# Quantitative EPR

## Expression for signal intensity

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

$I = \chi^{''} \eta Q \sqrt{P Z_0}$

where χ'' - imaginary component of effective susceptibility
η - sample filling factor
Q - loaded resonator quality factor
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 $\sqrt{P}$ will help find the non-saturating power setting.

## Standard materials

TEMPOL,4 - hydroxy - 2,2,6,6 - tetramethyl piperidinyl - 1-oxyl
C9H18NO2, 172.24 g/Mole
CAS 2226-96-2

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.

10 mM stock of TEMPOL in water can be prepared and then diluted to desired concentration.

### References

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