13.4
红外光谱仪
红外（IR）分光光度计主要有两种：色散型红外光谱仪和傅里叶变换红外（FTIR）光谱仪。在色散红外光谱仪中，热丝产生的红外辐射光束通过镜子分成两束平行的等强度光束。其中一束穿过样品，另一束则作为参考光束。随后，光束穿过单色仪，单色仪将辐射分离成不同频率的连续光谱。单色仪由一个快速旋转的光束斩波器组成，…
在色散型红外光谱仪中，由热丝产生的辐射经反射镜分为两条平行且强度相等的光束，分别通过参比池和样品池。
其单色器由一个光束斩波器组成，该斩波器交替地将光束导向一个缓慢旋转的衍射光栅棱镜，从而改变辐射的频率或波长。
热电偶探测器检测两束光的强度比。
最后，检测器的信号被放大，记录仪绘制出光谱。
傅里叶变换红外光谱仪包含一个干涉仪，其中辐射光依次通过分束器、固定镜和移动镜，然后到达样品。
分束器将光束分离为偏转辐射和未偏转辐射，分别投射到固定镜和移动镜上。
两束光在分束器处重新合并，产生包含宽波长或频率范围的干涉图样或干涉图。
检测器检测经样品调制的干涉图，计算机则通过傅里叶变换运算提取各个频率成分。
View the full transcript and gain access to JoVE Core videos
Q1: How does a dispersive IR spectrometer separate different wavelengths of radiation?
A dispersive IR spectrometer uses a monochromator containing a rapidly rotating beam chopper and diffraction grating to separate radiation into different frequencies. The beam chopper alternates sample and reference beams toward the grating, which varies the wavelength of radiation reaching the thermocouple detector. This process records the spectrum in the frequency domain as the grating rotates.
Q2: What is the role of the beam splitter in an FTIR spectrometer?
In an FTIR spectrometer, the beam splitter is a mirror positioned at 45° to incoming radiation that separates the beam into deflected and undeflected radiations. These separate beams travel to fixed and moving mirrors, then recombine at the splitter. The recombination creates an interferogram containing both constructive and destructive interference patterns due to path length differences.
Q3: How does constructive interference differ from destructive interference in FTIR?
Constructive interference occurs when the peaks of two waves align, increasing the amplitude of the combined wave. Destructive interference happens when one wave's peak aligns with another's trough, canceling each other and reducing amplitude. Both interference patterns are present in the interferogram produced by the FTIR beam splitter and mirrors.
Q4: What does the thermocouple detector measure in a dispersive IR spectrometer?
The thermocouple detector measures the intensity ratio between the sample and reference beams after they pass through the monochromator. This ratio is then amplified and recorded to create the final spectrum. The detector's measurement allows the spectrometer to quantify how much infrared radiation the sample absorbs at each wavelength.
Q5: How does the Fourier transform process extract frequency information in FTIR?
The detector in an FTIR spectrometer captures the interferogram modified by the sample, which contains a wide range of wavelengths and frequencies combined together. A computer then applies the Fourier transform operation to mathematically separate and extract the individual absorption frequencies from this complex interferogram data.
Q6: Why are FTIR spectrometers preferred over dispersive IR spectrometers?
FTIR spectrometers are preferred because they operate faster and provide greater sensitivity compared to dispersive IR spectrometers. The FTIR design allows simultaneous measurement of all wavelengths through the interferogram, whereas dispersive spectrometers measure frequencies sequentially as the diffraction grating rotates.
Q7: How do the reference and sample beams function in a dispersive IR spectrometer?
In a dispersive IR spectrometer, infrared radiation from a hot wire is divided by mirrors into two parallel equal-intensity beams. One beam passes through the sample while the other serves as a reference. The thermocouple detector compares their intensities after passing through the monochromator, enabling measurement of sample absorption.