What’s the Appropriate Infrared Detector for FTIR Analysis and How Do Thermoelectrically Cooled MCT Detectors Compare?

Fourier Transform Infrared (FTIR) spectroscopy is a powerful analytical technique used to obtain the infrared spectrum of absorption or emission of a sample, whether solid, liquid, or gas. Central to the functioning of FTIR spectroscopy are infrared detectors, which measure the intensity of infrared light after it interacts with the sample.  Let’s delve into the working principles of infrared detectors in FTIR analysis, compare thermoelectrically cooled (TEC) MCT (Mercury cadmium telluride) detectors with traditional IR detectors, and discuss the scenarios in which different IR detectors should be used in research.

Working Principles of Infrared Detectors in FTIR Analysis

FTIR spectrometers utilize an interferometer to modulate the infrared light from the source. This modulation produces an interference pattern called an interferogram, which encodes information across a broad range of infrared wavelengths simultaneously.

In FTIR analysis, an interferometer splits the incoming infrared beam into two paths, which are recombined to produce an interference pattern. This modulated light interacts with the sample, which absorbs specific wavelengths based on its molecular composition. The light, now carrying sample information, reaches the infrared detector, which converts the infrared radiation into an electrical signal.

Infrared detectors can be thermal, like thermocouples, bolometers, and pyroelectric detectors, which measure temperature changes due to absorbed radiation, or photon detectors, such as MCT detectors, which detect infrared photons by generating electron-hole pairs and offer higher sensitivity and faster response times but often require cooling to reduce noise. It’s important to note that thermoelectrically cooled (TEC) MCT detectors do not require liquid nitrogen for cooling and therefore there is no need to maintain LN₂ access or manage safety-related concerns.

The detector’s electrical signal is processed by a computer using a Fourier Transform to convert it into an infrared spectrum. This spectrum, displaying the intensity of infrared light as a function of wavelength or wavenumber, is analyzed to identify absorption bands corresponding to different molecular vibrations, providing detailed information about the sample’s molecular composition.

Comparing Thermoelectrically Cooled (TEC) MCT Detectors with Traditional IR Detectors

Thermoelectrically cooled (TEC) MCT detectors represent an advancement over traditional IR detectors, offering several benefits:

1. Sensitivity and Noise: Traditional MCT detectors require cryogenic cooling (e.g., with liquid nitrogen) to reduce thermal noise and achieve high sensitivity. TEC MCT detectors use a thermoelectric cooler, which is more convenient and safer, eliminating the need for cryogenic liquids. TEC MCT detectors still achieve high sensitivity by maintaining a stable, low operating temperature, however the sensitivity that can be achieved depends on the wavelength range of the TEC MCT detector. TEC MCTs will perform better than a thermal detector for low throughput applications.
2. Convenience and Safety: TEC MCT detectors are more user-friendly and safer to operate. The absence of cryogenic liquids simplifies the setup and maintenance, making these detectors more suitable for routine laboratory use.
3. Performance: TEC MCT detectors maintain high performance with excellent signal-to-noise ratios, fast response times, and broad spectral range detection. They are particularly useful in applications requiring fast data acquisition. TEC-MCTs provide immediate results without time spent cooling the detector before using.  There is no time limitation caused by dewar hold times (i.e., the need to refill a detector dewar for long measurements is eliminated)

When to Use Different IR Detectors in Research

Choosing the appropriate IR detector for FTIR analysis depends on the specific requirements of the research application:

Thermal Detectors: These detectors are suitable for applications where high sensitivity and fast response times are not critical. They are ideal for general-purpose FTIR spectroscopy, educational settings, and applications where cost-effectiveness is important. Thermal detectors are robust and can cover a wide spectral range.

Traditional MCT Detectors: When high sensitivity and fast response times are required, traditional MCT detectors are preferred. They are ideal for advanced research applications, such as detecting low-concentration analytes, studying fast kinetic processes, and performing detailed molecular analysis. However, the need for cryogenic cooling can be a limitation in terms of convenience and safety.

TEC MCT Detectors: These detectors offer a balance between performance and convenience. They are suitable for some high-sensitivity applications without the hassle of cryogenic cooling. TEC MCT detectors are ideal for routine high-performance FTIR spectroscopy in research laboratories, quality control, and industrial applications where speed, ease of use, and safety are paramount.

Summary

Infrared detectors are integral to FTIR spectroscopy, converting modulated infrared light into electrical signals for spectrum generation and analysis. Thermoelectrically cooled MCT detectors offer significant advantages over traditional IR detectors, combining high speed with operational convenience. TEC-MCTs do not require liquid nitrogen for cooling and therefore no need to maintain LN₂ access or manage safety-related concerns. These detectors enable high speed FTIR materials analysis while maintaining spectral quality, and have long been the detectors of choice to measure quickly and collect quality spectra, because of their relatively wide spectral range and high sensitivity.

Resources

• FTIR Spectroscopy Academy
• IR Detector Options for the Thermo Scientific Nicolet iS50 FTIR Spectrometer
• FTIR spectrometers and FTIR microscopes
• VIDEO: Thermo Scientific Nicolet iS50 FTIR Spectrometer