Chemical-shift referencing and resolution stability in gradient LC–NMR (acetonitrile:water)

Chemical-shift referencing and resolution stability in methanol:water gradient LC–NMR

“…All of these issues make referencing more difficult to rigorously control. We previously explained these issues in detail and showed how these issues (and several others) are manifested when acquiring LC-NMR data with solvent mixtures of acetonitrile and water [1]. Here we show how similar effects can be observed in mixtures of methanol and water.…”

“…This figure shows that the hydroxyl resonance moves as much as 0.1 ppm relative to the methyl resonance during the 0-to-100% solvent gradient. This is much less than the 2.4-ppm movement seen in our previous study for the water resonance relative to acetonitrile in an acetonitrile:water (0-100%) solvent gradient [1]. This striking difference explains some of the differences Fig.…”

“…It is also influenced by how much the changes in solvent composition move each resonance (which is not under user control). We used the Scout-Scan technique [1,9,11] to automatically re-optimize these frequencies for every increment (spectrum) of the pseudo-2D data acquired during on-flow LC-NMRÃ experiments. The Scout-Scan technique first takes a small-tip-angle 1 H spectrum without using solvent suppression, and then analyzes that spectrum both to set the transmitter on the desired resonance and to calculate the resulting frequency offset(s) for the solvent signal(s) to be suppressed.…”

“…The Scout-Scan technique first takes a small-tip-angle 1 H spectrum without using solvent suppression, and then analyzes that spectrum both to set the transmitter on the desired resonance and to calculate the resulting frequency offset(s) for the solvent signal(s) to be suppressed. Next, it creates a shaped pulse that excites all these resonances, and then resets the parameters to do a signal-averaged solvent-suppressed 1 H spectrum and starts acquisition. The entire Scout-Scan process takes just a few seconds.…”

“…We previously evaluated how the use of acetonitrile:water as a mobile phase affects the NMR methodology needed for running solvent-gradient LC-NMRÃ experiments [1]. (The term ''LC-NMRÃ" is used here to generally refer to all the various hyphenated permutations of the technique [i.e., LC-NMR, LC-NMR-MS, and LC-UV-NMR-MS, etc.]…”