Fourier Transform IR spectroscopy (FTIR) - How it works?

Summary

This video explores Fourier Transform Infrared (FTIR) Spectroscopy, a technique that offers superior accuracy and speed compared to traditional Dispersive IR Spectroscopy. The video explains the core principles of FTIR, including the use of Fourier Transformation to convert time-domain data into frequency-domain spectra. It delves into the instrumental components, such as the source, Michelson interferometer, and fast-response detectors. FTIR’s ability to rapidly measure infrared spectra makes it a powerful tool for analyzing chemical and molecular structures, with applications in fields like chemistry, biology, and material science.

Takeaways

• 😀 FTIR (Fourier Transform Infrared) Spectroscopy converts time-domain data into frequency-domain data using Fourier transformation.
• 😀 Unlike dispersive IR spectroscopy, FTIR uses an interferometer instead of a monochromator to split and combine beams.
• 😀 The principle behind FTIR involves producing a path difference between two optical pathways, causing constructive or destructive interference.
• 😀 Constructive interference increases amplitude, while destructive interference reduces it, and both are critical in creating the interferogram.
• 😀 The interferogram is initially plotted as amplitude versus optical path difference in the time domain, then converted into the frequency domain via Fourier transformation.
• 😀 FTIR typically uses sources like globar and nichrome for the mid-IR region, and tungsten halogen or mercury vapor lamps for other IR regions.
• 😀 The Michelson interferometer in FTIR consists of a beam splitter, two mirrors (fixed and movable), and a helium-neon laser for accurate mirror positioning.
• 😀 The mobile mirror’s position determines the optical path difference, which is essential for accurate spectral measurement.
• 😀 FTIR offers high accuracy and precision compared to conventional IR spectroscopy, and the conversion to frequency domain allows for precise wave number measurements.
• 😀 Fast-response detectors, like pyroelectric detectors or photon detectors (e.g., mercury-cadmium telluride), are used in FTIR for quick data acquisition.

Q & A

What is FTIR spectroscopy?

FTIR (Fourier Transform Infrared Spectroscopy) is a technique used to obtain the infrared spectrum of absorption or emission of a sample. It works by converting the time domain data into the frequency domain using Fourier transformation, allowing for highly accurate and precise measurements of the infrared spectra of solids, liquids, or gases.

How does FTIR differ from traditional dispersive IR spectroscopy?

FTIR differs from traditional dispersive IR spectroscopy in that it uses Fourier transformation to convert time-domain data into frequency-domain data, which increases its accuracy and speed. Additionally, FTIR uses an interferometer (Michelson interferometer) instead of a monochromator, making it faster and more precise.

What is the principle behind FTIR spectroscopy?

The principle behind FTIR is the measurement of the interference caused by a path difference between two beams of infrared light. This path difference leads to either constructive or destructive interference, which is recorded as an interferogram and later converted to a frequency domain spectrum using Fourier transformation.

What role does the Michelson interferometer play in FTIR?

The Michelson interferometer in FTIR splits the incoming infrared light into two beams using a beam splitter. One beam is directed towards a fixed mirror, and the other towards a movable mirror. The difference in path lengths of the beams causes interference, which is used to produce the interferogram that is later transformed into the IR spectrum.

What are constructive and destructive interference in the context of FTIR?

Constructive interference occurs when the two beams of light are in phase, leading to an increase in amplitude. Destructive interference occurs when the beams are out of phase, leading to a decrease in amplitude or complete cancellation. These interference patterns are used to generate the interferogram in FTIR spectroscopy.

Why is Fourier transformation used in FTIR?

Fourier transformation is used in FTIR to convert the time-domain interferogram into the frequency domain. This allows for the measurement of wave numbers and the creation of an IR spectrum. The Fourier transformation improves the accuracy and resolution of the data compared to traditional methods.

What types of radiation sources are used in FTIR?

In FTIR, the radiation source depends on the region of the infrared spectrum being measured. Common sources include a globar for the mid-IR region, a tungsten halogen lamp for the near-IR region, and a high-pressure mercury vapor lamp for the far-IR region.

What is the function of the beam splitter in the Michelson interferometer?

The beam splitter in the Michelson interferometer splits the incoming infrared light into two beams of equal intensity. One beam is directed to a fixed mirror, while the other is directed to a movable mirror. This splitting allows for the creation of an interference pattern, which is essential for generating the FTIR spectrum.

What detectors are commonly used in FTIR spectroscopy?

FTIR spectroscopy typically uses fast-acting detectors like pyroelectric detectors (e.g., deuterium triglycine sulfate) and photon detectors (e.g., semiconductor detectors filled with mercury cadmium telluride). These detectors provide quick response times, which is crucial for accurate measurements in FTIR.

What is an interferogram in FTIR, and how is it converted into a spectrum?

An interferogram is a plot of amplitude versus optical path difference in the time domain. It is produced by measuring the interference of infrared light beams in the Michelson interferometer. The interferogram is then converted into a spectrum using Fourier transformation, which results in a frequency-domain plot of transmittance versus wave number.