User:Sdriegel/sandbox

Benchtop NMR Spectrometers
Benchtop NMR spectrometer refers to a nuclear magnetic resonance (NMR) spectrometer that is significantly more compact and portable than the conventional equivalents. Instead of requiring dedicated infrastructure and extensive installations these instruments can be moved as necessary (e.g., to the benchtop or fumehood). These spectrometers offer improved workflow even for novice users as they are simpler and easy to use. They differ from relaxometers in that they can be used to measure spectra and are not limited to the determination of relaxation variables (e.g., T1 and T2).

Types of NMR Spectrometers
A NMR spectrometer is comprised of five components: i) a magnet to generate a uniform magnetic field; ii) a radio frequency generation unit; iii) a probe; iv) a phase-sensitive detector and v) a data processing unit.

Although the magnetic field must be homogenous and reproducible, the method with which the field is generated is not important. Three magnet classes have been used: permanent magnets, electromagnets, or super-conducting solenoids.

Permanent Magnets
This first generation harnessed magnetic field as was generated by large iron magnets. Although permanent magnet were ultimately replaced by stronger magnetic fields, there has been a recent renaissance in this technology. The development of benchtop NMR spectrometers explores permanent rare-earth magnets (e.g., samarium or neodymium). . Spectrometers built with permanent magnets have lower resolution (< 90 MHz) than spectrometers built of other types of magnets but they function at ambient temperatures, do not require skilled maintenance personal hence they are significantly more versatile and affordable.

Electromagnets
The second wave of instruments augmented the magnetic field by coiling permanent magnets with electrical wires. This medium-resolution second generation was later replaced with the more powerful superconducting magnets that are now ubiquitous.

Super-conducting Magnets
Current spectrometer technology uses superconducting wires cooled with cryogens (i.e., liquid helium (bp = 4.2 K) and liquid nitrogen (bp = 77 K)) to generate a magnetic field. These cryogens are dangerous, are becoming increasingly expensive,  and require a dedicated, trained staff to handle for regular system maintenance. Regardless, these NMR spectrometers provide the most homogenous magnetic fields and the greatest spectral resolution. They are however, far more complex, costly (can exceed 2 million USD per unit) and sterically encumbered. Installation of these spectrometers requires extensive planning and once the magnet has been initialized, it remains in a central NMR facility where it can be regularly monitored. Generally, these spectrometers function as a walk-up and use instrument, are accessed by a centralized Internet sign-up system (e.g., Faces Scheduling System), or the staff may offer access as a service.

Applications
NMR spectroscopy can be used for chemical analysis, reaction monitoring, and quality assurance/quality control experiments. Higher-field instruments enable unparalleled resolution for structure determination, particularly for complex molecules. Cheaper, more robust, and more versatile medium and low field instruments have sufficient resolution for reaction monitoring and QA/QC analyses. As such permanent magnet technology offers the potential to extend the accessibility and availability of NMR to institutions that do no have access to super-conducting spectrometers (e.g., beginning undergraduates or small-businesses).

Available Benchtop NMR Spectrometers
Development of this new class of spectrometers began in the mid-2000’s leaving this one of the last characterization techniques to hit the benchtop.

Magritek
New Zealand based Magritek is one of the oldest commercialized benchtop technologies, offering an array of compact NMR and MRI spectrometers. The Magritek Ultra-Compact spectrometer offers very good sensitivity and resolution at 42.5 MHz spectrometer, but its portability is limited due to the weight of the machine. Samples are introduced into the magnetic field in standard 5 mm NMR tubes and the spectrometer is controlled through an external computer.

Picospin
In 2009, Picospin patented a smaller (7 x 5.75 x 11.5”), lighter (10.5 lbs), slightly higher field (45 MHz) spectrometer with good resolution and mid-to-low-range sensitivity. Instead of the traditional static 5 mm NMR tubes, the Picospin spectrometer uses a flow-through system that requires sample injection into a 1/16” or 1/32” capillary. To insure the flow-through system remains functional, the sample must be filtered diligently. This spectrometer is also controlled through an external computer.

Nanalysis
Nanalysis NMReady offers the highest field (60 MHz) for a benchtop instrument and the first all-in-one unit. The spectrometer is controlled by a touchscreen computer that is contained within the same enclosure as the magnet. This spectrometer offers good sensitivity and resolution and is compatible with standard 5mm NMR tubes, sample preparation and measurement.