Benchtop nuclear magnetic resonance spectrometer
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A Benchtop nuclear magnetic resonance spectrometer (Benchtop NMR spectrometer) refers to a Fourier transform nuclear magnetic resonance (FT-NMR) spectrometer that is significantly more compact and portable than the conventional equivalents, such that it is portable and can reside on a laboratory benchtop. This convenience comes from using permanent magnets, which have a lower magnetic field and decreased sensitivity compared to the much larger and more expensive cryogen cooled superconducting NMR magnets. Instead of requiring dedicated infrastructure, rooms and extensive installations these benchtop instruments can be placed directly on the bench in a lab and moved as necessary (e.g., to the 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 high resolution NMR spectra and are not limited to the determination of relaxation or diffusion parameters (e.g., T1, T2 and D).
This first generation of NMR spectrometers used large Electromagnets weighing hundreds of kilograms or more. Slightly smaller permanent magnet systems were developed in the 1960s-70s at proton resonance frequencies at 60 and 90 MHz and were widely used for chemical analysis using continuous wave methods but these permanent magnets still weighed hundreds of kilograms and could not be placed on a benchtop. Superconducting magnets were developed to achieve stronger magnetic fields for higher resolution and increased sensitivity. However, these superconducting magnets are expensive, large, and require specialized building facilities. In addition, the cryogens needed for the superconductors are hazardous, and represent an ongoing maintenance cost.[unreliable source?] As a result, these instruments are usually installed in dedicated NMR rooms or facilities for use by multiple research groups.
Since the early 2000s there has been a renaissance in permanent-magnet technology and design, with advances sufficient to allow development of much smaller NMR instruments with useful resolution and sensitivity for education, research and industrial applications. Samarium–cobalt and neodymium magnets in particular are strong enough for instruments up to 90 MHz. These smaller designs, which operate with magnet temperatures from room temperature to 45oC, allow instruments to be made small enough to fit on a lab bench, and safe to operate in a typical lab environment. They require only single phase local power and with UPS systems can be made portable and can perform NMR analyses at different points in the manufacturing area.
Disadvantage of Small-Size Magnets and Method to Overcome It
One of the biggest disadvantages of low-field (0.3-1.5T) NMR spectrometers is the temperature dependence of the permanent magnets used to produce the main magnetic field. For small magnets there was a concern that the intensity of external magnetic fields may adversely affect the main field, however the use of magnetic shielding materials inside the spectrometer eliminates this problem. The currently available spectrometers are easily moved from one location to another, including some that are mounted on portable trolleys with continuous power supplies. Another related difficulty is that currently available spectrometers do not support elevated sample temperatures which may be required for some in-situ measurements in chemical reactions.
A recent paper suggests that a special experimental setup with two and more coils and synchronous oscillators may help overcome this problem  and allow it to work with unstable magnetic field and with affordable oscillators.
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 sensitivity and 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 not have access to super-conducting spectrometers (e.g., beginning undergraduates or small-businesses).
Many automated applications utilizing multivariate statistical analyses (chemometrics) approaches to derive structure-property and chemical and physical property correlations between 60 MHz 1H NMR spectra and primary analysis data particularly for petroleum and petrochemical process control applications have been developed over the past decade.
Available Benchtop NMR Spectrometers
Development of this new class of spectrometers began in the mid-2000s leaving this one of the last molecular spectroscopy techniques to be made available for the benchtop.
New Zealand- and Germany-based Magritek's Spinsolve instrument, operating at 80 MHz, 60 MHz and 42.5 MHz, offers very good sensitivity and resolution less than 0.5 Hz and weighs under 73 kg, 60 kg and 55 kg respectively. The ULTRA model has an even higher resolution of 0.2 Hz with a lineshape of 0.2 Hz/ 6 Hz/ 12 Hz comparable to high field NMR specifications. 1D Proton, 19F Fluorine, 13C Carbon and 31P Phosphorus as well as T1 and T2, and 2D HETCOR, HMBC, HMQC, COSY and JRES spectra can be measured. The magnet is stabilised with an external lock, which means it does not require the use of deuterated solvents. Samples are measured using standard 5 mm NMR tubes and the spectrometer is controlled through an external computer where standard NMR data collection and processing takes place.
In 2009, picoSpin, based in Boulder, Colorado, launched a small (7 x 5.75 x 11.5”) 45 MHz spectrometer with good resolution (< 1.8 Hz) and mid-to-low-range sensitivity that weighs 4.76 kg (10.5 lbs) and can acquire a 1D 1H or 19F spectrum. picoSpin was acquired by Thermo Fisher Scientific in December 2012, and subsequently renamed the product Thermo Scientific picoSpin 45. Instead of the traditional static 5 mm NMR tubes, the picoSpin 45 spectrometer uses a flow-through system that requires sample injection into a 1/16” or 1/32” capillary. Deuterated solvents are optional due to the presence of a software lock. It needs only a web browser on any external computer or mobile device for control as the spectrometer has a built-in web server board; no installed software on a dedicated PC is required. In August 2013 a second version was introduced, the Thermo Scientific picoSpin 80, that operates at 82 MHz with a resolution of 1.48 Hz and weighs 19 kg.
Calgary, AB, Canada based Nanalysis Corp offers two NMReady 60 MHz benchtop NMR instruments that weigh 25 kg. The spectrometers are all-in-one units, controlled by a touchscreen computer that is contained within the same enclosure as the magnet. The NMReady 60e model performs 1D 1H and 19F experiments as well as T1, T2, JRES and COSY. In addition, the NMReady 60Pro is a dual nuclei instrument that can also be tuned to 13C, 31P, 11B, and 7Li and perform DEPT, HSQC, HMBC, with options for additional experiments like signal suppression. The magnet is stabilized with an internal 2H lock so the use of deuterated solvents is recommended but not required. These spectrometers offer resolution < 1.2 Hz, use standard 5 mm NMR tubes, and are compatible with most third-party software suites.
In 2013, Oxford Instruments launched a 60 MHz spectrometer called Pulsar which is a high resolution (<1 Hz), benchtop, cryogen-free NMR analyser. It incorporates a 60 MHz rare-earth permanent magnet. 19F or 1H measurements are made on a single probe. The magnet and spectrometer are in two separate boxes with the magnet weighing 149 kg and the electronics weighing 22 kg. Pulsar requires a standard mains electrical supply and uses standard 5mm NMR tubes. Instrument control comes from the SpinFlow workflow package, while the processing and manipulation of data is achieved using Mnova NMR software from Mestrelab.
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