Common element test methods

1. Infrared absorption spectrometry

Analysis principle: absorption of infrared light energy, causing vibration and rotational energy level transition of molecules with dipole moment change

Representation of spectrum: relative transmitted light energy varies with transmission light frequency

Information provided: the location, intensity and shape of the peaks, and the characteristic vibrational frequencies of functional groups or chemical bonds

2. NMR

Analysis principle: in an external magnetic field, the nucleus with nuclear magnetic moment absorbs RF energy and produces a nuclear spin level transition 

Representation of spectrogram: change of absorbed light energy with chemical shift

Information provided: chemical shifts, intensities, splitting fractions and coupling constants of peaks, as well as information on the number of nuclei, chemical environment, and geometric configuration 

When the frequency of the external RF field is the same as that of the nuclear spin precession, the energy of the RF field can be effectively absorbed. Therefore, for a given nucleus, in a given external magnetic field, only the energy provided by a specific frequency RF field can be absorbed, thus forming a nuclear magnetic resonance signal. 

3. Mass spectrometry

Analysis principle: molecules are bombarded by electrons in vacuum to form ions, which change according to different m / E through an electromagnetic field

Information provided: mass number and relative kurtosis of molecular ions and fragment ions, and information on molecular weight, element composition, and structure

The working process of mass spectrometer is as follows:

The analyzer of FT-ICR mass spectrometry is a cavity with a uniform (superconducting) magnetic field. The ions make cyclotron motion in a circular orbit perpendicular to the magnetic field. The cyclotron frequency is only related to the magnetic field strength and the mass to charge ratio of ions. Therefore, ions with different mass charge ratios can be separated and the mass charge ratio related spectra can be obtained.  

4. Gas chromatography

Analysis principle: each component in the sample is separated between the mobile phase and the stationary phase due to different partition coefficients

The expression method of spectrogram: the change of effluent concentration after column with a retention value

Information provided: the retention value of the peak is related to the thermodynamic parameters of the components, which is the qualitative basis

5. Gel chromatography GPC

Analysis principle: when the sample passes through the gel column, it separations according to the molecular hydrodynamic volume and the macromolecule first flows out.

The expression method of spectrogram: the change of effluent concentration after column with a retention value

Information provided: average molecular weight and distribution of polymers

According to the properties of the gel used, it can be divided into gel filtration chromatography (GFC) using the aqueous solution and gel permeation chromatography (GPC) using organic solvents.

Only according to size separation, large components are first washed out

The stationary phase of chromatography is a porous gel, and only components whose diameter is smaller than pore size can enter the gel channel. Large components cannot enter the gel pores and are blocked, and can only pass through the gap between the gel particles, so the largest component is first washed out. 

The group can enter most of the gel holes and stay in the chromatographic column for a long time, which will be washed out more slowly. The solvent molecule is the smallest volume and can enter all the gel holes, so it is finally washed from the chromatographic column. This is also the biggest difference from other chromatographic methods. 

6. NMR

Analysis principle: in an external magnetic field, the nucleus with nuclear magnetic moment absorbs RF energy and produces a nuclear spin level transition

Representation of spectrogram: change of absorbed light energy with chemical shift 

Information provided: chemical shifts, intensities, splitting fractions and coupling constants of peaks, as well as information on the number of nuclei, chemical environment, and geometric configuration

When the frequency of the external RF field is the same as that of the nuclear spin precession, the energy of the RF field can be effectively absorbed. Therefore, for a given nucleus, in a given external magnetic field, only the energy provided by a specific frequency RF field can be absorbed, thus forming a nuclear magnetic resonance signal.