What Is Infrared Spectroscopy? Core Principles, Components, & Applications

What Is Infrared Spectroscopy? Core Principles, Components, & Applications

Infrared (IR) spectroscopy is a fundamental tool of modern chemical analysis. A technique centered around the absorption of infrared light by molecules, it offers a window into the molecular world that reveals a wide range of details about chemical structures and compositions. IR spectroscopy’s ability to identify and analyze many different materials—from organic compounds to inorganic substances—makes it indispensable across various scientific fields that rely on chemical analysis.

The importance and significance of this technique lie in its ability to detect vibrations of molecular bonds. When infrared light interacts with a sample, specific frequencies are absorbed, corresponding to the vibrational energies of the molecular bonds within the sample. This absorption creates a unique spectral fingerprint for each substance, providing insights into its molecular structure and characteristics.

Our post aims to provide a comprehensive look at IR spectroscopy and will include:

  • An overview of the technique's core working principles.
  • The different types of IR spectrometers.
  • And the technique’s vast array of applications.

Whether you are a seasoned researcher or new to the field of spectroscopy, this post will give you a stronger understanding of IR spectroscopy, its role in chemical analysis, and its applications across different fields of study.

Fundamentals of IR Spectroscopy

Infrared (IR) spectroscopy is an analytical technique for studying molecular vibrations and chemical bonds. At its core, IR spectroscopy involves the absorption of infrared light by molecules, leading to increased vibrational energy that can be measured and analyzed.

Understanding these fundamentals of IR spectroscopy is key to leveraging its capabilities in chemical analysis, helping scientists and researchers reveal detailed insights into the molecular composition of various substances.

  • Molecular Vibrations & IR Absorption: IR spectroscopy explores how molecules absorb infrared light and vibrate at frequencies characteristic of their structure. Different types of chemical bonds and atoms absorb specific frequencies, reflecting their vibrational energies. This absorption provides a detailed view of the molecular structure, including information about the electronic environment surrounding these bonds. The unique vibrational patterns allow for the identification and characterization of various substances.
  • Types of Vibrational Modes: The technique detects two primary types of molecular vibrations: stretching and bending. Stretching involves atoms moving along the bond axis and can be symmetric or asymmetric, while bending includes motions like scissoring, rocking, wagging, and twisting. These vibrations, with their distinct IR absorption frequencies, offer detailed insights into molecular geometry, isomeric states, and bond types. This level of detail makes IR spectroscopy invaluable for studying complex molecular architectures and conformations.
  • The IR Spectrum & Molecular Fingerprinting: Each molecule produces a unique IR spectrum, acting as a molecular fingerprint. This spectrum plots the intensity of absorbed IR radiation against frequency or wavelength. The IR spectrum helps identify functional groups and impurities and provides quantitative data, enabling concentration measurements of specific compounds in mixtures. The spectrum's intensity and pattern of absorption bands are necessary for understanding molecular structures and compositions.
  • Functional Group Identification: IR spectroscopy excels in identifying functional groups and differentiating isomers, making it essential in organic and medicinal chemistry. By analyzing the absorption bands, chemists can determine the presence or absence of functional groups, which are crucial for quality control and ensuring the integrity of chemical products.
  • Complementing Other Spectroscopic Methods: IR spectroscopy forms part of a comprehensive analytical approach, often used alongside other spectroscopic methods like NMR and mass spectrometry. This synergy allows for a more complete understanding of a sample, combining structural, dynamic, and compositional information for complex analyses.

IR spectroscopy plays a vital role in various scientific fields through its detailed molecular analysis, enhancing our understanding of molecular structures and interactions. It's a cornerstone technique in the pharmaceutical, materials science, and environmental chemistry industries.

IR Spectroscopy vs. IR Spectrometry

It's important to understand the distinction between IR spectroscopy and IR spectrometry in analytical chemistry, as they represent different aspects of the same scientific technique.

IR Spectroscopy refers to the study and application of the technique itself. 

IR spectroscopy encompasses the theoretical aspects of how infrared light interacts with matter, IR spectra interpretation, and molecular information extraction. It involves understanding the principles behind the vibrational modes of molecules and how these vibrations translate into specific absorption patterns in the IR spectrum.

IR Spectrometry, in contrast, is more focused on the measurement process and the instrumentation used for IR spectroscopic analysis. It involves the practical aspects of how IR spectrometers are designed, operated, and maintained. IR spectrometry covers the development and application of devices that measure the intensity and wavelength of IR radiation absorbed by a sample.

Spectroscopy is the domain of chemists and researchers analyzing the spectra, while spectrometry is more concerned with engineers and technicians designing, building, and maintaining the instruments. For instance, discussions about improvements in spectral resolution or detector sensitivity fall under the purview of IR spectrometry.

While these terms have distinct meanings, they are often used interchangeably in many scientific discussions without causing confusion. The context usually makes it clear whether the focus is on the scientific study (spectroscopy) or the measurement technique and its instrumentation (spectrometry).

Comparing Other Spectroscopy Techniques

IR spectroscopy is one of several powerful spectroscopic techniques used in chemical analysis, each with unique strengths. Understanding how IR spectroscopy compares with other methods can provide insights into its specific applications and advantages.

Each of these spectroscopic techniques offers unique insights into the properties of substances. The choice of technique depends on the specific requirements of the analysis, such as the type of sample, the nature of the information needed, and the experimental conditions:

  • UV-Visible Spectroscopy: UV-Visible spectroscopy measures a sample's absorption of ultraviolet or visible light. While IR spectroscopy is focused on vibrational transitions related to molecular bonds, UV-Visible spectroscopy involves electronic transitions. UV-Visible is often used for analyzing compounds that absorb light in the visible spectrum, like dyes and pigments. In contrast, IR is more suited for studying molecular structures and functional groups.
  • Nuclear Magnetic Resonance (NMR) Spectroscopy: NMR spectroscopy provides information about the atomic environment in molecules, especially hydrogen and carbon atoms. Unlike IR, which is sensitive to bond vibrations, NMR detects the magnetic properties of atomic nuclei. NMR is invaluable for determining the structure of organic compounds, complementing the bond-specific information obtained from IR spectroscopy.
  • Mass Spectrometry (MS): MS analyzes the mass-to-charge ratio of ions to identify and quantify molecules in a sample. While IR spectroscopy provides insights into molecular structures through vibrational analysis, MS offers detailed information about molecular weights and compositions. The two techniques are often used together for comprehensive chemical characterization.
  • Near-Infrared Spectroscopy (NIR): Similar to IR, NIR spectroscopy involves the absorption of near-infrared light. NIR is typically used for quantitative analysis in agricultural, pharmaceutical, and food industries. While IR provides detailed molecular fingerprints, NIR is better suited for rapid, non-destructive analysis and is often used in process control and quality assurance.
  • Raman Spectroscopy: While IR spectroscopy measures the absorption of IR radiation, Raman spectroscopy detects scattered light. Raman is complementary to IR, as it can analyze molecules that are IR inactive due to symmetry. Raman is also less influenced by water, making it advantageous for analyzing aqueous solutions.

Types of IR Spectrometers

The variety of IR spectrometers available caters to a wide range of analytical needs, each offering unique capabilities.

Each type of IR spectrometer is designed to meet specific analytical requirements, from high-resolution laboratory analysis to rapid on-site testing. The selection of the appropriate spectrometer type is crucial for obtaining accurate and relevant spectral data for the specific application at hand:

  • Fourier Transform Infrared (FTIR) Spectrometers: FTIR spectrometers are renowned for their efficiency and accuracy. Utilizing the Fourier Transform technique, they convert the interferogram obtained from the IR radiation into a detailed spectrum. These spectrometers are known for their rapid scanning capabilities, high signal-to-noise ratio, and ability to collect spectral data across a wide range simultaneously. FTIR spectrometers are versatile, suitable for analyzing gasses, liquids, and solids, and are used in applications ranging from chemical identification to environmental monitoring and pharmaceutical quality control.
  • Portable IR Spectrometers: Designed for mobility and ease of use, portable IR spectrometers have opened the door to in-field chemical analysis. They are invaluable for applications that require immediate results or where sample transportation is impractical. These spectrometers are widely used in on-site environmental analysis, quality assurance in manufacturing industries, and forensic science for crime scene investigations. Their compact design, though offering slightly reduced sensitivity compared to laboratory-based spectrometers, provides the flexibility needed for fieldwork.
  • Dispersive IR Spectrometers: The traditional design of dispersive IR spectrometers involves separating IR light into its component wavelengths using prisms or diffraction gratings. Although FTIR spectrometers have largely supplanted them due to lower speed and sensitivity, dispersive IR spectrometers are still valuable for educational purposes and in settings where high-resolution spectral data is not critical. They are simpler in design and can be more cost-effective, making them accessible for basic IR spectroscopic studies.
  • Near-Infrared (NIR) Spectrometers: NIR spectrometers specialize in the near-infrared region of the electromagnetic spectrum. They are particularly effective for quantitative analysis in industries like agriculture for soil and crop analysis, pharmaceuticals for drug formulation, and food processing for quality control. NIR spectrometers excel in rapid, non-destructive testing, providing real-time analysis crucial for industrial process control.
  • Attenuated Total Reflectance (ATR) Spectrometers: ATR spectrometers, often used as an accessory with other IR spectrometers, are ideal for analyzing complex samples. The ATR technique allows for direct analysis of solids, pastes, and viscous liquids without extensive sample preparation. This method is particularly effective for studying surface layers and thin films and samples that are too absorbent for traditional transmission methods.

Components of an IR Spectroscopy System

An IR spectroscopy system comprises several key components, each playing a vital role in identifying and analyzing infrared spectra.

Every component of an IR spectroscopy system must work harmoniously to produce accurate and reliable spectral data. The configuration of these components can vary depending on the specific type of IR spectrometer and the intended application:

IR Light Source

The heart of the system is the IR light source, which emits a broad spectrum of infrared radiation. The quality and stability of the IR source are crucial for consistent spectral output. Sources like Globar (silicon carbide) and Nernst Glower (rare-earth oxides) are commonly used for their wide spectral range and stable output.

Sample Interface

This component is where the sample interacts with the IR light. Depending on the type of spectrometer and sample, this could be a transmission cell, an ATR crystal, or a diffuse reflectance accessory. The sample interface must be carefully chosen to suit the nature of the sample (solid, liquid, or gas) and the specific requirements of the analysis.


The detector in an IR spectrometer captures the IR radiation that has interacted with the sample. Different types of detectors are used, such as thermocouples, bolometers, and photovoltaic detectors, each with varying sensitivity and response times. The choice of detector impacts the quality of the spectral data, particularly in terms of sensitivity and noise levels.

Monochromator/Diffraction Grating

In dispersive IR spectrometers, a monochromator or diffraction grating separates the IR light into its component wavelengths. This separation is essential for producing a detailed spectrum. In FTIR spectrometers, an interferometer performs a similar function by creating an interferogram that is later transformed into a spectrum.

Data Acquisition & Analysis Software

Modern IR spectrometers are equipped with sophisticated data acquisition, processing, and analysis software. This software transforms raw data into interpretable spectra, peak identification, quantitative analysis, and even the comparison of spectra to known reference materials.

Optics & Pathlength

The optical system, including mirrors and beam splitters, directs the IR light through the spectrometer. The path length, or the distance the light travels through the sample, can be crucial, especially for liquid and gas samples. Adjusting the path length can help optimize the intensity of the spectral bands for clearer analysis.

Sample Preparation in IR Spectroscopy

Effective sample preparation is a critical step in IR spectroscopy, impacting the accuracy and interpretability of the resulting spectra.

Proper sample preparation in IR spectroscopy is as necessary as the measurement itself. It is essential for obtaining reliable, representative spectra and for accurate interpretation of the molecular information contained within the sample:

Solid Samples

Preparing solid samples often involves creating pellets or thin films. When making pellets, the sample is typically ground into a fine powder and mixed with a non-absorbing medium like potassium bromide (KBr). The mixture is then pressed under high pressure to form a transparent pellet through which the IR beam can pass. For thin films, the sample is often dissolved in a suitable solvent and then deposited onto an IR-transparent substrate, where the solvent evaporates, leaving a thin layer of the sample.

Liquid Samples

Liquids are generally placed in specially designed IR cells or cuvettes made from materials like calcium fluoride or barium fluoride, which do not absorb IR radiation. The choice of cell path length is important to avoid excessive absorption; a short path length is ideal for highly absorbing samples. Careful handling is necessary to prevent air bubbles and ensure a uniform sample thickness.

Gaseous Samples

Analyzing gasses with IR spectroscopy requires cells with much longer path lengths than those for liquids or solids. This is because gasses at standard pressure and temperature have lower molecular densities. Long-path gas or multipass cells, which effectively increase the path length using reflective mirrors, are commonly used. These cells allow the IR beam to pass through the gas sample multiple times, enhancing the absorption signal.

ATR (Attenuated Total Reflectance)

ATR simplifies sample preparation significantly. Samples in various states—solid, liquid, or semi-solid—can be analyzed directly by placing them on the surface of an ATR crystal. The IR light penetrates only a few micrometers into the sample, making ATR ideal for studying surface properties and for samples that are difficult to prepare in other ways.

Minimizing Contaminants

Maintaining purity during sample preparation is paramount. Contaminants can introduce unwanted absorption features or mask important aspects of the sample's IR spectrum. Clean, dry, and IR-transparent materials and tools should be used to handle samples.

Sample Size & Thickness

The optimal sample size and thickness depend on the sample's IR absorption strength. Overly thick samples can lead to band saturation, where absorption peaks are too strong to measure accurately, while very thin samples may produce weak, difficult-to-detect signals. Achieving the right balance is key for a clear and informative IR spectrum.

Troubleshooting & Calibration in IR Spectroscopy

Effective troubleshooting and calibration are crucial for maintaining the precision and accuracy of IR spectroscopy, often involving collaboration with equipment manufacturers and their technicians:

  • Calibration of the Spectrometer: Regular calibration, essential for accurate wavelength and intensity measurements, often requires standard materials with known absorption peaks. Working with the manufacturer can ensure appropriate calibration standards and procedures, maintaining the spectrometer's accuracy over time.
  • Dealing with Sample Issues: Challenges in sample preparation can significantly impact results. Manufacturers often provide guidelines and training on best practices for sample preparation, whether for solid, liquid, or gaseous samples, to avoid common pitfalls like contamination or uneven sample thickness.
  • Detector Performance: Detector issues, such as reduced sensitivity or increased noise, can degrade spectral quality. Engaging with the manufacturer's technical support can help identify whether maintenance or replacement is needed, ensuring the detector operates at optimal performance.
  • Optical Alignment: Proper alignment of the spectrometer's optical components is critical. Manufacturers can offer technical assistance and routine service checks to ensure optimal alignment, efficient IR light transmission, and consistent spectral output.
  • Handling Baseline & Background Issues: Manufacturers can provide software updates and technical support to address baseline fluctuations or drift, which are often caused by environmental factors or instrument instability. Their expertise can be invaluable in fine-tuning instrument settings for stable and accurate baseline correction.
  • Environmental Factors: Temperature and humidity control can be crucial, especially in sensitive measurements. Manufacturers can advise on the best practices and necessary equipment (like environmental chambers) to mitigate environmental variations' impact on the sample and the spectrometer.
  • Manufacturer Assistance: Regular communication with the equipment manufacturer and their technicians can be one of the most effective ways to troubleshoot and calibrate IR spectroscopy instruments. Their expertise and knowledge of the system can provide tailored solutions to specific challenges, ensuring the longevity and accuracy of the equipment.

Effective collaboration with the manufacturer and routine maintenance and calibration ensure the IR spectroscopy system functions optimally, providing precise and reliable data crucial for various analytical applications.

Applications of IR Spectroscopy

IR spectroscopy's diverse applications underscore its critical role in various scientific and industrial contexts. The versatility and precision of IR spectroscopy in providing molecular-level analysis make it an invaluable tool across these fields, enhancing our understanding and capabilities in diverse areas of science and industry:

Chemical & Pharmaceutical Industries

Beyond basic identification and quantification, IR spectroscopy is instrumental in understanding the interactions and stability of different pharmaceutical compounds. It facilitates the study of complex chemical reactions and mechanisms, playing a key role in drug development and synthesis. This technique also aids in patent formulation studies and counterfeit drug detection, ensuring the integrity and efficacy of pharmaceutical products.

Environmental Analysis

IR spectroscopy is pivotal in assessing the impact of industrial activities on the environment. It enables the detection of greenhouse gases and the monitoring of air and water pollution levels. This technique is also crucial in studying the effects of various pollutants on ecosystems, aiding in developing strategies for environmental remediation and sustainability.

Food Quality Control

In the food industry, IR spectroscopy is used to detect contaminants, analyze nutritional components, and monitor changes during food processing. It provides a rapid, non-destructive means of evaluating food quality, including detecting spoilage and verifying product labels, thereby ensuring consumer safety.

Medical Diagnostics

IR spectroscopy's role extends to the early detection of diseases through analyzing biological fluids, offering a non-invasive diagnostic tool. It also contributes to research in personalized medicine by analyzing patient-specific responses to various treatments, paving the way for more targeted therapeutic approaches.

Material Science

The application of IR spectroscopy in material science includes the analysis of wear and degradation in industrial materials. It's essential in studying corrosion, aging, and the mechanical properties of materials under various environmental conditions, contributing significantly to developing more durable and efficient materials.

Forensic Science

Beyond analyzing typical forensic samples, IR spectroscopy assists in reconstructing crime scenes by determining the origin of unknown substances. It's used to analyze paint chips, plastic fragments, and other trace evidence, providing crucial information that can link suspects to crime scenes.

Art & Heritage Conservation

In art conservation, IR spectroscopy helps determine the age and authenticity of artworks. It assists conservators in choosing appropriate restoration materials and methods by providing insights into the chemical composition of both the original and degraded materials.

Agricultural Science

IR spectroscopy aids in precision agriculture by analyzing soil health and composition, optimizing fertilizer use, and monitoring plant stress. It's also used in the quality control of agricultural products, ensuring the safety and quality of produce from farm to table.

Equip Your Lab with New IR Spectroscopy Instruments

Infrared spectroscopy is an indispensable analytical tool in many different scientific and industrial domains, including pharmaceutical research, environmental monitoring, and more. This is due to its ability to provide a number of molecular insights that other techniques might not be able to identify and analyze. Embracing the full potential of IR spectroscopy can enhance research and analytical capabilities.

However, if you require a new IR spectrometer to support your chemical analysis, consider leasing IR spectroscopy equipment with Excedr. Our leasing program provides access to cutting-edge technology and comprehensive support throughout the lease's duration.