1、 Introduction: Revolutionary Breakthrough in Infrared Spectroscopy Technology

In the field of analytical chemistry, the technology that can accurately analyze the molecular structure of substances has always occupied a core position. The emergence of Fourier Transform Infrared Spectrometer (FTIR) has changed the traditional pattern of infrared spectroscopy analysis. Compared to dispersive infrared spectrometers that rely on prisms or gratings for spectral analysis, FTIR has achieved a triple leap in detection speed, sensitivity, and resolution through the mathematical principle of Fourier transform, becoming a multidisciplinary analysis tool in chemistry, materials, biology, environment, and other fields. From basic research in the laboratory to quality control in industrial production lines, from celestial component analysis in space exploration to disease screening in clinical diagnosis, FTIR, with its unique ability of "molecular fingerprint recognition," continues to push the boundaries of human understanding of the material world.

2、 The core principle of Fourier transform infrared spectrometer: the "decoding" process from interferogram to infrared spectrum

The working logic of FTIR originates from the interference phenomenon of Michelson interferometer and the mathematical transformation of Fourier transform. Its core process can be summarized as three steps: "interference generates signal - Fourier transform analysis - spectral presentation results":

The generation of interference signals: The infrared light source built into the instrument (usually a silicon carbide rod, deuterium lamp, or laser light source) emits continuous wavelength infrared light, which is divided into two beams by a beam splitter: one beam is transmitted to a fixed mirror, and the other beam is reflected to a movable mirror. Two beams of light converge again after reflection, and due to the optical path difference, interference effects occur, forming an "interference pattern" containing all wavelength information - this is a time-domain signal, manifested as a fluctuation curve of light intensity with optical path difference, seemingly irregular, but actually containing the absorption characteristics of substances for infrared light of different wavelengths.

The decoding function of Fourier transform: interferograms cannot directly reflect the molecular structure of substances, and Fourier transform operations need to be performed through computers. This mathematical process converts time-domain interference signals into frequency-domain spectral signals, that is, the variation of light intensity with optical path difference is transformed into the variation of light intensity with wave number (or wavelength), ultimately forming the infrared absorption spectrum we see.

The recognition logic of "molecular fingerprints": The chemical bond vibrations (stretching, bending, etc.) of different molecules have specific frequencies and only absorb infrared light of corresponding wavelengths, thus forming characteristic absorption peaks on the spectrum. For example, hydroxyl (- OH) has a strong absorption peak in the range of 3200-3600 cm ⁻¹, while the characteristic peak of carbonyl (C=O) is located in the range of 1600-1800 cm ⁻¹. By comparing the position, intensity, and shape of the characteristic peak, the functional group composition and molecular structure of the substance can be accurately determined.

3、 Core component: FTIR's "precision organ" and technological advantages

The high performance of FTIR relies on the collaborative work of the four core components, and the technical characteristics of each component directly determine the analytical capability of the instrument:

Infrared light source: It is necessary to provide a continuous spectrum with stable intensity and a wide wavelength coverage range (usually 4000-400 cm ⁻¹, i.e. the mid infrared region). Silicon carbon rods are suitable for routine analysis, while laser sources (such as quantum cascade lasers) can achieve high-resolution detection and meet the needs of trace component analysis.

Michelson interferometer: The "heart" of the instrument, consisting of a beam splitter, a fixed reflector, and a movable reflector. The beam splitter needs to have a high transmission/reflection ratio for infrared light, and commonly used materials are germanium or potassium bromide; The precision requirement for movable mirrors (with a movement step length of nanometers), ensuring precise control of optical path difference, is the key to achieving high resolution.

Detector: responsible for capturing interference signals and converting them into electrical signals, requiring high sensitivity and fast response capability. Common types include mercury cadmium telluride detector (MCT), deuterated glycine sulfate detector (DTGS), etc. - MCT detector has high sensitivity and is suitable for trace analysis; The DTGS detector has good stability and low cost, and is widely used in routine detection.

Computer and software systems: undertake Fourier transform operations, spectral data processing and analysis tasks. Modern FTIR software not only supports basic functions such as spectral acquisition, baseline correction, and peak area calculation, but also integrates a large standard spectral library (such as the Sadtler spectral library), which can quickly identify unknown substances through spectral comparison. It also supports advanced applications such as multi-component quantitative analysis and dynamic process monitoring.

Compared to dispersive infrared spectrometers, FTIR has significant core advantages: ① Fast detection speed (single detection only takes a few seconds, even milliseconds), suitable for monitoring rapid reaction processes; ② High sensitivity (interference signal superposition effect reduces noise, detection limit can reach ppm level), capable of detecting trace components; ③ High resolution (able to accurately distinguish absorption peaks with close wavelengths), suitable for complex mixture analysis; ④ High energy utilization efficiency (no need for splitting, infrared light participates in detection across all bands), reducing light source losses.

4、 Multi domain application: a versatile analytical tool from scientific research to industry

In the field of chemistry and chemical engineering: ① Identification of unknown compound structures, determining functional groups through characteristic peaks (such as determining whether organic compounds contain hydroxyl, amino, carbonyl, etc.); ② Reaction process monitoring, real-time tracking of spectral changes in reactant reduction and product generation, and optimization of reaction conditions; ③ Purity analysis, which determines the purity of a substance by the intensity of impurity characteristic peaks (such as impurity content detection in drugs).

In the field of materials science: ① polymer analysis, identifying the materials of plastics, rubber, and fibers (such as distinguishing between polyethylene and polypropylene), and detecting the content of additives; ② Thin film and coating analysis, determination of coating thickness, uniformity of composition, and evaluation of material aging degree; ③ Characterization of nanomaterials, analysis of surface functional groups and chemical bonding modes of nanoparticles.

In the field of biomedicine: ① drug research and development, rapid screening of drug molecules, optimization of formulation formulas, and testing of drug stability; ② Clinical diagnosis: auxiliary diagnosis of diabetes, liver disease and other diseases (such as the correlation between blood glucose concentration and specific spectral peak intensity) through infrared spectral characteristics of blood and urine; ③ Microbial identification, quickly distinguishing the types of bacteria and fungi (there are differences in the cell wall composition spectra of different microorganisms).

Environmental monitoring field: ① Air pollutant detection, real-time monitoring of gas concentrations such as formaldehyde, VOCs (volatile organic compounds), sulfur dioxide, etc; ② Water quality analysis, detecting pollutants such as organic matter and heavy metal complexes in water; ③ Soil pollution screening to determine the type and content of harmful substances such as petroleum hydrocarbons and pesticide residues in soil.

In the field of food and agriculture: ① Analysis of food ingredients, determination of protein, fat, moisture, starch and other contents (such as protein content detection in milk powder); ② Testing of food additives and harmful substances, such as detecting nitrite and preservatives in food; ③ Quality assessment of agricultural products, using spectral characteristics to determine fruit maturity and grain mold degree.

5、 The development trend of Fourier transform infrared spectrometer technology: intelligence, miniaturization, and integration

With the advancement of technology, FTIR is developing towards higher efficiency, convenience, and precision, presenting three major trends:

Intelligence and automation: Integrating artificial intelligence (AI) algorithms to achieve automatic analysis of spectral data, rapid identification and quantitative analysis of unknown substances; Combining robot technology to build an automated detection platform (such as a high-throughput sample detection system) suitable for large-scale sample analysis (such as quality control in pharmaceutical production lines).

Miniaturization and portability: Traditional FTIR has a large volume and relies on laboratory environments, while portable FTIR has achieved on-site detection capabilities through the development of micro interferometers and lightweight detectors (such as environmental emergency monitoring, cultural relic on-site restoration analysis, food security inspection, etc.). Some portable instruments weigh only a few kilograms and can be operated handheld, meeting the needs of outdoor operations.

Upgrade of integrated technology: Integrate with other analytical techniques to expand application boundaries. For example, GC-FTIR (Gas Chromatography Fourier Transform Infrared Spectroscopy) combines the separation ability of gas chromatography with the qualitative advantages of FTIR to accurately analyze trace components in complex mixtures; LC-FTIR (liquid chromatography Fourier transform infrared spectroscopy) solves the analysis problem of non-volatile compounds; In addition, the combination of FTIR with Raman spectroscopy and mass spectrometry further enhances the comprehensiveness and accuracy of substance analysis.

6、 Conclusion: Future prospects of molecular analysis technology

The Fourier transform infrared spectrometer, with its unique working principle and performance, has become a bridge connecting the microscopic molecular world with macroscopic material analysis. FTIR has always played an irreplaceable role in molecular structure analysis in basic scientific research, quality control in industrial production, and food safety and environmental monitoring in the field of people's livelihood. With the continuous breakthroughs in intelligent, miniaturized, and integrated technologies, FTIR will demonstrate its potential in more emerging fields such as space exploration, precision medicine, and new energy materials, providing more powerful analytical tools for humans to solve complex problems. In the future, FTIR will not only become a "standard configuration" in laboratories, but also enter various scenarios of production and life, becoming a core technological support for safeguarding health, ensuring safety, and promoting innovation.