Processing: phase correction

The previous post mentioned that the fourier transform produces complex data - a set of "real" data points and a set of "imaginary" data points. Both datasets are equally legitimate - real and imaginary are simply labels to distinguish the two orthogonal datasets. Two datasets 90^o out of phase with each other are collected to enable quadrature detection, the discrimination of positive and negative frequencies.

Processing: the fourier transform

Rather than scanning a frequency range, like the original continuous wave spectrometers, modern NMRs use a radio frequency pulse to excite a range of frequencies at once then monitor the decay of the resultant signal with time. After acquiring the signal, the fourier transform is used to process it from the time domain to the more familiar frequency domain. Pulsed fourier transform NMR is much faster than continuous wave NMR, enabling more extensive signal averaging and thus increasing sensitivity per unit time.

How tuning and matching impacts sensitivity

Tuning and matching is the process of optimizing the frequency and resistance of the probe to suit your sample. Our probes need to be tuned and matched manually by turning the rods at the bottom of the probes. With the 1.7 mm probe the sample volume is small enough that the tuning and matching does not vary much from sample to sample and you can get away without doing it. For the 5 mm probe, however, the larger volume results in a significant difference in the tuning and matching when switching between aqueous and organic solvents. To automate data acquisition, ideally we would use an automated tune and match module to enable the computer to  adjust the rods, but these are expensive. As a workaround we could just skip the tuning and matching and accept a loss in sensitivity. I decided to measure the loss in sensitivity by acquiring spectra of organic solvents when the probe was tuned and matched for an aqueous sample.

1D 13C experiments

There are several different types of 1D ¹³C experiments available on our spectrometers. The spectra shown below were recorded using 100 mg/ml of cholesteryl acetate in CDCl₃. All spectra were recorded with 128 scans and are plotted with the same vertical scale to enable comparison of the sensitivity.

Strong coupling

Several samples recently run on the SSPPS NMR spectrometers showed spectra that could only be explained by invoking strong coupling. Strong coupling is a distortion of the relative intensities of the lines in a multiplet due to the scalar coupling approaching the chemical shift difference. The lines in the multiplet closest to the coupled resonance increase in intensity, while the lines on the far side decrease. This makes the multiplet appear to "lean" towards its coupling partner. Some examples can be found on Hans Reich's pages at the University of Wisconsin¹, and a very thorough description of strong coupling and its origin can be found in Neil Jacobsen's book².

Signal-to-noise, pulse width and the relaxation delay

In the previous post I reported data showing signal-to-noise increased in a less than linear fashion with increasing number of scans. In fact, the increase was much less than the expected square root increase. To further probe this relationship I have recorded a series of experiments with an increasing number of scans and measured the signal-to noise in each.

Scans vs slices

When running a multi-dimensional NMR experiment you have to decide how many scans and how many slices (or rows, or increments) to use. If your time is limited, as it normally is, increasing one means reducing the other. So which is better more scans or more slices?

NOESY and ROESY

Unlike nearly all other NMR experiments, NOESY and ROESY record correlations transferred through space via the nuclear Overhauser effect (noe) instead of using through-bond scalar coupling. This makes NOESY and ROESY experiments useful for determining stereochemistry and is the basis for the solution structure determination of biomolecules.

Indirect detection

The previous post examined ¹³C sensitivity using direct detection of ¹³C. Here I look at obtaining ¹³C chemical shifts using indirect detection.

Carbon sensitivity

I am often asked "Which is the best magnet for acquiring ¹³C spectra?"
The short answer is "The magnet with the 5mm TCI probe".