A noise-reduction scheme for computer-controlled EPR spectrometers

Abstract

Improving the signal-to-noise ratio, S/N, of an EPR signal has traditionally involved signal averaging or simply increasing the scan time and detector time constant. Either technique results in a substantial increase in the time required for data acquisition. When rapidly decaying samples or large numbers of samples must be measured, this large increase in time per sample may be unacceptable. In this note we report on a signal averaging technique which is useful and time efficient when only selected regions of the spectrum are crucial to the experiment. Such a situation occurs in spin-labeling experiments where measurement of the spectral parameters such a AL and A,, depends on only a few selected peaks in the spectrum (I). All that is needed in such situations is to minimize the noise in the selected spectral regions where splittings or amplitudes will be measured. Signal-to-noise levels elsewhere are not critical as long as the general shape is clear and artifacts or impurity signals can be detected. The key, then, to rapidly obtaining a low-noise spectrum is to signal average only in the spectral regions to be measured. Furthermore, strong, clearly defined features such as a narrow line, require less signal averaging than poorly defined features such as a low, broad peak. Such a scheme is easily implemented on most computer operated spectrometers. Our implementations are on a Varian El09 with an E900 Data System and on an early Bruker ER 200 series machine. The essential spectrometer requirement is for an external input, magnetic field advance which can be computer controlled. We use the spectrometer external analog ramp input which is in turn driven by an external digital-to-analog converter; however, the technique should be adaptable to systems with a direct digital field advance. Via the analog ramp, the computer can advance the field rapidly in unimportant spectral regions and slowly, with signal averaging, in important regions. Programs which advance the field at a fixed clock rate (typically by slaving the computer to the spectrometer) sacrifice much of the power of the computer and are unable to perform the manipulations described here. Because it is compatible with the techniques described below, we also mention reverse field sweeping which is useful for spectra with a weak, rapidly decaying highfield line, a frequent occurrence in spin-labeled cell work. This simple expedient can greatly enhance the S/N of the high-field line. Variable rate sweep and signal averaging are implemented as follows: A fairly rapid, base scan time, tb, and an appropriate nondistorting signal detector time constant, t,,