HIGH SPEED SPINNER FOR NMR SPECTROMETER

Abstract: An NMR spinner having separate drive and stabilizing jets incorporates a valve to auto­ matically divert a portion of the stabilizing pressure to the drive jets in order to maintain the bearing when drive pressure is otherwise reduced.

Description: Field of the Invention This invention relates generally to nuclear magnetic resonance (NMR) analytic instruments and, in particular, to means for rotating samples within such instruments. The examination of the radio frequency (rf) magnetic resonance spectra of samples is facilitated by the high speed rotation of the sample, by which means residual gradients and inhomogeneities in the magnetic field are averaged azimuthally. Solid phase samples exhibit resonance lines which are inherently broadened by static dipolar interactions between nucleii. Furthermore, the chemical shift for nuclear moments within solids often depends on the inclination of the molecule or lattice with respect to the polarizing field. In a polycrystalline material where the orientations of magnetic moments of interest may be randomly oriented, this angular dependence yields a broadening of the resonance line. In liquids and gases, thermal motions tend to average these effects so that sharp resonance lines are observed. For solids, an effective averaging may also be realized by the practice of "magic angle spinning". For this technique, the spinning axis is inclined at an angle ϑ with respect to the polarizing magnetic field satisfying the conditions 1 -3 cos² ϑ = 0. The magic angle is the solution of that condition, for which ϑ = 54° 44′ Further, the spin rate should be high in proportion with the strength of the inter-nuclear interaction or spread of chemical shift values, which is achieved with spinning rates of the order of several KHz. Finally, the spin rate must be relatively stable over the duration of the experiment. Several items of prior art are known for spinning apparatus. One example is given in U.S. 4,275,350, which apparatus employs separate air bearings for the three functions of radial stabilization, rotation and levitation of the rotor for the stator. Another item of prior art is the work of Doty et al, "Design of High Speed Cylindrical NMR Sample Spinners", Rev. Sci. Inst., vol. 52, December 1981, pp. 1868-1875. Brief Description of the Invention The present invention utilizes an automatic valve arrangement which in normal operation isolates the drive gas pressure source from the radial stabilizing gas source. When the normally higher pressure drive gas pressure falls below the radial bearing gas pressure, the valve opens to bleed a portion of the radial bearing gas to the drive jets to maintain the bearing until the spinner can be controllably stopped. Brief Description of the Figures FIG. 1a - 1c shows a rotor assembly for the spinner of the present invention. FIGS. 1c - e shows a stator assembly for the present invention. FIG. 2 is a section through the component of FIG. 1d. FIG. 3 shows the dependence of spin rate on drive gas pressure. Detailed Description of the Invention The spinner unit is shown as disassembled constituents in FIGS. 1a - e. The rotor assembly comprises the end cap 10 and the rotor body 12. The latter has a cavity 14 therein to contain the sample and is closed by insertion of end cap 10. Preferably, there are grooves 16 machined into the end cap for retaining resilient seals 18 to provide reliable closure of cavity 14. Rotor body 12 is preferably constructed from Macor, Al₂O₃, partially stabilized zirconia or Torlon. End cap 10 is preferably constructed from PMMA, Kel-F or Torlon. The length of the rotor body 12 may include a portion 19 which may be arranged to cooperate in a known manner with a light source and light sensor, not shown, to monitor spinning rate during operation. The stator assembly comprises three pieces as shown in FIGS. 1c - e. The top piece 20 and bottom piece 22 each contain twelve radially directed jets 24. These jets each comprise 0.3mm apertures. Stator body 26 contains a bore 28 with ports 30 to communicate with annular regions 32 to provide pressurized gas to radial jets 24 for stabilizing the axis of rotor body 12 in operation. It should be apparent that when assembled, top and bottom pieces 20 and 22 define bore 34 through which rotor body 12 is inserted for operation. An access port 35 is cut into stator body 26 to permit insertion of the coil or coils employed for excitation and detection of nuclear magnetic resonance in the sample contained within rotor body 12. Top piece 20 has inner conical surface 36 which accepts the outer conical profile 38 of end cap 10. Twelve jets 40 are distributed annularly on the inner conical surface 36 to provide pressurized gas which acts on flutes 42 of the rotor assembly for levitating and rotating the rotor assembly. The pressurized gas for jets 40 is furnished from bore 44. These jets comprise apertures of 0.4mm diameter. In typical operation, the gases in the respective bores 28 and 44 are maintained at different pressures. In a usual arrangement for such dual jet spinner apparatus, the rotation, or drive pressure may be several bars greater than the radial bearing pressure. When the drive pressure in such apparatus is reduced to 0, the result is often a severe impact of the rotor assembly against the stator. In the present invention this is prevented by disposing a simple check valve between the two pressurized bores supplying the respective jets. When the pressure of the normally high pressure jets falls below the pressure at the low pressure jets, the check valve opens to bleed a portion of the low pressure gas into the formerly high pressure region. In a practical situation this permits maintenance of the levitation of the rotor and slow rotation to occur thereby avoiding damage by sudden impact. It should be noted that such damage frequently entails the removal, disassembly and replacement of portions of the stator assembly. In the present apparatus a check valve is established between the volume of bore 28 and the drive jets 40 by means of aperture 46 and flapper 48. Flapper 48 is constructed from a thin piece of plastic conforming to the surface in which aperture 48 is formed. Aperture 48 is 1mm in length and 0.5mm in diameter. Flapper 48 is glued or otherwise affixed on one side so that it opens whenever the pressure from bore 44 falls below the pressure then obtaining in bore 28 (usually about 1 bar). One example of the present invention is capable of stable operation (± 1 Hz) from 250 Hz to 9500 Hz and contains 225 microliter of sample measuring 25mm x 7mm o.d. Spinning rate as a function of drive pressure is shown in FIG. 3. The stator further utilizes a special lining 50, machined from a stable nonconducting material such as Macor, Al₂O₃ or PSZ. This lining 50 is press fitted into the top and bottom pieces and the stator is then assembled. The linings 50 are then bored out to 7 +0.02/-0.00mm. It is occasionally necessary to disassemble the stator in order to service the components, replace an RF coil, or the like. This construction assures reproducible operation and close tolerances which contribute to the excellent stability of operation of the spinner. In the above description, Macar refers to a machinable glass, made by Dow Corning Corporation, Midland, Michigan, United States of America. Torlon is a polyimide made by Amoco Corporation, Chicago, Illinois, United States of America. Kel-F is a polyfluorinated hydrocarbon made by Dupont Company, Wilmington, Delaware, United States of America. PMMA is polymethylmethacrylate and PSZ is partially stabilized zirconia.

Claims: 1. Apparatus for spinning a sample about an axis comprising rotor means for containing a sample and for rotation in response to pressurized gas directed along the outer surface thereof, first gas supply means for supplying gas at a first pressure to cause rotation and levitation of said rotor means, second supply means for supplying gas at a second pressure to maintain radial stability of said rotor means, valve means for bleeding a portion of said gas from said second supply means to said first supply means for maintaining said first pressure at a valve sufficient to prevent said rotor from impacting against said stator in the absence of gas at said first pressure from said first gas supply means. 2. The method of preventing damage to an NMR sample spinner having separate gas pressure sources supplying gas bearings for respectively stabilizing said spinner and for rotating said spinner one said pressure higher than the other in operation, comprising (a) sensing the reduction of said higher pressure to a value below said other pressure, (b) automatically opening a valve between said separate gas pressure sources, and (c) limiting the gas pressure drop from said step of opening said valve.

Inventors: DAUGAARD, PREBEN, JAKOBSEN, HANS J., LANGER, VAGN
Applicants: VARIAN ASSOCIATES, INC.
Application year: 1987
Main classification: G01R33
IPC codes: G01R33