*4.3.4 Nuclear magnetic resonance (NMR)*

*Phytochemicals in Human Health*

conjugated chains.

*4.3.2 Infrared spectra (IR)*

625 cm<sup>−</sup><sup>1</sup>

ranging from 4000 to 625 cm<sup>−</sup><sup>1</sup>

between 960 and 900 cm<sup>−</sup><sup>1</sup>

*4.3.3 Mass spectrometry (MS)*

formulas, and fragment structures of molecules.

UV spectra could provide the following information: (1) the compounds show no UV absorption at 220–800 nm, indicating the compounds were aliphatic hydrocarbons, aliphatic cyclic hydrocarbons, or their simple derivatives. (2) The compounds show strong absorption at 220–250 nm, indicating that the compounds possess conjugated diene, α,β-unsaturated aldehyde, or ketone substructures. (3) The absorption at 250–290 nm is moderately strong, indicating that the compounds possess benzene rings or aromatic heterocycles. (4) Weak absorption at 250–350 nm indicates the presence of carbonyl or conjugated carbonyl groups. (5) Strong absorptions at above 300 nm indicate that the structures possess long

Generally, UV spectrum can only provide part of the structural information, rather than the whole structural information of a compound, so it can only be used as an auxiliary method to identify the structures. It possesses practical value to determine the structures of phytochemicals with conjugated substructures.

IR is caused by the vibration-rotational energy level transition of the molecule,

region, and the absorption of characteristic functional groups such as hydroxyl, amino, carbonyl, and aromatic rings occurs in this region. The region of 1250 to

In a mass spectrometer, mass and strength information of molecular and fragment ions is recorded after the molecules are ionized and enter into the collector under the action of electric and magnetic fields. The abscissa represents the mass-to-charge ratio (m/z) and the ordinate represents the relative intensity in a MS spectrum. Unlike IR, UV, and NMR spectra, MS is mass spectrum, which characterizes fragment ions, not an absorption spectrum. Its role is to determine weights,

With the rapid development of modern techniques, new ion sources have emerged in recent years, which make MS play more important role in determining the molecular weights, elemental composition, detecting functional groups by cleavage fragments, identifying compound types, and determining carbon skeletons [25]. In the structural analysis, the information of molecular weights could be obtained on the basis of molecular ion peaks, and the molecular formula could be obtained by high-resolution mass spectrometry (HR-MS). Fragment ion peaks, combined with molecular ion peak, could be applied to conjecture chemical structures. Tandem mass spectrometry even can isolate and analyze the mixed ions again. According to the types of ion sources, common mass spectrometry could classified as electron impact mass spectrometry (EI-MS), chemical ionization mass spectrometry (CI-MS), field desorption mass spectrometry (FD-MS), fast atom bombardment mass spectrometry (FAB-MS), matrix-assisted laser desorption mass spectrometry (MALDI-MS), electrospray ionization mass spectrometry (ESI-MS), tandem mass spectrometry (MS–MS),

. The region above 1250 cm<sup>−</sup><sup>1</sup>

 is fingerprint region, and the peaks appear mainly due to the stretching vibrations of C-X (X = C, O, N) single bonds, and various bending vibrations. IR is mainly used for the determination of functional groups and the types of aromatic ring substitution. In some cases, IR can also be used to determine the configuration of plant chemical constituents. For example, there is a significant difference

for 25R and 25S spirostanol saponins.

is functional group

**60**

and so on.

With the birth of Fourier transform spectrometer, the great progress of radionuclide research such as 1 H, 13C, 15N, 19F, 31P, and the advancement of two-dimensional and three-dimensional nuclear magnetic technology, NMR has become the most important spectroscopic method to determine chemical structures. Particularly, hydrogen spectrum and carbon spectrum are most widely used. During the operation of nuclear magnetic resonance spectrometer, compound molecules are irradiated by electromagnetic waves in a magnetic field, energy level transitions occur after the atomic nuclei with magnetic distance absorb a certain amount of energy, and then NMR spectrum is obtained by mapping the absorption strength with the frequencies of the absorption peaks. It can provide structural information about the type and number of hydrogen and carbon atoms in the molecule, the modes they are connected, the surrounding chemical environment, configuration, and conformation [26].
