FTIR Spectroscopy and Graph Plotting Tutorial: Principles, Instrumentation & Origin Software Guide

Table of Contents

• Introduction to FTIR
• Why FTIR?
• Principle of FTIR
• Instrumentation of FTIR
• Advantages of FTIR
• Applications of FTIR
• Limitations of FTIR
• Understanding FTIR Graphs
• Why Plot FTIR Data in Origin Software
• References

Introduction to FTIR

• Fourier Transform Infrared (FTIR) spectroscopy is the preferred method of infrared spectroscopy.
• In infrared spectroscopy, IR radiation is transmitted through a sample.
• The sample absorbs some of the infrared energy while some of it passes through.
• The resulting spectrum provides a molecular fingerprint of the sample by showing its absorption and transmission characteristics.
• Each unique molecular configuration produces a distinct infrared spectrum, much like human fingerprints.
• This uniqueness makes infrared spectroscopy valuable for a wide range of analyses.
• FTIR spectrometers are widely used in fields such as organic synthesis, polymer science, petrochemical engineering, the pharmaceutical industry, and food analysis.
• FTIR spectrometers can also be hyphenated with chromatography, allowing them to be used in studying chemical reaction mechanisms and detecting unstable compounds.

Why FTIR?

• Infrared spectroscopy has been a reliable technique for material analysis in laboratories for nearly seventy years.
• An infrared spectrum acts as a fingerprint for a sample, where absorption peaks represent the vibrational frequencies of atomic bonds within the material.
• Each material has a unique atomic composition, so no two compounds produce the same infrared spectrum.
• This uniqueness allows infrared spectroscopy to provide positive identification (qualitative analysis) of any material.
• The peak magnitudes in the spectrum indicate how much of the material is present, enabling quantitative analysis when combined with modern software algorithms.
• Fourier Transform Infrared (FTIR) spectrometry addresses the limitations of traditional dispersive instruments, especially the time-consuming scanning process.
• FTIR allows simultaneous measurement of all IR frequencies using an optical device called an interferometer, which embeds all IR frequencies into a single signal.
• The signal is typically captured within a few seconds, making the process very fast.
• Most interferometers include a beamsplitter that divides the incoming IR beam into two paths—one fixed and one variable due to a moving mirror.
• The interference between these two beams produces a signal called an interferogram, which captures all frequencies at once.
• Since the interferogram cannot be interpreted directly, a mathematical technique called Fourier transformation is used to decode it into a usable frequency spectrum.
• FTIR spectroscopy is preferred over dispersive or filter methods for several reasons:
• It is non-destructive.
• It offers accurate measurements without needing external calibration.
• It allows fast scanning—collecting a scan every second.
• It enhances sensitivity by averaging multiple scans to reduce random noise.
• It provides higher optical throughput.
• It features a simplified mechanical design with only one moving part.

Principle of FTIR

• The principle of FTIR is based on the use of a Michelson interferometer.
• The Michelson interferometer consists of three main components: a beam splitter, a moving mirror, and a stationary (fixed) mirror.
• The beam splitter divides the incoming light beam into two separate beams.
• One beam reflects off the stationary mirror, while the other reflects off the moving mirror.
• These two beams are then recombined by the beam splitter.
• As the moving mirror moves back and forth (reciprocating motion), the optical path length of the beam reflected from it changes.
• This variation in path length leads to a continuous change in the phase difference between the two beams.
• When the two beams recombine, their phase differences cause constructive and destructive interference, producing an interference pattern.
• This pattern of interference light is detected and recorded as an interferogram.
• The interferogram represents the intensity of the combined light beams, with the optical path difference plotted along the horizontal axis.

Instrumentation of FTIR

• FTIR instrumentation includes several key components that work together to generate and analyze infrared spectra.

The Source: 

• A broadband emitter provides the IR radiation. Common sources include:
• Mid-IR ceramic sources
• Far-infrared mercury lamps
• Near-infrared halogen lamps

The Interferometer: 

• The core of an FTIR spectrometer, composed of:
• A beamsplitter (semi-transparent mirror) that divides a collimated light beam into two optical paths
• A stationary mirror
• A moving mirror
• Half of the light is transmitted to the moving mirror, and the other half is reflected to the stationary mirror.
• Both mirrors reflect the light beams, which then recombine at the beamsplitter before entering the sample chamber and reaching the detector.

The Sample:

• The recombined beam enters the sample compartment.
• Depending on the analysis type, light is either transmitted through or reflected from the sample’s surface.
• The sample absorbs specific frequencies of IR energy that are characteristic of its molecular structure.

Detector:

• Measures and converts the transmitted or reflected light into an electrical signal.
• The detector’s material and design determine the sensitivity and range of data that can be captured.
• The detector first converts the IR beam into photons, which are then converted into electrical signals readable by a computer.
• Common detectors include:
• Room temperature DLATGS (Deuterated L-alanine doped triglycine sulfate) for standard applications.
• Cooled liquid nitrogen detectors for high-sensitivity applications.
• Silicon photodiodes for near-infrared and visible IR applications.
• Silicon far-infrared bolometers for far-infrared range analysis.

Advantages of FTIR

• FTIR has higher speed because all frequencies are detected simultaneously, allowing most measurements to be completed within seconds. This is known as the Felgett Advantage.
• It offers high sensitivity due to multiple factors:
• More sensitive detectors are used.
• Greater optical throughput reduces noise levels.
• Fast scanning allows multiple scans to be added together (coadded) to reduce random measurement noise to any desired level.
• FTIR provides high accuracy and reproducibility, making it well-suited for background subtraction.
• It is a highly reliable technique for positively identifying nearly any material.
• Instruments include a HeNe laser for internal wavelength calibration, known as the Connes Advantage. This allows for self-calibration, eliminating the need for manual user calibration.
• FTIR can capture IR data from very small sample sizes.
• It has high optical throughput, referred to as the Jaquinot Advantage, meaning the radiant power reaching the detector is significantly higher than in dispersive instruments.
• FTIR delivers high precision, as the laser functions as a stable reference signal and timekeeper.
• Internal synchronization ensures reliable results unaffected by external factors like sunlight or temperature changes.

Applications of FTIR

• FTIR spectroscopy is widely used to analyze industrially manufactured materials during quality control processes.
• It serves as a common first step in material analysis by detecting changes in absorption band patterns, indicating alterations in material composition or contamination.
• It is employed in drying processes for polymers, photoresist materials, and polyimides.
• FTIR investigates the interaction between matter and electromagnetic radiation, producing a spectrum for analysis.
• FTIR spectrum analysis aids in diagnosing various organ diseases and quantifying biomolecules like proteins, nucleic acids, and lipids.
• It is a valuable method for assessing fuel stability variations in biodiesel/antioxidant mixtures.
• In failure analysis, FTIR helps detect breakdowns, oxidation, and the presence of uncured monomers.
• FTIR is used in high-resolution experiments.
• It supports trace analysis of raw materials and final products.
• It enables analysis of reactions occurring on the microsecond time scale.
• It is used for analyzing chromatographic and thermogravimetric sample fractions.
• FTIR identifies reaction components and is useful in kinetic studies of chemical reactions.
• It aids in compound identification by comparing the unknown substance’s spectrum with reference spectra (fingerprinting).
• FTIR is used for identifying functional groups in unknown compounds, such as ketones, aldehydes, carboxylic acids, and more.

Limitations of FTIR

• The molecule must be infrared-active; at least one vibrational motion must change the molecule’s net dipole moment when exposed to IR radiation for absorption to be detected.
• FTIR provides minimal elemental information for most samples, making it less suitable for detailed elemental analysis.
• The material being analyzed must be transparent in the spectral region of interest; opaque or highly absorbing materials can hinder accurate measurements.

Understanding FTIR Graphs

What Does an FTIR Graph Show?

• The FTIR spectrum is a graph of infrared light absorption (or transmittance) versus wavenumber (cm⁻¹).
• Each peak represents a specific vibrational mode of a chemical bond.
• Helps to identify functional groups and assess molecular structure.

Axis Details:

X-axis: 

• Wavenumber (cm⁻¹).
• Ranges from 4000 to 400 cm⁻¹.
• Higher wavenumbers = stronger bonds or lighter atoms (e.g., O–H, N–H).
• Lower wavenumbers = heavier bonds (e.g., C–C, C–Br).

Y-axis:

• Usually % Transmittance or Absorbance
• Transmittance: how much IR light passed through
• Absorbance: how much was absorbed by sample

Important Regions:

Functional Group Region (4000–1500 cm⁻¹):

• Most diagnostic bands for identifying functional groups.

Fingerprint Region (1500–400 cm⁻¹):

• Highly specific to individual molecules.
• Used for compound identification through spectral matching.

Common Peaks to Recognize:

• O–H stretch (broad, ~3200–3600 cm⁻¹).
• C=O stretch (sharp, ~1700 cm⁻¹).
• C–H stretch (2800–3000 cm⁻¹).
• N–H stretch (3300–3500 cm⁻¹).
• C–O stretch (~1000–1300 cm⁻¹).

 Interpretation Tips:

• Sharpness, intensity, and shape of peaks matter.
• Compare experimental spectrum with standard reference spectra.
• Use peak position, width, and height to deduce molecular structure.

Why Plot FTIR Data in Origin Software

Advantages of Using Origin:

• Professional, high-resolution plots for research or publication
• Allows custom labeling, color schemes, and peak annotation
• Easy to zoom into specific regions (e.g., fingerprint region)

Key Features for FTIR Plotting:

• Import FTIR data directly from Excel or CSV files.
• Customize axes: reverse X-axis (standard for wavenumber).
• Add vertical lines to mark important peaks (e.g., C=O, O–H).
• Overlay multiple spectra (e.g., control vs treated sample).
• Use Baseline correction and peak deconvolution tools.

Organize & Save:

• Save your project with multiple graphs in different folders.
• Reuse templates for similar spectra.
• Export high-quality figures (JPG, PNG, TIFF, PDF) for papers.

Enhanced Data Presentation:

• Add titles, legends, sample IDs to make it viewer-friendly.
• Use color coding for time-course or concentration-based studies.
• Highlight degraded, shifted, or new peaks for biodegradation analysis.

References

• LibreTexts Chemistry. (n.d.). Operation of an FTIR Spectrometer. Retrieved from https://chem.libretexts.org
• JASCO Inc. (n.d.). Fundamentals of FTIR Spectroscopy: Instrumentation Overview. Learning Center. Retrieved from https://jascoinc.com
• University of Barcelona. (n.d.). Applications of FTIR Spectroscopy. Retrieved from https://diposit.ub.edu
• University of Alaska Fairbanks. (n.d.). FTIR Instrument Instructions and Operational Guide. Retrieved from https://instrumentalanalysis.community.uaf.edu
• e-PG Pathshala (UGC). (n.d.). FTIR Spectroscopy – Module on Principles and Applications. Retrieved from https://epgp.inflibnet.ac.in