscopy." Instrumental Analysis (CHEM 311), LibreTexts.
https://chem.libretexts.org/C…

[11] Fahelelbom, K.M. et al. "Recent Applications of Quantitative Analytical FTIR Spectroscopy in Pharmaceutical, Biomedical, and Clinical Fields: A Brief Review." Reviews in Analytical Chemistry, 2022, 41(1).
https://www.researchgate.net/…

[12] Newport. "IR Absorption Spectroscopy." Technical Note.
https://www.newport.com/n/ir-…

[13] JASCO. "Theory of FTIR Spectroscopy." Learning Center.
https://jascoinc.com/learning…

[14] Rockmore, D. "The FFT: An Algorithm the Whole Family Can Use." Stanford CS339 Lecture Notes.
https://web.stanford.edu/clas…

[15] TechHistoryLab. "The FFT: How Gauss Discovered It 160 Years Before."
https://techhistorylab.com/ga…

[16] Metrohm. "Five Myths about Online Dispersive NIR, FT-NIR, and FT-IR Spectroscopy." Blog.
https://www.metrohm.com/pl_pl…

[17] Dabrowska, A. et al. "Mid-IR Dispersion Spectroscopy vs FT-IR." Applied Spectroscopy, 2024.
https://journals.sagepub.com/…

[18] Jaggi, N. & Vij, A. "Chapter 9: Fourier Transform Infrared Spectroscopy."
https://www.researchgate.net/…

Preview of Next Episode: Ep 12 — The Michelson Interferometer: Heart of FTIR
Today we learned why FTIR won. In the next episode, we will dive into the "heart" of FTIR—the Michelson interferometer—explaining in detail how interferograms are generated, the relationship between moving mirror travel distance and resolution, the selection of beamsplitter materials, and the precise mechanism of He-Ne laser referencing.

This article is licensed under CC BY-NC-SA 4.0. Images are from public domain or online resources with attributed sources; copyright belongs to the original authors.