New methods in mixture analysis
The quest for a complete understanding of mixtures
is a challenge which has stimulated the development of several
techniques. One of the most powerful NMR-based techniques is known
as Diffusion-Ordered SpectroscopY (DOSY), in which it is possible
to distinguish the NMR spectra of chemical species with different
hydrodynamic radii, i.e. with different self-diffusion
coefficients. It allows the study of intact mixtures, providing
information on the interactions within the mixture and saving time
and money compared to other techniques. Unfortunately, DOSY is not
very effective when signals overlap and/or the diffusion
coefficients are very similar. This drawback has led to the
development of new methods to overcome this problem. The present
investigation is focused on developing some of these.Most DOSY
datasets show multiplet phase distortions caused by J-modulation.
These distortions not only hinder the interpretation of spectra,
but also increase the overlap between signals. The addition of a
45o purging pulse immediately before the onset of acquisition is
proposed as a way to remove the unwanted distortions.Most DOSY
experiments use 1H detection, because of the higher sensitivity
which is generally achieved. However, acquiring spectra with other
nuclei such as 13C can reduce overlap problems. Two new sequences
have been developed to maximize the sensitivity of heteronuclear
DOSY experiments.In order to increase resolving power, it is also
possible to incorporate another variable into diffusion experiments
as a further dimension. If this results in an approximately
trilinear dataset (each dimension varying independently), it is
possible to extract physically meaningful information for each
component using multivariate statistical methods. This is explored
for the cases where the new variable is either the relaxation
behaviour or the concentration variation (which can be measured
during a reaction or in a set of samples with different
concentrations for each component). PARAllel FACtor (PARAFAC)
analysis can obtain the spectra, diffusional decay and relaxation
evolution or kinetics for each of the components.In a completely
different approach, the separation power of liquid chromatography
has been combined in a novel way with the NMR potential for
elucidating structures. NMR has been used previously as a precise
way to measure average flow velocities, even in porous media. Using
this capability to detect the different average velocities of
solutes that occur in chromatographic columns ought to provide a
new way of analysing mixtures with the same potential as LC-NMR,
but faster and more simple. In such a flow system, a
chromatographic column is introduced into the NMR probe and a 2D
dataset is acquired and Fourier transformed to obtain the velocity
distribution for each of the detected NMR signals.