Reference chemical shifts

reference corrects the chemical shift referencing of a spectrum. Give the dimension and an old => new pair: where a reference signal currently appears, and where it should appear.

using NMRTools, CairoMakie
spec = exampledata("2D_HN")

One dimension

moved = reference(spec, H1, 4.72 => 4.70)
first(dims(spec, F1Dim)), first(dims(moved, F1Dim))

(11.07119, 11.05119)

The dimension can be given as a nucleus (H1, N15, …), a dimension type (F1Dim, …) or a number:

reference(spec, H1, 4.72 => 4.70)
reference(spec, F1Dim, 4.72 => 4.70)
reference(spec, 1, 4.72 => 4.70)

The axes move, not the data:

fig, ax, _ = nmrplot(spec[:, Near(121.0)]; title="before and after referencing", xlims=(7.5, 9))
nmrplot!(ax, moved[:, Near(121.0)])

The applied offset is recorded in each dimension's :referenceoffset metadata.

Indirect referencing

By default, referencing one dimension also references every other frequency dimension, using IUPAC Xi ratios. Correcting the ¹H reference of an HSQC therefore corrects its ¹⁵N axis too:

moved[F2Dim, :referenceoffset]

-0.022631555309522773

Pass indirect=false to reference only the given dimension:

only1h = reference(spec, H1, 4.72 => 4.70; indirect=false)
only1h[F2Dim, :referenceoffset]

0.0

The Xi ratios depend on the reference compound: DSS for aqueous samples, TMS for organic solvents. NMRTools decides which from the solvent in the spectrum's metadata, or you can choose:

reference(spec, H1, -0.12 => 0; aqueous=true)     # DSS
reference(spec, H1, -0.12 => 0; aqueous=false)    # TMS

The ratios are listed in Nuclei, coherences and Xi ratios.

Several dimensions

Give a vector of dimensions and a vector of pairs:

reference(spec, [H1, N15], [4.72 => 4.70, 120.0 => 118.5])

Water referencing

For aqueous samples, reference(spec) references to the water signal, whose chemical shift depends on temperature:

δ(H₂O) = 7.83 − T / 96.9

where T is the temperature in kelvin, read from the spectrum's metadata unless given:

watershift(spec[:temperature])

4.9713952528379775

reference(spec)                           # temperature from metadata
reference(spec; temperature=298.15)       # temperature in K

Bruker spectrometers typically set the water signal to 4.7 ppm when locking, so the correction is applied relative to this default.

In a pipeline

loadfid("expt/1") |> autoprocess |> reference(H1, 4.72 => 4.70)