Studies on the separation of 99mTc from large excess of molybdenum
National Centre of Nuclear Research, Radioisotope Centre POLATOM, Otwock, Poland
[Received 12 VI 2015; Accepted 16 VII 2015]
Abstract
BACKGROUND: Due to aging and unexpected prolonged shutdown of nuclear reactors producing 99Motor 99Mo/99mTc generators it was necessary to explore the alternative methods of technetium-99m production. The first choice were the accelerators. Three years ago IAEA (International Atomic Energy Agency) initiated the Coordinated Research Project „Accelerator-based Alternatives to Non-HEU production of Mo-99 /Tc-99m” aimed at direct production of 99mTc in proton accelerators using the 100Mo(p,2n)99mlTc reaction. POLATOM is participating in this enterprise together with the Heavy Ion Laboratory of Warsaw University and the Institute of Nuclear Chemistry and Technology.
MATERIAL AND METHODS: 99Mo/99mTc solutions and pure 99mTc used for generators production or milked from ready to use generators were used in experiments. Commercial chromatographic and laboratory-prepared columns were used for separation. The peristaltic pumps were used for solutions delivery onto the columns. Radioactivity of eluted 99Mo and 99mTc was measured using high resolution gamma spectrometry or ionisation chamber in case of high radioactivity. For separation, three different chromatographic methods were used, one based on ion exchange and two on extraction.
RESULTS: Synthetic mixtures simulating the real solutions were used. 99mTc is quantitatively bound in the Dowex-1×8 column whereas molybdenum is only slightly retained and totally rinsed with 2M NaOH. 99mTc is eluted with TBAB. The elution yield has been reproducible and amounted to 78%. The AnaLig Tc-02 resin column was used for 99mTc retention. Residual Mo was removed by rinsing with 2M NaOH and 99mTc eluted using small volume of water. The recovery was equal to about 85%. Using C-18 column coated with PEG over 80% of 99mTc was recovered in about 50 mL of water. The reduction of volume was necessary.
CONCLUSIONS: The recovery of 99mTc was the highest using AnaLig Tc-02 resin. Time of 99mTc separation is the shortest for AnaLig Tc-02 resin and it is not higher than 100 minutes and it can further be shortened.
KEY words: 99mTc, molybdenum, Dowex-1×8, TBAB, AnaLig Tc-02, PEG, 99mTc separation
Nuclear Med Rev 2015; 18, 2: 65-69
Background
The 99mTc is the most frequently used isotope for medical diagnosis. From the about 30 million examination procedures performed yearly about 95% apply this isotope. Practically, till now the total supply comes from the 99Mo/99mTc generators in which 99Mo is the fission product of 235U. There are worldwide no more than 6 nuclear reactors producing significant amounts of 99Mo covering these needs. The reactors are mostly approaching the end of their production lives and the reliable delivery of 99Mo can be seriously distorted in coming years. There are alternative methods of 99Mo production. One of the most promising is the use of proton accelerators which are already quite widely used in hospitals for production of such isotopes as 18F, 11C, 13N, 150.99mTc can abundantly be produced in the nuclear reaction 100Mo(p,2n)99mTc. This method of 99mTc production is known since 1971 [1]. A number of studies on the cross section of this reaction has been carried out [2-6]. The simultaneous production of undersirable Tc isotopes have also been taken into account [7]. The required 99mTc product using protons of energy in the range from 16 to 20 MeV can constitute about 80%. The rest is long lived 99Tc (T1/2 = 2.1 × 105 years).
First separations of Tc from other elements has been undertaken in 1960s [8, 9]. This topic still requires improvement. Good summary of separation methods is given in [10]. There is a number of patents on 99mTc separation [11-14].
In last few years several papers, mostly from Canadian laboratories, were published on experimental production of 99mTc and its separation, even in curie quantities [15-17]. The 99mTc with its half live of 6 hours must be quickly separated from the bulk of molybdenum. The irradiated 100Mo has to be dissolved in the shortest possible time and 99mTc separated. It is expected that the cyclotron production of 99mTc will be implemented in Poland in near future.
Materials and methods
Materials and instrumentation:
- sodium molybdate, ammonium carbonate, molybdenum trioxide, tetrabutyloammonium bromide (TBAB), hydrogen peroxide 30% solution, Dowex-1×8 resin (Cl-form, 100-200 mesh); analitycal grade, Sigma-Aldrich;
- dichloromethanefor HPLC, sodium hydroxide chemical grade; Merck;
- polyethylene glycols PEG-2000, PEG-3000, PEG-4000, PEG-6000, chemical grade; Fluka AG;
- C-18 column OASIS HB, Waters;
- AnaLig Tc-02 resin (60-100 mesh); IBC Advanced Technologies;
- 0.9% sodium chloride solution for injection; Polpharma;
- generator 99Mo/99mTc, (Polgentec, POLATOM);
- Na299MoO4 solution, NTP South Africa;
- acidic alumina column Pak Vacc 1cc (100 mg); Waters;
- Bond Elut Reservoir with two 20 μm polypropylene frits, volume 1.5 mL; Varian;
- Dose Calibrator CRC-55tR, Capintec;
- HPGe (High Purity Germanium) gamma spectometer GX1520, Canberra;
- MasterFlex Peristaltic Pump, Cole Palmer;
- Total Organic Carbon (TOC); Sievers 900 GE Healthcare.
Separation methods
Following three methods have been evaluated for 99mTc separation from large molybdenum excess:
- anion exchange on Dowex-1×8 [15, 18];
- adsorption on AnaLig Tc-02 resin [17];
- C-18 column modified with PEG (ABEC procedure) [15, 19].
Separation on Dowex-1×8
After soaking of 100 to 140 mg of resin in 5 mLof 0.5 M NaOH, the suspension was introduced in a column. Before depositing the solution onto bed, the column was washed with 5 mL of 0.5 M NaOH. 10 mL of Na2MoO4 80 mg/mL solution containing 10 MBq of 99Mo in equilibrium with 99mTc was introduced onto the column with a flow rate in the range from 0.6 to 1.0 mL/min. Eluate from column was collected in 1 mL portions. Activity of 99Mo in collected aliquots was measured in a HPGe gamma spectrometer. Before the 99mTc elution the column was rinsed using 10 mL of 0.9% NaCI. As an eluent 5 mL of TBAB (tetrabutyloammonium bromide) solution of 0.2 mg/mL in CH2CI2 was used. The eluate was collected as 1 mL aliquots and measured like 99Mo fractions. The yield of adsorption and desorption processes was calculated as a ratio of measured activities of 99mTc in collected aliquots to the total activity of 99mTc loaded onto the column. Diagram of the process is shown in Figure 1.
Figure 1. Diagram of sorption and elution process on Dowex-1×8
Separation on AnaLig Tc-02
The columns were filled with about 100 mg of resin suspension. To 10 mL of Na2MoO4 solution of 120 mg/mL in 2 M NaOH, the tracers of 99Mo (80 to 100 kBq) or 99mTcO4- (170 to 240 MBq) were added. The solution was delivered on the column using a peristaltic pump with the flow rate of 0.2 mL/min. To the column outlet, a flexible tube was connected enabling the collection of eluate to vials for gamma spectrometry measurements of 99Mo with the use of HPGe detector. In next step, the column was rinsed with 3 mL of 2M NaOH. For 99mTc elution 5 to 10 mL of water was used and the eluate was collected in 1 mL portions to 10 mL vials. The flow rate was 0.5 mL/min. Due to high activities of 99mTc the vials were measured in dose calibrator.
The diagram of the separation and desorption processes is shown in Figure 2.
Figure 2. Diagram of separation process using AnaLig Tc-02 resin
Separation on C-18 column modified with PEG
The aqueous biphasic extraction chromatography (ABEC) with C18 (OASIS HB) column coated with polyethylene glycol as extractant was used. Around tenfold excess of polyethylene glycol with molecular weight in the range from 2000 to 6000 was loaded on the column containing 0.1 g of C-18 resin in water. The excess of PEG unbound with resin was washed with water. The solution of ammonium molybdate (0.05 g of Mo) with about 2 MBq of 99Mo in 10 mL of 3M ammonium carbonate solution was loaded on the column. The column was washed with 10 mL 3M ammonium carbonate and then 99mTc was eluted with ten 5 mL fractions of water. Elution yield and content of 99Mo in technetium fraction were determined by gamma spectrometry with HPGe detector. The content of polyethylene glycol in 99mTc fraction was measured by TOC method.
Results and disscusion
Separation on Dowex-1×8
In four experimental runs using Dowex resin, the 99mTc yield was above 77%. Total retention of 99mTc was achieved for the flow rate of 0.6 mL/mln. Increasing flow rate to 2 mL/mln during column loading and elution resulted in decreasing the elution yield for more than 10%. The elution is fastest at the beginning and more than 90% of 99mTc is contained in the first 5 mL of TBAB solution. The elution yield was higher when the contact of TBAB solution with Dowex resin was increased to about 10 minutes. On the other hand this has no effect on the amount of 99mTc retained on the column. Results of 99mTc recovery are presented in Table 1.
Table 1. Elution efficiency of 99mTc
| Dowex-1×8 mass [mg] |
99mTc in 0.9% NaCl solution | 99mTc eluted [%] | 99mTc retained on column [%] |
|---|---|---|---|
| 125 | nd | 77.5 | 22.5 |
| 133 | nd | 78.6 | 21.4 |
| 139 | nd | 78.5 | 21.5 |
| 140 | nd | 79.1 | 20.9 |
nd – not detected
Separation on AnaLig Tc-02
Table 2 shows the results of 99mTc retention on column from 2M NaOH solution and 99mTc recovery from this column after elution with water. 99mTc is retained almost completely on the column. Only 0.025 % of technetium is lost in these steps of separation. The recovery of 99mTc amounted to approximately 85% and all technetium was contained in 2 mL of water solution.
Table 2. 99mTc activity distribution in retention and elution process using AnaLigTc-02 resin
| Experiment | 99mTc in solution loaded on column [MBq] | 99mTc in solution | 99mTc eluted from column [MBq] |
99mTc recovery [%] | |
|---|---|---|---|---|---|
| Leaving column [kBq] | 2 M NaOH rinsing [kBq] | ||||
| 1 | 172 | 10 | 20 | 144 | 83.7 |
| 2 | 235 | 20 | 30 | 204 | 86.5 |
| 3 | 176 | 10 | 20 | 149 | 84.6 |
| 4 | 235 | 20 | 20 | 203 | 86.3 |
The 99Mo activity distribution from the AnaLig Tc-02 column loaded with 99Mo in equilbrium with technetium is shown in Table 3. It can be seen from this table and Figure 3 that all molybdenum was found in eluate and in rinsing solutions. The activity of 99Mo measured in 99mTc water fractions was from 1 to 8 Bq.
Table 3. 99Mo activity distribution in 99mTc recovery process
| 99Mo in solution loaded on column [kBq] | 99Mo not retained on [kBq] | 99Mo in 2M NaOH rinsing solution [kBq] | 99Mo in water eluted 99mTc 1 mL fractions [Bq] | |||
|---|---|---|---|---|---|---|
| 1 | 2 | 3 | 4 | |||
| 94 | 90.0 | 3.80 | 1 | nd | nd | nd |
| 96 | 92.6 | 3.35 | 5 | nd | nd | nd |
| 89 | 86.0 | 2.86 | nd | nd | nd | nd |
| 102 | 97.8 | 4.10 | 8 | nd | nd | nd |
nd – not detected
Figure 3. 99mTc separation process on AnaLig Tc-02 resin
Separation on C-18 column modified with PEG
Typical elution profile of 99Mo and 99mTc from C-18 column coated with PEG is presented in Figure 4. Blue line represents the 99Mo eluted during solution loading on the column while the red line represents 99mTc eluted from the column with water after successful 99Mo loading. The 99mTc elution profile improves with decrease of PEG molecular weight as can be seen in Figure 5.
Figure 4. Typical elution profile of 99Mo and 99mTc from OASIS HLB Plus column coated with PEG
Figure 5. Influence of PEG molecular weight on the elution profile of 99mTc from OASIS HLB Plus column coated with PEG
Using the C-18 column In-house modified with polyethylene glycol allowed to separate 99mTc from excess of Mo with over 80% yield. The highest elution yield of 99mTc > 80% were obtained for polyethylene glycol with low molecular weight (PEG-2000 and PEG-3000) . With Increasing the molecular weight of PEG, elution yields of 99mTc drops to around 70% for PEG-6000 as can be seen in Table 4.
Table 4. Elution yield of 99mTc and TOC content in 99mTc solution obtained from OASIS HLB Plus column coated with PEG
| PEG | Elution yield of 99mTc (50 mL of water) [%] | TOC content in 99mTc solution [μg/mL] |
|---|---|---|
| PEG-2000 | 82.5 | 25.7 ± 2.6 |
| PEG-3000 | 80.1 | 22.1 ± 1.9 |
| PEG-4000 | 73.6 | 9.8 ± 1.3 |
| PEG-6000 | 71.3 | 3.0 ± 0.3 |
Assuming that TOC method quantifies the level of PEG residue in 99mTc solution, the PEG contamination was lower than 0.005% and significantly decreased with increasing PEG molecular weight. The contamination of 99mTc with 99Mo was lower than 0.01% for all experiments. Despite the high separation yield, the volume of 99mTc eluate is rather high and further post-elution concentration might be required.
Conclusion
The separation yield of 99mTc for all 3 resins used is above 75%. In comparison to technetium-99m half-life the separation times were relatively short and were not higher then 100 minutes. The highest concentration of 99mTc has been achieved with the use of AnaLig Tc-02 resin. The concentration of 99Mo in recovered 99mTc is below 0.01% using AnaLig Tc-02 and C-18 column modified with PEG resins.
Acknowledgements
This project was supported by the grant ALTECH PBS1/A9/2/2012 awarded by the National Centre for Research and Development in Poland within the Applied Science Program.
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Correspondence to:
Wioletta Wojdowska, PhD
National Centre of Nuclear Research
Radioisotope Centre POLATOM
Andrzej Soltan 7 St., 05-400 Otwock, Poland
Tel: +48 22 273 1910
E-mail: wioletta.wojdowska@polatom.pl
