When evaluating a benchtop NMR instrument there are several key performance characteristics that have a very significant impact on how the instrument will perform in your lab. These key performance characteristics are:
The spectral resolution, which is directly related to the magnet and determines the width or shape of the NMR lines (often called lineshape), and in turn the ability to separate signals in the spectrum
The sensitivity which determines the limits of detection (LOD) and quantitation (LOQ), and in turn how long sample measurements take
The stability of the magnet and instrument over time, which impacts the ability to make longer measurements, and the overall ease of use of the spectrometer
Before I examine these performance characteristics in more detail, it’s worth emphasising from the outset that the biggest aspect of a benchtop NMR system’s design that dictates how well the system performs is the “quality” of the magnetic field produced by the magnet. By “quality” we are referring to how uniform the magnetic field is over the sample volume, often referred to as the B0 homogeneity. To illustrate the importance of this key aspect of magnet design, Figure 1 shows a series of spectra collected under varying degrees of B0 homogeneity.
Figure 1. Effect of static magnetic field (B0) homogeneity on the NMR spectrum. As the homogeneity gets worse, both the resolution and sensitivity are negatively affected. The series of spectra on the left are shown at the same scale and show how the 2 peaks can no longer be resolved, and the signal intensity decreases. The series on the right is the same spectra shown with the peak intensities normalised which how the signal-to-noise ratio is decreasing (the noise is increasing) when the field is less homogeneous.
Dr Catherine Santai is an Associate Professor of Chemistry & Biochemistry and Program Lead of the Integrative Sciences program at Harrisburg University of Science & Technology. The program utilizes a number of analytical techniques teaching undergraduates about their use, giving them the experience ahead of entering research or industrial roles in later life. So far, the Magritek 60 MHz Spinsolve Benchtop NMR Spectrometer has been used in the Organic Chemistry and Biochemistry laboratory sessions. These provide invaluable hands-on lessons about NMR techniques and analysis of a variety of compounds. NMR is used alongside FTIR (Fourier transfer infrared), AAS (atomic absorption), UV-VIS (ultraviolet – visible) and fluorescence spectroscopies.
At the Achema conference this week in Frankfurt we have a joint booth with Corning Advanced Flow Reactors. We are running a live Spinsolve Benchtop NMR reaction monitoring setup in combination with a Corning Advanced Flow Reactor. If you are Achema, please come and visit us at our joint booth with Corning AFR Hall 9.2 / Booth A32 and see the powerful combination of Corning Flow Reactors and Magritek Benchtop NMR.
The best-selling Magritek 80 MHz Spinsolve benchtop NMR is also available with the X-channel set to 31-Phosphorus. 31P NMR spectroscopy is routinely used by chemists to determine structure and measure impurities. When looking for impurities it is important to know the lower limit of detection (LOD). The LOD is the lowest concentration of a molecule that can be distinguished from the absence of that molecule.
In NMR it is the sensitivity that determines the LOD for a particular substance, and the higher magnetic field of an 80 MHz magnet brings a number of advantages including increased sensitivity. We thought it would be interesting to determine the LOD for tetramethylphosphonium chloride with different acquisition times. We defined the LOD as an NMR peak with signal height that was 3 times the noise level, i.e. an SNR of 3.
There are two naturally occurring NMR active nuclei of Boron, 11B (80.1%) and 10B (19.9%). Both nuclei are quadrupolar with spin of greater than ½. 11B has a spin of 3/2 and 10B is spin 3. In terms of sensitivity, 11B is the better nucleus to use as it has a higher natural abundance, a higher gyromagnetic ration, and a lower quadrupole moment. A Spinsolve benchtop NMR spectrometer with a proton frequency of 60 MHz can be configured to measure the 11B NMR signal which has a frequency of 19.2 MHz.
The 11B NMR spectrum of a 0.23 M solution Sodium tertraphenylborate in MeOH-d4 is shown below. The first spectrum shows the excellent sensitivity of Spinsolve using just 8 scans to acquire a spectrum in only 16 seconds.
Magritek is delighted to announce its participation in EUCOMC XXIII in Helsinki, Finland 16-20 June 2019. You will have the chance to talk to our scientists about the capabilities and applications of the Spinsolve Benchtop NMR system. We are looking forward to seeing you at this event!
In my first two posts on using 1D and 2D NMR methods to assign the peaks of quinine (Figure 1), I looked at the 1H and 13C spectra.
Figure 1. Structure of quinine
In this post, I’m moving on to look at the 1H-13C HSQC spectrum. It’s worth spending a brief moment recapping what HSQC is all about and what info it gives you. In a nutshell, the HSQC experiment correlates proton and carbon chemical shifts over one chemical bond. Another way to put this is that a cross-peak in an HSQC spectrum says, “The proton with this chemical shift is directly attached to the carbon with that chemical shift”. By convention, HSQC spectra are presented with 1H shifts along the horizontal axis and 13C shifts along the vertical axis.
Some variants of HSQC also encode into the phases of the cross-peaks additional information about how many hydrogen atoms are attached to each carbon atom. This is sometimes referred to as multiplicity or DEPT editing. In the multiplicity-edited HSQC spectrum, it is conventional for the CH and CH3 groups to have positive phase, and the CH2 groups to have negative phase, just as in a DEPT-135 spectrum. Figure 2 shows the multiplicity-edited HSQC (“HSQC-ME”) spectrum of our 400 mM quinine sample. The CH2 signals are shown in blue and the CH and CH3 signals in red.
The permitted hydrocarbon content of discharged water from offshore oil and gas exploration is becoming increasingly limited by more stringent legislation. This creates the demand for measurement methods that are sensitive enough to detect contaminants at ppm level, but also compact and robust to field conditions. The group of Professor Mike Johns at the University of Western Australia in Perth has developed a benchtop NMR method to quantify the hydrocarbon content in water at the ppm level.