Electronically tunable surface-coil-type resonator for L-band EPR spectroscopy
Electronically tunable surface-coil-type resonator for L-band EPR spectroscopy
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DOI：
10.1006/jmre.1999.1927
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发表时间：
2000-01-01
影响因子：
2.2
通讯作者：
Swartz, HM
中科院分区：
文献类型：
--
作者：
Hirata, H;Walczak, T;Swartz, HM
传统电子顺磁共振（EPR）波谱仪中的自动频率控制（AFC）电路自动将微波源调谐到谐振器的谐振频率。对于足够稳定的样品，电路可以令人满意地工作，使得谐振结构中的几何关系不会发生显着变化。然而，当在小型啮齿动物体内实验中测量 EPR 信号时，信号源和表面线圈检测器之间的距离可能会迅速变化。当传统的 AFC 电路在这些条件下保持振荡器调谐到谐振器时，所产生的频率变化可能超过 +/- 5 MHz，并显着改变 EPR 信号的位置。这种转变会对光谱产生不可接受的影响，尤其是当实验者处理窄 EPR 谱线时。动物的运动还会导致谐振器和 50 欧姆传输线的不匹配。这种不匹配的直接结果是噪声增加；基线位置的变化；并且很有可能过度驱动信号前置放大器，从而导致 EPR 信号丢失。因此，我们设计、构建和测试了一种新型表面线圈谐振器，使用变容二极管将谐振频率调谐到固定频率振荡器，并将谐振器与 50 欧姆传输线进行电容匹配。通过测量植入大鼠体内的锂酞菁的 EPR 光谱，测试了自动匹配系统的体内性能。通过使用和不使用自动匹配系统测量 EPR 光谱来评估光谱仪的稳定性和灵敏度。在使用自动匹配系统的体内实验中，发现光谱仪的整体实验性能显着提高。在所有测试的实验环境下，50 欧姆传输线和谐振器之间都保持了良好的匹配。这应该使我们现在能够进行以前不可能的实验。 (C) 2000 年学术出版社。
The automatic frequency control (AFC) circuit in conventional electron paramagnetic resonance (EPR) spectrometers automatically tunes the microwave source to the resonance frequency of the resonator. The circuit works satisfactorily for samples stable enough that the geometric relations in the resonance structure do not change in a significant way. When EPR signals are measured during in vivo experiments with small rodents, however, the distance between the signal source and the surface-coil detector can change rapidly. When a conventional AFC circuit keeps the oscillator tuned to the resonator under those conditions, the resultant frequency change may exceed +/- 5 MHz and markedly shift the position of the EPR signal. Such a shift results in unacceptable effects on the spectra, especially when the experimenter is dealing with narrow EPR lines. The animal movement also causes a mismatching of the resonator and the 50-ohm transmission line. Direct results of this mismatching are increased noise; shifts in the position of the baseline; and a high probability of overdriving the signal preamplifier with consequent loss of the EPR signal. We therefore designed, built, and tested a new surface-coil resonator using varactor diodes for tuning the resonance frequency to the fixed frequency oscillator and for capacitive matching of the resonator to the 50-ohm transmission line. The performance of the automatic matching system was tested in vivo by measuring EPR spectra of lithium phthalocyanine implanted in rats. Stability and sensitivity of the spectrometer were evaluated by measuring EPR spectra with and without the use of the automatic matching system. The overall experimental performance of the spectrometer was found to significantly improve during in vivo experiments using the automatic matching system. Excellent matching between the 50-ohm transmission line and the resonator was maintained under all experimental circumstances that were tested. This should allow us now to carry out experiments that previously were not possible. (C) 2000 Academic Press.