DOSY Experiments - Emory University

nugflag
nugcal_[1-5]
dosyproc
dosybypoints
'n' uses simple mono- or multi-exponential fitting to estimate
diffusion coefficients
'y' uses a modified Stejskal-Tanner equation in which the exponent
is replaced by a power series.
a 5-membered parameter array summarizing the results of the
calibration of non-uniform field gradients. Used if nugflag='y',
requires a preliminary NUG-calibration by the Doneshot_nugmap
sequence. The values are taken from the probe file at the time of
the data acquisition
'discrete' - invokes monoexponential fitting with dosyfit if ncomp=1,
and multiexponential fitting with the external programme SPLMOD
if ncomp>1.
'continuous' invokes processing with the external programme
CONTIN and gives a continuous distribution in the diffusion
domain.
'n' divides the spectrum into individual peaks, creating one crosspeak
for each individual peak found in the 1D spectrum
'y' performs a diffusion fit for every point in the displayed region of
the spectrum that lies above the selected threshold.
4.4 Convection and Convection-Compensation in Diffusion
Experiments
Convection within the sample tube can seriously affect diffusion experiments, in particular at
elevated temperatures. Convection currents are caused by small temperature gradients in the
sample and result in additional signal decay that can be mistaken for faster diffusion.
The convection conditions are described by the Rayleigh-Bénard equation:
R a
gβR
∂T
=
νχ
∂z
4
Ra = 67 for insulating walls
Ra = 216 for conducting
where g is the gravitational acceleration, ν is the viscosity, χ the thermal diffusivity, β the
expansion coefficient of the liquid, R the internal diameter of the NMR tube and ∂T/∂z the
temperature gradient along the sample axis. When the critical Rayleigh number (Ra) is exceeded
convection will occur. Convection typically causes the following anomalies in diffusion
experiments:
Anomalously large diffusion coefficients (D)
D values that are not independent of gradient duration (δ) and the diffusion delay (del)
Stejskal-Tanner plots that show periodicity
Irregular (non-Arrhenius) temperature dependence of D
A simple calculation based on the Rayleigh-Bénard equation indicates that, for a solvent like
chloroform, a temperature gradient of as little as 0.05 K/cm is sufficient to cause convection flow.
In general, larger temperature gradients are needed for more viscous solvents.
In a typical DOSY experiment, a uniform sample flow velocity v introduces a phase modulation of
the signal:
S zi
2 2 2
G ⎟
⎞ zi = S(
0)
exp( −Diγ
δ ( Gzi)
Δ)
* exp( iγδG
Δ))
⎜
⎛ v
⎝ ⎠
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