System Performance

Resolution

CW Performance

Bruker BioSpin is dedicated to consistently delivering the world's top spectrometer performance.  Through constant hardware development, the ELEXSYS and EMX CW-EPR spectrometers offer the highest sensitivity and highest resolution available.

field_homogeneity
CW-EPR spectrum of the pereylene radical used for magnetic field homogeneity tests. A modulation amplitude of 10 mG at 10 kHz was used.

Magnetic Field Homogeneity

Another important property in CW EPR spectroscopy is the magnetic field homogeneity at the sample.  The presence of an inhomogeneous field manifests as an increase in the observed linewidth in the CW-EPR spectrum.  For narrow line EPR species (ΔBpp<100 mG), a high homogeneity is required to obtain meaningful CW-EPR spectra.

The magnetic field homogeneity is tested using the perylene radical.  A sample tube filled to 25 mm in height is placed in the EPR resonator.  The magnet homogeneity in the x- and z-directions is checked by moving the resonator laterally and observing the intensity in the EPR spectrum.  At the position of maximum lateral homogeneity (maximum intensity), the homogeneity in the y-direction is observed from the linewidth of the central feature in the EPR spectrum (ΔBpp=30 mG).
 

Magnetic Field Resolution

Galvinoxyl_FieldResolution
X-band CW-EPR Spectrum of the galvinoxyl radical. A modulation of 20 mG at 20 kHz was used.

The field resolution of a CW-EPR spectrometer depends on the homogeneity of the magnetic field, the stability of electromagnet power supply, and the purity of construction materials (to reduce ferromagnetic contamination).

An example of the field resolution performance is evident in the CW-EPR spectrum of the galvinoxyl radical.  A super-hyperfine splitting of 48 mG (4.8 µT) is observed in the spectrum.  By defining the minimum peak resolution to be the full-width at half maximum of the hyperfine line, a field resolution of 24 mG (2.4 µT) is demonstrated in the galvinoxyl radical spectrum.  This very high field resolution is further evidenced by the deep peak-to-trough amplitudes of the super-hyperfine lines .

 

N@C60 Field Resolution
CW EPR signal of N@C60 in degassed CS2. The measured linewidth is 15 mG. A modulation amplitude of 5 mG at 20 kHz was used.

An extreme test of the field resolution is best demonstrated using a sample with very narrow linewidths.  The N@C60 species is such a sample. The sample was dissolved in degassed CS2 to eliminated broadening due to dissolved O2.  The CW-EPR spectrum was collected using 20 000 points over a 20 G field range with a modulation amplitude of 5 mG at 20 kHz.  The measured peak to peak linewidth for the center line was 15 mG.

largesweep_resolution
Q-band CW-EPR Spectrum of Oxygen at 1.6×10-2 mbar. 100 mG resolution (Sweep range = 14 000 G, Points = 131 000)

Field Sweep Resolution

Very high resolution magnetic field sweeps are available with the ELEXSYS E500 and the EMXplus spectrometers.  With sweep ranges up to ±17 000 G composed of up to 256 000 points, the highest degree of experimental flexibility is maintained allowing both large and small sweep ranges to achieve the highest field resolution possible.
  
The Q-Band CW-EPR spectrum of atmospheric oxygen at reduced pressure illustrates this exceptional field sweep resolution.  The EPR spectrum was collected with a 100 mG field resolution over a 14 000 G sweep range.  The narrow EPR lines (300-500 mG), which are spread over at least an 8000 G range, are faithfully recorded.