Improved performance of a varian FT-80A NMR spectrometer at low operating frequencies

Abstract

A method to extend the multinuclear capability of a Varian FT-80A nuclear magnetic resonance spectrometer recently has been described (I). The reported changes extended the lower operating frequency to 2.5 1 MHz (lo3Rh). Troublesome baseline distortions were encountered at operating frequencies between 2.5 and 5.3 MHz. These distortions were caused by leakage of the radiofrequency transmitter pulse into the receiver preamplifier. As a part of the original modification, a calibrated rf attenuator was used to establish 45, 22, and 11 degree flip-angle pulses by reducing the transmitter rf amplitude. This approach reduced the transmitter-toreceiver leakage, and, hence, reduced anomalies in the Fourier-transformed spectra. Operating with a flip-angle pulse of less than 90” did not always provide optimum results, especially for broader resonance lines of some quadrupolar nuclei. The primary defense against pulse breakthrough in the FT-80A gated preamplifier (schematic 87-144-766-O 1) is a network of PIN diodes (Hewlett-Packard type 50823039) controlled by gated bias currents. This circuit isolates the preamplifier when the transmitter rf pulse is applied by establishing a high impedance between the sample probe and the preamplifier. An ideal PIN diode provides a variable rf resistance controlled by a direct current or low-frequency control signal. The performance of a real PIN diode, however, is limited by both rf amplitude and frequency because of rectification effects (2). The effects are more serious at low frequencies because the period of the rf carrier is closer to the lifetime of the charge carriers in the diode intrinsic layer (3). There is sufficient time for these charges to be influenced by the changing rf voltage. The carrier lifetime of the 5082-3039 PIN diode is 100 nsec. Therefore, radiofrequencies below 10 MHz have an increasing modulation effect on the device and, hence, a reduction in isolation of the FT-80A preamplifier. The effect of this interference on the NMR signal is shown in Figs. 1 and 2. (For these spectra, the spectrometer crystal filter was bypassed to minimize baseline distortions.) The inserts shown in Fig. 2 indicate the magnitude of pulse breakthrough compared to the strength of the observed signal. The transient overload of the receiver due to pulse breakthrough prematurely filled the word length of the computer memory, thus limiting the number of accumulations and hence the achievable signal-to-noise ratio.