Chemistry Reference
In-Depth Information
The selectivity of the solvent-impregnated resins generally follows from the known
selectivity of the liquid/liquid extraction system, although exceptions have been
noted. Horwitz, Dietz, and coworkers at Argonne National Laboratory pioneered
the development of these extraction chromatographic materials and their analytical
uses.
26-29
The weight distribution ratios,
D
w
, and column capacity factors,
k
′, for
radionuclides and potential interferences are well characterized in the literature.
The weight distribution ratio,
D
w
, is the ion quantity per weight of resin divided by
the ion quantity per volume of solution at equilibrium, typically in milliliters per
gram. For radioactive isotopes, this is determined in a batch contact experiment
where the activity of the solution is known before,
A
o
, and after equilibrating with
the resin material,
A
s
, according to
D
w
= ((
A
o
-
A
s
)/
W
)/(
A
s
/
V
), where
W
is the weight
of the resin and
V
is the volume of the solution. The capacity factor,
k
′ =
D
(
V
s
/
V
m
), which indicates the number of free column volumes to peak maximum, is
given by the volume distribution ratio,
D
, times the phase volume ratio, where
V
s
is the volume of the stationary phase and
V
m
is the volume of the mobile phase.
30,31
The volume distribution ratio,
D
, as normally determined in liquid-liquid extrac-
tion, can be obtained by dividing the milliliter of organic extractant or solution per
gram of resin into
D
w
. The validity of the relationships between
D
,
D
w
, and
k
′ have
been verified experimentally.
4
Extraction chromatography can be differentiated from other forms of chroma-
tography as follows. Classical chromatography utilizes differences in distribution
ratios to achieve separation as species migrate at different rates down a column.
Ion chromatography binds all appropriately charged ions, which are then gradually
eluted by increasing the eluting power of the mobile phase. Extraction chroma-
tography selectively binds particular species or groups of species according to the
complexants or extractants used in the immobilized organic phase. After the wash
step, the selected species are abruptly released by changes in the mobile phase or by
carrying out reaction chemistry (e.g., redox reactions) on the sorbed species. Thus,
this separation format relies on the selective uptake and release properties of the
separation material, rather than on the high chromatographic efficiency of a long
separation column.
Extraction chromatographic methods generally provide columns with high capac-
ities, tolerate high levels of potential interferences and a variety of sample matrixes,
allow flexibility in sample loading conditions, and work at low pressures. These are
the features that are well suited for radiochemical separations in general and auto-
mated radiochemical separations in particular.
9.3
AUtoMAtIon APPRoACHes
9.3.1 f
l u i d i C
m
e t H o d S
Fluidic approaches move samples, reagents, and eluents from place to place entirely
through a system of pumps, valves, and tubing. Fluidic methodology from the field of
flow injection (FI) analysis has been adapted to the needs of automated radiochem-
istry. In its original form, FI used a multichannel peristaltic pump and an injection
valve in a continuous forward flow paradigm to mix the sample with reagents and
