Environmental Engineering Reference
In-Depth Information
subatomic particles (protons, neutrons, and electrons) have spin. In some atoms
(e.g., 12C, 16O, 32S) these spins are paired and cancel each other out so that the
nucleus of the atom has no overall spin. However, in many atoms (1H, 13C, 31P,
15N, 19F) the nucleus does possess an overall spin [
12
].
The nuclear magnetic resonance phenomenon can be described in a nutshell as
follows. If a sample is placed in a magnetic field and is subjected to radiofrequency
(RF) radiation (energy) at the appropriate frequency, nuclei in the sample can
absorb the energy. The frequency of the radiation necessary for absorption of
energy depends on three things. First, it is characteristic of the type of nucleus
(e.g., 1H or 13C). Second, the frequency depends on chemical environment of the
nucleus. For example, the methyl and hydroxyl protons of methanol absorb at
different frequencies and amide protons of two different tryptophan residues in a
native protein absorb at different frequencies since they are in different chemical
environments. The NMR frequency also depends on spatial location in the magnetic
field if that field is not everywhere uniform. This last variable provides the basis for
magnetic resonance imaging (MRI), for self-diffusion coefficient measurements,
and for coherence selection—topics which will not be discussed further in this
introductory chapter. For diffusion coefficient measurements and for imaging, the
magnetic field is made to vary linearly over the sample. However, for most
spectroscopic purposes we generally wish the magnetic field to be as homogeneous
as possible over the sample. The homogeneity requirements for NMR spectroscopy
are rather stringent; the magnetic field should vary no more than 10 parts per billion
(ppb) over the sample volume. After absorption of energy by the nuclei, the length
of time and the manner in which the nuclei dissipate that energy can also be used to
reveal information regarding a variety of dynamic processes [
13
].
13.3 Nanoparticle Tracking Analysis (NTA)
with NanoSight
The NTA method measures the diffusion coefficients of individual particles and
builds the distribution one particle at a time. This compares favorably with an
ensemble measurement of the combined light scattering intensity of a population of
particles. Consequently, rather than presenting an ideal curve driven by a range of
assumptions, the result from NTA is a true high resolution.
This provides a series of key advantages of the NTA method: overcomes
intensity-biased results seen in DLS/PCS; size is calculated particle by particle;
direct measurement without modeling or assumptions; an absolute method not
requiring calibration; provides high-resolution measurement of distribution;
requires no information about collection angle, wavelength, or solvent refractive
index; minimal sample preparation, with automation capability and immune to
interference from dirt and aggregates [
14
].
