American Journal of Anal yt ical Chemistry, 2011, 2, 104-108
doi:10.4236/ajac.2011.22011 Published Online May 2011 (
Copyright © 2011 SciRes. AJAC
Effects of Cross Linking on the Chromatographic Nitrogen
Isotope Separation
Xunyue Liu1, Xingcheng Ding1*, Tatsuya Suzuki2, Masao Nomura2, Yasuhiko Fujii2
1Institute of Nuclear Agricultural Scienc e, Zhejiang University, Hangzhou, China
2Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology, Tokyo, Japan
Received November 15, 2010; revised January 26, 2011; accepted Februar y 9, 2011
The effects of cross linking with porous cation exchange resin were studied for nitrogen isotope separation.
The displacement chromatography was conducted by the resin with cross linking rang from 20% to 40%. A
sharp adsorbed ammonium band was maintained for each operation. Enriched 15N isotopes with 0.93% were
obtained by 20% cross linking resin and two meters chromatographic operation which started from the natu-
ral abundance of ammonium molecule. The effect of cross linking percentage on the height equivalent to a
theoretical plate (HETP) was evaluated in the present work and the HETP value is proportional to the cross
linking percentage. HETP value of 0.036 cm was obtained at the present system by using 20% cross linking
Keywords: Cross Linking, Chromatography, Nitrogen Isotope, Ion Exchange Resin
1. Introduction
Enriched nitrogen isotope 15N has been used in various
scientific and technological fields as a non-radioactive
tracer [1]. In nuclear engineering, nitride fuel is the most
appropriate fuel for the innovative nuclear reactors be-
cause of its outstanding characteristics such as high
melting point, high thermal conductivity, high breeding
ratio, etc. for fast breeder reactors (FBRs). FBRs with the
nitride fuel of highly enriched in 15N isotope are consid-
ered as one of the promising directions for the develop-
ment of safe atomic energy [2]. Natural nitrogen, how-
ever, cannot be used as the material of the nitride fuel,
due to the neutron reactivity of the major isotope 14N.
The major isotope 14N, is converted to the radioactive
isotope 14C in nuclear reactors whose half life is more
than 5000 years by the reaction of 14N (n, p) 14C. The
present urgent problem is lowering the price of nuclear
fuel which is mainly affected by the large expense in 15N
enrichment from the natural abundance of 0.365% up to
the required 99.9% [3]. For the sake of neutron economy
and to avoid environmental risk of 14C, innovative tech-
nologies are needed for the production of highly 15N en-
riched nitrogen.
Nitrogen isotope separation by ion exchange chroma-
tography has been studied since Urey et al. made ex-
periments between gaseous ammonia and ammonium
salts in aqueous solution using a cation exchange resin
[4]. Separation and concentration of a stable isotope from
an isotopic mixture with natural occurrence is a very
complex problem and usually, the isotopic separation
coefficients of natural abundance are very low. Recently,
many studies have been focused on nitrogen isotope
separation [5-10] and chemical exchange method in iso-
tope separation had been proved to be the most promis-
ing method for light element isotope separation since
firstly performed by Spedding et al. [11]. In this process,
strong acidic cation exchange resin was packed in
colums and ammonium ions were adsorbed in resin
phase. Then, a NaOH solution was fed into the columns
as an eluent. During this process, the light isotope 14N
was enriched at the front boundary of ammonium ion
adsorption band and the heavy isotope 15N was enriched
at the rear boundary region. In general, cation exchange
process is a promising technique to produce highly en-
riched isotope due to the nature of small HETP value.
Among the operating parameters in cation exchange
process, cross linking is know as a decisive factor on the
process of chromatographic isotope separation. In this
paper, as a continue work of our previous work, we
studied nitrogen isotope separation by cation exchange
resin with different percentage of cross linking to evalua-
X. Y. LIU ET AL.105
tion the chromatographic performance of 15N isotope
2. Experimental
In the present work the experiments were performed by
the cation exchange resin which was synthesized from
the law material of styrene, the synthetic method was
given in Figure 1 and Figure 2 was the microscope
photographs of the synthesized resin. The detailed ex-
perimental conditions were shown in Table 1. The syn-
thesized resins were packed uniformly in a jacketed glass
column (1.0 m long, 8 mm I. D) and was preliminarily
conditioned to H+ form with 2.0 mol/dm3 HCl solution,
then rinsed with pure water to remove free H+ ions. One
meter ammonium adsorption band was formed in the
column by feeding (0.2 ± 0.1) mol/dm3 NH4OH and was
eluted by the eluent of ca. (0.2 ± 0.1) mol/dm3 NaOH
solutions which fed at the flow rate of (1.0 ± 0.1) ml/min.
The ammonium adsorption band moved down in the
columns at the speed of 5.2 m/day. The thermostated
water was circulated through the jacket to maintain the
temperature at (308 ± 1.0) K throughout the experiments.
The effluent out of the last column was collected in
fractions by a fraction collector. Prior to the sampling, a
certain volumes of HCl solutions were put into the sam-
pling tubes to neutralize or acidify the sampled fractions.
The sampled fractions of effluent were subjected to con-
centration analysis of ammonium ion by Yokogawa-
Hokusin IC-100 ion chromatography analyzer (Tokyo,
Japan). Nitrogen isotopic abundance ratios of 14N and
15N in the samples were measured by two mass spec-
trometers ESCO EMD-05S double focusing mass spec-
trometer for gas samples and IsoPrime EA mass spec-
trometer, which accepts solid samples such as NH4Cl.
When ESCO EDM-05 was used, the samples collected in
the chemical form of NH4Cl were converted to N2 sam-
ples by KBrO solutions prior to the analysis by mass
spectrometry and the isotopic abundance of 15N was cal-
culated from the ratio of the peak height.
Styrene Divynylbenzene(DVB) Styrene/DVB resin
esin Ion-exchan
Figure 1. Synthetic method of the cation exchange resin.
(a) (b)
Figure 2. The microscope photographs of the synthesized resin: (a) Particle size: 60 - 100 mesh, cross linking 20%; (b) Parti-
cle size: 100 - 250 mesh, cross linking 40%.
Copyright © 2011 SciRes. AJAC
Table 1. Experimental conditions.
Cation exchange resin: Strongly acidic type, cr oss linking 20% - 40%, particle size 60 - 250 mesh
Feed solution: (0.2 ± 0.1) mol/dm3 NH4OH
Effluent solution: (0.2 ± 0.1) mol/dm3 NaOH
Column diameter: 8 mm
Flow rate: (1.0 ± 0.1 ) mL/min
Migration distance: (2.0 ± 0.05) m
Band velocity: (5.2 ± 0.2) m/d
Temperature: (308 ± 1.0) K
3. Results and Discussion
Three different percent cross linking resins were per-
formed for nitrogen isotope separation. The chroma-
tographic concentration curve and isotopic enrichment
profile of 20% cross linking resin as an example was
given in Figure 3. It could be seen in Figure that the pro-
files of ammonium ion bands were very sharp at the
boundaries and 15N was apparently enriched at the rear
boundary. It is necessary to maintain the sharp ammo-
nium band in order to obtain highly enriched 15N; the
results clearly indicate that the present chromatographic
operation conditions were suitable for nitrogen isotope
separation. Since the chromatographic operations were
conducted in a reverse breakthrough manner, the isotopic
abundance ratios in the middle band fractions of the
chromatogram were equal to the isotopic abundance in
the original feed solution. This means no remixing oc-
curred in the middle band during the present operation.
When different cross linking was synthesized, high
cross linking means high divynylbenzene (DVB) materi-
als contained in the resin structure and this means the
percentage of ion exchange functional group was de-
creased in the same amount resin. Figure 4 was the rela-
tionship between 15N enrichment percentage and cross
linking. Enriched 15N isotopes were decreased from 0.93
to 0.68 when compared with the cross linking of twenty
and forty percentage. Since high cross linking means low
exchange capacity, it is reasonable that the 15N enrich-
ment percentage is reduced with the increase of cross
linking at the same given migration distances. SQS-6
resin was the commercial resin and with 8% cross link-
ing which used in References [8-10], 15N enrichment
percentage was 1.56% when SQS-6 resin was performed
under the same condition, and low cross linking resin has
much higher enrichment ability than high cross linking
The separation of nitrogen isoto pes b y means of cation
exchange resin is based on the isotopic fractionation be-
tween ammonia in aqueous solution and ammonium ion
in the ion exchange resin as shown below:
R14NH4+ + 15NH3 = R15NH4+ + 14NH3 (1)
where R represents the fixed anion in the resin.
In order to evaluate the performance of the 15N en-
richment in displacement chromatography, the height
equivalent to a theoretical plate (HETP) is in troduced. In
a transient state, as in the case of the present work, HETP
is usually obtained by a computer calculation fitting
Figure 3. Chromatographic concentration curve and iso-
topic enrichment profile of 20% cross linking resin.
Copyright © 2011 SciRes. AJAC
X. Y. LIU ET AL.107
Figure 4. Relationship between enriched 15N and cross
based on a cascade theory. However, Fujii et al. pro-
posed more convenient equations to analyze chroma-
tographic data on isotope separation [12]. These equa-
tions are used in the present work and the HETP is de-
termined by the following equatio n for the transient state
isotope separation:
1exp 1
When the enrichment extent is not so large, i.e.
 (3)
where H is the HETP, the separation coefficient, k is the
slope coefficient, Ro is the original isotope atomic frac-
tion, L is the migration length.
The observed HETP of different cross linking was
plotted in Figure 5. It is seen in the figure that cross
linking can affect the HETP value obviously. HETP val-
ue proportionally increase with the cross linking at the
present work and low cross linking has much advantage
for HETP. HETP value of 0.036 cm was obtained at the
present system by using 20% cross linking resin. Previ-
ously, Aida et al. reported the boron isotope separation
by using commercially available anion exchange resin
(Diaion WA-21, 60 - 80 mesh) and got the value of
HETP was 0.18 cm [13]. Ohwaki et al. reported nitrogen
isotope separation by using HITEC-H1 cation exchange
resin (porous strong acid type, cross linking 30%, particle
size 37 - 77 um) and the value of HETP was 0.019 cm
[7,14]. In our previous work [8-10], SQS-6 resin (8%
cross linking, particle size 60 - 80 um) was applied to
nitrogen isotope separation and the value of HETP was
0.02 cm. Comparing with above mentioned HITEC-H1
Figure 5. Observed HETP values and cross linking (Note:
SQS-6 was the different type resin which used by our pre-
viously experiments, its detail information can see the Ref-
erences [7-10]).
and SQS-6 resin, the present resin has quite large HETPs.
Sugiyama and his co-worker studied nitrogen isotope
separation by using cryptand resin [15], the HETP value
was evaluated (0.5 ± 0.2) cm. The present resin has much
advantage than that cryptand resin. HETP values are
small in the present operation when the enrichment of
15N is steadily proceeding and the present resin can be
used for the nitrogen isotope separation.
4. Conclusions
Chromatographic nitrogen isotope separation was con-
ducted by using cation exchange resin with different
cross linking percentage cation exchange resin. It was
found that a sharp boundary was obtained for each dif-
ferent cross linking chromatographic operation and it is
important for nitrogen isotope enrichment. With the
same migration distance, low cross linking resin has the
large n itrogen iso top e en r ichment capacity. The values of
HETP of the present study have shown increase with
high cross linking, the present 20% cross linking resin
has quite large HETP value compare with other type of
cation exchange resins but much smaller than the cryp-
tand resin in Reference [15].
5. Acknowledgements
The present paper was partially financially supported by
the Special Fund for Agro-scientific Research in the
Public Interest (201103 007).
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