Page 264 - Radiochemistry and nuclear chemistry

248 Radiochemistry and Nuclear Chemistry

ion exchange resin. Following sorption (e.g. of M+), the metal is eluted from the resin bed
by passage of a solution (eluant) through the column. The eluant may contain a complexing
anion or another metal ion (e.g. M 3+) which displaces M + through competition with it for
positions on the resin. The metals are eluted by complexing agents in an order depending
on their complex formation properties, as e.g. is illustrated for the lanthanides and actinides
in Chapter 16 (Fig. 16.7).
In principle, liquid partition chromatography (LPC) is a liquid-liquid extraction where one
of the liquid phases is stationary and attached to a supporting material, and the other liquid
phase is mobile. It can be carried out with either the aqueous or the organic phase
stationary; in the latter case the technique is referred to as reversed phase LPC. The
aqueous phase can be made stationary by adsorption on silica gel, cellulose powder, etc.
In order to make the organic phase stationary, beads (usually 50 - 200 #m) of PVC, teflon,
KeI-F, etc., are being used.
Reversed phase LPC has been useful in radiochemistry for separating individual elements,
e.g. lanthanides or actinides. It has also been used for separation of macro amounts of
actinides. Instead of using columns in partition chromatography, a sheet of paper may be
used to hold the stationary phase (paper chromatography) or an adsorbent coated on a glass
plate (thin-layer chromatography). This technique has an advantage over column separations
because the positions of the radioactive species are easily identified on the sheet, either
simply by autoradiography (w or by scanning instruments. Paper chromatography is
further described in an example in w and illustrated by Figures 9.7 and 9.8.

9.3. Analytical chemistry

9.3.1. Radiometric analysis

The term radiometric analysis is often used in a broad sense to include all methods of
determination of concentrations using radioactive tracers. In a more restricted sense it refers
to a specific analytical method which is based on a two-phase titration in the presence of
a radioactive isotope. The endpoint of the titration is indicated by the disappearance of the
radioisotope from one of the phases. Figure 9.4 illustrates two cases, (a) the determination
of Ag + in a solution by titration with NaI solution containing 129I- (~/--y tlh 1.57 • 107
y),and (b) the determination of Fe 2+ in an aqueous solution, to which trace amounts of
radioactive 55Fe2+ (EC t,/: 2.73 y) has been added. In case (a) the AgI precipitate is radio-
active but the solution has little radioactivity until all the Ag + has been precipitated. The
activity of the solution is measured by a liquid flow GM-detector (Ch. 8). In the latter case
(b) a two-phase liquid-liquid analytical technique is used (w the titrant contains a
substance (oxine) which extracts Fe(II) from the aqueous to the chloroform phase. The
radioactivity of the organic phase is followed by liquid scintillation (sampling) to determine
the end point of the titration.
Radiometric analysis is simple and rapid. Nevertheless, it is rarely used in analytical
routine work, as a large number of multiple-element "instrumental" techniques are readily
available (though the instruments usually are more expensive). Its most extensive use is for
calibration of other techniques, and in analytical comparative techniques (e. g. environmental