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Principle of Fourier Transform Infrared Spectroscopy (FTIR) Analysis


The basic principle of Fourier Transform Infrared Spectroscopy (FTIR)

FTIR spectroscopy can be classified into Fourier Transform Infrared Spectroscopy (FTIR) and Dispersive Infrared Spectrophotometer based on the spectroscopic technique. The main difference lies in the specifications of the interferometer and computer. Currently used interferometers are mostly of the Michelson type, which transmits signals from the light source to the computer in the form of interference patterns for Fourier transformation mathematical calculations, ultimately restoring the interference patterns into spectra. A Michelson interferometer mainly consists of a light source, fixed mirror, moving mirror, beam splitter, and detector.

Figure 1: Schematic diagram of Michelson interferometer configuration

According to the above equation, by recording the interference pattern and performing Fourier cosine transformation through the basic equation of Fourier Transform Spectroscopy, the intensity at any wave number can be obtained. However, this transformation process is highly complex and cumbersome, requiring the use of computers. In other words, Fourier Transform Infrared Spectroscopy (FTIR) became practical only after the emergence and development of computers, as they are essential for carrying out the required mathematical computations.

Fourier Transform Infrared Spectroscopy (FTIR) instrument structure and product introduction

The Fourier Transform Infrared Spectroscopy (FTIR) instrument is composed of components such as an infrared light source (Ever-Glo), a Michelson interferometer, a sample chamber, a detector, and a computer.

Figure 2: The product image of ThermoFisher Nicolet iS Series

Nicolet Instruments, a part of ThermoFisher's Fourier Transform Infrared (FTIR) spectroscopy series, introduces the Nicolet iS Series, an intelligent research-grade FTIR spectrometer. All connections on its optical platform utilize plug-and-play sensor chip design, enabling the computer system to automatically recognize the current optical component configuration (Bench) and select the required parameters (Experiment parameter). It can also automatically identify and execute transitions between different spectral ranges (Range) and change different accessory experimental modules (Accessory). The Nicolet FTIR offers a variety of accessories including transmission, reflection, diffuse reflection, attenuated total reflection (ATR), and fiber optic probes. It also provides interfaces and accessories necessary for various serial techniques (Microscope/FTIR, GC/FTIR, TGA/FTIR). Its intelligent diagnostic system continuously monitors the parameters of each optical and electronic component, indicating any abnormalities. In case of instrument malfunction, the system identifies the faulty component and provides guidance on resolving the issue.

Advantages of Fourier Transform Infrared Spectroscopy

1.The scanning speed of Fourier Transform Infrared Spectroscopy (FTIR) instruments is several hundred times faster than that of dispersive instruments. Within any measurement time, FTIR can obtain all frequency information of the infrared light source, meaning that all spectral signals can be obtained simultaneously. The scanning time is not related to the scanning range. The scanning speed is mainly determined by the speed of the moving mirror. One scan of the moving mirror can collect all information. This advantage makes it particularly suitable for use in conjunction with gas phase analyzers (TGA, GC), as well as for tracking rapid chemical reactions and studying chemical reaction kinetics. For stable samples, multiple scans are generally used in a single measurement, and the interferograms are accumulated and averaged, which improves the signal-to-noise ratio. Under the same total measurement time and resolution conditions, the signal-to-noise ratio of FT-IR is several tens of times higher than that of dispersive instruments, which is also an advantage of fast scanning.

2.High resolution is one of the main performance indicators of infrared spectrometers, referring to the instrument's ability to distinguish between closely spaced absorption peaks. Typically, the resolution of prism-based spectrometers is around 3 cm-1 at 1000 cm-1, while grating-based spectrometers can achieve around 0.2 cm-1 at 1000 cm-1. Fourier Transform Infrared Spectroscopy (FTIR) can achieve resolutions of 0.1 to 0.005 cm-1 across the entire spectral range (4000-400 cm-1). Its resolution is related to the optical path difference of the interferometer; the larger the optical path difference, the better the instrument's resolution. This means that the longer the distance moved by the moving mirror, the better the resolution, but this also increases the scanning time.

3.Fourier Transform Infrared Spectroscopy (FTIR) high wavenumber precision and accuracy

4.Fourier Transform Infrared Spectroscopy (FTIR) research spectrum range wide.A Fourier Transform Infrared Spectrometer (FTIR) can study the full spectrum range of near-infrared, mid-infrared, and far-infrared regions (27,000 cm^-1 to 20 cm^-1) simply by automatically switching the instrument's components (such as different beam splitters and light sources). This is highly advantageous for the analysis of inorganic compounds and organometallic compounds.

Figure 3: The Fourier Transform Infrared Spectrometer (FTIR) spectrum range chart

The application of Fourier Transform Infrared Spectroscopy (FTIR) technology in chemical analysis

In addition to its general spectral measurement functions, Fourier Transform Infrared Spectroscopy (FTIR) possesses advantages such as fast scanning speed and high resolution. It also has the capability to measure instantaneous spectral changes, perform differential spectral techniques, and conduct low-order spectral measurements. FTIR technology finds widespread applications in the field of analytical chemistry, such as surface characterization, qualitative compound analysis, investigation of reaction mechanisms, quality control (QC/QA), life science research, geological surveys, and environmental monitoring.

FTIR instruments are also used in various coupled techniques, such as Gas Chromatography - Fourier Transform Infrared Spectroscopy (GC/FTIR), High-Performance Liquid Chromatography - Fourier Transform Infrared Spectroscopy (HPLC/FTIR) for mixture analysis and identification, Thermal Gravimetric Analysis - Fourier Transform Infrared Spectroscopy (TG-IR) for identifying products after the vaporization of polymers, and Microscope/FTIR for measuring small/sample quantities and analyzing product defects.