= DPPH electron spin resonance experiment
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The University of Iowa's <Electron Spin Resonance, University of Iowa laboratory protocol> (revised July 2007, Experiment and Resonance Data Analysis, printed pp. 2–6) specifies <DPPH> on cotton in a tube inside an RF coil, between <Helmholtz coils>. A bridge monitors RF absorption; an <oscilloscope> records this output and the voltage across a $1\,\Omega$ current-monitor resistor. Sweeping the coil current passes through the resonant <magnetic field>. The protocol describes two dips when the sweep revisits resonance; these are repeated crossings, not two distinct spectral lines.
Its calibration replaces the sample with a <Hall effect> probe, maps the <magnetic field> in $2\,\mathrm{mm}$ steps, and accounts for sample dimensions. At least four RF <frequencies> test the resonance relation. Sweep speed is adjusted to obtain comparable passages. Only a local peak-plus-background fit is prescribed, because the full shape is not Gaussian. The protocol also flags unexplained trailing spikes and possible electronics contributions. These controls constrain field and sweep errors; an empty-tube/cotton blank would be an additional background control, not a reported measurement in this protocol.<Electron Spin Resonance, University of Iowa laboratory protocol>[Electron Spin Resonance, revised July 2007: printed pp. 4–7, sweep behavior, field mapping, local fitting and anomalous spikes]{ref}
For an isolated effective <spin one-half> with dimensionless positive factor $g$, the basic resonance model is
$$
\Delta E=g\mu_{\rm B}|B|=h\nu,\qquad
g=\frac{h\nu}{\mu_{\rm B}|B_{\rm res}|}.
$$
Here $\mu_{\rm B}$ is the <Bohr magneton>, $h$ is the <Planck constant>, $\nu$ is the applied <frequency>, and $B_{\rm res}$ is the calibrated resonant <magnetic field>. A change in <frequency> should shift the resonance field proportionally in this model. The factor $g$ describes the electronic <magnetic moment> coupling, not the species' net <electric charge>.
A resonance supports the presence of detectable <paramagnetism>. It does not, by itself, identify an unknown <radical (chemistry)>: other species exhibiting <paramagnetism>, impurities, overlapping responses, and interactions with <atomic nuclei> can complicate interpretation. Conversely, absence of a detectable resonance need not mean absence of <free radicals>; a low <concentration>, a short lifetime, or an unsuitable measurement range can conceal them. The occupancy and chain diagrams are original bookkeeping illustrations; neither is a measured or simulated <DPPH> spectrum.
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