Broadband Transmission Spectrometer
The transmission instrument is one of the two EPR spectrometers based on the 17 Tesla superconducting magnet.
In this instrument, microwaves travel along a 10 mm cylindrical waveguide through the sample compartment and further towards the bolometer detector. The transmission instrument may use one of three microwave sources:
- Gunn source operating at 95±3, 190±6, 275±9, 380±12 and 475±15 GHz
- Gunn source operating at 110±3, 220±6, 330±9, 440±12 and 550±15 GHz
- New high-power source operating at 26±2, 52±4, 104±8, 208±16, 312±24 and 403-448 GHz
The frequency of the new source is stable within 24 kHz in the 300 GHz range, while the accuracy of the Gunn diode generators is of the order of 1 MHz at their base frequency.
Although the absolute sensitivity of the transmission device is lower than that of the quasi-optical system, samples for the transmission apparatus may be very large (0.5 mL and above), thus offsetting the sensitivity
disadvantage. Samples are also substantially larger than in W-Band, so that spectra are often better than those obtained on the Bruker W-Band machine if enough sample is available. Thin Teflon vessels with an outer diameter of 9 mm and a depth of up to 10 mm or more are
used as sample containers. Standard X-Band 3 mm and 4 mm quartz tubes can also be used for samples requiring sealing. Gelatin capsules up to the size ‘00’ (8 mm diameter) were also successfully employed. The sample temperature can be controlled over the range of 3K to 300K.
The instrument does not employ resonance cavity. The superconducting magnet has been factory-calibrated, but for accurate measurements usage of g standards is recommended (for
example DPPH or phosphorus doped into silicon). The superconducting magnet must be swept if a wide magnetic field range is required for a
spectrum. For samples requiring a sweep width of no more than 2000 gauss, an additional sweep coil is used while the superconducting magnet
is put in the persistent mode (i.e., drawing no current from the power supply). The transmission instrument also works well for liquid solutions of samples dissolved in organic solvents of low dielectric constant. Frozen solutions in most organic solvents and even aqueous solutions usually are no problem. Transition metal complexes, some compounds of the f-electron metals (gadolinium) as well as organic radicals have been studied on this spectrometer. The sensitivity is sufficient for studies on some enzymes (the manganese-containing oxalate decarboxylase, for example).
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For more information, contact Andrew Ozarowski.
