How NMR Spectroscopy is different from UV- Visible Spectroscopy?

The key objective of both the spectroscopic techniques is identification and quantification of the components of samples. However, there are intrinsic differences between the two which need to be clearly understood before deciding between them for your analysis requirements.

UV-Visible Spectroscopy

UV可见光谱基于样品与波长区域190-900nm的光形式的电磁辐射相互作用。这种相互作用导致从地面状态激发电子到不稳定的更高能量状态。吸收程度与梁路径中存在的吸收物种的浓度直接成比例。

NMR光谱学

Nuclear magnetic resonance on the other hand does not involve any transitions of the electronic levels. The magnetic properties of the nuclear particles are exploited to seek information on the structure of molecules. Transitions between spin states of nuclear particles are achieved by placing the sample in a uniform magnetic field.

吸收的基础

UV-visible spectroscopy is dependent on presence of chromophoric groups in molecules, degree of conjugation in molecular structure or complexation reaction between metal ions and ligands which tends to produce coloured compounds.

另一方面，核磁共振涉及在外部磁场的影响下旋转核颗粒之间的转变。核的旋转角动量是中子的旋转角动量的矢量和，并存在于细胞核中的质子。具有奇数质子和中子的核的细胞核具有角动量的半积分或倍数，并产生NMR吸收

Sample Handling

用于UV-可见分析的样品在由石英制成的比色板或细胞中拍摄，一般路径长度为1厘米。另一方面，NMR样品包含在5mm外径的玻璃管和15-20cm的长度。

任何使用的示例是可溶性的溶剂for UV – visible analysis and a blank solution is taken as reference in the beam path. In case of NMR the solvent needs to be aprotic and carbon tetrachloride is a convenient choice. However, due to limited solubility of analytes other deuterated solvents such as deuterated chloroform,deuterated benzene, deuterated acetone and deuterated dimethylsulfoxide are commonly used.

不需要参考标准管。可以使用诸如四甲基硅烷（TMS）的内标。

反应速度

与NMR相比，UV - Vis光谱是一种快速技术。激发的UV可见光谱学中的毫微微秒的顺序，而在NMR分析中，响应时间越长，甚至高达一分钟或更长

Sample Excitation

Sample and reference are placed inside the spectrometer compartment along the beam path in case of UV –visible analysis. However, in case of NMR the sample tube is exposed to a magnetic field in a cavity between the poles of a magnet. Permanent magnets, electro magnets and superconducting magnets have been in use in NMR spectrometers. Modern high-resolution spectrometers use superconducting magnets.

The sample tube inside the magnet cavity is rotated using an air turbine so that the sample experiences a uniform magnetic field. In addition a coil around the tube generates radio frequency for excitation and detection of NMR signals.

Spectral Response

UV - 可见光谱被记录为沿y轴和沿x轴的波长吸光度的刻度上的吸收带。另一方面，NMR光谱用X轴记录，X轴表示PPM单元和Y轴的化学位移，为吸收强度。参考化合物TMS具有零值，化学位移从左右增加。

The line widths decrease sharply from UV–Vis to NMR. The comparatively long life times of excited nuclear spin states helps NMR provide a wealth of information about the structure of molecules. NMR finds several applications in analysis of multi component mixtures of organic compounds and structure elucidation through identification of common functional groups.