Benchtop NMR spectrometer

A 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 benchtop.^[1] Instead of requiring dedicated infrastructure, rooms and extensive installations these 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).

Magnet development

This first generation of NMR spectrometers used large permanent magnets weighing hundreds of kilograms or more. These were replaced by electromagnets and later by superconducting magnets to achieve stronger magnetic fields for higher resolution and sensitivity studies. However, these instruments are expensive, large, and require specialized building facilities.^[2] In addition, the cryogens needed for the superconductors are hazardous,^[3] and represent an ongoing maintenance cost.^[4][5] 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,^[6] with advances sufficient to allow development of much smaller NMR instruments with useful resolution and sensitivity for education, research and industrial applications.^[7] 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 45^oC, 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 to be fully portable and can perform NMR analyses at different points in the manufacturing area.

Applications

NMR spectroscopy can be used for chemical analysis,^[8][9] reaction monitoring,^[10] 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.^[11] 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^[12] 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.^[13][14] A large number of applications are available online from Process NMR Associates.

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.

Spinsolve

New Zealand- and Germany-based Magritek's Spinsolve instrument, operating at 42.5 MHz, offers very good sensitivity and resolution less than 0.7 Hz and weighs 55 kg. 1D Proton, 19F Fluorine and 13C Carbon as well as T1 and T2, and 2D HETCOR, HMBC, HMQC, COSY and JRES spectra^[15] 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.

picoSpin

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) ^[16] and can acquire a 1D 1H or 19F spectrum. picoSpin was acquired by Thermo Scientific in December 2012, and subsequently renamed the product picoSpin 45.^[17] 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 picoSpin 80, that operates at 82 MHz with a resoluton of less than 1 Hz and weighs 19 kg.

NMReady

Nanalysis NMReady offers a 60 MHz benchtop NMR instrument that weighs 20 kg.^[18] The spectrometer is an all-in-one unit, controlled by a touchscreen computer that is contained within the same enclosure as the magnet. 1D 1H and 19F experiments and typical T1, T2 relaxation experiments can be measured. The magnet is stabilized with an internal 2H lock which requires the use of deuterated solvents in the sample. This spectrometer offers good sensitivity and resolution < 2 Hz and is compatible with standard 5 mm NMR tubes, sample preparation and data collection.

Pulsar

In 2013 Oxford Instruments launched a 60MHz spectrometer called Pulsar. Pulsar is a high resolution (<1 Hz), benchtop, cryogen-free NMR analyser. It incorporates a 60MHz rare-earth permanent magnet. 19F or 1H measurements are made on a single probe. 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.^[19]

Fourier 60

The Fourier 60 is a benchtop NMR spectrometer, sold by Bruker, operating at 60 MHz. Probe heads are available for ¹H, ¹ H/¹³C, and ¹⁹F. The spectrometer accepts standard 5 mm OD NMR sample tubes and weighs 145 kg, so although more compact than a superconducting spectrometer, it pushes the boundaries of what can be considered 'benchtop'.^[20]

References