When I was in graduate school, electron paramagnetic resonance (EPR) instruments were the size of a Volkswagen Beetle and weighed about four tons. Sample size was about a milliliter. The only analytical assays I recall were the Emit line of clinical diagnostics marketed by a joint venture between Syntex and Varian Associates called “Syva.”1
After hearing about a smaller, more sensitive EPR instrument recently developed at NIST (Gaithersburg, Md.), I called Jason Campbell, an electrical engineer in the Semiconductor and Dimensional Metrology Division at NIST, to find out more. He explained that the old unit’s resonator cavity had been replaced with a short, thin, wire probe that detects changes in current induced by variations in the magnetic field, which produces a phase shift and absorption.2 The probe wire is part of a sensitive bridge circuit that picks up the characteristic EPR absorptions and dispersions. Using frequencies in the microwave region, the circuit can detect changes as small as 1 ppb. The new unit offers a 20,000× improvement in detection sensitivity in a more compact package.
The large electromagnet in early EPR machines has been replaced with a small, permanent magnet mounted on the probe head, which scans the sample from just a few microns above the surface. The technique is similar to atomic force microscopy and enables characterization of specific structures on the sample surface. Campbell expects the EPR instrument will eventually help in the characterization of one-atom defects in semiconductors. He also expects that it will be useful for studying biological surfaces and membranes: after all, with one unpaired electron, NO is both a great EPR probe and physiologically important.
Potential applications of the EPR instrument include chemistry, biochemistry and biology. NIST hopes to attract a commercial partner and will be attending the EPR Symposium, July 26‒31, in Snowbird, Utah.
For more information, please contact Dr. Campbell at 301-975-8308; [email protected]
NIST has made electron spin resonance useful for exploring tiny objects for the first time, potentially enabling the decades-old technique to spot defects on computer chip surfaces or view the workings of proteins on the surface of cells. (Credit: NIST)
References
- Eaton, G. and Eaton, S. Foundations of Modern EPR.
- Campbell, J.P.; Ryan, J.T. et al. Electron spin resonance scanning probe spectroscopy for ultrasensitive biochemical studies. Anal. Chem. 2015, 87(9), 4910‒16; DOI 10,1021/acs.analchem.5b00487, May 5, 2015.
Robert L. Stevenson, Ph.D., is Editor Emeritus, American Laboratory/Labcompare; e-mail: [email protected].