Application of quantitative EPR

Abstract

Although the high sensitivity and broad dynamic range of EPR make it an attractive analytical technique for species with unpaired electrons, the precision and accuracy of spin concentration measurements have often been low. The marked improvement in quantitative EPR is a result of improvements in instrumentation and greater attention being given to operating procedures. In 1962 the lower limit for the EPR determination of polycyclic aromatic hydrocarbons was reported to be ca . 3|ig. Using the same method these compounds have now been determined down to nanogram levels. Aromatic nitro compounds present at submicrogram levels can also be quantified by EPR. A free radical assay technique has been applied to the determination of drugs in body fluids; a morphine concentration in urine of 0.5 |ig cm -3 is detectable. Molybdenum has been determined in sea water with a relative precision of 4.7 % at the 11 pg 1 -1 level and a detection limit of 0.46 pg 1 -1 . By using the EPR signal of [FeF 6 ] 3- the total iron content of solutions containing Mn II , Fe II and Fe III has been measured, the analytical range being 10 -6 -10 -2 M with a detection limit of 6 x 10 -7 M. Diamagnetic metal ions can be estimated by EPR with the aid of spin-labelled chelating reagents, e.g. Zn II over the linear range 10 -6 -10 -3 M with a detection limit of 5.5 x 10 -7 M. Nitrite ion can be determined by EPR with a precision of 0.9% at the 0.5p.p.m. level.