Chemical Shift Referencing
Chemical shift δ is usually expressed in parts per million (ppm) by frequency, because it is calculated from:
Since the numerator is usually in hertz, and the denominator in megahertz, delta is expressed in ppm.
The detected frequencies (in Hz) for 1H, 13C, and 29Si nuclei are usually referenced against TMS (tetramethylsilane) or DSS, which is assigned the chemical shift of zero. Other standard materials are used for setting the chemical shift for other nuclei.
Thus, an NMR signal that absorbs at 300 Hz higher than does TMS at an applied frequency of 300 MHz has a chemical shift of:
Although the frequency depends on the applied field the chemical shift is independent of it. On the other hand the resolution of NMR will increase with applied magnetic field resulting in ever increasing chemical shift changes.
Read more about this topic: Chemical Shift
Famous quotes containing the words chemical and/or shift:
“We are close to dead. There are faces and bodies like gorged maggots on the dance floor, on the highway, in the city, in the stadium; they are a host of chemical machines who swallow the product of chemical factories, aspirin, preservatives, stimulant, relaxant, and breathe out their chemical wastes into a polluted air. The sense of a long last night over civilization is back again.”
—Norman Mailer (b. 1923)
“The frantic search of five-year-olds for friends can thus be seen to forecast the beginnings of a basic shift in the parent-child relationship, a shift which will occur gradually over many long years, and in which a child needs not only the support of child allies engaged in the same struggle but also the understanding of his parents.”
—Dorothy H. Cohen (20th century)