EPR system(Electron paramagnetic resonance)
is a magnetic resonance technique that originates from the magnetic moments of unpaired electrons and can be used to qualitatively and quantitatively detect the unpaired electrons contained in atoms or molecules of matter and to explore the structural properties of their surroundings. For free radicals, the orbital magnetic moment plays almost no role, and the vast majority (more than 99%) of the total magnetic moment is contributed by the electron spin, That is why electron paramagnetic resonance is also known as electron spin resonance (ESR).
Electron paramagnetic resonance spectroscopy
is one of the modern means of testing the properties of high-tech materials and is a spectroscopic method for detecting samples with unpaired electrons. It allows us to obtain meaningful information on the structure and dynamics of substances even in the context of chemical and physical reactions that are carried out without affecting them. It has been widely used in many fields such as physics, chemistry, biology, biochemistry, medicine, environmental science, geological prospecting, etc. EPR is an ideal technique to compensate for other analytical means.EPR system in quantum manipulation and quantum computing
EPR system has the advantages and prospects of classical computing in quantum computing. The method of quantum manipulation and quantum computing with EPR is to make a chip of spintronics material and manipulate and encode the single electron spin state in the outer atomic layer by applying microwave pulses to it, and perform quantum operations using the electron spin state encoding. Due to the advantages of long coherence time, fast logic gate operation, and single quantum bit readout for solid-state quantum computing of spin, it has become a hot spot for research.EPR system for direct detection and analysis of free radical intermediates
The g-factor of the corresponding absorption peak in the obtained EPR spectrum is calculated and compared with the standard value to estimate which radical it is, and then the radical is eliminated chemically to verify the above inference.
Currently, some radicals are stable at room temperature and can be directly obtained by EPR spectroscopy, for example, the EPR signal of the negative ion radical Sc3 C2 including C80 formed by the reaction of fullerene C80 with metal Sc. The radical intermediates in the electron transport chain of the photosynthesis reaction were studied in combination with low-temperature techniques. A very characteristic study is the development of EPR-specific in situ electrochemical radical reaction cells to characterize radicals for electrode reactions. The measurement of the formation of intermediate radical products from irradiation of carbon-containing inorganic compounds is the essence of the EPR archaeological dating method, which can be applied as a reference for the siting of large hydroelectric power plants and building complexes.EPR detection method for transient radicals and its applications
The combination of radical capture technology and EPR has the advantages of high detection sensitivity, high specificity and reliable analytical results, and is widely used for the detection of transient radicals with short lifetimes and low steady-state concentrations, and is widely used in many studies involving cellular and even animal systems and chemical reaction mechanisms. The experimental method for EPR detection of transient radicals is to first design and synthesize a probe molecule capable of capturing the radicals, which must be able to rapidly capture the transient radicals generated during the reaction, and then analyze the molecular structure of the captured reaction adducts by EPR, and infer and identify the structures of the components corresponding to the peaks on the EPR spectra one by one.
In addition, EPR spectroscopy of paramagnetic ion complexes and EPR for pharmaceutical applications are also available.
The basic working principle of the EPR system is as follows:
In Electron paramagnetic resonance, an electron is an elementary particle with a certain mass and a negative charge, which is capable of two kinds of motion; one is in an orbit around the nucleus, and the other is a spin on an axis passing through its center. As the motion of the electron generates a moment of force, a current and a magnetic moment are generated in the motion. In the applied constant magnetic field H, the magnetic moment of electrons acts like a tiny magnetic rod or needle, and since the spin quantum number of electrons is 1/2, there are only two orientations of electrons in the external magnetic field: one parallel to H, corresponding to the low energy level, with the energy of -1/2gβH; one inverse parallel to H, corresponding to the high energy level, with the energy of +1/2gβH, and the energy difference between the two energy levels is gβH. If the direction perpendicular to H If the electromagnetic wave of frequency v is added in the direction perpendicular to H, the electron of the low energy level will absorb the energy of the electromagnetic wave and jump to the high energy level, which is called electron paramagnetic resonance. In the above basic conditions for electron paramagnetic resonance, h is Planck's constant, g is the spectral splitting factor (referred to as g factor or g value), and β is the natural unit of electron magnetic moment, called Bohr magneton. With the g value of free electron = 2.00232, β = 9.2710×10-21 erg/Gauss, h = 6.62620×10-27 erg-sec, substituting into the above equation, the relationship between electromagnetic wave frequency and resonance magnetic field can be obtained: (Gauss) = 2.8025 (MHz).
The following 3 microwave frequencies are commonly used in electron paramagnetic resonance spectrometry
Electron paramagnetic resonance detection objects can be divided into two main categories.
a. Substances that have unpaired electrons (or single electrons) in the molecular orbitals. Such as free radicals (molecules containing a single electron), double and multiple radicals (molecules containing two or more single electrons), triplet molecules (also have two single electrons in the molecular orbitals, but they are very close together and have strong magnetic interactions with each other, unlike double radicals), etc.
b. Substances with single electrons
in the atomic orbitals, such as atoms of alkali metals, transition metal ions (including iron, palladium, and platinum ions, which have unfilled 3d, 4d, and 5d shells in that order), rare earth metal ions (with unfilled 4f shells), etc.
There is more to getting accurate EPR measurements
than simply putting the sample into the resonant chamber.
a. The resonant cavity has a so-called superior value, i.e. Q. The Q value is the ratio of the maximum value of the electromagnetic energy stored in the resonant cavity in one cycle multiplied by 2πν (ν is the frequency) to the energy consumed by the cavity per unit time, reflecting the energy consumption of the electromagnetic wave. the larger the Q value, the higher the signal peak value.
b. If not done properly, the Q value is not the same for each test, so that the obtained EPR signal intensity does not reflect the concentration of free radicals in the sample. When using the peak of the signal peak to represent the free radical concentration in quantitative measurements, it is important to pay attention to the size of the Q value. Under normal conditions, the Q value should be stable when the experimental parameters of the EPR are correctly adjusted.
c. After the sample is placed in the EPR quartz cuvette, it is important to ensure that the depth in the resonant cavity where the cuvette is located is the same for each measurement and that the cuvette is vertical and does not deviate too much in one direction. Ideally, the sample should be placed at the location where the microwave magnetic field is strongest and the electric field is weakest, because the magnetic resonance must interact with the magnetic field of the electromagnetic wave, while the interaction of the electric field can only lead to non-resonant losses in the medium.
d. For the working environment in which the EPR instrument is located, drastic temperature and humidity changes as well as air circulation should not occur to ensure a relatively smooth baseline of the spectrum.