Quantitative EPR

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Contents 1 Expression for signal intensity 2 Parameters affecting spectra 2.1 CW EPR 2.1.1 Microwave power 2.1.2 Time constant 2.1.3 Conversion time 2.1.4 Modulation frequency 3 Standard materials 3.1 References

Expression for signal intensity

Signal intentity in CW EPR at the absence of saturation is expressed as:

where χ^'' - imaginary component of effective susceptibility
η - sample filling factor
Q - loaded resonator quality factor
P - irradiation power
Z₀ - characteristic impedance of the transmission line

Parameters affecting spectra

CW EPR

Microwave power

For quantitative measurements power must be set at a level that does not significantly saturate the transition. In the absence of saturation, intensity of spectrum should be proportional to the square root of microwave power.

Recording a series of spectra at different power levels and plotting it vs the will help find the non-saturating power setting.

Time constant

Conversion time

Modulation frequency

Standard materials

TEMPOL (4-hydroxy-2,2,6,6-tetramethylpiperidinyloxy) dissolved in water was proposed[1] as a primary standard for determination of concentration of free radical species by EPR. Concentraion of TEMPOL standard solutions can be determined by measurement of absorbance at 436.68 nm and using molar absorptivity of 13.2±0.1 LM^-1cm^-1. Concentration of free radical species was found at ~98% of expected based on the weight of dissolved material.

References

1. Yordanov, ND and Ranguelova, K. Quantitative electron paramagnetic resonance and spectrophotometric determination of the free radical 4-hydroxy-2, 2, 6, 6-tetramethylpiperidinyloxy. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 56(2):373--378, 2000. BibTeX [yordanov]