Spectrometer microwave frequency

Deuteration of desired molecules or molecular groups allows selective study of intermolecular interactions. Moreover, for commonly used X-band EPR spectrometers (microwave frequency about 9 GHz, magnetic field about 3400 G), the ESEEM signal resulting from interaction with deuterium nuclei substantially exceeds that for interaction with protons. Note that in addition to using deuterium nuclei, nuclei also can be used for studying intermolecular interactions in certain favorable cases. ... 

This brief anecdote should serve to illustrate that its extensively interdisciplinary character is not only a strength of bio-EPR but also its Achilles heel. When the production of significant results requires comparable input efforts from different disciplines, there is an increased chance for the occurrence of time-wasting misunderstandings and errors. A less anecdotic example is the claim—frequently found in physics texts—that sensitivity of an EPR spectrometer increases with increasing microwave frequency. Although this statement may in fact be true for very specific boundary conditions—for example, when sensitivity stands for absolute sensitivity of low-loss samples of very small dimensions—when applied in the EPR of biological systems it can easily lead to considerable loss of time and money and to frustration on the part of the life science researcher, because it is simply not true at all for (frozen) solutions of biomolecules. 

Good X-band resonators mounted into a spectrometer and with a sample inside have approximate quality factors of 103 or more, which means that they afford an EPR signal-to-noise ratio that is over circa three orders of magnitude better than that of a measurement on the same sample without a resonator, in free space. This is, of course, a tremendous improvement in sensitivity, and it allows us to do EPR on biomolecules in the sub-pM to mM range, but the flip side of the coin is that we are stuck with the specific resonance frequency of the resonator, and so we cannot vary the microwave frequency, and therefore we have to vary the external magnetic field strength. 

To evaluate set-up costs we assume that we have to start from scratch. From our previous discussion about microwave frequencies it should be obvious that we want a cw X-band spectrometer as the central (frequently only) facility. What exactly is a complete spectrometer The answer depends a bit on the type of experiments planned, but for all cases the minimum requirements would be a basic spectrometer (bridge + resonator magnet control unit) and a frequency counter. 

In rough-tuning, the spectrometer is put at the tune mode. A low microwave power (e.g., 25 dB, 0.63 mW) is applied to generate the model pattern , which is the microwave power reflected from the resonator as a function of the microwave frequency. Microwave frequency is adjusted to... 

To fine-tune the cavity, the spectrometer is put in the operate mode. Adjust the microwave frequency, the iris position (resonator parameter), and the reference arm current ( bias ) so that the analog indicators for the automatic frequency control ( AFC ) and the diode always stay at the center as the microwave power is increased from minimum (e.g., 50 dB, 2 fiW) to maximum (e.g., 0 dB, 200 mW). This indicates that at all power levels, the majority of microwave power is stored in the resonator and very little is reflected. Adjust the signal phase to let the diode indicator reach the maximum, and then decrease the bias if necessary to put diode back to center again. 

Most e.s.r. spectrometers operate in the so-called X-band microwave frequency range 9.5 GHz is a typical operating frequency, and it is convenient because the magnetic fields required for resonance are in the range accessible to conventional electromagnets. Spectrometers operating up to 40 GHz have been used but they are less suitable for gas phase studies, mainly because of the smaller size of the resonant cavity. 

Several techniques allow further elucidation of the information contained in an EPR spectrum. Through single-crystal EPR it is possible to determine the orientations of the g and A tensors relative both to each other and to the internal coordinates of a structurally defined active site. The use of several microwave frequencies can be particularly informative. While the spectrum shown in Fig. 2 (middle) was taken with an X-band spectrometer (u = 9 GHz), Q-band (v 35 GHz) and S-band (u = 3 GHz) should also be employed. The high microwave frequency leads to increased resolution by spreading the g values over a wider magnetic field range with little effect on hyperfine splitting. Low... 

The EMR studies were performed in the range of MFs from 1000 to 5500 G using X-band EPR spectrometer Bruker EMX-8/2. The commercial gas-flow cryostat was used to achieve temperature in the range of -100° - 90° C. Ferrofluid spectra in a quartz flat cell and PVP films in a quartz tubes. Microwave frequency power did not exceed 0.1 mW. 

A. Performance of ESR Spectrometers In general the most desirable features in a spectrometer are high resolution and sensitivity. Resolution of an ESR spectrometer depends basically on the time stability of the magnetic field and the microwave frequency as well as the homogeneity of the magnetic field. The spectrometer sensitivity, expressed in terms... 

EPR spectra are recorded at 293 K and 77 K with a Bruker EMX spectrometer using the X-band microwave fi-equency. A dual cavity is used and the g-values are determined by measuring the magnetic field, H, and the microwave frequency. All the thermal treatment of the samples are carried out in a microflow reactor, which is assembled with a quartz EPR tube to allow the introduction of the solid into the resonance cavity without exposure to air. 

Electron spin resonance spectrometers are available commercially, and typical operating conditions are a frequency/of 10 ° cps (x band), and a magnetic field H of 3600 gauss. Sometimes the measurements are made at other frequencies such as 2.4 X 10 cps (k band). The samples may be examined as a function of the microwave power, the microwave frequency, the modulation conditions, and the temperature. 

The nuclear Larmor frequency v increases with magnetic field and thus with the microwave frequency of the EPR spectrometer, whereas A" and F" are molecular parameters and thus are independent of the field (although, as described below, they are dependent on the g value, or position within the EPR spectrum). This typically means that when the ENDOR transitions from two different types of nuclei (e.g., H, N) are overlapping for one microwave frequency range, they may be resolved by changing to a different microwave frequency, as shown in Fig. 5. 

The basic components of an ESR spectrometer are shown in Fig. 4 [15]. The microwave bridge suppUes microwaves at a fixed frequency and chosen power, however, the microwave frequency is tuneable over a Hmited frequency range. The microwave source is a klystron or a gundiode. If one wishes to obtain ESR spectra at different frequencies, then a wide range of microwave sources need to be called in. The most commonly used and commercially available frequency is... 

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