Flow-through NMR probe having a replaceable NMR flow tube

Electricity: measuring and testing – Particle precession resonance – Spectrometer components

Reexamination Certificate

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C324S322000, C324S300000

Reexamination Certificate

active

06177798

ABSTRACT:

FIELD OF THE INVENTION
The invention in general relates to the field of chemical analysis, more particularly to the fields of high performance liquid chromatography (HPLC) and nuclear magnetic resonance spectroscopy (NMR), and still more particularly to a flow-through NMR flow tube used for the on-line coupling of the two analytical techniques. Specifically, the invention pertains to a flow-through NMR probe having replaceable NMR flow tube assembly and a method for the efficient insertion and removal of an NMR flow tube in a flow-through NMR probe. The invention allows for ease of replacement of an NMR flow tube in a flow-through configuration to optimize performance and minimize downtime for the coupled HPLC-NMR analytical techniques without requiring major modification of the probe structure.
BACKGROUND OF THE INVENTION
An NMR apparatus is most often characterized in gross by cylindrical symmetry. A typical NMR magnet is of the superconducting variety and is housed in a dewar which includes a room temperature cylindrical bore in which a very carefully controlled homogeneous magnetic field is sustained by operation of the superconducting magnet in the interior of the dewar. An NMR probe holds a sample placed in the uniform magnetic field. The housing for the probe is typically cylindrical to fit within the bore of the magnet and the sample is generally positioned along the central axis of the probe. A coil is disposed close to the sample within the probe to apply an exciting radio frequency (RF) magnetic field to the sample. The resultant resonance signal of the sample is picked up by the coil and delivered to a receiver circuit. The receiver circuit generates an output signal. A computer takes the Fourier transform of the signal to obtain an NMR spectrum.
HPLC is widely used to separate organic mixtures for analysis. Although ultraviolet, infrared and mass spectroscopy have been used for qualitative analyses of HPLC eluents, NMR spectroscopy generally provides unequaled structural information and has sample requirements more reasonably matched to HPLC. Efforts to couple these two analytical techniques, however, have been hampered by the low sensitivity of the NMR detector. Recent improvements in NMR detection cells for use in flow-through NMR probes have allowed for high resolution, high sensitivity and ease of use in HPLC-NMR coupled analyses. See, for example, U.S. Pat. No. 5,867,026, entitled “Flow Tube for NMR Probe” disclosing an improved flow-through NMR detection cell and method of manufacture. Such improved flow tube designs have led to increased acceptance and usage of sample placement for NMR spectrometers using fluid injection methods and have created further interest in flow-based automatic sample measurement. As these techniques become more routinely used and accepted, the minimization of downtime for the NMR and the optimization of system performance for efficient measurement throughout become increasingly advantageous.
Current NMR flow tube assemblies, including the NMR sample flow tube together with its various connectors and associated tubing for attachment to an HPLC, are delicate, difficult to handle and not well suited for exchange in the field. Removal and insertion of such assemblies in the NMR probe is risky and expensive, at least in part because the flow tubes (and attached connectors) are positioned and secured to the NMR probe within nested assemblies of coils, dewars, and support structures. Many present designs require significant mechanical interaction with these closely mated subassemblies. Electrical manipulations are often needed to exchange the flow tube, such as unsoldering and resoldering of the RF and pulsed field gradient coils. Some designs have RF circuitry directly attached and secured to the flow tubes. There is an additional cost and risk associated with exchange of the flow tube in these designs because of the directly secured RF circuitry.
Other flow tube assembly designs that promise exchangeability of the NMR flow tube require significant modification of the NMR probe or subassemblies to accommodate the removable flow tube. Such designs generally utilize fully integrated flow tube assemblies having parts and associated tubing that are permanently bonded together with chemical adhesives. See, e.g., Barjat et al., “Adaptation of Commercial 500 MHz Probes for LCNMR,” Journal of Magnetic Resonance, Series A 119, 115-119 (1996). These NMR flow tubes offer the advantage of a high filling factor due to their slender construction and consistent outside diameter, however, such fully assembled and permanently bonded structures do not allow rapid exchange of the flow tube or associated tubing, for cleaning or optimizing individual applications. They also lack mechanical reproducibility due to difficulties in controlling the adhesive-assembly process. Moreover, the possibility of contact between the analytical solutions and the adhesives used in bonding the parts of fully such integrated assemblies can cause chemical compatibility problems and sample contamination.
Ease-of-exchange of the NMR flow-tube is important. Users often wish to change or exchange the flow tube assembly since flow tubes and attached tubing can become clogged over time or reach the point where cleaning protocols are insufficient. The flow tubes or tubing may break and require replacement or the user may wish to incorporate a post-probe sample collector. Moreover, users may wish to optimize the sample chamber of the flow tube for various applications, for example, if research shifts to samples where quantities are limited. What is needed is an NMR flow tube assembly of inert construction that permits the simple and efficient removal and insertion of an NMR flow tube in a flow-through NMR probe with a minimum amount of probe modification while maintaining a high filling factor.
SUMMARY OF THE INVENTION
The invention described here provides a cost-effective, reliable and robust NMR flow tube assembly that allows for ease of replacement of an NMR flow tube in a flow-through NMR probe while maintaining good NMR sensitivity. The invention requiring only modest modification of the NMR probe assembly and can be assembled without chemical adhesives. The invention further provides a method for inserting and removing an NMR flow tube in a flow-through NMR probe that helps minimize downtime and optimize performance for sample measurement in an HPLC-NMR environment.
In accordance with one aspect of the present invention, a flow-through NMR probe is provided which comprises a lower insulator and an upper insulator supported at a distance apart within the probe. The various probe subassemblies, including coils, dewars and support structures necessary for sample spectroscopic measurements, are typically positioned between the upper and lower insulators in the probe. Each insulator has an opening or aperture aligned along the central axis of the probe. An upper and lower insulator ring are positioned in the upper and lower insulators to help align an NMR flow tube in the probe. A guide tube is attached at a first end to the bottom of the lower insulator for communicating along the central axis of the probe through the opening in the lower insulator and toward the opening in the upper insulator. The guide tube extends downward to the base of the probe. An NMR flow tube, attached to inflow tubing at an inlet end by a detachable connector and fitted with a lower spacer ring may be inserted through the guide tube into the probe.
Using the guide tube, the flow tube is slid into position through the lower and upper apertures and the various probe subassemblies. The upper insulator ring is fitted over the outlet end of the flow tube. The outlet end of the flow tube is attached to an outflow tube by a detachable connector. The connector is then seated in a key or notch in the top of the upper insulator ring to position the NMR flow tube for spectroscopic measurements.
In accordance with a second aspect of the invention, the guide tube may be provided with an inner diameter at the first end

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