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An Expert’s 5-Step Guide: How Does FTIR Work in 2025?

December 25, 2025

Abstract

Fourier Transform Infrared (FTIR) spectroscopy is a powerful analytical technique used for the identification and quantification of molecular compounds in a vast array of sample types, including solids, liquids, and gases. The methodology operates by irradiating a sample with a broadband infrared light source. The core of an FTIR instrument is the Michelson interferometer, which modulates the infrared beam, creating a unique signal called an interferogram. This interferogram, a plot of light intensity versus the optical path difference generated by a moving mirror, contains information about all infrared frequencies simultaneously. The sample absorbs specific frequencies of infrared radiation corresponding to the vibrational energies of its molecular bonds. The attenuated light, in the form of the interferogram, is then detected. A mathematical process known as the Fourier Transform is applied to the interferogram, converting this time-domain signal into a frequency-domain spectrum. The resulting infrared spectrum plots absorbance or transmittance against wavenumber, producing a unique molecular "fingerprint" that allows for detailed chemical analysis.

Key Takeaways

  • FTIR collects data for all infrared frequencies at once, offering significant speed and signal-to-noise advantages.
  • The Michelson interferometer is the central component that modulates the infrared light to create an interferogram.
  • A mathematical Fourier Transform converts the raw interferogram data into a readable infrared spectrum.
  • To understand how does FTIR work, one must grasp the interaction between IR light and molecular vibrations.
  • The resulting spectrum is a unique molecular fingerprint used for both qualitative and quantitative analysis.
  • Proper sample preparation is paramount for obtaining a high-quality, interpretable FTIR spectrum.
  • FTIR offers higher throughput and wavenumber accuracy compared to older dispersive infrared techniques.

Table of Contents

Step 1: The Genesis of the Signal – Generating and Guiding Infrared Light

To embark on the exploration of how an FTIR spectrometer functions, we must begin at the very origin of the process: the creation of light. It is not just any light, but a specific band of electromagnetic radiation that holds the key to unlocking molecular secrets. This initial step involves generating a stable, continuous, and broad range of infrared frequencies and carefully guiding this energy toward the analytical heart of the instrument. The quality and stability of this initial beam are foundational to the precision of the final measurement.

The Nature of Infrared Radiation

Before we can appreciate how the instrument manipulates light, we must first develop a feel for the light itself. The infrared (IR) portion of the electromagnetic spectrum resides just beyond the red light visible to the human eye, spanning a wavelength range from approximately 780 nanometers to 1 millimeter. In the context of molecular spectroscopy, we are most concerned with the mid-infrared region, typically defined as 2,500 to 25,000 nanometers, or, in the more common units of spectroscopists, 4000 to 400 cm⁻¹ (wavenumbers).

Why this specific region? The answer lies in the world of molecular mechanics. Molecules are not static structures; their atoms are in constant motion, connected by bonds that behave much like springs. These bonds can stretch, bend, rock, and twist. Each of these vibrational modes has a characteristic frequency, a natural resonance at which it prefers to oscillate. As it happens, the energy required to excite these fundamental molecular vibrations corresponds precisely to the energy of photons in the mid-infrared range (nanoAnalytics, 2025). When a molecule is struck by an infrared photon possessing the exact energy of one of its vibrational modes, the photon is absorbed, and the amplitude of that specific vibration increases. This selective absorption is the fundamental physical phenomenon upon which FTIR spectroscopy is built. Imagine tapping a series of tuning forks, each with a different pitch. Only the sound wave that matches a fork's specific pitch will cause it to resonate. In the same way, only IR light of the right frequency (or energy) will be absorbed by a specific molecular bond.

Sourcing the Light: The Infrared Emitter

The goal of the IR source is to produce a bright, stable beam of light that contains all the frequencies within the mid-infrared range needed for the analysis. The instrument does not scan through individual frequencies one by one; instead, it uses a broadband source that emits them all simultaneously. This is a crucial point that we will return to when discussing the advantages of FTIR.

The most common type of source used in modern FTIR spectrometers is an inert solid heated to incandescence, typically between 1,000 and 1,800 °C. At these temperatures, the material glows brightly, emitting a continuous spectrum of radiation that approximates a theoretical black-body radiator. The choice of material is critical for ensuring longevity and a consistent output across the desired spectral range.

One of the most prevalent sources is the Globar, a rod of silicon carbide (SiC). When an electric current is passed through it, it heats up to around 1,200 °C, emitting strong radiation from about 5,500 to 400 cm⁻¹. Its output is particularly robust in the lower frequency "fingerprint" region, which is rich with information. Another common source is the Nernst glower, a cylinder made from a mixture of rare-earth oxides (like zirconium, yttrium, and erbium). It operates at higher temperatures (around 1,500 °C) and provides intense radiation, though its output can be weaker than a Globar below 600 cm⁻¹. Some instruments, particularly those designed for the near-infrared (NIR) region, might use a tungsten-halogen lamp, which is more familiar as a type of household lighting but provides excellent intensity at higher frequencies (shorter wavelengths).

The stability of this source is paramount. Any fluctuation or drift in the source's temperature or output intensity during a measurement will be encoded into the signal, appearing as noise or artifacts in the final spectrum. High-quality FTIR instruments employ sophisticated power supplies and feedback mechanisms to maintain the source at a constant temperature, ensuring the light bathing the sample is as steady as possible.

Guiding the Beam: The Role of Mirrors and Optics

Once generated, this divergent cone of infrared light cannot simply be left to its own devices. It must be collected, collimated (made into a parallel beam), and precisely directed through the rest of the optical path. Unlike visible light optics, which can use glass lenses, mid-infrared radiation is absorbed by glass. Therefore, FTIR instruments rely almost exclusively on mirrors for focusing and directing the beam. These mirrors are typically coated with a highly reflective material like gold, which offers excellent reflectivity (>98%) across the entire mid-IR range (Newport, 2025).

The use of mirrors, particularly off-axis parabolic mirrors, is a point of elegant optical engineering. A parabolic mirror has the unique property of taking light from a point source at its focus and reflecting it as a perfectly parallel (collimated) beam. This collimated beam is essential for the proper functioning of the interferometer, the next stop on our journey. Any divergence in the beam as it passes through the interferometer can degrade the instrument's resolution (Newport, 2025).

The entire optical path, from the source to the detector, is often enclosed and may be purged with a dry, inert gas like nitrogen or dry air. This is because both water vapor (H₂O) and carbon dioxide (CO₂) are present in the ambient atmosphere and are strong absorbers of infrared light. If not removed from the optical path, their absorption signals would be superimposed on the sample's spectrum, potentially obscuring important features. Imagine trying to listen to a quiet conversation in a noisy room; the atmospheric absorptions are like background chatter that needs to be silenced to hear the sample's "voice" clearly. Modern instruments, like the advanced FTIR spectrometers, often feature a hermetically sealed and desiccated optical bench to permanently eliminate this interference, ensuring exceptionally stable and reproducible results over the instrument's lifetime (Anton Paar, 2025).

This first step, while seemingly straightforward, lays the groundwork for everything that follows. A powerful, stable, and well-collimated beam of broadband infrared radiation is now prepared and aimed at the heart of the system: the interferometer.

Step 2: The Heart of the Instrument – Modulation by the Michelson Interferometer

We have now generated a steady, broad beam of infrared light. If we were to simply pass this light through a sample and onto a detector, we would get a single reading of total light intensity. We would know that some light was absorbed, but we would have no way of knowing which frequencies were absorbed. This is the central problem that FTIR solves, and it does so using a device of remarkable ingenuity: the Michelson interferometer. This is the stage where the magic truly begins, where the light is encoded with information that will allow us to unravel its spectral content. To fully grasp how does FTIR work, one must develop an intuition for the function of this elegant optical engine.

Anatomy of the Interferometer

The Michelson interferometer, as implemented in an FTIR spectrometer, consists of three primary optical components:

  1. A Beamsplitter: This is a semi-reflective mirror, often made of a material like potassium bromide (KBr) coated with a thin layer of germanium. Its job is to split the incoming collimated infrared beam into two separate beams of roughly equal intensity. Approximately 50% of the light passes straight through to a fixed mirror, while the other 50% is reflected at a 90-degree angle toward a moving mirror.
  2. A Fixed Mirror: This is a stationary, highly reflective mirror. Its sole purpose is to reflect its portion of the beam directly back toward the beamsplitter.
  3. A Moving Mirror: This mirror is identical to the fixed mirror in reflectivity, but it is mounted on a mechanism that allows it to travel back and forth along the axis of the light beam. The movement of this mirror must be exceptionally smooth and precise. In modern instruments, this is often achieved using frictionless air bearings or highly precise mechanical drives.

The two beams, one having traveled to the fixed mirror and back, the other to the moving mirror and back, are recombined at the beamsplitter before proceeding toward the sample and detector (Newport, 2025). It is in this recombination that the crucial phenomenon of interference occurs.

The Dance of Waves: Optical Path Difference and Interference

Let's trace the journey of a single frequency (a monochromatic wave) of light. When the moving mirror is at a position where its distance from the beamsplitter is exactly the same as the fixed mirror's distance, the two beams travel identical path lengths. This position is called the Zero Path Difference (ZPD). When the two beams recombine at the beamsplitter, they are perfectly in phase—crest aligns with crest, and trough with trough. This results in constructive interference, and the recombined beam has a maximum intensity.

Now, imagine the moving mirror moves a small distance. The beam traveling to it and back now has a longer path to cover than the beam that went to the fixed mirror. The difference in the distances these two beams travel is called the Optical Path Difference (OPD).

What happens when the OPD is exactly half of the light's wavelength (λ/2)? The two recombining waves are now perfectly out of phase—the crest of one wave aligns with the trough of the other. They cancel each other out, resulting in destructive interference, and the intensity of the recombined beam drops to a minimum (ideally zero).

As the mirror continues to move, the OPD increases. When the OPD reaches one full wavelength (λ), the waves are back in phase, and we see constructive interference again. This pattern of alternating high and low intensity repeats as the mirror moves. If we were to plot the intensity of the recombined light at the detector as a function of the mirror's position (the OPD), we would see a perfect cosine wave. The frequency of this cosine wave is directly related to the frequency of the original light. A high-frequency (short wavelength) light source would produce a rapidly oscillating intensity pattern, while a low-frequency (long wavelength) source would produce a slowly oscillating pattern.

This is the genius of the interferometer: it converts the high frequencies of infrared light (on the order of 10¹³ to 10¹⁴ Hz), which are too fast for any detector to follow directly, into much lower frequency intensity modulations (typically in the range of hundreds or thousands of Hz) that a detector can easily measure. Each infrared frequency is effectively "tagged" with a unique, lower modulation frequency determined by the speed of the moving mirror.

Feature FTIR Spectroscopy Dispersive IR Spectroscopy
Operating Principle All frequencies measured simultaneously (interferometry) Frequencies measured sequentially (monochromator with a grating/prism)
Speed Very fast; a full spectrum is typically acquired in seconds. Slow; scanning through all frequencies can take several minutes.
Signal-to-Noise Ratio High (Fellgett's Advantage), due to simultaneous measurement of all frequencies. Lower, as each frequency is measured for only a small fraction of the time.
Throughput (Energy) High (Jacquinot's Advantage), as no restrictive slits are needed. Low, as narrow slits are required to achieve good resolution.
Wavenumber Accuracy Extremely high and precise (Connes' Advantage), internally calibrated by a laser. Lower, dependent on the mechanical accuracy and calibration of the grating.
Resolution Determined by the maximum travel distance of the moving mirror. Easily adjustable. Determined by the grating ruling and slit width. Not easily changed.
Stray Light Minimal effect, as stray light is typically not modulated by the interferometer. Can be a significant problem, leading to inaccuracies in absorbance.

The Conductor's Baton: The Helium-Neon Laser

A critical question arises: how does the instrument know the precise position of the moving mirror at any given instant? Without this information, the OPD is unknown, and the resulting data is meaningless. The solution is another piece of optical elegance: a second, parallel interferometer is built into the system that uses a single-frequency Helium-Neon (HeNe) laser as its light source.

Because the laser's wavelength (632.8 nm) is known with extreme accuracy and stability, its own interference pattern—a pure sine wave—serves as an internal ruler. As the moving mirror travels, the laser detector sees a sinusoidal signal. The zero-crossings of this sine wave provide highly precise trigger points for the data acquisition system to sample the main infrared signal (Newport, 2025). This ensures that the IR interferogram is sampled at exact, evenly spaced intervals of OPD. This laser-referenced timing, known as the Connes' Advantage, is the reason for the exceptional wavenumber accuracy and precision of FTIR instruments, a feature that older dispersive instruments could never achieve. It is like having a metronome of unparalleled accuracy ticking alongside the measurement, ensuring every data point is captured at the perfect moment.

The interferometer has now performed its primary function. It has taken the broadband IR beam and modulated it, creating a complex wave in which every frequency of the original light is encoded as a specific modulation. This modulated beam, now containing a wealth of information, continues its journey toward the sample.

Step 3: The Moment of Truth – Interaction with the Sample and Detection

The infrared beam, now elegantly modulated by the interferometer, has been prepared for its most important task: to interrogate the sample. This is where the chemistry happens. The light that has been so carefully generated and encoded will now pass through or reflect off the sample, and in doing so, it will be imprinted with the sample's unique molecular signature. The final part of this stage is the capture of the resulting light by a detector, which translates the optical signal into an electrical one, creating the raw data of the experiment: the interferogram.

Probing the Molecule: The Absorption Process

The modulated beam from the interferometer is focused onto the sample. As we discussed earlier, molecules are not static. The chemical bonds within them are constantly vibrating in various ways—stretching, bending, and twisting. Each of these vibrational modes has a specific, quantized energy level, meaning it can only absorb energy of a particular frequency.

When the broadband infrared light encounters the sample, the molecules within it will selectively absorb photons that have the exact energy (frequency) needed to promote a bond from its ground vibrational state to an excited vibrational state (Gasmet Technologies Oy, 2020). For a vibration to be "infrared active," meaning it can be observed by this technique, it must cause a change in the molecule's dipole moment. A dipole moment is the separation of positive and negative charge within a molecule. Symmetrical vibrations, like the stretching of the N≡N bond in a nitrogen molecule, do not change the dipole moment and are therefore IR-inactive. Asymmetrical vibrations, like the stretching of a carbonyl (C=O) group, create a significant change in the dipole moment and produce very strong absorption signals.

So, what does this mean for our beam of light? Frequencies that correspond to the sample's vibrational modes are absorbed and removed from the beam. Frequencies that do not match any vibrational modes pass through the sample unaffected. The light that emerges from the sample is therefore an attenuated version of the original beam, with "gaps" in the spectrum at the frequencies where the sample absorbed energy. The interferometer has ensured that this absorption information is encoded within the modulations of the recombined beam.

The location of the sample is a key design choice. In most modern instruments, the sample is placed after the interferometer but before the detector. This is the most common configuration. Placing the sample here means that any thermal energy the sample itself might emit will not be modulated by the interferometer and thus will not appear in the final spectrum, reducing potential artifacts.

Capturing the Signal: The Infrared Detector

The light that has passed through the sample now strikes the detector. The detector's job is to convert the fluctuating intensity of the infrared light into a corresponding electrical signal. The ideal detector should be sensitive across the entire mid-infrared range, respond very quickly to changes in light intensity, and have a linear response (meaning a doubling of light intensity produces a doubling of the electrical signal).

The most common detector in standard laboratory FTIR instruments is the Deuterated Triglycine Sulfate (DTGS) detector. This is a pyroelectric detector, which means it functions based on temperature changes. As the modulated infrared beam strikes the DTGS crystal, the absorbed energy causes its temperature to fluctuate rapidly. This temperature change alters the crystal's electrical polarization, generating a voltage that is proportional to the intensity of the incoming light. DTGS detectors have the advantage of operating at room temperature and providing good sensitivity across the entire mid-IR range, making them excellent general-purpose detectors (Newport, 2025).

For applications requiring higher sensitivity or a faster response time, a Mercury Cadmium Telluride (MCT) detector is often used. MCT is a photoconductive detector. When photons strike the MCT material, they excite electrons into the conduction band, changing the material's electrical resistance. This change in resistance is measured as the signal. MCT detectors are significantly more sensitive than DTGS detectors but have a more limited spectral range and, critically, must be cooled to liquid nitrogen temperature (77 K) to minimize thermal noise. This cooling requirement adds complexity and cost, but for demanding applications like trace gas analysis or FTIR microscopy, the increase in performance is indispensable.

Assembling the Data: The Interferogram

The electrical signal from the detector is amplified and then digitized by an Analog-to-Digital Converter (A/D converter). As we learned, the timing for this digitization is precisely controlled by the HeNe reference laser, ensuring data points are collected at exact intervals of the moving mirror's travel.

The resulting digital data stream is a plot of detector signal intensity versus optical path difference. This plot is the interferogram. It is the raw, fundamental measurement produced by the FTIR instrument (Shimadzu, 2024).

An interferogram from a broadband source has a characteristic shape. It features a very large, sharp signal at the center, which corresponds to the Zero Path Difference (ZPD) point. This is called the centerburst. At ZPD, all frequencies are interfering constructively, so their intensities add up to produce a massive signal. As the mirror moves away from ZPD in either direction, the different frequencies rapidly go out of phase with one another. The interferogram signal drops off quickly from the centerburst, decaying into a complex, oscillatory pattern of lower amplitude "wings." All of the spectral information is contained within this intricate pattern. The high-frequency components of the spectrum contribute to the fine, rapidly changing details in the interferogram, while the low-frequency components contribute to the broader, more slowly varying features.

At this point, we have a complete set of raw data. We have successfully translated the pattern of infrared absorption by a sample into a digital signal. However, this signal is in the "time domain" (or more accurately, the OPD domain). To a human, it is not directly interpretable. The next step is to use the power of mathematics to translate this encoded message into a language we can understand: a spectrum.

Step 4: The Language of Frequencies – Decoding the Signal with Fourier Transform

We have reached a pivotal moment in our journey. The instrument has done its physical work, producing an interferogram that holds the sample's spectral information in an encoded form. Now, the process moves from the realm of optics and hardware to the domain of computation and algorithms. The task is to convert the complex, time-domain interferogram into a familiar frequency-domain spectrum, where the y-axis represents light intensity (or absorbance) and the x-axis represents wavenumber. This transformation is achieved through a powerful mathematical procedure known as the Fourier Transform (FT). It is this computational step that gives the technique its name and is the key to unlocking the information hidden within the interferogram.

From Time to Frequency: The Fourier Transform Algorithm

The relationship between an interferogram and its corresponding spectrum is what mathematicians call a Fourier pair. One can be transformed into the other via the Fourier transform integral. In essence, the Fourier transform is a mathematical tool that deconstructs a complex waveform into the sum of the simple sine and cosine waves that make it up.

Think of a musical chord played on a piano. Your ear hears a single, complex sound. A skilled musician, or a computer with the right algorithm, can listen to that chord and identify the individual notes (frequencies) that were played to create it—a C, an E, and a G, for example. The Fourier transform does exactly this for our interferogram. The interferogram is the "chord," a superposition of all the cosine waves generated by each infrared frequency passing through the interferometer. The FT algorithm "listens" to this complex signal and determines the intensity of each individual "note" or frequency that contributed to it.

In the early days of interferometry, performing this calculation was a monumental task, taking hours or even days on the best computers available. The revolution came with the development of the Fast Fourier Transform (FFT) algorithm by Cooley and Tukey in 1965. The FFT is a highly efficient method for computing the Fourier transform, reducing the computation time by orders of magnitude. The advent of the FFT, combined with the explosive growth of affordable computing power, is what made modern, rapid-scan FTIR spectroscopy a practical reality (Newport, 2025).

The computer in the FTIR instrument digitizes the interferogram, creating an array of numbers. It then applies the FFT algorithm to this array, and the output is another array of numbers representing the intensity of light at each discrete wavenumber. This result is called a single-beam spectrum. It contains information not only from the sample but also from the instrument itself (the source's emission profile, the beamsplitter's efficiency) and the environment (any residual atmospheric gases). To get the final spectrum of just the sample, we must perform a ratio calculation. First, a background interferogram is collected with no sample in the beam path. This is transformed to get a background single-beam spectrum (I₀). Then, the sample interferogram is collected and transformed to get the sample single-beam spectrum (I). The final transmittance spectrum (T) is the ratio I / I₀. This can then be converted to the more commonly used absorbance spectrum (A) using the formula A = -log(T).

Refining the Data: Apodization and Phase Correction

The theoretical Fourier transform assumes that the interferogram is measured from an OPD of zero to infinity. In reality, the moving mirror can only travel a finite distance. This means we must abruptly truncate the interferogram at some maximum OPD. This sudden cutoff is like cutting off a sound wave in the middle of a note; it introduces artifacts into the transformed spectrum. Specifically, it causes the baseline around sharp absorption peaks to show a series of ripples or "side-lobes," which can distort the spectrum and obscure small features.

To mitigate this, a process called apodization is used. The term literally means "removing the feet." Before the Fourier transform is performed, the interferogram is multiplied by an apodization function. This function has a value of 1 at the centerburst (ZPD) and smoothly tapers to zero at the maximum OPD (Shimadzu, 2024). Instead of cutting the signal off abruptly, it gently fades it out. This process significantly reduces the spurious ripples in the spectrum. However, there is a trade-off. Apodization slightly broadens the spectral peaks, which means it causes a small reduction in the effective resolution. Various apodization functions exist (e.g., Triangular, Happ-Genzel, Beer-Norton), each offering a different compromise between ripple suppression and resolution loss. The choice of function depends on the specific requirements of the analysis.

Another computational refinement is phase correction. In an ideal world, we would collect a data point exactly at the Zero Path Difference. In practice, this is impossible. Furthermore, the electronics and optics of the instrument introduce small, frequency-dependent delays, causing a "phase error" in the interferogram. This results in an asymmetric interferogram and, if uncorrected, leads to distorted, asymmetric peak shapes in the final spectrum. The instrument's software uses a small, double-sided portion of the interferogram around the centerburst to calculate a phase correction function. This function is then applied during the Fourier transform process to correct for these errors, ensuring that the final spectral peaks are symmetrical and accurately shaped (Newport, 2025).

The Limits of Observation: The Nyquist Theorem and Spectral Range

The use of a HeNe laser for sampling provides incredible precision, but it also imposes a fundamental limit on the spectral range of the instrument. The Nyquist theorem is a fundamental principle of signal processing that states that to accurately represent a sine wave, you must sample it at a rate of at least two points per cycle.

In FTIR, the highest frequency of light that can be accurately measured is determined by the sampling frequency, which is set by the HeNe laser. A standard HeNe laser produces a trigger pulse for every full cycle of its own interferogram. This means the highest IR frequency (or shortest wavelength) the instrument can "see" is the one that produces exactly two data points per cycle of its modulation. This frequency is called the Nyquist frequency. For a standard HeNe reference at 632.8 nm, this limits the shortest measurable wavelength to approximately 1.26 µm, or about 7900 cm⁻¹. Any light with a higher frequency than this will be "aliased" or "folded" back into the spectrum at an incorrect, lower frequency, creating spectral artifacts. To prevent this, optical and electronic filters are used to remove light with frequencies above the Nyquist limit before it reaches the detector. Some systems use "oversampling," triggering on both the positive and negative zero-crossings of the laser signal, effectively doubling the sampling rate and pushing the short-wavelength limit down to the laser wavelength itself, around 633 nm (15,800 cm⁻¹) (Newport, 2025).

Through this intricate sequence of mathematical operations—apodization, Fourier transformation, and phase correction—the raw data from the detector is converted into a clean, accurate, and interpretable spectrum. The encoded message has been deciphered.

Step 5: The Molecular Fingerprint – Interpreting the Final Infrared Spectrum

The journey from a glowing infrared source to a set of data points on a computer screen is now complete. The final product of this intricate process is the infrared spectrum, a graphical representation of how a substance interacts with infrared light. This spectrum is not merely a collection of lines; it is a rich and detailed map of a molecule's structure. It is a unique "molecular fingerprint" that, with the proper knowledge, can be used to identify a substance, determine its purity, quantify its concentration, and even investigate the chemical changes it undergoes. This final step is about reading and understanding the language of the spectrum.

The Anatomy of a Spectrum: Transmittance vs. Absorbance

An FTIR spectrum is typically plotted with wavenumber (in units of reciprocal centimeters, cm⁻¹) on the x-axis and either percent transmittance (%T) or absorbance (A) on the y-axis. The x-axis is conventionally plotted with high wavenumbers (high energy) on the left and low wavenumbers (low energy) on the right.

  • Transmittance: A transmittance spectrum shows the percentage of the initial light intensity that passes through the sample at each frequency. A value of 100%T means all the light passed through (no absorption), while a value near 0%T means most of the light was absorbed. In a transmittance spectrum, the baseline is at the top (100%), and absorption features appear as "peaks" pointing downwards.
  • Absorbance: An absorbance spectrum is mathematically derived from the transmittance spectrum (A = -log(T) or A = log(1/T)). In this format, the baseline is at the bottom (zero absorbance), and absorption features appear as peaks pointing upwards. The great advantage of the absorbance scale is that it is directly proportional to the concentration of the absorbing species, a relationship described by the Beer-Lambert Law. For this reason, absorbance is the preferred format for nearly all quantitative analysis and is the most common format used for spectral interpretation and library searching.

Reading the Map: Functional Group and Fingerprint Regions

An infrared spectrum can be broadly divided into two main regions, each providing different types of information.

Wavenumber Range (cm⁻¹) Region Name Type of Vibrations Typical Information Gained
4000 – 2500 cm⁻¹ X-H Stretching Stretching of single bonds to hydrogen (O-H, N-H, C-H) Presence of key functional groups like alcohols, amines, carboxylic acids, and alkanes/alkenes/alkynes.
2500 – 2000 cm⁻¹ Triple Bond Region Stretching of C≡C and C≡N bonds Identification of alkynes and nitriles.
2000 – 1500 cm⁻¹ Double Bond Region Stretching of C=O, C=N, and C=C bonds Strong, sharp peaks here are characteristic of carbonyls (ketones, aldehydes, esters, amides).
1500 – 400 cm⁻¹ Fingerprint Region Complex vibrations (bending, rocking, twisting, wagging) A unique, complex pattern of peaks specific to the entire molecule. Used for definitive identification.

1. The Functional Group Region (4000 – 1500 cm⁻¹): This higher-energy portion of the spectrum is dominated by the stretching vibrations of specific chemical bonds, particularly those involving hydrogen (O-H, N-H, C-H) and triple or double bonds (C≡N, C=O). These vibrations are relatively isolated and characteristic of specific functional groups. For example, a broad, strong peak centered around 3300 cm⁻¹ is a tell-tale sign of an alcohol's O-H group. A sharp, intense peak around 1715 cm⁻¹ almost certainly indicates the presence of a carbonyl (C=O) group, found in ketones, aldehydes, and esters. By examining the peaks in this region, a chemist can quickly deduce which functional groups are present in an unknown molecule, much like identifying the types of instruments playing in an orchestra.

2. The Fingerprint Region (1500 – 400 cm⁻¹): This lower-energy region is far more complex. It contains a dense and intricate pattern of peaks arising from the complex bending, rocking, and wagging vibrations of the molecule's entire carbon skeleton, as well as stretching vibrations of single bonds between heavier atoms (C-C, C-O, C-N). These vibrations are highly coupled, meaning the motion of one part of the molecule influences many others. The result is a unique pattern of peaks that is characteristic of the molecule as a whole. No two different compounds (except for enantiomers, which are mirror images) will have the exact same fingerprint region. This is why it is called the "fingerprint" region. While the functional group region can tell you that a molecule is, for example, a ketone, the fingerprint region can distinguish between acetone, cyclohexanone, and acetophenone.

From Spectrum to Answer: Qualitative and Quantitative Analysis

With a spectrum in hand, the analyst can perform two main types of analysis.

  • Qualitative Analysis (Identification): This is the process of identifying an unknown substance. The analyst examines the functional group region to propose a class of compound and then compares the entire spectrum, especially the fingerprint region, to a database of known spectra. Modern FTIR software comes with extensive digital libraries containing hundreds of thousands of reference spectra (analysers.nl, 2025). The software can perform a search, comparing the unknown spectrum to the library entries and providing a "hit list" of the most likely matches, ranked by a quality score. This allows for rapid and confident identification of materials, from polymer pellets and pharmaceutical tablets to unknown contaminants.

  • Quantitative Analysis (Concentration): This involves determining how much of a specific substance is present in a mixture. Because absorbance is proportional to concentration (Beer-Lambert Law), the height or area of a specific absorption peak can be used to measure the amount of the corresponding compound. The analyst first creates a calibration curve by measuring the absorbance of a series of standards with known concentrations. The absorbance of the unknown sample is then measured, and its concentration is determined by referencing the calibration curve. This is a widely used technique in industrial quality control, for example, to measure the concentration of an additive in a polymer or the amount of water in an oil sample.

The ability to extract this wealth of chemical information—rapidly, non-destructively, and with high precision—is the ultimate reason why understanding how does FTIR work is so valuable. It transforms a complex interaction of light and matter into clear, actionable answers.

The Decisive Edge: Why FTIR Surpasses Dispersive Techniques

To fully appreciate the significance of Fourier Transform Infrared spectroscopy, it is instructive to compare it to the technology it largely replaced: dispersive infrared spectroscopy. For many decades, dispersive instruments were the standard for IR analysis. These instruments work much like a prism splitting white light into a rainbow. They use a diffraction grating to physically separate the broadband IR light into its component frequencies and then scan through these frequencies one by one, using a narrow slit to select a small frequency range to pass to the detector at any given moment. While functional, this method suffers from several inherent limitations that FTIR technology overcomes, providing three major advantages that cemented its dominance.

Fellgett's Advantage: The Power of the Multiplex

The most significant advantage of FTIR is the multiplex or Fellgett's Advantage. As we have seen, in an FTIR instrument, all frequencies of the spectrum are measured simultaneously in every scan of the moving mirror. In contrast, a dispersive instrument measures only one narrow band of frequencies at a time.

Imagine you have one minute to measure a spectrum consisting of 1000 individual frequency points. In a dispersive instrument, you could spend only 1/1000th of a minute (0.06 seconds) measuring the signal at each point. In an FTIR instrument, you spend the entire minute collecting information from all 1000 points at once.

The consequence of this is a dramatic improvement in the signal-to-noise ratio (S/N). The signal in a spectrum increases linearly with measurement time, while random noise increases only with the square root of the measurement time. Because FTIR measures all frequencies for the entire duration of the scan, it achieves a much higher S/N in the same amount of time compared to a dispersive instrument. Alternatively, an FTIR can achieve the same S/N as a dispersive instrument in a fraction of the time (Newport, 2025). This advantage means FTIR is far superior for analyzing samples that produce weak signals (e.g., trace contaminants, very thin films) or for applications that require very rapid data acquisition, such as monitoring chemical reactions in real-time.

Jacquinot's Advantage: The Throughput Supremacy

The second key benefit is the throughput or Jacquinot's Advantage. Dispersive spectrometers require narrow mechanical slits to achieve good spectral resolution. The slit's job is to isolate a small band of frequencies, but in doing so, it physically blocks a large portion of the light from the source, drastically reducing the amount of energy (throughput) that reaches the detector. This is particularly problematic in the infrared region, where sources are inherently less intense than in the visible or UV regions.

FTIR instruments, on the other hand, do not require resolution-limiting slits. The resolution is determined by the maximum travel distance of the moving mirror, not by a physical aperture. They use a large, circular aperture, allowing a much greater amount of the infrared beam's energy to pass through the instrument and reach the detector. This higher throughput directly translates to a stronger signal at the detector, which further contributes to the superior signal-to-noise ratio of FTIR compared to dispersive instruments (Newport, 2025). More light simply means a better, cleaner measurement.

Connes' Advantage: The Precision of the Laser

The third, and arguably most critical advantage for high-performance applications, is the wavenumber accuracy or Connes' Advantage. As we discussed, FTIR instruments use an internal HeNe laser as a constant reference for the position of the moving mirror. This provides an extremely precise internal wavelength calibration for every single scan. The resulting wavenumber axis of an FTIR spectrum is accurate to within ±0.01 cm⁻¹ or better.

Dispersive instruments rely on the precise mechanical calibration of the diffraction grating's rotation. This calibration is susceptible to mechanical wear, backlash, and thermal drift, making it difficult to achieve and maintain high wavenumber accuracy. Spectra taken on different dispersive instruments, or even on the same instrument on different days, could show slight shifts in peak positions, making precise comparisons challenging. The internal laser reference of FTIR ensures unparalleled accuracy and long-term stability. This allows for highly reliable spectral subtraction, where the spectrum of a solvent can be cleanly removed from a solution's spectrum, and makes searching spectral libraries far more robust and dependable.

Together, these three advantages—Fellgett's speed/S/N, Jacquinot's energy throughput, and Connes' wavenumber accuracy—represent a fundamental leap in performance. They are the reasons why FTIR has become the undisputed standard for infrared spectroscopy, enabling a vast range of applications in research and industry that were previously impractical or impossible.

Practical Dimensions: Sampling Techniques and Instrument Care

Understanding the theoretical underpinnings of how does FTIR work is essential, but for the laboratory professional, this knowledge must be paired with a practical mastery of sample handling and instrument operation. The quality of an FTIR spectrum is often more dependent on how the sample is prepared than on the instrument itself. A poorly prepared sample will yield a poor spectrum, regardless of how advanced the spectrometer is. Furthermore, proper care and maintenance are vital for ensuring the long-term performance and reliability of these sophisticated optical instruments.

The Art of Sampling: Preparing for Analysis

FTIR can analyze samples in virtually any form—solids, liquids, and gases—but each requires a specific approach. The primary goal of any sampling technique is to present the sample to the infrared beam in a way that allows for an appropriate amount of light absorption. If the sample is too thick or concentrated, it will absorb all the light, resulting in "flat-topped" peaks and an unusable spectrum. If it is too thin or dilute, the absorption will be too weak to detect clearly.

1. Transmission: This is the classic method, where the IR beam passes directly through the sample.

  • For Liquids: A few drops of the liquid can be pressed between two polished salt plates (typically KBr or NaCl, which are transparent to IR light), creating a thin capillary film. For quantitative analysis, a cell of a known pathlength is used.
  • For Solids: The most common method is the KBr pellet. A small amount of the solid sample (1-2 mg) is finely ground with about 200 mg of dry, powdered potassium bromide. The mixture is then pressed under high pressure in a die to form a small, transparent pellet. The KBr acts as an inert matrix that holds the sample particles in the beam path. Alternatively, a mull can be prepared by grinding the solid with a drop of a mulling agent (like Nujol, a mineral oil) to create a paste, which is then spread between salt plates.

2. Attenuated Total Reflectance (ATR): ATR has become the most popular sampling technique in modern labs due to its simplicity and versatility. It is one of the most powerful FTIR sample preparation tools. An ATR accessory uses a high-refractive-index crystal, typically diamond, zinc selenide (ZnSe), or germanium (Ge). The IR beam is directed into the crystal in such a way that it undergoes total internal reflection at the crystal surface. This creates an "evanescent wave," a small field of IR energy that penetrates a very short distance (typically 0.5 – 2 µm) into the space just beyond the crystal surface (nanoAnalytics, 2025).

To analyze a sample, one simply presses it firmly against the ATR crystal. The sample interacts with the evanescent wave, and the reflected light carries the absorption information back into the spectrometer. ATR is ideal for analyzing solids (powders, films, plastics) and liquids (including aqueous solutions, which are difficult to do by transmission) with minimal to no sample preparation. Just press and measure. The choice of crystal is important: diamond is extremely robust and chemically inert, making it an excellent general-purpose choice, while germanium has a shallower penetration depth, making it ideal for highly absorbing samples like carbon-filled polymers.

3. Reflectance: These techniques are used for analyzing surfaces and films.

  • Specular Reflectance: Used for smooth, reflective surfaces like mirrors or films on metallic substrates. The IR beam reflects off the surface at an angle, and the spectrum reveals the composition of any coating on the surface.
  • Diffuse Reflectance (DRIFTS): Used for rough, non-reflective solids and powders. The sample is placed in a cup, and the IR beam illuminates it. The diffusely scattered light is collected by mirrors and directed to the detector. This technique is excellent for analyzing powders with minimal sample preparation and is often more sensitive than the KBr pellet method.

Maintaining the Instrument: Best Practices

An FTIR spectrometer is a precision instrument that requires a clean and stable environment to perform optimally.

  • Environment: The instrument should be located in a temperature- and humidity-controlled room, away from sources of vibration (like pumps or centrifuges). As mentioned, atmospheric water and CO₂ are the enemies of good IR spectroscopy.
  • Purging and Desiccation: The optical compartment of the spectrometer contains components like the KBr beamsplitter, which are hygroscopic (they absorb water from the air). Moisture can permanently damage these optics. To prevent this, the instrument should be kept powered on to maintain a stable internal temperature, and the optical bench should be continuously purged with dry air or nitrogen. Many modern instruments feature a sealed and desiccated design, which greatly reduces the need for external purging, but the desiccant cartridges still need to be monitored and replaced periodically (Anton Paar, 2025).
  • Cleaning: Sample accessories, especially ATR crystals and transmission cells, must be cleaned thoroughly between samples to prevent cross-contamination. A solvent like isopropanol or acetone on a soft, lint-free tissue is typically used. Care must be taken not to scratch the delicate optical surfaces.

By mastering these practical aspects of sampling and instrument care, an analyst can ensure they are obtaining the highest quality data possible, allowing the powerful capabilities of the FTIR technique to be fully realized.

Frequently Asked Questions (FAQ)

What does FTIR stand for and what does it do? FTIR stands for Fourier Transform Infrared. It is a type of spectroscopy, which is an analytical technique that measures the absorption of infrared light by a sample. It is used to identify the chemical composition of a substance (qualitative analysis) and to determine the concentration of its components (quantitative analysis) by analyzing the vibrations of its molecular bonds.

How does an FTIR spectrometer work in simple terms? In simple terms, an FTIR spectrometer shines a special kind of light (infrared) through a sample. Molecules in the sample absorb specific colors (frequencies) of this light, which correspond to the way their chemical bonds vibrate. The instrument measures which colors are absorbed, creating a unique pattern called a spectrum. This spectrum acts like a "molecular fingerprint," allowing scientists to identify the substance. The "Fourier Transform" part is a mathematical step that converts the raw instrument signal into this readable fingerprint.

What is the main difference between FTIR and older IR techniques? The main difference lies in how they measure the spectrum. Older dispersive IR instruments measure one frequency of light at a time, slowly scanning through the entire range. An FTIR measures all frequencies simultaneously using a device called an interferometer. This gives FTIR three major advantages: it's much faster (Fellgett's Advantage), it gets a stronger signal (Jacquinot's Advantage), and it is far more accurate in its frequency measurement (Connes' Advantage).

Why is a background scan necessary in FTIR? A background scan is performed without the sample in the beam path to measure the spectrum of the instrument and the environment. This includes the light source's output, the detector's response, and any absorption from atmospheric gases like carbon dioxide and water vapor. By taking the ratio of the sample spectrum to the background spectrum, all these extraneous signals are mathematically removed, leaving only the spectrum of the sample itself.

What kinds of materials can be analyzed with FTIR? FTIR is incredibly versatile and can be used to analyze a vast range of materials. This includes organic compounds, polymers, pharmaceuticals, petrochemicals, foods, and biological samples. It can analyze samples in solid form (powders, films, plastics), liquid form (solvents, oils, solutions), and gaseous form. Specialized sampling techniques, like ATR, make it easy to analyze even difficult samples like dark plastics, pastes, and fabrics with minimal preparation.

What is the "fingerprint region" and why is it important? The fingerprint region is the complex area of an IR spectrum typically below 1500 cm⁻¹. While the region at higher frequencies shows peaks from specific functional groups (like C=O or O-H), the fingerprint region contains a dense pattern of peaks from the vibrations of the entire molecular skeleton. This pattern is unique to each specific molecule. Therefore, while two different molecules might have a C=O group, their fingerprint regions will be different, allowing for definitive identification.

Is FTIR a destructive or non-destructive technique? For the most part, FTIR is considered a non-destructive technique. When using methods like transmission or reflectance, the infrared light simply passes through or reflects off the sample, leaving it chemically unchanged. The sample can be recovered after the analysis. Techniques that require sample preparation, like making a KBr pellet, involve physically altering the sample (grinding and pressing), but the chemical nature of the substance is not changed by the measurement itself.

Conclusion

The journey through the workings of a Fourier Transform Infrared spectrometer reveals a symphony of physics, engineering, and computation. From the generation of a broadband infrared beam to its intricate modulation within the Michelson interferometer, every step is a testament to precision. The interaction with the sample, where molecular bonds absorb their characteristic frequencies, is the heart of the chemical inquiry. The subsequent detection and mathematical transformation of the interferogram into a clear spectrum represent a powerful fusion of hardware and software. The resulting spectrum, a unique molecular fingerprint, provides chemists and material scientists with an unparalleled tool for identification and quantification. The inherent advantages in speed, sensitivity, and accuracy have firmly established FTIR as an indispensable technique in modern analytical laboratories. Its ability to probe the vibrational world of molecules offers profound insights into the composition and structure of matter, making it a cornerstone of scientific discovery and industrial quality control.

References

analysers.nl. (2025). FTIR operating principle. Analysers.nl. https://www.analysers.nl/en/ftir-operating-principle

Anton Paar. (2025). FTIR instruments. Anton Paar GmbH.

Gasmet Technologies Oy. (2020, June 10). FTIR technology guide. Gasmet. https://www.gasmet.com/white-papers/ftir-technology-white-paper/

nanoAnalytics. (2025). Infrared spectroscopy (FTIR). nanoAnalytics GmbH.

Newport. (2025). Introduction to FTIR spectroscopy. Newport Corporation.

Newport. (2025). FT-IR spectroscopy definitions of characteristics. Newport Corporation. https://www.newport.com/n/ft-ir-spectroscopy-definitions-of-characteristics

Shimadzu. (2024, March 15). Fourier transform and apodization. Shimadzu Corporation. https://www.shimadzu.com/an/service-support/technical-support/ftir/tips_and_tricks/apodization.html

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