质谱（mass spectrum）
Mass spectrometry (MS), 2010-05-22, 22:597.4.1 principle
Mass spectrometry is one of the important methods to determine the structure of organic compounds developed in the 40s of this century. The mass spectrometer mainly consists of four parts
Ionization variable arc magnetic field ion collector recorder
In mass spectrometry, the sample is converted to vapor in the ionization chamber. The ionization chamber is a high vacuum (10-8 mmHg), an electron source in the ionization chamber, the electron beam can emit high energy, organic compounds by high-energy electron beam bombardment, they can lose an outer electrons into high molecular ions with positive charge on a.
M + e, M+ + 2e
These energetic molecules can also be cleaved into a variety of positively charged cations, negatively charged anions, and uncharged fragments. There is also an accelerator in the ionization chamber, where the positive ions can be accelerated and enter the strong magnetic field below, while the negative ions and neutral fragments are not accelerated and evacuated by vacuum pumps. The magne
tic field in mass spectrometry is an arc magnetic field, which enters a positive ion in a strong magnetic field. Because of their different mass of m/z, when they are moving along the arc track, m/z is different
Ions vary in degrees of curvature. When the positive ion acts as an arc, the centripetal force (Hen) is balanced with the
centrifugal force mv2/R.
H is the magnetic field intensity, and R is the radius of curvature of the ion motion. E, m and V are the charge, mass and velocity of ion motion, respectively.
Since the ion is accelerated in an electric field whose electric field is V, its kinetic energy is equal to that of potential energy:
Combine the above two formula to get:
Cationic mass small, arc radius is small, and large mass cation, arc radius, so, according to the detected signal, you can know the quality of debris, that is to say, which is split into compound ions, according to these pieces, you can put the structure information for organic compounds the.
7.4.2 mass spectrum
The mass spectra of m/z are in abscissa, and the ordinate is the relative abundance of ions. The maximum ion is 100% (base peak), and the maximum abundance ions are not necessarily molecular ions.
Information provided by 7.4.3 mass spectrometry
Mass spectrometry is usually used in two ways:
(a) confirm whether the two compounds are identical;
(b) help determine the structure of the new compound.
Mass spectrometry can determine the structure of a new compound or an unknown substance from the following aspects
(1) given the correct molecular weight (molecular ion peak m/z), containing an odd number of N atomic compounds, the molecular ion peak is odd, and the rest are all even numbers.
(2) according to the abundance and variety of fragments, it is possible to point out which structural units exist in the molecule.
(3) according to isotopic abundance, the number of atoms such as C, Cl, Br, N and so on can be calculated.
(4) given a structural formula, at least it can be reduced to very few possibilities.
7.4.4 applications
7.4.4.1 for element composition
Because organic molecules contain isotopic elements, there is a M+1 peak near the molecular ion peak. According to the relative abundance of molecular ion peaks and isotopic peaks and the relative isotopic abundances of elements (Table 7.1) in mass spectra, the number of atoms in the compound can be calculated by pressing down.
Table 7.1 isotopic abundance of common elements in organic
compounds element Abundance /% carbon
12C 100
13C 1.08 hydrogen
1H 100
2H 0.016 nitrogen
14N 100
15N 0.38 oxygen
16O 100
17O 0.04
18O 0.20
fluorine
19F 100
sulfur
32S 100
33S 0.78
34S 4.40
chlorine
35Cl 100
37Cl 32.5
bromine
79Br 100
81Br 98
iodine
127I 100
For example, in nature, carbon has two isotopes. The abundance of isotope 13C relative to 12C is 1.08 (100-1.08) =1.092%,
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