1.4. TMS as 0 PPM
Although converting to a frequency domain makes interpreting the data simpler it introduces another problem. The position of the signals along the x-axis is dependent on the value of ΔE for those nuclei. One of the factors that affects ΔE is the exact strength of the applied external magnetic field. Different NMR spectrometers have different field strengths. Even two spectrometers designed to produce the same field strength will have small variations (i.e. 500.01 MHz vs. 400.99 MHz). As a result, no two instruments would result in identical positions for the signals along that axis even if the exact same sample were tested in each. This would cause significant issues with communication and structure verification. To counter this organic chemists use specific molecules’ signals as standards. Then, further converting the frequency domain into a parts-per-million (PPM) scale removes variability in positions (Figure 1.9). The result is consistency in values across all spectrometers.
Figure 1.9 – Example of an NMR Spectrum with PPM Domain.
PPM values are unitless but are typically represented using the lowercase delta (δ) symbol followed by the value itself. The position along the axis for each signal is then referred to as the signal’s “chemical shift”.
By far the most common type of NMR spectroscopy chemists use is 1H NMR. This determines values for all protium isotopes in the sample being tested. For reasons of tradition, this is spoken aloud as “proton NMR” rather than “protium NMR”. For 1H NMR spectroscopy the compound used as a standard is tetramethylsilane (TMS; Figure 1.10). This compound produces only one signal, which has its value set as δ0.00 ppm. Crucially, TMS is not added to every sample that is tested on the spectrometer. The machine is calibrated by running data collection for TMS once at its specific magnetic field strength. Then that value is set as zero for all subsequent spectra.
Figure 1.10 – Tetramethylsilane as the Reference Standard Signal at δ0.00 ppm in 1H NMR Spectroscopy.
