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ANALYTICAL SCIENCES FEBRUARY 2000, VOL. 16
2000 © The Japan Society for Analytical Chemistry
241
Evaluation of Dissolved Species of Lanthanum in the Solutions
Containing Different Amino Acids by Cation-Exchange
Chromatography Coupled with Electrospray Ionization
Mass Spectrometry
Qiuquan WANG, Jianli LIU, Benli HUANG,† Limin YANG, Xuming GUO, and Xiaoru WANG
Department of Chemistry and the MOE Key Laboratory for Analytical Sciences, Xiamen University,
Xiamen 361005, P. R. China
Dissolved species of lanthanum in the solutions which contained ethylenediamine tetraacetic acid (EDTA) and Ltryptophan, respectively, were evaluated by electrospray ionization mass spectrometry. The stability of the species of La
complexes during the cation-exchange chromatographic separation process was discussed. The results indicated that the
speciation of lanthanum in the solution was remarkably influenced by the dissociation of the La species, which is in turn
depended on the cation-exchange column selected.
(Received October 12, 1999; Accepted November 18, 1999)
Because the separation and purification methods of rare-earth
elements (REEs) are well developed, REEs have recently been
widely used in many fields of industry and agriculture. The
widespread application of REEs as additives in fertilizers,
especially in China,1 inherently leads to their residues in the
environment, accumulation in organisms, and entering into the
food chain. Moreover, the bioavailability and toxicity of REEs
mainly depend on its dissolved species, which can be partly
assimilated by plants and animals.2 Thus, the speciation
analysis of REEs in the environment and organisms is becoming
ever more important today. The study of the interaction of
REEs with humic substances and proteins is a useful way to
understand their behavior in the environment or during the
biological process of an organism.
Electrospray ionization (ESI), as a soft source for mass
spectrometry (MS), has recently been used to evaluate the
chemical species of dissolved metal ions; it provides
information on the interaction between metal ions and organic
ligands, which forms the basis of many essential biochemical
processes.3–10 On the other hand, because of the complexity of a
real sample, the chromatographic separation of the target
metallic element species was usually necessary for its
speciation.11–15 In this study, the interaction of lanthanum with
ethylenediamine tetraacetic acid (EDTA), which has donor
atoms of nitrogen and oxygen, and may be considered as a
model ligand existing in the environment, and with Ltryptophen (Try) as the basic unit of a protein, were studied by
the cation-exchange HPLC coupled with ESI-MS. The stability
of La species during a cation-exchange chromatographic
separation process is also discussed in this paper for obtaining
accurate information on lanthanum speciation.
†
To whom correspondence should be addressed.
E-mail: blhuang@xmu.edu.cn
This paper was presented at ASIANALYSIS V, Xiamen, China,
May 4 - 7, 1999.
Experimental
Reagents and chemicals
Lanthanum oxide (La2O3) was obtained from Changchun
Institute of Applied Chemistry of the Chinese Academy of
Sciences; purity >99.9999%. La stock solution (1.00 mg cm–3)
was prepared by dissolving an appropriate amount of La2O3,
which had been ignited at 850˚C for 4 h, in 2 mol dm–3 HNO3,
and then diluted with doubly deionized Milli-Q water to 100 mL
in a polyethylene bottle. Ligand stock solutions (5×10–3 mol
dm–3) were prepared by dissolving EDTA-Na2 (A. R., Shanghai
Reagent Factory) with doubly deionized Milli-Q water, and Try
(purity >99%, Shanghai Bo-Ao Bio-Technology Co.) with a 1:1
water–methanol solution in acid-washed polyethylene bottles.
HPLC-grade methanol was obtained from Shanghai Wusong
Chemical Factory. An ammonium acetate–acetic acid (A. R.,
Shanghai Reagent Factory) solution (1×10–3 mol dm–3, pH 5 – 6)
was used as a buffer. Formic acid (A. R., Shanghai Reagent
Factory) and α-hydroxyisobutyric acid (α-HIBA, Hunan
Institute of Rare-earth Metals of China, A. R.) solutions were
used as mobile phases for HPLC separation. Samples were
prepared by combining the calculated amount of stock and
buffer solutions to give an La concentration of 1×10–4 mol dm–3
and appropriate ligand concentrations, respectively.
Apparatus
An Orion 828 pH meter was used for pH measurements; a
Shimadzu HPLC-10Avp system was used for La species
separation; a Finnigan LCQ MS spectrometer with the ESI
source was used for MS measurements.
Mass spectrometry
Samples were analyzed by continuous-injection ESI-MS using
a 250-µL glass syringe on a Finnigan syringe pump connected
to the ESI probe by a 100-µm-i.d. fused-silica capillary, flow
rate 3 µL min–1. Mass analysis was performed using an ion-trap
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ANALYTICAL SCIENCES FEBRUARY 2000, VOL. 16
Fig. 1 Mass spectra of the species of La-EDTA (H2Y2–) and La-Try (HL) complexes in the solution. LaEDTA complex, 1:1, pH 4.98; La-Try complex, 2:1, pH 5.87.
mass spectrometer fitted with an N2-assisted electrospray probe.
The experimental conditions were as follows: sheath gas flow
rate, 0.65 L min–1; auxiliary gas flow rate, 0.06 L min–1; ion
spray voltage, 3.5 kV; capillary temperature, 200˚C; capillary
voltage, 27 V; tube lens offset, 55 V. Mass spectra were
scanned in the m/z range of 50 – 2000.
HPLC separation
The La species separation was performed on a Shimadzu
HPLC-10Avp. The column was a TSK-GEL cation exchange
column (4.6-mm i.d.×5.0 cm length), and the mobile phases
were ammonium formate–formic acid (1×10–3 mol dm–3, pH 5.0)
solution for the elution of La-complexes (first 5 min), and αHIBA (0.5 mol dm–3, pH 7.0) solution for the elution of La3+
(from 5.01 to 15 min). The sample amount was 20 µL; flow
rate, 0.5 mL min–1; column temperature, 298±1 K.
Results and Discussion
In order to obtain sensitive detection of the target species, La
species in solutions containing EDTA-Na2 and Try were
detected in the negative-ion mode. Moreover, because the
dissociation of the target species in the ESI-MS interface should
be considered,5 the cone voltage of the electrospray ion source
was set at 27 V in this study.
Distribution of the La species in the solutions containing EDTANa2 and Try
The La species distribution in a 1:1 solution of La: EDTA-Na2
containing ammonium acetate (NH4Ac)–acetic acid (HAc)
buffer at pH 4.98 is shown in Fig. 1. H2Y2– stands for EDTA2–;
it has six donor atoms (4O, 2N) forming chelates with
lanthanum. The species of [LaY]– was detected at m/z 426.9,
[LaHYNO3·H2O]– at m/z 509.0, [H(LaY)2]– at m/z 854.9,
[Na(LaY)2]– at m/z 876.9, [Na2(LaY)2Ac]– at m/z 876.9, and
[(NaAc)2(LaY)2]– at m/z 1040.8. From the results shown in Fig.
1, La dominantly existing as [LaY]H and its dimer are
Fig. 2 Interaction between REE species and a strong cationexchange resin during their separation.
associated with such inorganic ions as NO3–, Ac– and Na+ at pH
4.98 in the solution. Similarly, the La species in a 1:2 La:Try
(HL) solution containing the buffer at pH 5.87 was detected as
[La(NO3)4]– at m/z 386.9, [LaL(NO3)3]– at m/z 528.0,
[LaL2(NO3)2]– at m/z 669.0, [LaL3NO3]– at m/z 810.0,
[LaL4·HNO3·HAc]– at m/z 1134.8, [(LaL2)2(NO3)3]– at m/z
1276.0, and [La2L5(NO3)2]– at m/z 1416.9. Because tryptophen
is a bidentate anionic ligand, lanthanum was coordinated by a
nitrogen atom in indolenine and oxygen in hydroxyl of
tryptophen to form complexes. However, there was still “free”
lanthanum ([La(NO3)4]–) in the solution, in agreement with the
result from chemical equilibrium calculation.
Stability of the La species during the process of cationexchange chromatographic separation
For metal-ion speciation in complicated environmental and
biological samples, cation-exchange chromatography was
frequently used to separate those species having different
charges. Lanthanum is a typical hard-acid element, which may
form stable complexes with those ligands having hard donor
atoms, such as oxygen and nitrogen, in the samples, while it
may also strongly interact with the functional group on the
cation-exchange resin during the chromatographic separation
ANALYTICAL SCIENCES FEBRUARY 2000, VOL. 16
243
Fig. 3 Chromatogram of La-EDTA (1:0.8) species and their MS spectra. Stationary-phase, cationexchange resin (–SO3–); mobile phase, 1×10–3 mol L–1 HCOOH (0 – 5 min); 1×10–3 mol L–1 HCOOH (5 – 10
min, 100% – 0), 0.5 mol L–1 α-HIBA (5 – 10 min, 0 – 100%); detection, ESI-MS.
Fig. 4 Chromatogram of the La-Trp (1:2) species and their MS spectra. Stationary phase, cation-exchange
resin (–SO3–); mobile phase, 1×10–3 mol L–1 HCOOH (0 – 5 min); 1×10–3 mol L–1 HCOOH (5 – 10 min, 100%
– 0), 0.5 mol L–1 α-HIBA (5 – 10 min, 0 – 100%); detection, ESI-MS.
process (Fig. 2). The dissociation of La species in the separation
process must be considered for obtaining an accurate result
when a cation-exchange chromatography is employed for
speciation. Here, the stability of the La species in a solution
containing EDTA-Na2 or L-tryptophen was investigated.
Samples of La:EDTA-Na2 (H2Y2–) (1:0.8) at pH 3.76 and
La:Try (HL) (1:2) at pH 5.87 were first separated by cationexchange chromatography, and then determined by ESI-MS.
The results are shown in Figs. 3 and 4. Clearly, La and EDTA
formed very stable chelates, did not dissociate during the cationexchange chromatographic separation, and could be separated
from the “free” lanthanum. However, in the case of La:Try
(HL) (1:2) at pH 5.87, even La and Try could form complexes;
the interaction between La and the functional group (–SO3–) is
thus much stronger than that between La and Try. That is, the
stability of (–SO3–)3-La is higher than that of the La-Try
complex. All of the La species in the form of a complex
dissociated during the separation process, and only “free”
lanthanum could be detected. Thus, for achieving La speciation
in different samples, the types of ligands in the samples and the
stability of the species formed should be carefully considered.
In conclusion, chromatographic separation coupled with ESIMS detection may be a practical tool for the speciation of rare-
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earth elements. Further studies of the species stability, and
suitable selections of the stationary phases used for the species
chromatographic separation are under consideration.
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Acknowledgement
This work was supported in part by the National Nature Science
Foundation of China (No.29735160) and a Grant-in-Aid of
Scientific Research for Returned Scholars from the Ministry of
Education of China. Partial financial support from the Xiamen
Municipal Government for the HPLC is greatly appreciated.
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