Background removal procedure for rapid scan EPR

Abstract

In rapid scan EPR the changing magnetic field creates a background signal with components at the scan frequency and its harmonics. The amplitude of the background signal increases with scan width and is more significant for weak EPR signals such as are obtained in the presence of magnetic field gradients. A procedure for distinguishing this background from the EPR signal is proposed, mathematically described, and tested for various experimental conditions.

Figure 1

Demonstration of the background removal procedure. The data were obtained for 2 tubes containing BDPA in the presence of a magnetic field gradient, with scan widths of 20 G (a) Absorption signals recorded with center fields offset by 2.0 gauss; (b) symmetric parts of the two signal obtained by combining the up and down half cycles (Eq. 3); (c) Interchange of the second half cycles for traces 1 and 2; (d) shift of the signal towards each other by half the center field offset. The red line is the background signal calculated by subtracting the two shifted traces.

Figure 2

Values of log(a_k) (Eq. (7) for the background in the absorption (red squares) and dispersion (blue circles) signals. The lines connect the points for the discrete values.

Figure 3

(a) Comparison of the experimental signal (green) with the reconstructed background (solid blue), and the off-resonance background (dotted red). (b) Spectra obtained by subtraction of the reconstructed background (trace 1, blue) or the off-resonance background (trace 2, red) from the experimental signal (Fig. 1b, trace 1).

Figure 4

Comparison of absorption (a,b) and dispersion (c,d) spectra of BDPA obtained by subtraction of the reconstructed background (blue curves) or off-resonance background subtraction (red curves), with spectra of BDPA with negligible noise and background components (green curves). Data with a center field offset of 0.4 G were used to calculate spectra in (a, c) and data with a center field offset of 2 G were used to spectra in (b,d).

Figure 5

Absorption and dispersion rapid-scan signals for trityl-CD₃ (a,c), that show the deviation of the baseline from a constant (dashed) line. Deconvolution followed by the background removal procedure gave the blue (solid) lines (b,d). Subtraction of an off-resonance background trace gave the red (dashed) lines (b,d), which is nearly superimposable on the trace obtained by the background removal procedure.