Journal of Nuclear and Radiochemical Sciences, Vol. 8, No.1, pp. 11-19, 2007

Titanium Molybdate Gels as Matrix of 99Mo/ 99mTc Generators
F. Monroy-Guzman,* O. Cortes Romero, and H. Díaz Velázquez
Instituto Nacional de Investigaciones Nucleares Carretera México-Toluca, 52750 Edo. de Mexico, Mexico
Received: November 1, 2006; In Final Form: March 26, 2007
This work deals with 99Mo/ 99mTc generators produced from titanium 99Mo-molybdate, first synthesized and thenirradiated. This method avoids the use of special equipment for the synthesis of radioactive material and reduces
the process time which usually is at least 7 h. The effect of four parameters of titanium molybdate gel synthesison the performance of 99Mo/ 99mTc generators and some physico-chemical properties of these gels were evaluated:
gel irradiation, titanium solution concentration, effect of the washing gels, and their drying temperature. The data
reveal that: (1) the irradiation of the titanium molybdate gels did not influence significantly the generator
performance, (2) the dehydration of titanium molybdate gels, the presence of occluded salt in the gel duringdrying, and the washing of the gels after drying improved the diffusion of the 99mTcO4 – ions and thus the generator
performance, and (3) the gels had a three-dimensional structure with cavities which allowed the diffusion of the99m
TcO4 – ions inside the gel and were directly connected with the performance of the titanium 99Mo-molybdate99
Mo/ 99mTc generators.

1. Introduction
The development of new techniques to produce radioactive
generators is of great importance in nuclear medicine, considering that radioisotopes such as 99mTc, which are widely used as
diagnostic agent, are mainly produced by generator systems.1,2
In these systems, the 99Mo is normally adsorbed onto an alumina column and the less strongly bound 99mTcO 4 – is eluted
with isotonic saline solutions at intervals. However, these generators present an inherent disadvantage: the limited adsorption
capacity of alumina for Mo(VI) requires the use of very highspecific activities of 99Mo (~103 TBq/g).2-4 To avoid this drawback, alternative methods of generator preparation using low or
medium specific activities have been proposed and developed.
In particular, the incorporation of 99Mo into insoluble precipitates is the most studied procedure. Amorphous materials
formed for the junction of molybdates with atoms like zirconium, titanium, cerium, thorium, tin, etc. have been examined
as 99mTc generator matrices.2-19 In this case, the content of Mo
inside the matrix is about 30% and it is possible to use low or
medium specific activities of 99Mo (~ Bq/g).2,20 Consideringthat the properties of these generators depend basically upon
their preparation conditions, factors as the pH of 99Mo-molybdates, the molar ratios of X:Mo (X = Zr, Ti, Th, etc.), the concentrations, the order of reactive addition, the drying
temperature of the final product, etc., must be carefully studied
and determined to reproduce the properties of the gel generators.2-19
Our laboratory has developed and evaluated the gel formulation of zirconium molybdate gels and its effect on the performance of 99Mo/ 99mTc generator.16,18 The results have shown thetechnical viability of these 99 Mo/ 99mTc gel type generators.
However, the highest 99mTc elution efficiencies attained havebeen about 82%. In addition, the synthesis of these gels using
99
MoO3 requires the use of specialized installations in order tohandle radioactive materials and at least a period of 7 h processing time.16,18 In order to simplify the production process ofthese 99Mo/ 99mTc gel type generators and taking advantage of*Corresponding author. E-mail: fmg@nuclear.inin.mx Fax:+(52) (55) 53.29.73.22.

our previous experience with the zirconium molybdate generators, we propose in this work to develop a generator based ontitanium-molybdate gels which can be irradiated after being
synthesized. The short half life of the only radioisotope produced for the titanium, 51Ti (t1/2 = 5.79 m), during the irradiation of the titanium molybdate gels and the analogue chemical
properties of the titanium and the zirconium make feasible thepreparation of titanium-molybdate 99Mo/ 99mTc generators by
this approach.
Vanaja et al8 and Nieto and Osso21 have reported the preparation of a 99mTc generator using neutron irradiated titanium
molybdate. The preparation conditions of the titanium molybdates in both works were different: Vanaja et al used TiCl4, andsuggests that (1) the irradiation of the gels induced the reduction of 99mTcO4 – directly or via Ti(III), formed by reduction ofTi(IV), (2) the decomposition of the gel forms soluble Mo species if the gels are irradiated at neutron fluence more than
2×1013 n cm –2 s –1, and (3) the yields of 99mTc depends on thewater content of the titanium molybdate8; while Nieto and Osso
synthesized the gels from TiCl3 and TiO2, obtaining best resultswith TiCl 3 : a yield of 99mTc about 75% and a good quality
99m
Tc.21
In order to explain the influence of the preparation conditions of the gels on the generator performance, systematic studies on titanium 99Mo-molybdate gel 99Mo/ 99mTc generators weredone in this work. The effect of five parameters of titanium
molybdate gel synthesis on the 99Mo/ 99mTc generator performance were evaluated: concentration of the titanium (IV andIII) solutions, Ti:Mo molar ratios, effect of the gels washed andthe drying temperature, and irradiation of the titanium molybdate gels. The gel physico-chemical properties were also determined in order to establish the connection among the chemistryof the reactant solutions, the gel properties, and the generator
performance.
2. Experimental Section
2.1. Preparation of titanium molybdate gels. The titanium molybdate gels were prepared by adding molybdate solution (80 drops min –1 ) to TiCl 4 or TiCl 3 solutions. The
molybdate solutions were prepared from MoO3 (1 g) which is© 2007 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 06/30/2007

12

GuzmanJ. Nucl. Radiochem. Sci., Vol. 8, No. 1, 2007

dissolved in 2 M NH4 OH. The pH of this solution is thenadjusted to 4.5 by adding 4 M HC1. The TiCl3 (10% wt Sigma
Aldrich) solutions were dissolved in H 2O and 6 M HCl. The
TiCl4 solutions were prepared dissolving TiCl4 (99% purity
FLUKA) in HCl. The dilution was performed in a N2 atmosphere by means of a special device. This solution was then dissolved in concentrated HCl and the titanium concentration wasquantified by spectrophotometric measurements with hydrogen
peroxide at 410 nm.22-24
The gel was stirred and the pH value was adjusted with 7 M
NH4OH and dried in the air for 1 day at 40 − 60 °C using an
infrared lamp and for a period of 20 h at 40 °C in a stove. The
resulting gel was crushed in an agate mortar. One portion ofthis gel was placed on a funnel to be washed using 200 mL of
distilled water and then dried in a stove for 24 h at 40 °C. The
gels were synthesized in duplicate at different conditions andfour parameters were evaluated: concentration of the titanium
solution (series A and E4), Ti:Mo molar ratios (series B), theeffect of the washed gels (series C, E), and the drying temperature (series D). Table 1 shows the preparation conditions oftitanium molybdate gels for each series.
2.2. Irradiation of titanium molybdate gels and preparation of 99Mo/ 99mTc generators. The gels were irradiated for 2
h at a neutron fluence of about 1.61×1013 n cm–2 s–1 in the Triga
Mark III Reactor (Mexico). After irradiation, about 1.5 g oftitanium 99Mo-molybdate gels (~4.9 MBq/g) were added into a
glass column (12 mm × 70 mm) containing a bed of acid alu-

mina (1 g). The column was finally washed with 50 mL ofsaline solution and eluted every 24 h using 0.9% NaCl for 1
week.
2.3. Elution of generators and eluate analysis. The 99Mo
and 99mTc activities were measured in a CRC-10R Capintec
dose calibrator and an HPGe detector (Canberra 7229P) coupled to a PC-multichannel analyzer (ACUSPECT-A, Canberra).
Elution profiles were obtained by collecting 250 µL fractionsof 0.9% NaCl solution. The radiochemical purity of the 99mTc
eluate was determined by paper chromatography using 1 CHR(Whatman®) paper as solid phase and 85% methanol as mobile
phase. The Rf of 99mTcO4 – was 0.66−0.72. Titanium, molybdenum, and aluminium concentrations in eluates were determinedby the hydrogen peroxide method measured at 420 nm and 470
nm by the thiocyanate method, and at 535 nm by the aluminon
method, respectively.23-26
2.4. Gel characterization. Gels characterization wasachieved by X-ray diffraction (XRD), scanning electron
microscopy (SEM), thermogravimetry, infrared spectrometry,and neutron activation analysis (NAA). The X-ray diffraction
patterns were obtained on a Siemens D500 diffractometer for 1
h, and scanned from 2.5° to 70° with steps of 0.02°. SEM
imaging was performed by means a Philips XL30. Digital
images were obtained at 3000X magnifications in randomly
selected fields. The thermogravimetric analyses were performed using a Phillips unit at a heating rate of 10 °C min –1
under a nitrogen atmosphere. The infrared measurements were

TABLE 1: Preparation conditions of titanium molybdate gels

40
40
40
40
40
40
40
40
40
40
40
40
40
40
40
40
100
200
400

pH
molybdates
4.5
4.5
4.5
4.5
4.5
4.5
4.5
4.5
4.5
4.5
4.5
4.5
4.5
4.5
4.5
4.5
4.5
4.5
4.5

TiCl4
(M)
0.83
0.56
0.3
0.10
0.83
0.83
0.83
0.56
0.56
0.83
0.83
0.56
0.56
0.3
0.3
0.56
0.56
0.56
0.56

pH
gel
5.4
5.4
5.4
5.4
5.4
5.4
5.4
5.4
5.4
5.4
5.4
5.4
5.4
5.4
5.4
5.4
5.4
5.4
5.4

washed

Ti:Mo

T(°C)

pH
molybdates

TiCl3
(M)

pH
gel

E1LA
E1LB
E1LC
E1LD
E2LA
E2LC
E3LA
E3LB
E3LC
E4LA
E4LB
-

1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1

40
40
40
40
40
40
40
40
150
400
40
40
40

4.5
4.5
4.5
4.5
4.5
4.5
4.5
4.5
4.5
4.5
4.5
4.5
4.5

0.35/HCl
0.35/HCl
0.35/HCl
0.35/HCl
0.35/H2O
0.35/H2O
0.35/H2O
0.35/HCl
0.35/HCl
0.35/HCl
0.35/HCl
0.35/H2O
0.7/HCl

1
5.9
8.5
9.0
5.8
8.5
9.0
5.9
5.9
5.9
8.5
8.5
8.7

Series

Gel

Ti:Mo

T(°C)

A

A1
A2
A3
A4
B1
B2
B3
B4
B5
C1
C2
C3
C4
C5
C6
D1
D2
D3
D4

1:1
1:1
1:1
1:1
1:1
1:2
2:1
1:1
2:1
2:1
2:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1

Seriesunwashed

E1

E1A
E1B
E1C
E1D
E2A
E2B
E2C
E3A
E4A
E4B
E4C

B

C

D

E2
E3
E4

washing
yes
yes
yes
yes
yes
yes
yes
yes
yes
no
yes
no
yes
no
yes
yes
yes
yes
yes

Titanium Molybate Gels as Matrix of 99Mo/ 99mTc Generators

J. Nucl. Radiochem. Sci., Vol. 8, No. 1, 2007Tm

taken on a Nicole Magna-IR spectrometer 550 with the samples pressed in KBr pellets.16
The gel concentrations of titanium and molybdenum were
determined by means of instrumental neutron activation analysis (INAA). The procedure described in previous works wasused in the Mo determination.20 In the case of titanium, 20 mgof each gel and TiO2, used as reference material, were irradiated in the Triga Mark III reactor at a neutron fluence of about
1.65×1012 n cm 2 s–1 for 30 s. Titanium was determined by the
330 keV γ-ray of 51Ti by means of a coaxial HPGe detector(7229P, CANBERRA).
3. Results
3.1. Acid-base behavior of titanium molybdate gels. The
Mo/ 99mTc generators performance depends on gel physicochemical properties and thus on their preparation conditions.
In particular, the gel pH can benefit or damage the formationof indissoluble gels. Our experiments showed that the gels prepared with 0.83 M TiCl4 formed indissoluble gels without theneed to fix their pH, whereas the gels prepared at other TiCl4
concentrations had to adjust their pH values to induce precipitation. Figure 1 shows the typical acid-base behaviour of thetitanium molybdates prepared in this work. The shape of thecurve is similar to that of a strong acid and indicates that thegel can perform as a strong-acid cation exchanger. It is important to consider that the ammonium ions (NH4Cl) were alreadypresent in the system before adding the ammonium hydroxide
to the gel, as a result of the dissolution of the molybdenumoxide. The presence of the ammonium chloride results in alower and slightly more sloping curve in the early titration
stages.27 However, the pH change produced by the presence of
this salt is less drastic in the strong-acid exchanger.
According to Figure 1, the “apparent” pK of the titanium
molybdate gel is about 1.8, which is considered as the acid-baseequilibrium of the gel and the ammonium chloride, and acapacity of 10.7 meq g–1 gel. The acid-base behavior of titanium molybdate gels could thus be expressed by reaction (1):99

→ [Tiy (NH4)2MoxOz] + H2O.
[TiyNH4MoxOzHr] + NH3 ←(1)

The neutralization of the charges by the increased pH values
causes the decrease of zeta potential and the increase of theelectrolytic concentration in the solution. Thus, both effects
promote the flocculation of the titanium molybdates. The
intrinsic precipitation of the gel prepared from 0.83 M TiCl4
can be then explained by the ionic strength of the system which
is sufficient to allow to the particles to approach each other and
12

10

pH

8

6

4

2

0
0

2

4

6

8

10

12

meq. NH4OH
Figure 1. Acid-base behavior of titanium molybdate gel.

14

13

aggregate, whereas in the case of the gels prepared with TiCl4
concentrations of less than 0.83 M, this ionic strength has toincrease by the addition of the NH4OH solutions to induce theprecipitation of the gels. In order to allow the precipitation of
the gels and to simplify the interpretation of the results, thegels pH was fixed to 5.4 by adding NH4OH solutions. Under
these conditions the ionic strength was about 1.
3.2. Performances of 99Mo-molybdate titanium generators. Table 2 shows the performances of the titanium 99Momolybdate gel generators obtained for the series A, B, C, D,
and E. These data are the average of six elutions performed oneach generator.
3.2.1. Effect of the Ti(IV) concentration (Series A). An
increase of the TiCl4 concentration during the preparation ofgels causes the augmentation of the elution efficiency and thedecrease of the eluate pH values. All these gels showed 99Mo
breakthroughs of more than 0.1% (see Table 2). The gels prepared at higher TiCl4 concentrations presented higher 99Mo
breakthroughs. The radiochemical purity was more than 90%
except for the gel prepared at a higher TiCl4 solution concentration which presented 44% of radiochemical purity. The elution
volumes (~3 mL) and the Al contents (<10 ppm) of the 99mTc
eluates were not changed by the variation of TiCl4 concentration during the preparation of the gels.
3.2.2. Effect of the Ti:Mo molar ratio (Series B) . An
increase of the Ti in the Ti:Mo molar ratio of the gels produced
the decrease of efficiency and 99Mo breakthroughs at the two
TiCl 4 concentrations used (see Table 2). However, these
decreases presented different rates in each TiCl4 concentration.
At 0.83 M TiCl4 , efficiency and 99 Mo breakthroughs werereduced in 60% and 95%, respectively, regarding the best efficiency (82%) and its Mo breakthrough value (0.13%), while at
0.56 M TiCl4 the decreases were about 20% and 60%, respectively. The increase of Ti produced also an increase of radiochemical purity in the case of the 0.83 M TiCl4 solutions, and adecrease when the 0.56 M TiCl4 solutions were used. The gels
prepared with 0.83 M TiCl4 solutions always presented the lowest radiochemical purity (< 65%). The eluate pH values tended
to 7.4 when the Ti:Mo molar ratio was 2:1 and increased to 7.7
at Ti:Mo molar ratio of 1:1 and a 0.56 M TiCl4 solution concentration. At a Ti:Mo molar ratio of 1:2, the efficiency decreased
and the 99Mo breakthroughs increased about 35% and 97%,
respectively, regarding gel values with 1:1 molar ratio and 0.83
M TiCl4 solution. The elution volume and the Al content weresimilar to those of series A: 3 mL and less than 10 ppm, respectively.
3.2.3. Effect of the washed gel (Series C). The washing of
the gels before drying and irradiation produced a decrease ofelution efficiency, 99Mo breakthroughs, and elution volume;and increase of pH values and radiochemical purity. The elution efficiency decreased to 35% after washing the gel, except
for the gel prepared with 0.56 M TiCl4 which presented an
opposite behavior and its efficiency increased by 15%. These
efficiencies were, in all the cases, relatively low (~30–70%).
The washing of the gels reduced the 99 Mo breakthroughs
between 55% and 95%, improving considerably the 99mTc eluate radionuclide purity. Nevertheless only the gel preparedwith a Ti:Mo molar ratio of 2:1 is within the limits establishedby the Pharmacopeia (< 0.015% 99Mo).28 The eluate pH valuesof the unwashed gels were more acidic than their washedhomologues which tended toward pH values between 7.4 and7.8. The elution volume was reduced about 30% in the washedgels remaining at about 3 mL. The radiochemical purity wasappreciably improved with the washing of the gels, it increased
from 2 to 90% in the gel prepared with 0.3 M TiCl4, but it was
reduced in the gel prepared with Ti:Mo molar ratio of 2:1 from90 to 65%.
3.2.4. Effect of the drying temperature of the gels (Series
D). The increase of the gel drying temperature had the follow-

14

GuzmanJ. Nucl. Radiochem. Sci., Vol. 8, No. 1, 2007

TABLE 2: Performance of 99Mo/ 99mTc gel generators based on titanium molybdate-99Mo
99m

99
Tc elution
Mo
Efficiency Breakthrough
(%)
(%)

Gel

Eluatevolume(mL)

99m
Tc
Radiochemical
purity (%)

Al3+
(ppm)

%Ti

%Mo

pH
Eluategels prepared from Ti(IV) solutions
A1
A2
A3
A4
B1
B2
B3
B4
B5
C1
C2
C3
C4
C5
C6
D1
D2
D3
D4

TiCl4 (M)
0.83
0.56
0.3
0.100
Ti:Mo TiCl4 (M)
1:1
0.83
2:1
0.83
1:2
0.83
1:1
0.56
2:1
0.56
2:1
0.83
2:1
0.83
1:1
0.56
1:1
0.56
1:1
0.3
1:1
0.3
T(°C)
40
100
200
400

82.4
69.0
42.1
55.8

0.132
0.113
0.249
0.171

2.8
3.0
3.0
3.0

44.51
95.40
90.14
97.42

<10
<10
<10
<10

19.90
21.30
20.08

38.12
36.56
38.35

7.1
7.8
7.8
7.8

82.4
32.6
53.5
69.0
55.5
52.8
32.6
58.4
69.0
62.2
42.1

0.132
0.006
0.246
0.113
0.046
0.121
0.006
0.254
0.113
0.768
0.249

2.8
3.0
3.5
3.0
3.0
5.2
3.0
4.3
3.0
4.3
3.0

44.51
65.66
71.36
95.40
77.40
90.36
65.66
77.16
95.40
2.64
90.14

<10
<10
<10
<10
<10
<10
<10
<10
<10
<10
<10

24.02
15.24
19.90
3.36
24.02
19.90
21.30
22.34

24.99
35.42
38.12
8.60
24.99
38.12
36.56
36.56

7.1
7.4
6.1
7.8
7.5
7.3
7.4
6.5
7.8
5.3
7.8

65.4
75.2
82.7
90.2

0.313
0.109
0.037
0.009

3.6
3.3
5.0
4.7

82.01
83.15
81.60
86.17

<10
<10
>10
<10

19.90
-

38.12
-

5.1
4.7
4.4
4.3

gels prepared from Ti(III) solutions

E1A
E1B
E1C
E1D
E1LA
E1LB
E1LC
E1LD
E2A
E2B
E2C

E2LA
E2LC

E3LA
E3LB
E3LC

E4A
E4B
E4C

Gel
pH
1
5.9
8.5
9.0
Gel
pH
1
5.9
8.5
9.0
Gel
pH
5.9
8.5
9.0
Gel
pH
5.9
9.0
Gel
pH
5.9
5.9
5.9
Gel
pH
8.5
8.5
8.7

TiCl3
(M)
0.35/HCl
0.35/HCl
0.35/HCl
0.35/HCl
TiCl3(M)
0.35/HCl
0.35/HCl
0.35/HCl
0.35/HCl
TiCl3(M)
0.35/H2O
0.35/H2O
0.35/H2O
TiCl3(M)
0.35/H2O
0.35/H2O
T(°C)
40
150
400
TiCl3
(M)
0.35/HCl
0.35/H2O
0.7/HCl

unwashed
94.8
90.1
72.8
64.7
washed<0.0001
0.00460.00280.0182

4.7
4.0
4.5
3.8

89.53
74.82
47.02
76.15

<10
<10
<10
<10

19.90
19.60
15.75
17.20

39.05
43.35
32.60
35.75

4.8
4.4
4.4
4.4

95.7
95.4
94.3
94.6
unwashed

0.0130<0.0001
0.01670.0090

3.8
3.7
4.0
3.8

92.54
94.15
94.89
76.51

<10
<10
<10
<10

-

-

5.5
5.9
5.7
6.0

80.2
67.5
69.2
washed

0.0266
0.04190.0152

4.5
4.0
4.1

95.57
96.36
94.56

<10
<10
<10

18.62
15.23
27.15

30.47
31.52
31.85

4.6
4.5
4.4

92.7
93.5
washed<0.0001
0.0210

3.9
3.9

96.35
67.09

<10
<10

-

-

6.3
6.5

95.4
95.0
95.5
unwashed<0.0001
0.00660.0042

3.7
3.7
3.6

94.15
72.00
62.24

<10
<10
<10

19.60
26.25
28.39

43.35
38.49
38.79

5.9
6.6
6.4

72.8
67.5
88.5

0.00280.04190.0008

4.5
4.0
4.4

47.02
96.36
52.10

<10
<10
<10

15.75
15.23
33.35

32.60
31.52
27.37

4.4
4.5
4.7

Titanium Molybate Gels as Matrix of 99Mo/ 99mTc Generators(a)washed gel
not washed gel
10

20

30

40

50

60

70

2θ

(b)
100

Weight (%)

90

washed gel

80
70
60
50

not washed gel
40
30
20
200

400

600

800

Temperature /°C(c)washed gel
O-H
Ti-O

Transmittanceing effects on the generator performance: rise of the elution
efficiency and the elution volume, reduction of the 99Mo breakthroughs and the eluate pH values, and no significant effect on
the radiochemical purity.
3.2.5. Gels prepared from Ti (III) solutions (Series E).
Series E comprises the titanium 99Mo-molybdate generators
prepared from TiCl 3 solutions. The effect of four synthesis
parameters on the generator performance was evaluated: thegel pH, the Ti (III) concentration, the washing, and drying temperature of the gel.
In the case of the unwashed gels, the increase of the gel pH
caused a decrease of the elution efficiency and the radiochemical purity, and an increase of the 99Mo breakthroughs. High
Ti(III) concentrations promoted higher elution efficiencies, andlower 99Mo breakthroughs and radiochemical purities. The
medium used in the preparation of Ti(III) solutions also
induced a variation in the generator performance. The gels
prepared from TiCl3 solutions diluted in H2O present lower elution efficiencies and higher 99Mo breakthroughs and radiochemical purities than those prepared with HCl. The volume
required to eluate the 99mTc in these generators is about 4 mL
with pH of around 4.4.
The washed gels improve considerably the elution efficiencies of the generators and the radionuclide and radiochemical
purity of the 99mTc eluates. In general, these parameters are notaffected by the variation of the gel pH, the Ti(III) concentration, or the drying temperature of the gel. These generators
have elution efficiencies higher than 90%, 99Mo breakthroughs
lower than 0.01%, radiochemical purities higher than 94%, andeluate pH of about 6. Accordingly, the titanium 99Mo-molybdate gels prepared from TiCl 3 solutions and washed before
being irradiated present the best performance of all generatorsand can be used in nuclear medicine.
3.3. Characterization of molybdate titanium gels. The
previous analysis of generator performance as a function of theconditions of gel preparation had shown that the Ti concentration, the Ti:Mo molar ratio, the washing of the gel, and the drying temperature changed the generator performance. In thepresent section, the effect of these parameters on some physicochemical properties of the gels was discussed.
3.3.1. Effect of gel washing. The analysis of the X-ray diffraction patterns of washed and unwashed titanium molybdategels (see Figure 2a) showed the presence of an amorphous
structure and a crystalline system identified as ammonium
chloride. This means that the ammonium chloride formed during the gel synthesis remained in the gel if the latter was notwashed.
The thermogravimetric analysis of washed and unwashedtitanium molybdate gels (see Figure 2b ) showed that the non
washed gel suffers a first weight-loss of approximately 5% at
50 °C while the washed gel loses about 15% at 70 °C. In thisfirst step, the gels lost weight by the elimination of water. In
the case of the washed gel, an anhydrous gel was formed after
the water loss, which was stable until the decomposition of thegel at about 770 °C. The unwashed gel had a large weight-loss(~ 55%) at about 250 °C by the decomposition of ammonium
chloride. Later it presented a slight weight-loss at about 345°C, probably caused by the elimination of intramolecular water,
forming the anhydrous gel, and at 775 °C the gel is destroyed.
It is important to note that more than 60% of the unwashed gel
weight corresponds to the ammonium chloride.
In general, the gels lose between 60 and 78% weight in thewashing and this loss increases with an increase of the TiCl4
concentration used for the preparation of the gel. The analysisof the washing water showed the presence of ammonium chloride as the principal component and Ti traces (~ 200 ppm).
The bulk of this salt was removed with about 40 mL of water
which had pH between 4.5 and 5.
The infrared spectra of the washed and unwashed gels are

15

J. Nucl. Radiochem. Sci., Vol. 8, No. 1, 2007

Mo-O-Mo

Ti-O

Mo-O

Ti-O

not washed gel
O-H

4000

N-H

N-H

3000

2000

1000

Frequency /cm-1(d)unwashed

washedFigure 2. Washing effect of titanium molybdate gels in (a) X-ray diffraction patterns, (b) thermograms, (c) infrared spectra, and (d)morphology.

16

GuzmanJ. Nucl. Radiochem. Sci., Vol. 8, No. 1, 2007

presented in Figure 2c to confirm the presence of the ammonium group in the unwashed gel and its disappearance in thewashed gel. Ammonium displays strong broad N–H stretchingbands in the region of 3300–3500 cm–1 which are located in thesame region as those for the O–H bands and a sharp bending
band at 1400 cm –1. It is important to note that the region
assigned to the aqueous bending of the unwashed gel is slightly
shifted toward lower frequencies. The infrared spectrum of
the washed gel loses the 3110 and 3027 cm–1 stretching bandsof the N–H bond and the 2800, 1650, and 1400 cm –1 bands
emerge, which express the Ti–O, O–H, and Mo–O groups,
respectively.16,29-31 The absorption band at 1750 cm–1 which disappears completely in the washed gel may be assigned to thedifferent arrangement of the Mo–O bond when the ammonium
chloride is present at high concentrations. This analysis was
also supported by the previously reported spectra of Na2MoO4,(NH4) 6Mo7O24, TiO2 , and MoO3 obtained in the same experimental conditions as the gels.16,31,32
Figure 2d shows the SEM images of the unwashed andwashed gels. In the unwashed gels, the ammonium chloride
salt is embeded into the gel and this presents a typical amorphous morphology, while the washed gel is formed as flakes orlayers with an appearance similar to clay.
3.3.2. Effect of the drying temperature on the gels. The
analysis of the change in the X-ray powder pattern during theheating of the gel shows (see Figure 3a) that in the gels heatedat 40 and 100 °C the ammonium chloride was the only crystalline phase identified in the diffractograms. At 400 °C, theammonium chloride disappeared and the gel tended to crystallize incipiently, although its amorphous nature was not eliminated. This fact agrees with the SEM images shown in Figure
3b. At 100 °C the ammonium chloride was distributed inside
(a)
NH4CI

400 °C

4. Discussion
NH4CI

200 °C

(b)

20

NH4CI

NH4CI

50

60

NH4CI

100 °C
10

the gel (white particles in the image), and at 200 °C the pores
got closer and salt particles were less visible. The morphologyof these gels presented a layer or a flake shapes as we havedescribed previously. At 400 °C the gel morphology was similar to that of glass, without the presence of the ammonium
chloride particles. The infrared spectra of these gels were similar to those presented in Figure 2c. In particular, the “fingerprint” region was not affected by the drying temperature.
3.3.3. Effect of gel irradiation. It is known that the irradiation of the inorganic materials can modify their structure andproperties depending on the irradiation conditions.32 Figure 4ashows the evident transformation of the gel morphology causedby irradiation. The gel prepared with the higher Mo concentration tended to form a glazed structure when it was irradiated,while the gel irradiated and prepared at Ti:Mo = 2:1 molar
ratios presented a morphology similar to that of clay, where theatoms were arranged as piled up sheets. In the case of the gel
prepared at 1:1 molar ratio, the irradiation also caused a glazed
appearance in the gel but less than that of the 1:2 gel. In thismatter, the amorphous nature of the gel was mainly caused by
the molybdenum. On the other hand, the infrared spectra(Figure 4b) of the irradiated and unirradiated gel (Ti:Mo = 1:1)
present practically the same functional groups, however the2915 and 2850, 692, 562, and 440 cm–1 bands, assigned to the
Ti–O group, disappear in the spectrum of the irradiated gel.
Thus the irradiation of the gel causes an arrangement changeof the Ti–O and Mo–O groups, where the presence and disposition of the Mo–O groups is dominant. In fact, the “fingerprint” region shape of the irradiated gel is very similar to thatof the MoO3 spectrum.16 Concerning the change of the X-ray
diffraction patterns, the both gels (Ti:Mo = 1:1), irradiated andunirradiated, present an amorphous structure.
The results of neutron activation analysis show that the Mo
and Ti percentages in the gels prepared from Ti (IV) solutions
are in average 40% and 20%, respectively,20 while those prepared from Ti (III) solutions are varied as a function of the gel
pH and of the drying temperature. When the pH of gels areadjusted to 1 and 5.9, the Mo and Ti contents are about 40%
and 20% and these percentages decrease to 30 and 15%,
respectively at pH 9. The Ti percentage of the gels dried at 150
and 400 °C increases about 27%. The Mo:Ti weight ratio of all
the gels is about 2:1.

30

40

2θ
100°C

70

We have suggested in previous works16 that the zirconium
molybdates are formed by the alkaline hydrolysis of the zirconium solutions in the presence of molybdates. The hydrous
zirconia (ZrO2·nH2O) was thus precipitated and the water molecules were displaced by the molybdate anions to form the zirconium molybdate.16 Considering that zirconium and titanium
have similar chemical properties, it is highly possible that titanium molybdate is also formed by the hydrolysis of TiCl4 or

200°C

Figure 3. Drying temperature effect of the titanium molybdate gel in (a) X-ray diffraction patterns and (b) morphology.

400°C

Titanium Molybate Gels as Matrix of 99Mo/ 99mTc Generators

17

J. Nucl. Radiochem. Sci., Vol. 8, No. 1, 2007(a)unirradiatedTi : Mo 1:1

unirradiatedTi : Mo 1:2

unirradiatedTi : Mo 2:1

irradiatedTi : Mo 1:1

irradiatedTi : Mo 1:2

irradiatedTi : Mo 2:1

(c)

(b)
Not irradiatedTransmittance

Mo-O-H
Mo-O

Ti-O

Mo-O

Ti-O

O-H

Irradiated gel

Ti-O

O-Mo-OMo-O

Not irradiated gel

Irradiated
O-H

4000

3500

3000

2500

2000

1500

1000

500

Frequency /cm-1

10

20

30

40

50

60

70

Angle 2θ / °Figure 4. Irradiation effect of the titanium molybdate gel in (a) morphology, (b) infrared spectra, and (c) X-ray diffraction patterns.

TiCl3 [hydrous titanium (IV) (TiO2·nH2O)] for the molybdate
solutions:(2x-y) –TiO2·xH2O + wMoxOz (OH) 8x-y-2z →–[TiO2·(x – w)H2O·MoxOz (OH) (2x-y)
8x-y-2z ] +wH2O.

According to the literature, the crystalline zirconium molybdate could have the formulae ZrMo2O7(OH)2⋅2H2O and a threedimensional structure built-up from regular zirconium
pentagonal bipyramids [ZrO3 (OH) 2O2)] and highly distorted
molybdenum octahedral [cis-MoO4 (OH)(H2O)].33-35 Since zirconium is tetravalent, the lattice carries a negative electriccharge: two elementary charges per zirconium atom. Thischarge is balanced by alkali or alkaline-earth cations which do
not occupy fixed positions, but are free to move in the channelsof the lattice framework. These ions act as counter ions andcan be replaced by other cations.27,34-35 Thus, these compounds
behave as a very narrow ionic sieve. In the case of the titanium
molybdate gels studied in this work, it was possible that theirstructures were similar to those of the zirconium molybdate

previously discussed. This assumption agreed with our NAA
results which show that the Mo:Ti ratio of the gels is about 2:1.
Therefore the crystalline titanium molybdate could have a formula and structure that is similar to that of the zirconium
molybdate: TiMo2 O 7 (OH) 2⋅2H 2 O with a three-dimensional
structure. These data were also supported by our SEM images(see Figures 2d, 3b, and 4a) which showed the presence of athree-dimensional framework with cavities in the gel. These
cavities allow the diffusion of ions inside the gel and are
directly connected with the performance of the titanium 99Momolybdate generators, in particular the 99mTc elution efficiency
and the Mo breakthrough. In addition, the infrared data showthat all the gels prepared present two types of bonded water:(1) in the form of hydroxyl bonded to a Mo atom (1400 cm–1)
and (2) water molecules present as hydration water inside thecrystal lattice or in an adsorbed state (3500 and 1650 cm –1).
These were congruent with the formula proposed for the crystalline titanium molybdate which contains hydroxyl groups andwater molecules. In the case of the precipitated titanium
molybdate gels, they could be represented by the empirical formula TiMoxO2+3x-y (OH)2y⋅zH2O.35

18

GuzmanJ. Nucl. Radiochem. Sci., Vol. 8, No. 1, 2007

Our infrared spectroscopy data had shown that the titanium
molybdate gels prepared at different conditions were practically constituted by the same functional groups and only variations on the intensity of the characteristic bands of each groupwere observed. These results proved that depending upon the
preparation conditions of the gels, the titanium pentagonal
bipyramids and molybdenum octahedral were packed differently in the framework, modifying the cavity sizes in the gel(see Figures 2b and 3b), its diffusion properties, and consequently the generator performance.
Considering that titanium molybdate gels act as cationexchangers, the 99mTcO 4 – ions produced in these generators
were not exchanged, and their elution was then governed by afree diffusion mechanism.16 During the elution, the 99mTcO4 –ions diffused within the gel cavities. If the size of the opening
was smaller than the diameter of the hydrated 99mTcO4 – ion, it
would stay captured in the gel and caused a decrease of the elution efficiency of the generator. According to this hypothesis,
the dehydration of the gels removed the hydration water allowing a better diffusion of the 99mTcO4 – ions and thus the generators prepared from these gels present higher elution efficiency(see Table 2).8 The presence of occluded salt (NH4Cl) in thegel (non washed gels Figure 2b) appeared to be essential duringdrying for the retention of an open pore structure which wascapable of a better free diffusion of the 99mTcO 4 – ions.2 This
fact agrees with the low efficiencies obtained in the generators
prepared from Ti(IV) solutions which were washed before drying (see Table 2). On the other hand, our data had also shownthat the washing of the gels after drying improved the elution
efficiency and the radionuclide purity (see Table 2, series E),
because the lattice cavities are free of NH4Cl and the 99mTcO4 –ions can diffuse easier across the gel.
The titanium solution concentration had an evident effect on
the gel generator performance: gels prepared from diluted titanium solutions present lower efficiencies and higher 99 Mo
breakthrough than those prepared from concentrated titanium
solutions. Considering that diluted solutions of tetrachloride
and trichloride titanium could form from hydrolysis of oxychloride titanium, these oxychlorides may block the formationof hydrous titanium (IV) which is essential to precipitate thetitanium molybdate gels. In this case, the reaction between thetitanium and molybdate solutions produces a mixture of titanium molybdate with titanium and molybdate species in solution or precipitated (oxy-hydroxide phases), causing an increaseof the 99Mo breakthrough in the eluates, especially if the gelswere not washed after drying.
The amorphous materials were less perturbed by the introduction of defects due to irradiation than those related withcrystal structure.36 The irradiation of the titanium molybdate
gels, shown in this work, did not influence appreciably the generator performance (see Table 2) although the irradiated titanium molybdate gels suffered a slight change in theirmorphology (amorphization), structure (arrangement of theTi–O and Mo–O groups) and sometimes in their color (seeFigure 4). These results showed the feasibility of producing
gel generators based on titanium molybdate which could be
irradiated after being synthesized. This approach will reduce
the preparation time of gel generators and eliminates the use of
specialized installations.
5. Conclusion
Titanium molybdate gels were amorphous materials which
act as cation exchangers, and their flocculation was favored by
the increase of the ionic strength. These gels could be represented by the empirical formula: TiMoxO2+3x-y (OH)2y⋅zH2O anda three-dimensional structure with cavities which allowed the
diffusion of ions inside the gel. Cavity sizes were directly
related to the performance of titanium 99Mo-molybdate genera-

tors. Depending upon the preparation conditions of the titanium molybdate gels, the titanium pentagonal bipyramids andmolybdenum octahedral were packed differently in the framework, modifying the cavity sizes in the gel, its diffusion properties and so the generator performance. The dehydration of
the titanium molybdate gels, the presence of occluded salt in
the gel during drying and the washing of the gels after drying
improved the diffusion of the 99mTcO 4 – ions, the elution efficiency and in general the generator performance. The irradiation of the titanium molybdate gels did not inf luencesignificantly the generator performances. It was feasible toproduce titanium 99Mo-molybdate generators preparing first
the titanium molybdate gels and later irradiating them. Thisapproach will reduce the preparation time of the gel generatorsand eliminates the use of specialized installations.
Practically, all the 99mTcO4 – eluates obtained from the generators prepared from gels washed before irradiation and from
TiCl3 solutions satisfied the Pharmacopoeias.28 The best generator performances were obtained using the gels preparedfrom 0.35 M HCl TiCl3 solutions, adjusted to pH 5.9, dried at
40 °C, and then washed.
Acknowledgements. This work was supported by the
CONACYT (J-33049). The authors indebted to Mr. Sabino
Hernández, to Mr. Francisco Jiménez Barreiro, to Ms. Leticia
Carapia, to the technical staff of reactor department, and to thechemical nuclear laboratory for performing the IR and TGA
analyses.
References(1) W. C. Eckelman and B. M. Coursay, Appl. Radiat Isot. 33,
10 (1982).
(2) J. V. Evans and M. E. Shying, Zirconium molybdate gel asa generator for technetium-99m AAEC/E59 (Australian
Atomic Energy Commission, 1984).
(3) J. V. Evans, P. W. Moore, M. E. Shying, and J. M. Sodeau,
Appl. Radiat. Isot. 38, 19 (1987).
(4) P. W. Moore, M. E. Shying, J. M. Sodeau, J. V. Evans, D. J.
Maddalena, and K. H. Farrington, Appl. Radiat. Isot. 38,
25 (1987).
(5) R. E. Boyd, Appl. Radiat. Isot. 42, 1027 (1997).
(6) M. A. El Absy, M. El Nagar, and A. I. Audah, J. Radioanal.
Nucl. Chem. Articles 183, 339 (1994).
(7) M. T. El Kolaly, J. Radioanal. Nucl. Chem. Articles 170,
293 (1993).
(8) P. Vanaja, N. Ramamoorthy, S. P. Iyer, and R. S. Mani,
Radiochim. Acta 42, 49 (1987).
(9) M. Tanase, K. Tatenuma, K. Ishikawa, K. Kurosawa, M.
Nishino, and Y. Hasegawa, Appl. Radiat. Isot. 48, 607(1997).
(10) L V. So, 1999 Workshop on the Utilization of Research
Reactors, Tokai & Mito, (Japan 2000), p187.
(11) D. V. S. Narasimhan, K. P. Vanaja, and R. S.Mani, J.
Radioanal. Nucl. Chem. Letters 85, 345 (1984).
(12) P. Saraswathy, S. K. Sarkar, R.R. Patel, S.S.Arora, and D.
V. S. Narashimhan, Radiochim. Acta 83, 97 (1998).
(13) P. Saraswathy, S. K. Sarkar, G. Arjun, S. K. Nandy, and N.
Ramamoorthy, Radiochim. Acta 92, 259 (2004).
(14) A. Sánchez-Ocampo and S. Bulbulian, Appl. Radiat. Isot.
42, 1073 (1991).
(15) IAEA-TECDOC-852, Procedures for production of 99mTc
generators based on 99Mo conversion to zirconium molybdate. Alternative technologies for 99mTc generators.
(IAEA, Vienna, 1995).
(16) F. Monroy-Guzman, V. Diaz-Archundia, and A. Contreras
Ramírez, Appl. Radiat. Isot. 59, 27 (2003).
(17) T. S. Jimenez Martinez and F. Monroy-Guzman, Proc. Int.
Joint Meeting LAS/ANS-SNM-SMSR, ed. SNM (Mexico,Titanium Molybate Gels as Matrix of 99Mo/ 99mTc Generators

2004) in CD.
(18) F. Monroy-Guzman, E. S. Barron Santos, and S.
Hernandez, Synthesis and Applications of Isotopically
Labelled Compounds Vol. 8, eds. D. C. Dean, C. N. Filer,
and K. E. McCarthy, Wiley, England (2004), p325.
(19) F. Monroy-Guzman, O.Cortés Romero, L. Longoria, and T.
Martínez Castillo, Synthesis and Applications of
Isotopically Labelled Compounds Vol. 8, eds. D. C. Dean,
C. N. Filer, and K. E. McCarthy, Wiley, England (2004),p321.
(20) F. Monroy-Guzman, H. Arriola, I. Ortega Álvarez, O.
Cortés Romero, and L. V. Díaz Archundia, J. Radioanal.
Nucl. Chem. Articles. 271, 523 (2007).
(21) R.C. Nieto and J.A.Osso Jr. Rev. Med. Nucl. Alasbimn J.
5, 1 (2002).
(22) W. T. Elwell and D. F.Wood, The analysis of the titanium,
zirconium and their alloys. (Kynoch Press, Great Britain,1961).
(23) O. Grossmann, Fresenius’ Z. Anal. Chem. 321, 442 (1985).
(24) M. K. T. Bassan, K.V. Iyer, and M. Sudersanan, Ind. J.
Chem. Tech. 10, 291(2003).

(25) R.A. Clark and G.L. Krueger, J. Histochem. Cytochem.

J. Nucl. Radiochem. Sci., Vol. 8, No. 1, 2007

19

33, 729 (1985).
(26) H. Perkampus, UV-Vis Spectroscopy and its applications,(Springer-Verlag, Berlin, 1992).
(27) F. Helfferich, Ion exchange, (Dover, New York, 1995).
(28) Farmacopea de los Estados Unidos Mexicanos (FEUM).
Vol. I, 8 Ed. (Secretaría de Salud, México, 2004).
(29) J. Chalmer and P.Griffths, Handbook of Vibrational
Spectroscopy. (John Wiley & Sons, Chinchest U.K. 2001).
(30) O. Cortés Romero, M. S. Dissertation (Universidad
Nacional Autónoma de México, Mexico, 2004).
(31) H. Díaz Velázquez, M. S. Dissertation (Universidad
Autónoma del Estado de México, Mexico, 2005).
(32) K. Nordlund, J. Keinonen, M. Ghaly, and R.S. Averback,Nature 398, 49 (1999).
(33) M. S. Dadachov, R.F.Howe, and R.M. Lambrecht,
Radiochim. Acta 86, 51 (1999).
(34) A. Clearfiel, Inorganic Ion Exchange Materials, (CRC
Press, USA, 1982).
(35) A. Clearfiel, J. Inorg. Nucl. Chem. 34, 2641 (1972).
(36) K. E. Sickafus, L. Minervini, R. W. Grimes, J. A. Valdez,
M. Ishimaru, F. Li, J. MacClellan, and T. Hartmann,
Science 289, 748 (2000).