The electron paramagnetic resonance (EPR or ESR) technique is the only method available for directly detecting unpaired electrons in samples. Among them, the quantitative EPR (ESR) method can provide the number of unpaired electron spins in a sample, which is essential in studying the reaction kinetics, explaining the reaction mechanism and commercial applications. Therefore, obtaining the unpaired electron spin numbers of samples by electron paramagnetic resonance techniques has been a hot topic of research.

**Two main quantitative electron paramagnetic resonance methods are available: relative quantitative EPR (ESR) and absolute quantitative EPR (ESR).**

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**Relative Quantitative EPR (ESR) Method**

The relative quantitative EPR method is accomplished by comparing the integrated area of the EPR absorption spectrum of an unknown sample with the integrated area of the EPR absorption spectrum of a standard sample. Therefore, in the relative quantitative EPR method, a standard sample with a known number of spins needs to be introduced.

The size of the integrated area of the EPR absorption spectrum is not only related to the number of unpaired electron spins in the sample, but also to the settings of the experimental parameters, the dielectric constant of the sample, the size and shape of the sample, and the position of the sample in the resonant cavity. Therefore, to obtain more accurate quantitative results in the relative quantitative EPR method, the standard sample and the unknown sample need to be similar in nature, similar in shape and size, and in the same position in the resonant cavity.

Quantitative EPR Error Sources

**Absolute Quantitative EPR (ESR) Method**

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The absolute quantitative EPR method means that the number of unpaired electron spins in a sample can be obtained directly by EPR testing without using a standard sample.

In absolute quantitative EPR experiments, to obtain the number of unpaired electron spins in a sample directly, the value of the quadratic integral area of the EPR spectrum (usually the first-order differential spectrum) of the sample to be tested, the experimental parameters, the sample volume, the resonance cavity distribution function and the correction factor are needed. The absolute number of unpaired electron spins in the sample can be directly obtained by first obtaining the EPR spectrum of the sample through the EPR test, then processing the EPR first-order differential spectrum to obtain the second-integrated area value, and then combining the experimental parameters, sample volume, resonant cavity distribution function and correction factor.

**CIQTEK Electron Paramagnetic Resonance Spectroscopy**

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The absolute quantification of unpaired electron spins of the CIQTEK EPR (ESR) spectroscopy can be used to obtain the spin number of unpaired electrons in a sample directly without the use of a reference or standard sample. The resonant cavity distribution function and correction factor are set before the instrument is shipped. After the spectroscopy is completed, the user only needs to enter the relevant parameters in the software to obtain the spin number of unpaired electrons in the sample directly. The user input parameters include: sample diameter, sample length, electron spin quantum number, test temperature, secondary integration area and the distance from the sample center to the top sample release clip position. This function allows the user to easily and quickly obtain the number of unpaired electron spins in the test sample.

CIQTEK Absolute Quantitative EPR (ESR) Function Interface

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The CIQTEK EPR (ESR) spectroscopy provides a non-destructive analytical method for the direct detection of paramagnetic materials. It can study the composition, structure, and dynamics of magnetic molecules, transition metal ions, rare earth ions, ion clusters, doped materials, defective materials, free radicals, metalloproteins, and other substances containing unpaired electrons, and can provide in situ and non-destructive information on the microscopic scale of electron spins, orbitals, and nuclei. It has a wide range of applications in the fields of physics, chemistry, biology, materials, industry, etc.