¿Qué es FTIR? Guía del experto sobre 7 aplicaciones clave para 2025

Resumen

Fourier-transform infrared (FTIR) spectroscopy is a potent analytical technique for the identification and quantification of chemical substances across solid, liquid, and gaseous states. The method operates by measuring the absorption of infrared radiation by a sample, which induces vibrational excitations in its molecular bonds. Unlike conventional dispersive spectroscopy, an FTIR spectrometer collects all spectral data simultaneously using a Michelson interferometer. The resulting signal, an interferogram, is a time-domain representation of the detector response. A mathematical process known as the Fourier transform is then applied to convert this interferogram into a frequency-domain spectrum, which plots absorbance or transmittance as a function of wavenumber. This approach affords significant advantages, including superior signal-to-noise ratio (Fellgett's advantage), higher throughput (Jacquinot's advantage), and exceptional wavelength accuracy. These benefits have established FTIR as an indispensable tool in diverse fields such as materials science, pharmaceuticals, environmental monitoring, and quality control, enabling rapid, non-destructive, and highly specific molecular analysis.

Principales conclusiones

• FTIR provides a unique chemical "fingerprint" for identifying unknown materials.
• The technique is non-destructive, preserving the sample for further analysis.
• It offers rapid results, often within minutes, for high-throughput screening.
• To understand what is FTIR, recognize its ability to analyze solids, liquids, and gases.
• Data from multiple wavelengths is collected at once, improving signal quality.
• Modern accessories simplify sample handling, requiring minimal preparation.
• FTIR is applicable across industries, from pharmaceutical quality control to environmental science.

Índice

• A Foundational Perspective on Light and Molecular Interaction
• The Heart of the Instrument: Deconstructing the FTIR Spectrometer
• From Interference to Insight: The Journey of an FTIR Signal
• Preparing for Discovery: A Guide to FTIR Sample Preparation Techniques
• Application 1: Pharmaceutical Quality Control and Development
• Application 2: Polymer and Plastics Industry
• Application 3: Environmental Monitoring and Analysis
• Application 4: Food and Beverage Science
• Application 5: Forensic Science and Investigation
• Application 6: Semiconductor Industry
• Application 7: Geosciences and Mineralogy
• Preguntas más frecuentes (FAQ)
• A Concluding Thought on Molecular Inquiry
• Referencias

A Foundational Perspective on Light and Molecular Interaction

To begin our exploration into the world of Fourier-transform infrared (FTIR) spectroscopy, we must first cultivate an understanding of the fundamental principles that govern it. This is not merely a technical exercise; it is an inquiry into the very nature of how matter and energy interact on a molecular scale. Think of it not as memorizing facts, but as learning a new language—the language that molecules speak through vibrations.

At its core, all spectroscopy is the study of the interaction between electromagnetic radiation and matter. The electromagnetic spectrum is a vast continuum of energy, from high-energy gamma rays and X-rays to low-energy microwaves and radio waves. Our own eyes are sensitive to only a tiny sliver of this spectrum, which we perceive as visible light. Just beyond the red end of the visible spectrum lies the infrared (IR) region, a form of energy that we often associate with heat. It is this specific region of light that holds the key to FTIR analysis.

The Vibrational Life of Molecules

Let us consider a molecule. It is not a static, rigid entity. Instead, it is a dynamic system of atoms connected by chemical bonds. A helpful, though simplified, analogy is to imagine these bonds as tiny springs connecting the atomic nuclei. Just as a spring can be stretched or compressed, these molecular bonds can vibrate. They can stretch, where the distance between two atoms increases and decreases. They can bend, where the angle between three connected atoms changes. There are even more complex motions like twisting, scissoring, rocking, and wagging.

Each of these vibrational modes has a characteristic, natural frequency, much like a guitar string has a specific pitch. For a molecule to absorb infrared light, a crucial condition must be met: the frequency of the incoming IR radiation must exactly match the natural frequency of one of the molecule's vibrational modes. When this resonance occurs, the molecule absorbs the energy from the light, causing the amplitude of that specific vibration to increase. This absorption event is what an FTIR spectrometer is designed to detect.

A second condition is that the vibration must cause a change in the molecule's dipole moment. A dipole moment is a measure of the separation of positive and negative charge within a molecule. Symmetrical molecules like nitrogen (N₂) or oxygen (O₂), where the charge is evenly distributed, do not have a dipole moment that changes during vibration. Consequently, they do not absorb IR radiation and are considered "IR inactive." This is quite fortunate, as it means the nitrogen and oxygen that make up most of our atmosphere do not interfere with FTIR measurements. Conversely, molecules like carbon dioxide (CO₂) or water (H₂O), which have an uneven charge distribution, are "IR active" and produce strong signals.

The Infrared Spectrum: A Molecular Fingerprint

When we pass a broad range of infrared frequencies through a sample, the molecule will only absorb the specific frequencies that correspond to its unique set of vibrational modes. If we plot the amount of light transmitted through the sample against the frequency of the light, we obtain an infrared spectrum. This spectrum is not a random collection of lines; it is a highly specific and reproducible pattern of peaks and troughs.

The x-axis of an IR spectrum is typically represented in units of "wavenumbers" (cm⁻¹), which are inversely proportional to wavelength and directly proportional to energy. Higher wavenumbers correspond to higher energy vibrations, such as the stretching of strong double or triple bonds. Lower wavenumbers correspond to lower energy vibrations, like the bending of bonds or vibrations of the entire molecular skeleton.

The resulting spectrum serves as a unique "molecular fingerprint." No two different chemical compounds will have the exact same infrared spectrum. While certain functional groups—like a carbonyl (C=O) group or a hydroxyl (O-H) group—will absorb in predictable regions of thespectrum, the overall combination of peaks across the entire spectrum is unique to the molecule as a whole. It is this uniqueness that makes FTIR an extraordinarily powerful tool for chemical identification. By comparing the spectrum of an unknown sample to a library of spectra from known compounds, we can often identify the substance with a high degree of confidence (Thain, 2024).

The region from roughly 4000 to 1500 cm⁻¹ is often called the "functional group region," as the absorptions here are characteristic of specific types of bonds (e.g., O-H, N-H, C=O). The more complex region below 1500 cm⁻¹ is known as the "fingerprint region." The absorptions here are due to intricate vibrational and rotational modes of the entire molecule, making this area particularly rich for distinguishing between similar compounds. Understanding what is FTIR begins with appreciating this power to generate a unique, identifying signature for almost any organic or inorganic compound.

The Heart of the Instrument: Deconstructing the FTIR Spectrometer

Having established the principles of molecular vibration, we can now turn our attention to the instrument that measures these phenomena: the FTIR spectrometer. To the uninitiated, it may appear as just another piece of laboratory equipment. However, within its housing lies an elegant optical system that represents a significant leap forward from older dispersive techniques. The ingenuity of FTIR does not lie in a new way of making molecules vibrate, but in a revolutionary method of measuring the light that interacts with them. The core of this revolution is an optical engine called an interferometer, an invention credited to Albert Michelson in the late 19th century.

An FTIR spectrometer consists of four primary components: the source, the interferometer, the sample compartment, and the detector. Let us examine each in turn to appreciate its role in the analytical process.

The Source: Generating the Infrared Light

The process begins with a source that emits a continuous, broad spectrum of infrared radiation. Unlike a laser, which emits a single wavelength, the FTIR source must provide all the frequencies that will be analyzed. The ideal source is a black-body radiator—an object that glows when heated, emitting radiation across a wide range of wavelengths.

For the most common mid-infrared (MIR) range (typically 4000 to 400 cm⁻¹), a Globar source is often used. This consists of a silicon carbide (SiC) rod that is electrically heated to around 1,100 °C. Another common option is an Ever-Glo source, which is a ceramic element. These sources are reliable and produce stable, intense radiation across the required frequency range. The continuous beam of IR light is then directed by a series of mirrors toward the heart of the instrument: the interferometer.

The Interferometer: The Engine of FTIR

The interferometer is what distinguishes an FTIR spectrometer from its predecessors. Most modern instruments use a design based on the Michelson interferometer. Imagine the beam of IR light from the source arriving at a special component called a beamsplitter. As its name suggests, the beamsplitter divides the light into two separate beams of roughly equal intensity.

• Beam 1 passes through the beamsplitter and travels to a flat, fixed mirror.
• Beam 2 is reflected by the beamsplitter at a 90-degree angle and travels to a flat, movable mirror.

The beamsplitter itself is a critical component, often made of a material like potassium bromide (KBr) coated with a substance like germanium (Ge) to achieve the desired 50/50 split of transmission and reflection.

Both mirrors reflect their respective beams back toward the beamsplitter. At the beamsplitter, the two beams recombine. When they recombine, they interfere with each other. This interference can be either constructive (the waves add up, creating a stronger signal) or destructive (the waves cancel out, creating a weaker signal). The nature of this interference depends entirely on the difference in the path length that the two beams have traveled.

The movable mirror is the key to the entire process. It scans back and forth along a very precise track. When the movable mirror is at the exact same distance from the beamsplitter as the fixed mirror, the path lengths of the two beams are identical. This position is called the Zero Path Difference (ZPD). At ZPD, all wavelengths of light in the two beams are perfectly in phase, leading to maximum constructive interference and the strongest possible signal at the detector.

As the movable mirror moves away from the ZPD, a path difference is introduced. For a given wavelength, when this path difference is exactly half a wavelength, the two beams will be perfectly out of phase, resulting in maximum destructive interference and a minimal signal. As the mirror continues to move, the signal will oscillate between maximum and minimum intensity.

Crucially, the frequency of this oscillation is different for every wavelength of light. High-frequency (short-wavelength) light will produce a rapidly oscillating signal as the mirror moves, while low-frequency (long-wavelength) light will produce a slowly oscillating signal. In this way, the movement of the mirror encodes the frequency information of all the different wavelengths of light into a time-dependent signal. This complex, composite signal is called an interferogram.

To ensure the utmost precision, the position of the moving mirror is constantly monitored by a secondary interferometer system using a helium-neon (HeNe) laser, which provides a highly accurate reference signal for timing the data collection .

The Sample Compartment: Where Interaction Occurs

After leaving the interferometer, the modulated IR beam, now containing all the encoded frequency information, passes into the sample compartment. This is where the light interacts with the material being analyzed. The compartment is designed to accommodate a wide variety of herramientas avanzadas de preparación de muestras para preprocesamiento FTIR, such as transmission cells for liquids and gases or attenuated total reflectance (ATR) accessories for solids and viscous liquids.

As the beam passes through or reflects off the sample, the molecules within the sample absorb energy at their characteristic vibrational frequencies. This absorption selectively removes specific frequencies from the IR beam. The beam that emerges from the sample is therefore an altered version of the original, carrying the "imprint" of the sample's molecular structure.

The Detector: Measuring the Final Signal

The final component is the detector, which measures the intensity of the infrared radiation that reaches it. The detector converts the light energy into an electrical signal. The choice of detector depends on the spectral range and the required sensitivity.

For standard mid-IR analysis, a deuterated triglycine sulfate (DTGS) pyroelectric detector is the most common. It operates at room temperature and offers good sensitivity and a broad spectral response. For applications requiring higher sensitivity or faster response times, a mercury cadmium telluride (MCT) detector is often used. MCT detectors are photonic detectors, meaning they are more sensitive, but they must be cooled with liquid nitrogen to reduce thermal noise.

The electrical signal from the detector, which represents the intensity of the interferogram as a function of mirror position, is then sent to a computer for the final, critical step: the Fourier transform.

Component	Function	Common Materials/Types	Key Characteristic
Source	Emits broad-spectrum infrared radiation.	Globar (SiC), Ceramic	Continuous, stable, high-intensity output.
Interferómetro	Splits, modulates, and recombines the IR beam.	Beamsplitter (KBr), Mirrors (Gold-coated)	Encodes wavelength information into an interferogram.
Sample Holder	Positions the sample in the IR beam path.	Transmission cells, ATR crystals (Diamond, ZnSe)	Enables interaction between IR light and sample.
Detector	Measures the intensity of the final IR beam.	DTGS (Pyroelectric), MCT (Photonic)	Converts light energy into an electrical signal.

From Interference to Insight: The Journey of an FTIR Signal

We have now followed the path of infrared light from its source, through the intricate dance of the interferometer, and past the sample to the detector. But the signal captured by the detector is not yet an infrared spectrum. It is an interferogram, a complex waveform that holds the spectral information in a coded format. The final step in the process, and the one that gives FTIR its name, is to translate this interferogram into the familiar spectrum of absorbance versus wavenumber. This translation is achieved through a powerful mathematical algorithm: the Fourier Transform.

Understanding the Interferogram

Let's pause and visualize the interferogram again. It is a graph of light intensity measured at the detector versus the optical path difference (OPD) created by the moving mirror.

• At Zero Path Difference (ZPD): All frequencies of light arrive at the detector in phase. Their amplitudes add together, creating a massive, sharp spike of intensity. This feature is called the "centerburst" and is the most prominent part of any interferogram from a broadband source.
• Away from ZPD: As the mirror moves, the different frequencies go in and out of phase at different rates. High-frequency light (large wavenumber) creates a high-frequency sine wave in the interferogram, while low-frequency light (small wavenumber) creates a low-frequency sine wave.

The interferogram measured by the detector is the sum of all these individual sine waves from all the frequencies present in the light source. It is a complex, overlapping pattern that typically decays in amplitude as the mirror moves farther from the centerburst. You can think of it like hearing a chord played on a piano. Your ear hears one complex sound, but a trained musician (or a Fourier transform) can deconstruct that sound into its individual notes (frequencies).

The Magic of the Fourier Transform

The Fourier transform is a mathematical procedure that decomposes a function of time (or in our case, mirror position) into the frequencies that make it up. It essentially converts the data from the "time domain" (the interferogram) to the "frequency domain" (the spectrum).

The discovery of a highly efficient algorithm for this calculation, the Fast Fourier Transform (FFT) by Cooley and Tukey in 1965, was a pivotal moment. It dramatically reduced the computational time required, making the routine use of FTIR feasible with the advent of affordable computers (Thain, 2024).

When the FFT is applied to the interferogram, it produces what is called a "single-beam spectrum." This spectrum shows the intensity of light at each wavenumber. However, this is not yet the final result we want. The single-beam spectrum contains contributions from the source (it doesn't emit perfectly evenly at all frequencies), the atmosphere in the instrument (water vapor and CO₂ have strong absorptions), and the detector (its response is not perfectly flat).

Creating the Final Spectrum: Background Subtraction

To isolate the absorption due to the sample alone, we must perform a two-step measurement:

1. Recoge un espectro de fondo: First, an interferogram is recorded with no sample in the beam path. The instrument is empty, or contains only the pure solvent if analyzing a solution. This is transformed into a single-beam background spectrum, I_B. This spectrum represents the instrument's response function.
2. Collect a Sample Spectrum: Next, the sample is placed in the beam path, and a new interferogram is recorded. This is transformed into a single-beam sample spectrum, I_S.

The final transmittance spectrum (T) is then calculated by taking the ratio of the sample spectrum to the background spectrum at each wavenumber:

T = IS / IB

This simple ratioing process effectively cancels out all the instrumental and atmospheric artifacts, leaving only the information related to the sample. The result is a clean spectrum showing the percentage of light transmitted by the sample at each wavenumber.

Most often, scientists prefer to work with absorbance (A) rather than transmittance, because absorbance is directly proportional to the concentration of the sample, a relationship described by the Beer-Lambert Law. Absorbance is calculated from transmittance as:

A = -log(T)

The final result is the familiar plot of Absorbance vs. Wavenumber (cm⁻¹), the molecular fingerprint that is the ultimate goal of the FTIR measurement. Every peak in this spectrum corresponds to a specific molecular vibration that has absorbed energy from the infrared beam, providing a wealth of information about the sample's chemical identity and structure.

Advantages Unlocked by the FT Method

This entire process, from interferogram to spectrum, is what gives FTIR its significant advantages over older dispersive methods, which measured one wavelength at a time using a grating.

• Fellgett's Advantage (Multiplex Advantage): The FTIR spectrometer measures all frequencies simultaneously, rather than sequentially. This means a complete spectrum can be obtained in the same amount of time it would take a dispersive instrument to measure just one small frequency range. To achieve the same signal-to-noise ratio, an FTIR is orders of magnitude faster.
• Jacquinot's Advantage (Throughput Advantage): Dispersive instruments require narrow slits to achieve good resolution, which severely limits the amount of light (throughput) that reaches the detector. An FTIR has no such slits and uses a large, circular aperture, allowing much more light energy to pass through the instrument. This results in a much higher signal-to-noise ratio.
• Connes' Advantage (Wavelength Accuracy): The use of a HeNe laser to constantly reference the mirror position ensures that the wavenumber scale of the spectrum is extremely accurate and reproducible, far exceeding the mechanical precision of a dispersive grating.

These three advantages combine to make FTIR a faster, more sensitive, and more precise technique, solidifying its role as a cornerstone of modern analytical chemistry.

Preparing for Discovery: A Guide to FTIR Sample Preparation Techniques

The most sophisticated Espectrómetro de infrarrojos por transformada de Fourier is of little use without a proper way to introduce the sample into the infrared beam. The method chosen for sample preparation is not a trivial detail; it can profoundly affect the quality and interpretation of the final spectrum. The goal of any sampling technique is to obtain a high-quality spectrum of the analyte while minimizing spectral artifacts and interference. The choice of technique depends on the physical state of the sample—whether it is a solid, liquid, or gas—and the specific information sought.

Fortunately, decades of innovation have produced a wide array of sampling accessories, making FTIR one of the most versatile analytical methods available. Let's explore the most common techniques.

Transmission Spectroscopy: The Classic Approach

Transmission is the oldest and most conceptually simple method. The infrared beam passes directly through the sample, and the detector measures the amount of light that is transmitted.

• For Liquids: Liquids can be analyzed in a transmission cell. These cells consist of two windows made of an IR-transparent material (like NaCl, KBr, or ZnSe) separated by a thin spacer. The path length of the cell (the thickness of the liquid layer) is critical. For mid-IR analysis, path lengths are typically very short, from 15 µm to 1 mm, because many organic liquids absorb strongly. A "neat" (undiluted) liquid is placed between two salt plates to form a thin film, or a solution is prepared using a solvent that has minimal IR absorption in the regions of interest (e.g., carbon tetrachloride or carbon disulfide, though their use is now limited due to toxicity).
• For Solids: For a solid to be analyzed by transmission, it must be made sufficiently transparent to IR radiation. The most traditional method is the KBr pellet. A small amount of the solid sample (1-2 mg) is finely ground with about 100-200 mg of dry, spectroscopic-grade potassium bromide (KBr). The mixture is then pressed under high pressure in a die to form a small, transparent disc. KBr is used because it is transparent to IR radiation and has a refractive index similar to many organic compounds, which reduces scattering. While effective, this method is labor-intensive, sensitive to moisture (KBr is hygroscopic), and can sometimes induce changes in the sample's crystal structure.
• For Gases: Gases are analyzed in a gas cell, which is essentially a tube with IR-transparent windows at both ends. Because gases have very low concentrations, these cells often have very long path lengths (from several centimeters to many meters) to ensure sufficient interaction with the IR beam. Multi-pass gas cells use internal mirrors to reflect the beam back and forth through the sample multiple times, achieving effective path lengths of 10 meters or more within a compact cell.

Attenuated Total Reflectance (ATR): The Modern Workhorse

Attenuated Total Reflectance (ATR) has become the most popular sampling technique for mid-IR spectroscopy, especially for solids and liquids. Its popularity stems from its speed, ease of use, and the minimal sample preparation required.

The ATR accessory uses a crystal with a high refractive index, such as diamond, zinc selenide (ZnSe), or germanium (Ge). The IR beam is directed into the crystal at an angle such that it undergoes total internal reflection at the crystal's top surface. Although the beam is "totally" reflected, an electromagnetic wave known as the onda evanescente penetrates a very short distance (typically 0.5-2 µm) into the space just above the crystal surface.

If a sample is brought into intimate contact with this surface, the evanescent wave can be absorbed by the sample at its characteristic frequencies. The reflected beam, now "attenuated" by the sample's absorption, is then directed to the detector. The resulting spectrum is very similar to a conventional transmission spectrum.

The advantages of ATR are numerous:

• Minimal Sample Prep: Solid powders, polymer films, viscous liquids, and even soft tissues can be analyzed simply by pressing them onto the crystal.
• Velocidad: A measurement can be taken in seconds.
• Reproducibilidad: The path length is effectively fixed by the properties of the evanescent wave, leading to highly reproducible results.
• Aqueous Samples: Because the path length is so short, even highly absorbing samples like water can be analyzed, a significant advantage over transmission methods.

The choice of ATR crystal is important. Diamond is extremely robust and chemically inert, making it a versatile choice. ZnSe is less expensive but softer and sensitive to acids. Ge has a higher refractive index, resulting in a shorter path length, which is ideal for highly absorbing samples like carbon-filled polymers.

Reflectance Spectroscopy: Analyzing Surfaces

Reflectance techniques are used when the sample is too thick or opaque for transmission, or when only the surface of a material is of interest.

• Specular Reflectance: This technique measures the light that is reflected directly from a smooth, mirror-like surface. It is useful for analyzing thin coatings on reflective substrates (like a polymer coating on a metal can). The angle of incidence is typically near-normal (close to 90°).
• Diffuse Reflectance (DRIFTS): This is the preferred method for rough-surfaced solids and powders. When the IR beam strikes a powdered sample, it is scattered in multiple directions. Some of the light penetrates individual particles and is absorbed before being scattered out again. A special set of mirrors collects this diffusely scattered light and directs it to the detector. DRIFTS is highly sensitive and is an excellent alternative to KBr pellets, requiring less sample preparation.

Technique	Sample State	Principio	Ventajas	Desventajas
Transmisión	Gas, Liquid, Solid	El haz IR atraviesa directamente la muestra.	Quantitative accuracy (Beer's Law), well-established.	Requires sample preparation (e.g., KBr pellet), difficult for opaque samples.
ATR	Liquid, Solid	Evanescent wave from an internal reflection interacts with the sample surface.	Fast, easy, minimal prep, good for aqueous samples.	Spectrum can differ slightly from transmission; requires good sample contact.
Diffuse Reflectance	Solid (Powder)	IR beam scatters within the sample, and the scattered light is collected.	High sensitivity, minimal prep for powders.	Not suitable for smooth surfaces, can be difficult to quantify.
Reflectancia especular	Solid (Smooth)	IR beam reflects off a smooth, shiny surface.	Good for thin films on reflective substrates, non-contact.	Requires a highly reflective surface, can have spectral distortions.

Application 1: Pharmaceutical Quality Control and Development

In the pharmaceutical industry, the stakes are exceptionally high. The identity, purity, and consistency of active pharmaceutical ingredients (APIs) and excipients are not just matters of quality, but of public health and safety. Here, FTIR spectroscopy serves as a vigilant gatekeeper and an insightful development tool. Its speed, specificity, and non-destructive nature make it invaluable at nearly every stage of a drug's lifecycle, from initial research to final product verification (Fahelelbom et al., 2022).

Raw Material Identification and Verification

Before a single tablet is pressed, every raw material that enters the manufacturing plant must be rigorously tested to confirm its identity. A mistake at this stage could have catastrophic consequences. Traditionally, this involved time-consuming wet chemical tests. Today, FTIR with an ATR accessory offers a vastly superior alternative.

A small sample of an incoming material—be it an API powder, a lactose excipient, or a magnesium stearate lubricant—is placed on the ATR crystal. Within a minute, a spectrum is generated. This spectrum is then compared against a digital library of reference spectra for approved materials. Sophisticated software algorithms calculate a "match value" or correlation coefficient. If the match is above a pre-defined threshold (e.g., 99.5%), the material is confirmed and accepted. If not, it is flagged for further investigation. This process is so rapid and reliable that it can be implemented for every single container of raw material, providing 100% identity assurance.

Polymorph and Salt Form Screening

The physical form of an API can have a dramatic impact on its therapeutic properties, including its solubility, bioavailability, and stability. A single chemical compound can often crystallize into multiple different forms, known as polymorphs. While chemically identical, polymorphs have different crystal lattice structures. These structural differences lead to subtle but distinct variations in their vibrational spectra.

FTIR is a primary tool for identifying and differentiating between polymorphs. During drug development, chemists synthesize and crystallize a new API under various conditions to discover all possible polymorphic forms. FTIR analysis provides the unique spectral fingerprint for each form, allowing researchers to select the most stable and effective polymorph for clinical development. This is a critical step, as an unintended polymorphic transformation during manufacturing or storage could render a drug ineffective or even harmful.

Quantitative Analysis and Content Uniformity

Beyond simple identification, FTIR can also be used for quantitative measurements. By creating a calibration model using samples of known concentration, FTIR can determine the amount of API in a final dosage form, such as a tablet or capsule. This is crucial for ensuring content uniformity—that every tablet contains the correct dose of medicine.

While High-Performance Liquid Chromatography (HPLC) remains the gold standard for final release testing, FTIR provides a much faster method for at-line or in-process monitoring. It can quickly check the blend uniformity of a powder mixture before tablet compression or analyze finished tablets to ensure they meet specifications, helping to catch manufacturing deviations in real-time.

Application 2: Polymer and Plastics Industry

Polymers are the backbone of modern materials science, forming everything from simple packaging films to high-performance aerospace composites. The properties of a polymer are dictated by its chemical structure—the type of monomer units, the presence of additives, and the degree of degradation. FTIR spectroscopy is arguably the most widely used technique for the characterization of polymeric materials, providing rapid and reliable answers to critical questions of identity, quality, and failure.

Polymer Identification and Blend Analysis

The first question a polymer chemist often asks is, "What is this plastic?" FTIR provides a swift and definitive answer. A small piece of an unknown plastic part can be analyzed directly using an ATR accessory. The resulting spectrum is a unique fingerprint of the polymer's backbone. The sharp, intense peak around 1730 cm⁻¹ immediately suggests an ester group, characteristic of polyethylene terephthalate (PET). A complex pattern in the fingerprint region might identify polycarbonate (PC), while the characteristic C-H bending modes can distinguish between polyethylene (PE) and polypropylene (PP).

By comparing the unknown spectrum to extensive commercial libraries, the base polymer can be identified in seconds. This is vital for quality control, competitor analysis, and recycling efforts, where sorting different plastic types is essential. FTIR can also be used to analyze polymer blends and copolymers, often revealing the relative proportions of the different components.

Additive and Contaminant Detection

Few polymers are used in their pure form. They are almost always compounded with additives that enhance their properties: plasticizers for flexibility, antioxidants for thermal stability, UV absorbers to prevent light degradation, and fillers like talc or glass fibers for strength. These additives are often present in small quantities, but their impact is enormous.

FTIR can be a powerful tool for detecting and identifying these additives. While the strong absorptions from the base polymer can sometimes mask the weaker signals from additives, spectral subtraction techniques can help. By subtracting the spectrum of a pure reference polymer from the spectrum of the formulated product, a difference spectrum is generated that reveals the characteristic peaks of the additives. Similarly, FTIR is excellent for identifying contaminants that may have been introduced during processing, which could compromise the final product's performance.

Degradation and Failure Analysis

When a plastic part fails in service—it becomes brittle, discolors, or cracks—it is crucial to understand why. FTIR is a key tool in failure analysis, helping to uncover the chemical changes that led to the material's breakdown.

• Oxidation: Many polymers are susceptible to oxidation when exposed to heat and air. This process creates carbonyl (C=O) and hydroxyl (O-H) groups within the polymer structure. The appearance and growth of absorption bands in the 1700-1750 cm⁻¹ (carbonyl) and 3200-3600 cm⁻¹ (hydroxyl) regions are clear indicators of oxidative degradation. The extent of oxidation can even be quantified by measuring the area of these peaks.
• Hydrolysis: Polymers with ester or amide linkages, such as PET or nylons, can be degraded by moisture, a process called hydrolysis. This breaks the polymer chains and can be monitored by changes in the FTIR spectrum.
• UV Degradation: Exposure to sunlight can also break polymer bonds. FTIR can track the chemical changes associated with this photodegradation, helping engineers to select appropriate materials or UV stabilizers for outdoor applications.

Application 3: Environmental Monitoring and Analysis

The health of our planet is a matter of increasing concern, and analytical chemistry plays a vital role in monitoring and mitigating environmental pollution. FTIR spectroscopy has emerged as a versatile and powerful tool for a range of environmental applications, from identifying pollutants in water and soil to analyzing atmospheric gases. Its ability to provide rapid, on-site or in-situ measurements makes it particularly valuable for environmental fieldwork.

Microplastics Identification

One of the most pressing environmental issues of our time is the proliferation of microplastics—tiny plastic particles less than 5 mm in size—in our oceans, rivers, and even drinking water. A critical step in studying this problem is identifying the type of plastic these particles are made of, as this helps to trace their source and understand their environmental fate.

FTIR, particularly when coupled with a microscope (FTIR microscopy), is the leading technique for this task (Scopetani et al., 2019). Individual microplastic particles, often less than a millimeter in size, can be isolated from a water or sediment sample and placed under the microscope. The instrument can then focus the IR beam onto a single particle and acquire its spectrum. By comparing this spectrum to a polymer library, the particle can be identified as polyethylene, polypropylene, polystyrene, or another common plastic type. This information is invaluable for environmental scientists working to understand the scale and sources of plastic pollution.

Analysis of Oil Spills and Soil Contamination

When an oil spill occurs, rapid identification of the source is crucial for legal and remediation efforts. The crude oils from different geographic locations have slightly different chemical compositions, resulting in unique FTIR fingerprints. By taking a sample of the spilled oil and comparing its spectrum to a library of reference oils, investigators can often pinpoint the origin of the spill.

Similarly, FTIR can be used to screen soil samples for contamination by petroleum hydrocarbons, pesticides, or other organic pollutants. While gas chromatography-mass spectrometry (GC-MS) is typically used for definitive quantification, FTIR provides a very fast screening method. A soil sample can be analyzed using a DRIFTS or ATR accessory to quickly determine if contamination is present, allowing for rapid site assessment and prioritization of areas for cleanup.

Atmospheric Gas Analysis

The composition of our atmosphere has a direct impact on climate and air quality. FTIR is an essential tool for atmospheric scientists studying greenhouse gases and pollutants. As we discussed earlier, molecules like carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O), and various industrial pollutants are all IR active.

Using long-path gas cells, researchers can measure the concentrations of these gases in an air sample with high precision. Open-path FTIR systems can even monitor gas concentrations over long distances in an industrial facility or across an urban area. An IR source is placed on one side of the area and a telescope-coupled FTIR spectrometer on the other. The instrument measures the absorption of the IR beam as it passes through the open air, providing real-time data on the concentrations and fluxes of various atmospheric components. This is vital for monitoring emissions from industrial stacks, tracking urban smog formation, and studying the global carbon cycle.

Application 4: Food and Beverage Science

The global food industry relies on consistent quality, safety, and authenticity. Consumers expect the products they buy to be exactly what the label says they are, and manufacturers need robust analytical tools to ensure this is the case. FTIR spectroscopy has become a workhorse in food science, offering rapid and non-destructive analysis for everything from assessing the quality of olive oil to detecting fraudulent adulteration of milk.

Edible Oils and Fats Analysis

The composition of edible oils and fats determines their nutritional value, cooking properties, and shelf life. FTIR is widely used to characterize these products. For example, the degree of unsaturation in an oil, a key nutritional parameter, can be determined by measuring the intensity of the =C-H stretching peak around 3010 cm⁻¹.

Perhaps more importantly, FTIR is a powerful tool for detecting adulteration. Extra virgin olive oil, a high-value product, is frequently adulterated with cheaper oils like sunflower or canola oil. These different oils have distinct fatty acid profiles, which result in subtle but measurable differences in their FTIR spectra. Using chemometrics—advanced statistical methods applied to spectral data—it is possible to build models that can detect adulteration at levels of just a few percent, protecting both consumers and honest producers (Rohman et al., 2020).

Dairy and Beverage Analysis

In the dairy industry, FTIR is used for the routine analysis of milk. A single measurement can simultaneously provide the concentration of fat, protein, and lactose, the key components that determine the milk's quality and price. This has largely replaced slower, traditional wet chemical methods, allowing for high-throughput analysis at milk collection centers. FTIR is also used to detect adulterants, such as added water or melamine, a nitrogen-rich compound illegally added to artificially inflate the apparent protein content.

For beverages like wine and fruit juice, FTIR can rapidly measure parameters like sugar content (Brix), acidity, and alcohol concentration. It can also be used for authenticity testing, for example, to verify that a product labeled as "100% orange juice" has not been diluted with cheaper sugar water or juice from other fruits.

Cereal and Grain Quality

The quality of wheat, corn, and other grains is critical for baking, brewing, and animal feed production. Key parameters include protein content, moisture, and starch properties. Traditionally, these are measured by laborious reference methods like the Kjeldahl method for protein.

NIR spectroscopy, a close relative of mid-IR FTIR, is particularly dominant in this area. Using diffuse reflectance, a whole grain sample can be analyzed in seconds with no sample preparation. Calibration models built using reference data allow the instrument to predict the protein and moisture content with high accuracy. This enables grain elevators and food processors to rapidly segregate incoming grain based on quality, ensuring the right raw material goes to the right process.

Application 5: Forensic Science and Investigation

In forensic science, physical evidence is often fragmentary and minute. The ability to identify an unknown substance from a tiny trace can be the key to solving a crime. FTIR spectroscopy, especially when coupled with a microscope, is a cornerstone of the modern forensic laboratory. Its non-destructive nature is a significant advantage, as it preserves precious evidence for other potential analyses.

Fiber and Paint Chip Analysis

Fibers are a common form of trace evidence, often transferred between a suspect, victim, and crime scene. A single fiber found on a victim's clothing could link them to a suspect's carpet or car upholstery. FTIR microscopy is the ideal tool for identifying the polymer type of a synthetic fiber (e.g., nylon, polyester, acrylic). The spectrum provides a definitive chemical identification that can be used to include or exclude a potential source.

Similarly, paint chips transferred during a hit-and-run accident or a break-in can provide a powerful evidentiary link. A multi-layered paint chip contains a wealth of information. FTIR microscopy can be used to analyze each layer individually, identifying the chemical composition of the binders, pigments, and fillers in the primer, basecoat, and clearcoat. This detailed chemical profile can then be compared to a sample from a suspect vehicle, potentially establishing a common origin.

Illicit Drug and Controlled Substance Identification

Law enforcement agencies regularly seize unknown powders, pills, and plant materials. Rapid and accurate identification is necessary for prosecution. FTIR with an ATR accessory is a standard screening tool in forensic drug chemistry. A small amount of a seized powder can be analyzed in seconds. The resulting spectrum is compared against a comprehensive library of controlled substances, including cocaine, heroin, methamphetamine, and a vast array of synthetic "designer" drugs.

While a positive FTIR match is typically considered a presumptive test and is often confirmed by GC-MS, its speed and specificity make it an invaluable first-line tool for field agents and lab technicians. Portable, handheld FTIR units are now commonly used by law enforcement for on-site identification of suspicious substances.

Document and Ink Analysis

Questions of document authenticity, such as determining if a signature has been forged or if a document has been altered, are common in forensic investigations. FTIR can be used to analyze the chemical composition of inks. Different pen manufacturers use different ink formulations, which result in distinct FTIR spectra. By analyzing the ink from different parts of a document, it may be possible to determine if different pens were used, suggesting a later alteration. It can also help to determine the relative age of ink entries, although this is a more complex and challenging application.

Application 6: Semiconductor Industry

The semiconductor industry operates at the frontiers of materials science and engineering, where purity and precision are paramount. The fabrication of microchips involves hundreds of complex steps, and a microscopic contaminant or a subtle deviation in material properties can lead to the failure of an entire batch of expensive wafers. FTIR spectroscopy is an essential quality control and troubleshooting tool in this demanding environment, used to monitor processes, characterize materials, and identify sources of failure.

Wafer Contamination and Thin Film Analysis

Silicon wafers are the foundation of all microchips. Their surfaces must be kept exceptionally clean throughout the manufacturing process. FTIR is used to detect and identify organic and inorganic contaminants on the wafer surface. Using specialized grazing-angle reflectance accessories, the instrument can detect monolayer or sub-monolayer levels of contamination, identifying residues from photoresists, cleaning solvents, or human contact.

During fabrication, thin films of various materials (e.g., silicon dioxide, silicon nitride, polysilicon) are deposited or grown on the wafer. The thickness and chemical composition of these films are critical to the device's performance. FTIR can measure the thickness of these films non-destructively by analyzing the interference patterns (fringes) in the reflectance spectrum. It can also provide information about the film's chemical structure, such as the concentration of hydrogen or nitrogen incorporated into a silicon nitride layer, which affects its electrical properties.

Gas Purity Monitoring

The manufacture of semiconductors relies on a wide variety of ultra-high-purity (UHP) gases for processes like chemical vapor deposition (CVD) and etching. Even trace levels of impurities in these gases, such as water vapor or hydrocarbons, can be detrimental to the process.

FTIR, equipped with long-path gas cells, is used to continuously monitor the purity of these process gases. The instrument can be set up to monitor a gas line in real-time, providing immediate alerts if the concentration of a critical impurity like moisture exceeds a specified threshold (e.g., parts-per-billion levels). This allows engineers to divert the contaminated gas and troubleshoot the source of the problem before it affects production, saving significant time and money.

Failure Analysis

When a microchip fails, FTIR is a key tool in the failure analysis workflow. FTIR microscopy can be used to analyze microscopic defects or particles that may have caused the failure. For example, if a particle is found at the site of an electrical short, FTIR can identify its chemical composition. If the particle is identified as stainless steel, it might point to a problem with a piece of equipment; if it's identified as a specific polymer, it might be traced back to a packaging material. This chemical information is crucial for implementing effective corrective actions.

Application 7: Geosciences and Mineralogy

The study of the Earth—its rocks, minerals, and geological processes—relies on the accurate identification and characterization of geological materials. FTIR spectroscopy provides geoscientists with a powerful tool for rapidly identifying minerals and analyzing their chemical composition. This information is fundamental to understanding the formation of rocks, the exploration for mineral resources, and the study of planetary science.

Mineral Identification

Every mineral has a unique crystal structure and chemical composition, which in turn gives it a unique vibrational spectrum. The mid-infrared region is particularly sensitive to the vibrations of common anionic groups found in minerals, such as silicate (SiO₄), carbonate (CO₃), sulfate (SO₄), and phosphate (PO₄). The position, shape, and number of absorption peaks related to these groups provide a definitive fingerprint for mineral identification.

A geologist with a rock sample can grind a small portion into a powder and analyze it using a DRIFTS or ATR accessory. By comparing the resulting spectrum to a library of mineral spectra, the primary minerals in the rock can be identified. For example, the strong, broad absorption band near 1000-1100 cm⁻¹ is characteristic of silicate minerals. The exact position and structure of this band can distinguish between different types of silicates, such as quartz, feldspar, and olivine. A sharp peak near 1430 cm⁻¹ would immediately identify a carbonate mineral like calcite.

Characterization of Water and Hydroxyl in Minerals

The presence and structural state of water (H₂O) and hydroxyl groups (OH) in minerals are of great importance to geologists. This "water" content can provide clues about the temperature and pressure conditions under which the mineral formed and can significantly affect the physical properties of rocks in the Earth's mantle.

FTIR is extremely sensitive to the vibrations of O-H bonds. The stretching vibration of hydroxyl groups typically appears as a sharp peak in the 3700-3500 cm⁻¹ region. The bending vibration of molecular water molecules produces a peak around 1630 cm⁻¹. By analyzing the position and shape of these peaks, geoscientists can not only quantify the amount of water in a mineral but also determine its structural environment—for example, whether it is present as molecular water trapped in channels or as hydroxyl groups directly bonded within the crystal lattice.

Remote Sensing and Planetary Science

The principles of FTIR are not confined to the laboratory. Reflectance spectroscopy is a primary technique used in remote sensing to map the mineralogy of the Earth's surface from airplanes and satellites. Instruments known as imaging spectrometers measure the reflected sunlight from the ground, and the absorptions in the infrared region reveal the dominant minerals present.

This same technique has been extended to explore other planets. Rovers on Mars and orbiters around the Moon and asteroids are equipped with infrared spectrometers. By analyzing the reflected IR radiation from the surfaces of these celestial bodies, scientists can map their mineral composition from afar. The detection of hydrated minerals (those containing water or hydroxyl) by FTIR spectroscopy on Mars was a landmark discovery, providing compelling evidence that liquid water once existed on the planet's surface and guiding the search for past or present life.

Preguntas más frecuentes (FAQ)

What is the main difference between FTIR and conventional IR spectroscopy?

The primary difference lies in the instrumentation and how the spectral data is collected. Conventional IR (dispersive) spectroscopy uses a grating or prism to separate light into individual frequencies and measures them one at a time. An FTIR spectrometer uses a Michelson interferometer to collect data from all frequencies simultaneously. The raw data is an interferogram, which is then mathematically converted into a spectrum using a Fourier transform. This simultaneous collection gives FTIR significant advantages in speed, signal-to-noise ratio, and wavelength accuracy.

Is FTIR a destructive or non-destructive technique?

FTIR is considered a non-destructive technique. In most modern sampling methods, such as Attenuated Total Reflectance (ATR), the infrared beam interacts only with the surface of the sample. The sample is simply pressed against a crystal and can be fully recovered after analysis, unchanged. This is a major advantage, especially when dealing with valuable or limited-quantity samples, as is common in forensics or failure analysis.

What types of materials can be analyzed by FTIR?

FTIR is incredibly versatile and can analyze a vast range of materials in solid, liquid, or gaseous states. It is particularly powerful for identifying organic compounds, polymers, and many inorganic materials. This includes pharmaceuticals, plastics, rubbers, paints, foods, fabrics, and minerals. The main limitation is that the material must have molecular bonds that are infrared-active, meaning their vibration causes a change in the molecule's dipole moment. Symmetrical molecules like N₂ and O₂ are not IR-active, and metals, which reflect IR light, cannot be analyzed directly.

Can FTIR be used for quantitative analysis?

Yes, FTIR is an excellent tool for quantitative analysis. According to the Beer-Lambert Law, the absorbance of a specific peak in the spectrum is directly proportional to the concentration of the chemical component causing that absorption. By preparing a series of calibration standards with known concentrations and measuring their absorbance, a calibration curve can be created. The concentration of an unknown sample can then be determined by measuring its absorbance and using the calibration curve. This is widely used in industries for quality control, such as measuring the concentration of an active ingredient in a pharmaceutical tablet.

How much sample is needed for an FTIR analysis?

Very little sample is required, which is another key advantage of the technique. For analysis using a modern ATR accessory, only a few milligrams of a solid powder or a single drop of a liquid is needed—just enough to cover the surface of the small ATR crystal (typically 1-2 mm in diameter). For other techniques like KBr pellets or diffuse reflectance, a few milligrams are also usually sufficient. This makes FTIR ideal for analyzing trace evidence or precious materials.

A Concluding Thought on Molecular Inquiry

Our journey through the landscape of Fourier-transform infrared spectroscopy reveals it to be more than a mere analytical device. It is a profound instrument of inquiry, a means by which we can listen to the subtle, silent vibrations that define the chemical world. From the integrity of the medicines we depend on to the composition of distant planets, the insights afforded by FTIR are woven into the fabric of modern science and technology. The elegance of its design, which transforms a complex interference pattern into a clear molecular fingerprint, is a testament to human ingenuity. As we continue to refine this technology and expand its applications, we enhance our ability to understand, create, and protect the world around us, one vibration at a time. The capacity to pose precise questions to matter and receive intelligible answers is a foundational capability for human flourishing, and FTIR stands as a premier tool in that essential dialogue.

Referencias

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