Page 20 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

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Encyclopedia of Physical Science and Technology En001f25 May 7, 2001 13:58

Analytical Chemistry 559

Four major distinctions can be made between methods of C. Separation Science
activation analysis. They include the type of radiation or
Separation science is a very broad term covering all tech-
particle bombardment used for sample excitation, the type
niques that employ chemical and/or physical methods to
of radiation observed in the final emission process, sample
isolate one or more species from some mixture. These
preparation by isolation of the species of interest, or di-
methods include solvent extraction, distillation, selective
rect sample observation without separation by an energy-
precipitation, crystallization, and chromatography.
discriminating device such as a γ -ray spectrometer.
Neutron activation analysis is the most common tech-
nique employed today, and the activity A induced in a
1. Principles of Chromatography
sample can be related to neutron flux φ and sample half-
life t 1/2 as follows: Chromatography is a very specialized, yet widely appli-
cable technique of separation science by which one can
effectively separate chemically similar substances in com-
A = Nσφ[1 − exp(−0.693t/t 1/2 )],
plex mixtures. There are three distinct physical methods
for attaining separation, yet all are characterized by their
where N is the number of sample nuclei, σ is their neu-
common application of a stationary and a mobile phase.
tron capture cross section, and t is the time of sample
The stationary phase provides the chemical surface, which
irradiation. From this equation, it can be seen that, after a
interacts with the components of the mixture to be sepa-
particular irradiation time, the activity reaches a saturation
rated. The difference in the chemistry of these compo-
level where the rate of formation and decay of the active
nents results in various degrees of molecular bonding or
species becomes equivalent. At any one neutron flux, the
entrapment to the stationary phase. If the mixture is carried
saturation is determined by the characteristic half-life of
across a bed of stationary phase by some mobile phase,
the sample, which consequently establishes the analysis
the molecules experiencing the least interaction with the
time required to attain maximum sensitivity.
stationary phase will tend to advance more quickly than
The most significant analytical feature offered by acti-
those retained on the bed by stronger molecular interac-
vation analysis is its inherent sensitivity, which is suitable tions. This results in the separation of similar species as
for trace analysis to values as small as 10 −12 g for some influenced by differences in migration rates across the sta-
species. However, such sensitivities are attainable for only
tionary bed. The three distinct separation strategies are
some elements, whereas others may require a total mass summarized below and all assume a common stationary
of 10 −4 g before being detectable.
phase.
γ -Ray spectroscopy. The ability to measure pho-
ton energy accurately at this part of the electromagnetic 1. Frontal chromatography. The mobile phase is the
spectrum is used to great advantage in the identification, samplemixture,andasthesamplemigratesdownthesepa-
discrimination, and quantification of both natural or in- ration bed, the least retained species are concentrated near
duced radioactive species. The principle of operation can the leading edge.
be considered as proceeding in three phases. An energetic 2. Displacement chromatography. A sample mixture is
photon interacts with a scintillation crystal–photocathode applied to one end of a stationary phase, and then a mobile
arrangement or a semiconductor material as the first step phase is introduced. The mobile phase selectively binds
in transduction to an electric signal. This electric signal to the stationary phase competing for the same binding
is amplified by conventional electronics so that the output sites as the sample. Separation is established as a relative
signal energy is proportional to that of the original photon. function of the intermolecular interactions of the sample
The second phase involves analysis of the energy of the and mobile-phase species with the stationary phase.
output signal so that it can be sorted with respect to a pre- 3. Elution chromatography. A sample mixture is ap-
assigned energy resolution into a defined energy level, or plied to one end of a stationary phase, which may already
“channel.” The last phase involves counting and display contain mobile phase. The mobile phase does not inter-
of the signals in any one channel for quantitative anal- act significantly with the stationary phase and only carries
ysis. Both multichannel and single-channel instruments the sample mixture through the stationary bed. The differ-
exist and are selected according to cost and flexibility. ences in selective binding of sample mixture components
Multichannel systems can monitor hundreds of discrete withthestationaryphaseretardsmigrationofsomespecies
wavelengths simultaneously and are commonly used for so that separation occurs. This is the most analytically use-
multielement analysis in conjunction with techniques such ful form of chromatography due to its tremendous power
as neutron activation analysis. of separation of very similar species.