A simple and inexpensive experiment is described which detects magnetic field effects on chemical reactions with high signal-to-noise ratio and high resolution. It consists in applying a small modulation field to the sample, whilst the main field it experiences is varied, with optical detection at the modulation frequency. It consequently measures the derivative of the normal MARY spectrum. It is shown by theoretical analysis that when using this method it is better to monitor reaction intermediates than products. The method is demonstrated by application to known systems in which additional features are sometimes observed, in particular a low-field feature which is shown to vary as electron hopping occurs. MARY spectra obtained using modulated light excitation of the sample and pulsed excitation with laser-induced fluorescence detection are provided for comparison. The method represents a general technique for studying field effects in systems containing low stationary state concentrations of radicals, produced by any method.