Two-dimensional imaging by the continuous-wave EPR

Abstract

To be able to perform a two-dimensional study of free radical distribution by the continuous-wave electron paramagnetic resonance method in the X-band, special coils producing a magnetic gradient of 8 G/mm have been designed and construeted. The EPR spectra recorded for this gradient were subjected to the procedure of deconvolution in order to elieit information on the concentration of the radical distribution. The data obtained were used as the souree file of the program reconstructing the image. The reconstruction was based on the iterative simultaneous algebraic reeonstruction teehnique (Andersen A.H., Kak A.C.: Ultrason. lmag. 6, 81-94, 1984). The quality of the generated images depends on the angle of the sample axis to the gradient direction set by a goniometer and on the deconvolution procedures applied. The first tests on artificially generated phantoms indicated a dependence of the obtained images on the magnetic field gradients apptied. The determined spatial distribution of radicals has confirmed their uniform distribution in the sample. The preliminary tests were performed for diphenyl-picrylhydrazyl. Having proved the reliability of the method, analogous measurements were also performed for plyphenylene sulphide PPS-V1 and indicated a homogeneous distribution of radicals in the whole volume of the sample. The images obtained confirmed the uniforro distribution of the radicals.

Two-dimensional imaging of two types of radicals by the CW-EPR method

“…5) indicates a homogeneous distribution of manganese in sample. The image suggests that the manganese is an impurity in the glass fibre or in the mineral fillers since our previous EPR studies of the polymer PPS did not show any manganese ions [14]. The EPR spectrum of pure glass fibre showed a single line with a width of 6 G, and did not show characteristic Mn 2+ lines.…”

“…The verification was made on the previously reported EPR data for three samples of DPPH placed in three holes with 1 mm diameter separated by 3 mm [14]. The depth of one of the holes was twice than that of the other two, so the signal should be twice as large.…”

“…The method can be directly compared with the procedure that was used previously [14], which was based on the so-called golden deconvolution (Eq. (1)), also known as algebraic deconvolution.…”

“…The 2D images were obtained by the CW method on a Bruker X-band EMX spectrometer, with gradient coils and a goniometer [14]. The gradient coils are 230 · 13 mm, made of copper wire of 1 mm in diameter.…”

“…The choice of the parameters a and b determines the quality of the y(n) projections obtained [14]. This method permits getting the projections from EPR spectra with hyperfine structure or those characterised by a low S/N ratio (<30).…”