CINCH NucWik

This specific exercise is used to highlight several important aspects of nuclear chemistry. Its main purpose is to show the student radioactive equilibrium and the mother-daughter relationship. This can then be discussed in several ways, how this relates to practices in hospitals, and why radon is a concern for many people living on uranium-containing soil.

The students should also learn how to determine the half-life of a nuclide from measurements with applied radiochemistry.

If T_{1/2, mother} ≥ T_{1/2, daughter} – that is, the half-life of the mother is not several orders of magnitude greater than the half-life of the daughter, and the mother decreases significantly during the application period – transient equilibrium is established (Figure 1).

A practical example of transient equilibrium can be a ⁹⁹Mo/^99mTc generator used in medical diagnostics.
^99mTc is eluted from commercially available generators containing ⁹⁹Mo.
With its half-life of 65.92 hours, ⁹⁹Mo remains capable of producing what can be a very pure product for about a week after activation.
^99mTc (T_1/2 = 6.00 hours) is the most widely used radio-tracer in nuclear medicine, not least because it combines well with a number of pharmaceutical substances used for investigation of many diseases.
The half-life of technetium is long enough to allow examination of patients using γ-cameras but short enough to minimize problems related to absorption of radioactivity by the patients.

When ion exchangers are used for analytical purposes, the solution passes through a less permeable ion exchange resin packed in a column. Typically, the column is operated by running the solution through the ion exchanger from top to bottom. To initiate the desired ion exchange process, the ion exchange resin must be in contact with ions that can be retained.
In a dynamic separation setup, the solution to be analysed is fed into the ion exchange column either by gravity flow or at a flow rate controlled by a low-pressure pump. The general separation process for the column method can be described with the following steps:

1. Conditioning
2. Application of sample - adsorption
3. Washing
4. Elution - rinsing with appropriate solution or solvent
5. (Regeneration)

Conditioning
The conditioning step prepares the column material for sorption and involves rinsing the ion exchange resin with an appropriate volume of a solution that ideally has the same chemical composition as the sample solution, except that it does not contain elements from the sample matrix or ions to be separated from each other.
The ion exchanger will thus swell and be brought into equilibrium with this solution.

Adsorption
The second step is the sample application. The method of selective adsorption is based on the choice of suitable adsorption conditions for one element, or for a smaller group of elements that are included in the sample matrix. The goal of this step is to bind desired elements. Oppositely charged ions in the solution bind to ionic groups in the ion exchange medium and become concentrated on the column.
Uncharged particles, or those with the same charge as the ionic group, pass through the column at the same rate as the flow of solution, and are eluted during or just after sample application (depending on the total volume of sample loaded).
To quantitatively separate a solution mixture containing two elements by the method of selective adsorption, the distribution coefficients of the ions adhering to the column and the ions passing through should give a separation factor greater than ~100 (Korkisch, Handbook of ion exchange resins: Their application to inorganic analytical chemistry 1989, 42).

Elution
Once the sample is loaded and the column is washed with the starting solution (so that all particles that were not bound are washed out of the column), it is common to change the elution conditions to wash out the bound analytes. This method is called gradient elution.
Often the analytes are eluted by increasing the ionic strength (salt concentration) of the elution solution or by changing the pH. As the ionic strength is increased, the salt ions (typically Na⁺ or Cl^-) compete with the bound components for charges on the surface of the exchanger medium, and one or more of the bound species begin to move down the column. Components with the lowest net charge at the selected pH will be the first to be eluted from the column as the ionic strength increases.

In this exercise, ^234mPa is milked from a generator system made with a uranyl acetate (UAc) solution containing naturally occurring uranium (uranium dioxide, UO₂²⁺).
Naturally occurring uranium is a natural composition of ²³⁸U (99.3%), ²³⁵U (0.7%), and trace amounts of ²³⁴U.

²³⁴Th (T_1/2 = 24.1 days), which is produced by the disintegration of ²³⁸U, is loaded onto a cation exchange column. Ideally, the UAc solution should be at least 240 days old to allow maximum ingrowth of ²³⁴Th.
²³⁴Th disintegrates to ^234mPa (T_1/2 = 1.16 min), and the protactinium has reached maximum ingrowth after about 12 minutes.
Once the generator is made, it can be used repeatedly to, for example, measure the half-life of ^234mPa.

UAc (or any other suitable uranyl salt), where ²³⁴Th is in radioactive equilibrium, is dissolved in 1 M HCl and washed through a cation exchange column. The hexavalent U is almost not adsorbed to strong cation exchangers from a 1 M HCl solution will therefore pass through the column undisturbed.

The tetrapositive thorium ion is more adsorbed on cation exchange resins than most other ions. This fact makes it possible to adsorb trace amounts of thorium from large solution volumes to a small amount of resin material. The following equation describes how thorium attaches to a sulfonated cation exchanger:

Th⁴⁺+ 4 R(SO₃^-X⁺) ↔ Th(RSO₃^-)₄ + 4 X⁺.

1. Vacuum is adjusted to 80-400 mbar.
   • This step is necessary to create vacuum for suction of the solvents through the system
2. Column is preconditioned with 2 mL 1M HCl.
3. ²³⁴Th is attached to the column by application of 1 mL UAc.
4. Column is washed with 2 mL 1M HCl to flush out the remaining U.
   • Some of the functionality is locked until this step is complete.
5. To check if there are still some U remaining in the eluate, add one drop of potassium ferrocyanide (K₄[Fe(CN)₆]) into the analysis chamber.
   • Potassium ferrocyanide will react with uranium to form uranyl ferrocyanide – a deeply red-colored solution. This reaction is often used for qualitative identify the presence of uranium.
6. If uranyl ferrocyanide is formed in the analysis chamber, continue flushing with HCl until the test is negative.
7. When the test gives no result and the contents of the analysis chamber have been emptied into a waste container, chloride ions from the previous step are washed out with 2 ml of 10% citric acid.
8. Presence of chloride ions is tested for one drop of silver nitrate.The test is carried out in the analysis chamber.
   • If chloride ions are present, a precipitate will form: AgNO₃ (aq) + Cl^- (ag) → AgCl (s) + NO₃^- (aq). Continue washing.
9. When the column is free from uranyl and chloride ions, ^234mPa ingrowth will begin automatically. The detector on the right side of the column registers the ingrowth, and the ingrowth curve can be observed on a plot. A countdown from 12 minutes is started (approx. 10 half-lives for ^234mPa) to allow the ingrowth to reach a saturation.
   • In the simulated program, you can choose to skip 2 minutes per click by using «skip»-button.
10. Adjust the pressure to a value between 800-2100 mbar (adjusted at the top manometer). Which value within the given range is selected does not matter.
    • This step is necessary to push the content of the column into the counting cup.
11. After 12 minutes, citric acid is added to the column, and the contents are then emptied into the counting cup. The counting cup is sent to the detector.
    • The time you spend between emptying the column and starting the disintegration measurement is recorded and the delay is reflected in the achieved disintegration rate measured by the detector (and in the measurement table).