GB/T 6040-2019General rules for infrared analysis (English PDF)

3.1 transmittance

ratio of radiant energy transmitted through the sample to incident radiant energy

3.2 absorbance

logarithm to base 10 of the ratio of incident intensity to transmitted intensity

3.3 sample thickness

distance that the radiation beam passes through the sample

Note: The sample thickness is usually expressed in centimeters.

3.4 standard sample

standard substance of known composition, of which the chemical structure and analysis wavelength are the same as or very close to those of the substance to be tested

3.5 baseline

straight line or curve drawn on the absorption spectrum in a certain way, which is used to represent the background absorption curve when the absorption peak does not exist

3.6 fundamental bands

absorption peak produced by a molecule transitioning from one energy level to an adjacent higher energy level after absorbing photons

3.7 overtone bands

absorption peak produced by a molecule transitioning by two or more energy levels after absorbing photons twice as energetic as the molecule

3.8 combination bands

absorption peak appeared at the sum or difference of the wave numbers of two fundamental bands

3.9 interferogram

graph with optical path difference as the horizontal axis and light intensity as the vertical axis using the signal obtained by Michelson interferometer

3.10 apodization

mathematical operation of overlapping appropriate parameters on the interferogram in order to reduce the distortion of the recovered spectrum caused by the limited scanning distance of the interferometer

3.11 fourier self-deconvolution

deconvolution of actual spectrum obtained by convolution, that is, changing the measured spectrum into interferogram, then multiplying the interferogram with a suitable apodization, and performing Fourier transform again

3.12 kubelka-munk transformation

method of transforming the spectrum measured by diffuse reflection method into absorption spectrum

3.13 kubelka-kroning transformation

method of transforming the specular reflection spectrum measured into absorption spectrum

3.14 specular reflection method

method for measuring surface-modified samples, resins, polymer films, etc. by using an infrared beam to irradiate on samples at a certain incident angle which is equal to the reflection angle

3.15 attenuated total reflection method

method for measuring high absorption sample or sample surface, when an infrared beam is incident on the crystal with high refractive index closely attached to the sample surface at an angle greater than the critical angle, total reflection occurs because the refractive index of the sample is lower than that of crystal, and infrared beam only enters the extremely shallow surface layer of the sample, resulting in the absorption of certain frequencies, the attenuated total reflection spectrum of the sample is obtained by measuring this attenuated radiation

Note: Attenuated total reflection is called ATR for short.

3.16 diffuse reflection method

method for measuring powder samples, when infrared light incident on the surface layer of powder sample, a part of the light will cause specular reflection outside the surface particles, this part of light fails to enter into the sample particles or to interact with the sample, so it will not load any information of the sample; while the other part will enter into the sample particles and finally emit from the sample after transmission, refraction or reflection on the inner surface of the particles. In this way, different beams emit from the powder sample in different directions after many times of transmission, refraction or reflection in different sample particles, and they combines into reflected light. The diffuse reflection spectrum of the sample is obtained by measuring the radiation of this part of reflected light interacting with the sample.

3.17 electric ratio method

method of comparing the electrical signals of the luminous flux of a sample beam and a reference beam in a multi-beam infrared spectrometer

4.1 Overview of apparatus

The structures of common infrared spectrometers are shown in Figures 1, 2 and 3. The infrared spectrometer may be roughly classified into Fourier transform infrared spectrometer with Fourier transform mode and dispersive infrared spectrometer with spectral mode according to photometric mode.

Fourier transform infrared spectrometer is the most common, the working principle of which is shown in Figure 1. Infrared light emitted from the light source is collimated into parallel beams, which then enter the interferometer for modulation to obtain an interference beam. The interference beam passes through the sample to become an interference beam containing spectral information which reaches the detector. In the detector, the interference beam is changed from optical signal to electrical signal and amplified by an amplifier. The signal enters a computer through an analog-to-digital converter, and the computer performs Fourier transform fast calculation to obtain the infrared spectrogram with wave number as X-coordinate, which is sent to the plotter through digital-to-analog converter to get the spectrogram.

Figure 1 Structure and working scheme of Fourier transform infrared spectrometer

Figure 2 Structure of dispersive infrared spectrometer (optical null method)

Figure 3 Structure of dispersive infrared spectrometer (electric ratio method)

4.2 Structure of infrared spectrometer

4.2.1 Fourier transform infrared spectrometer

4.2.1.1 Light source

The light source is a non-illuminating electric light source with the main purpose of generating infrared radiation, having the function of emitting infrared rays with stable, strong energy and continuous wavelength.

The light source consists of emitter, power source, etc.

a) The emitter is made of emitting materials such as silicon carbide, ceramics and rare-earth oxides.

b) The power source provides stable voltage and current to the light source.

4.2.1.2 Diaphragm

The diaphragm is a physical object that restricts the beam in the optical system, having the function of controlling the luminous flux.

Diaphragms are classified into continuous variable diaphragms and fixed aperture diaphragms.

4.2.1.3 Sample chamber

The sample chamber consists of a sample cell, a sample holder and another sample holder for assembly of accessories. Fourier transform infrared spectrometer is usually a single beam spectrometer with a sample holder in the optical path. The multi-beam infrared spectrometer is provided with a sample holder in the sample optical path and the reference optical path respectively.

4.2.1.4 Interferometer

Michelson interferometer, consisting of moving mirror, fixed mirror and beam splitter, is generally used and its optical system is shown in Figure 4.

4.2.1.5 Detector

The detector is used to detect the energy of infrared interference light passing through sample by transforming the incident intensity into an electrical signal.

Common detectors include deuterated triglycine sulfate (DTGS) detector and mercury chromium telluride (MCT) detector.

4.2.1.6 Amplifier

The amplifier is used to amplify the analog signal from the detector to facilitate the signal processing system.

4.2.1.7 A/D converter

The A/D converter is used to convert the analog signal into a digital signal in order to store the amplifier signal in the memory of the computer.