Modification of a Varian HA60-IL N.M.R. spectrometer for measurement of carbon-13 spectra by audio-sideband sweeping

Abstract

Conversion of a conventional modulated-field proton magnetic resonance spectrometer (Varian HA60-IL) for measurement of natural-abundance carbon-13 N.M.R. spectra by audiofrequency (a.f.) sideband sweeping with internal homopolar field-frequency lock is described. The manufacturers modified the basic instrument so that sweep, multiscan accumulation (signal averaging), and data plotting were under control of a minicomputer (Varian 620/i with 8-K( memory), supplied an ref. transmitter/receiver unit and probe of appropriate frequency (15.086 MHz), the probe being also tuned to accept the corresponding proton-decoupling frequency (60 MHz), and suggested use of ¹³C-isotopically enriched methyl iodide as internal homopolar lock sample. In practice, under the high r.f. and a.f. powers required, the a.f. range to be swept must be chosen to avoid regions where the baseline response is highly frequency-dependent. Choice of a suitable fixed lock a.f, then permits use of an inexpensive proton-decoupler based on a simple crystal oscillator whose centre frequency can be 'pulled' a few hundred hertz for optimum decoupling, noise-modulated, and appropriately amplified. Phase differences between lock and observed signals are corrected with a simple a.f. phase shifter in the analytical channel. Even in the 'best' baseline region, residual 'roll' accumulates, but usually satisfactory major corrections may be effected rapidly by means of a sextic polynomial curve- fitting subroutine. Integration of peak areas is not feasible. When the sample and its dielectric constant are changed, it is important to retune the probe receiver coil for maximum response.