FeSO4* 7 H2O 
spin trap used in conjunction with Fe₂(DETC) to detect NO in brain, kidney, liver
and other tissues.

References:

1. Vanin, A.F. et al., Iron diethyldi-thiocarbamate as spin trap for nitric oxide detection. Meth. Enzymol. 301, 269, 1999.
2. Dikalov S. and Fink B. ESR techni-ques for the detection of nitric oxide in vivo and in tissues. Methods Enzymol. 2005; 396:597-610