Tuning and matching is the process of optimising the frequency and resistance of the circuit that includes the detection coil. Every sample has a slightly different ionic content and so the probe should be tuned and matched for each sample. Modern NMR probes have automatic tuning and matching devices, but the Skaggs NMR Facility probes do not. Here, I look at the impact of poor tuning and matching on a 1D 1H spectrum. The NMR spectra were collected using a sample of cholesteryl acetate in chloroform-d using a room temperature 5 mm broad band inverse detection probe. The spectra were collected ..read more

Publications arising from work at the SSPPS NMR Facility are tracked on the Facility's Publications webpage. Along with breakdowns of the list by year, lab and journal, this page also reports the Altmetric score for each publication. This is a measure of the attention the article has received, predominantly on social media. To see how the Altmetric score compares with citations I prepared a few graphs. The Altmetric score is calculated from the number of times an article is mentioned in traditional media, on blogs, on Twitter, LinkedIn, Reddit, and various other sources. The score is provided ..read more

In an NMR experiment the acquisition time is the period used to record the signal. Changing this value can affect the quality and appearance of your spectrum. Typically, larger acquisition times are used for one-dimensional spectra, with shorter values used in multidimensional spectra. The impact of using different acquisition times on 1D 1H spectra are shown in this post. To record the spectra a halogenated cholestane in CDCl3 was used. Five spectra were collected with the acquisition time initially set to 0.143 seconds and then doubled for each successive spectrum. The spectra were processe ..read more

The SSPPS NMR Facility routinely uses a 1.7mm micro-cryoprobe that takes capillary NMR tubes with a diameter of 1.7mm instead of the standard 5mm NMR tubes. These capillary tubes require much less sample volume than a standard tube, but exactly how much should be used? And what happens if there is too little? Or too much? To determine the optimum volume for the 1.7mm NMR tubes a series of 1D 1H NMR spectra were recorded with different volumes of methanol-d4. Initially 20μl of methanol-d4 was added and a spectrum recorded, then solvent was added in 5μl aliquots up to a total volume of 65μl. Sp ..read more

The most commonly encountered coupling in NMR spectra is due to 1H, but other nuclei can induce splitting of signals as well. The previous post discussed how coupling to 13C at natural abundance produces the small 13C satellite peaks. The other heteronuclear couplings that are most likely to be observed are due to 19F and 31P. Examples of these in 1D 1H and  13C spectra are shown below. Trimethyl phosphate has three magnetically-equivalent methoxy groups attached to a central phosphate atom. The 1D 1H spectrum taken in D2O (shown below) shows what at first looks like a singlet, as might ..read more

Recently I was asked about the small peaks appearing symmetrically around a large peak in a 1D 1H spectrum. These peaks are called 13C satellites and arise from heteronuclear coupling to the small amount of 13C naturally present. The figure below shows a 1D 1H spectrum of o-dichlorobenzene in acetone-d6. The aromatic ODCB signals appear at 7.57 and 7.35 ppm, the residual undeuterated acetone signal at 2.05 ppm, and the chemical shift reference compound tetramethylsilane (TMS) at 0.00 ppm. I'm not sure what causes the peak at 3.59 ppm. Note the small peaks either side of the TMS signal, these ..read more

The receiver gain is one of the parameters that should be optimised before recording an NMR spectrum. Here I'll explain what the receiver gain is and why it needs to be optimised. When recording an NMR spectrum the coils in the probe detect a continuously varying voltage. This analog signal needs to be digitised, converted to a list of numbers, for processing by the computer. This is done by an analog to digital converter (ADC). The ADC has a limited range so if the analog signal is too large, part of that signal will be lost and fourier transforming such a truncated signal will produce artif ..read more

NMR titrations are often used to demonstrate binding, locate active sites, and measure affinities. By measuring changes in chemical shifts or linewidths as the concentration of one of the binding partners is changed, quantitative data, such as dissociation constants and dissociation rates, may be obtained. In most cases the systems studied involve 1:1 binding. Systems with multiple binding sites are more difficult to analyse but, with some simplifying assumptions, quantitative information, such as stoichiometry, may still be obtained. For a 1:1 interaction between a macromolecule, M, and a li ..read more

The HMBC experiment provides 1H-13C correlations over multiple bonds and is essential for assigning small molecules. The main drawback to the HMBC is that it does not identify the length of the correlation and in some cases may be ambiguous. A recent publication describes a method, called i-HMBC, for distinguishing two bond HMBC correlations from longer ones. The i-HMBC is essentially a standard HMBC experiment recorded with vastly increased resolution in the 1H dimension. The increased resolution allows small differences in the chemical shifts of peaks to be measured. Wang et al report that ..read more

One of the Skaggs NMRs is normally fitted with a 1.7mm cryoprobe. This piece of equipment is essential for our natural products and metabolomics users who have limited amounts of their samples. The capillary sample tubes for the 1.7mm cryoprobe use special shuttles that do not grip the tube. If the probe gets dirty then the tube can fail to enter the top of the probe and instead gets pushed up out of the shuttle. This becomes apparent when the sample fails to produce a lock signal. Repeatedly ejecting and inserting the sample tube can sometimes get the tube to drop into place by luck, but this ..read more