Why do we take the first derivative of EPR/ESR spectra? Apologies if this question is a bit too chemistry-flavoured.
In electron paramagnetic resonance spectroscopy, there's a practically ubiquitous convention of plotting the first derivative of the absorption with respect to field, rather than the simple absorption. Why is this?
 A: The derivative-like line shape is a result of the use of field modulation.  In order to get sufficient signal to noise, the B_0 field (large, static field) is modulated (usually at 100kHz) and a lock-in amplifier (or equivalent) is used to reject any frequencies beside 100kHz.  The result of this field modulation is that the signal that is obtained is not the value of the absorbance, rather the difference in the absorbances between the ends of the modulation amplitude (more or less) i.e. ~d(absorption)/dB. A slightly more detailed explanation (with a picture, figure 2.7) is here.
There are lots of caveats and such due to the fact that it's not a true derivative, and there is, therefore, some signal distortion.
There is currently (again) development of detection techniques which are not dependent on the modulation of the field for S/N.
A: The first derivative exploits minor differences in the absorption curve that when taken  yields a chracateristic curve for a particular species of analysis.  The subsequent EPR spectra can be analyzed and can  provide information about material and the properties based on the hyper fine splitting and other details. 
A: Sorry I'm answering my own question, but a major figure in biological EPR popped in and informed me that the reason EPR spectra are taken as first derivatives is largely historical - old EPR spectrometers had large linearly field-dependent baseline shifts which could be eliminated easily by taking the first derivative, which changed the constantly increasing shift into a simple vertical offset on the plot.
This said, Sam's answer hints at a secondary benefit of taking the first derivative, which is in amplifying minor asymmetries in the peaks.
