SHORT COMMUNICATION

Jeff E. Prest Æ Peter R. Fielden

The use of indium(III) as a complexing counter-ion to enablethe separation of chloride, bromide, and iodide using isotachophoresis

Received: 4 March 2005 / Revised: 12 April 2005 / Accepted: 20 April 2005 / Published online: 20 May 2005

� Springer-Verlag 2005

Abstract A new method has been devised to enable thedetermination of halide anions by isotachophoresis. Thismethod uses an electrolyte system that employs in-dium(III) as a counter-ion to manipulate the effectivemobilities of sample species by means of complexationreactions. This new procedure successfully enabled thesimultaneous determination of the halide ions chloride,bromide, and iodide when a 12 mmol L�1 nitrate-basedleading electrolyte containing 3.5 mmol L�1 in-dium(III) at pH 3.0 was used.

Keywords Isotachophoresis Æ Electrophoresis Æ Halideseparation

Introduction

The halide ions chloride, bromide, and iodide are animportant group of ions which have traditionally beendifficult to analyse using electrophoretic separationmethods. The determination of these species by isota-chophoresis (ITP) is particularly problematic. The majorlimitation in the determination of these ions is that theyare all highlymobile species. Thismakes it difficult to fulfilone of themajor requirements of the electrolyte system forisotachophoretic analysis, namely that the leading ionmust have a higher mobility than any ions of interest. Onesuitable species, the dithionate ion, has been identifiedwhich has a sufficiently high mobility for use as a leadingion in the determination of chloride [1]. However, the useof dithionate does not overcome the problem that chlo-ride, bromide, and iodide have very similarmobilities. Theabsolute mobilities of these species are 79.1·10�9,80.9·10�9, and 79.6·10�9 m2 V�1 s�1, respectively [2].

Thus to enable the separation of these species the use ofadditional electrolyte additives is required. Proposedmethods for manipulating the effective mobilities of ha-lide ions in isotachophoretic separations have involvedthe use of complexation with cadmium(II) [3], formationof host–guest complexes with a-cyclodextrin [4], interac-tionwith the neutral polymer polyvinylpyrrolidone (PVP)[5], and ion-pairing with tetradecyldimethylbenzylam-monium (Zeph) [6]. Of these approaches the most suc-cessful to date in enabling the simultaneous determinationof halide anions by ITP with an aqueous electrolyte sys-tem is that involving use of cadmium(II) [3]. Unfortu-nately this species is a highly toxic known carcinogen [7],and as such its use is undesirable.Analternative strategy isto use organic solvents such as methanol [8, 9]. Such anapproach does, however, complicate the preparation ofsamples and electrolytes and places limitations on theinstruments which can be used for the separations.

This paper describes a new method enabling iso-tachophoretic determination of halide ions with anaqueous electrolyte system. This method uses a novelapplication of indium(III) as a complexing counter-ionin the leading electrolyte to achieve the separation ofchloride, bromide, and iodide ions.

Experimental

A miniaturised poly(methyl methacrylate) separationdevice with an integrated conductivity detector was usedto perform the isotachophoretic analyses. The devicecomprised two channels—a 44-mm-long, 200-lm-wide,300-lm-deep separation channel and a 57-mm-long,300-lm-wide, 300-lm-deep injection channel. A sche-matic diagram of the layout is shown in Fig. 1. Fulldetails of the device have been previously described [10].Injection and solution transport was achieved by meansof a gravity feed hydrodynamic system controlled usingLabVIEW software (National Instruments, Austin, TX,USA). Full details of the instrumentation are publishedelsewhere [11]. Separations were performed using an

J. E. Prest (&) Æ P. R. FieldenSchool of Chemical Engineering and Analytical Science,The University of Manchester, PO Box 88,Manchester, M60 1QD, UKE-mail: j.prest@manchester.ac.uk

Anal Bioanal Chem (2005) 382: 1339–1342DOI 10.1007/s00216-005-3267-4

applied current of 40 lA, which was reduced to 20 lAfor detection of the samples.

The electrolyte system developed incorporated aleading electrolyte composed of nitric acid (2 mol L�1

volumetric standard, Riedel–de Haen, Gillingham,Dorset, UK) and indium nitrate (99.9%, Aldrich, Gill-ingham, Dorset, UK). The terminating electrolyte was20 mmol L�1 cyanoacetic acid (99%, Aldrich). Hy-droxyethylcellulose (HEC) (molecular weight ca.250,000, Aldrich) was added to the leading electrolyte tosuppress electroosmotic flow and the pH of the elec-trolyte was adjusted to 3.0 using glycylglycine (99+%,Acros, Loughborough, UK). Chloride and fluoridesamples were prepared using an ion standard solution(1000 mg dm�3, BDH, Poole, UK). Bromide, nitrite,phosphate, and sulfate samples were prepared usingsodium salts (>99%, Aldrich). Iodide (99.5%, BDH),carbonate, and nitrate (>99%, Aldrich) samples wereprepared using potassium salts. All samples were pre-pared using >18 MX water (Elga Maxima Ultra Pure,Vivendi Water Systems, High Wycombe, UK).

Results and discussion

Examination of metal-anion stability constants revealedthat indium(III) forms relatively stable complexes withhalide ions. The formation constants (log K1) of thechloride, bromide, and iodide complexes are 2.41, 2.01,and 1.64, respectively [12]. The hydrolysis of indium(III)occurs at a sufficiently high pH, p Ka1=3.54 [13], that itwas thought this species could be used to manipulate theeffective mobility of halide anions during isotachopho-retic analysis. Complexation between indium(III) andnitrate is negligible (log K1=0.18 [12]), enabling thisspecies to be used as the leading ion.

To investigate the use of indium(III) as a complexingcounter-ionic leading electrolyte component a series ofexperiments was performed to investigate the effect ofthis species on the mobilities of the chloride, bromide,and iodide ions. These experiments involved analysingthe halide ions individually using leading electrolytescontaining different indium(III) concentrations andrecording the relative step heights (RSH) observed. Inthis work the RSH was calculated as the ratio of thesample step height to the terminating ion step height. InITP step heights are related to mobility, thus the RSHgives an indication of the effective mobilities of thespecies. Figure 2 shows the results obtained by varyingthe indium(III) concentration from 0 to 4 mmol L�1. Inthe absence of indium(III) all three species have similareffective mobilities, which are greater than that of thenitrate leading ion, and thus no sample zones were ob-served. With increasing indium(III) concentration theeffective mobilities, of first chloride, then bromide, andfinally iodide, are retarded sufficiently so that they arelower than the effective mobility of the leading ion. Itcan be seen that indium has the most significant effect onthe mobility of chloride, because this species has thehighest RSH of the three halide ions studied. The ob-served trend corresponds to the order of the stabilityconstants for indium(III)–halide complexes, which ischloride>bromide>iodide. The leading electrolytecontaining 4 mmol L�1 indium(III) produced lower thanexpected RSH values. This result may be slightlyanomalous in that to enable the leading electrolyte tohave a pH 3.0, a higher leading ion concentration wasrequired than for the other electrolyte systems used.Thus, the leading ion concentration in the system con-taining 4.0 mmol L�1 indium(III) was 13.5 mmol L�1,whereas in all the other systems it was 12 mmol L�1. InITP the composition of the leading electrolyte governs

Fig. 1 Schematic diagram of miniaturised poly(methyl methacry-late) separation device

Fig. 2 Effect of indium concentration on the relative step heightsobserved for chloride, bromide, and iodide. The leading electrolyteis at pH 3.0, the leading ion is nitrate and the terminating ion iscyanoacetate. Error bars shown represent two standard deviationsbased on three replicate runs

1340

many of the properties of the separation system. Thusslightly changing the concentration of the leading ionwill introduce additional changes to the equilibrium ofthe complexation reactions involving the halide ions.

The results shown in Fig. 2 indicated that separationof the three-halide ions under investigation should bepossible. A leading electrolyte containing 3.5 mmol L�1

indium(III) was thus used for subsequent analyses. Suchan electrolyte system was selected because it should yielda higher-resolution separation than a system containing3.0 mmol L�1 indium(III) and allow the use of a lowerleading-ion concentration than one containing 4 mmolL�1 indium(III). Figure 3 shows an example of a sepa-ration of chloride, bromide, and iodide using a leadingelectrolyte containing 3.5 mmol L�1 indium(III). Thegood clear separation of all three species is clearlyapparent. This is a significant improvement on many ofthe strategies proposed for analysing these ions usingITP. Previously the use of Zeph [6] and PVP [5] enableddetermination of bromide and iodide and use of meth-anol [8, 9] enabled determination of bromide and chlo-ride. The use of a-cyclodextrin [4] only enabled analysisof iodide. Although separation of iodide, bromide, andchloride has been achieved by using cadmium(II) as acomplexing agent [3], to achieve a good separation ofthese ions required the use of a higher concentration ofthe complexing agent, 6 mmol L�1, than in the newmethod devised in this study. The approach developed inthis work also has the particular advantage in theanalysis of chloride that indium(III) forms more stablecompounds with this species than with bromide or io-dide whereas the converse is true with cadmium(II).Therefore, if chloride were the only species of interest alower indium(III) concentration, e.g. 1 mmol L�1, couldbe used.

Further experiments revealed there to be no interfer-ence with the determination of chloride, bromide, or io-dide from other common anions. Nitrate was the leadingion used in this work so that the presence of this species inany samplesmerely adds to the leading zone and thus doesnot produce a visible step. Indium(III) complexes sulfateto a greater degree than chloride, so this species always hasa lower effective mobility. When indium(III) concentra-tions greater than 2.5 mmol L�1 were used, sulfate wasslower than the terminating cyanoacetate zone and thusnot observed. Because of the low pH used, carbonate,fluoride, nitrite, and phosphate are only partially ionised,and so these ions have low effective mobilities and do not,therefore, impede the determination of the halide anions.The lack of interference from such substances means thedeveloped method could be used for analysis of watersamples and industrial process streams, an application towhich isotachophoresis is particularly well suited [11, 14].

Reasonably reproducible RSH values were achievedwith halide mixtures. The RSH (±SD, n=15) forchloride, bromide, and iodide were 0.529±0.033,0.304±0.027, and 0.148±0.031, respectively. Pre-liminary results from triplicate determinations of sixchloride samples at concentrations ranging from 5 to50 mg L�1 showed linearity was good (r = 0.999) andthe limit of detection (LOD) was 1.2 mg L�1. The LODwas calculated as the value of the intercept of theweighted linear regression line plus three times thestandard deviation of this value.

Conclusions

A new method for determination of the halide ions byisotachophoresis has been developed. This methodincorporates a novel application of indium(III) as acomplexing counter-ion which enables the separation ofthese species. Preliminary results show that an electro-lyte system containing 3.5 mmol L�1 indium(III) hasgreat potential and enables simultaneous determinationof chloride, bromide, and iodide. Application of thisnew complexing agent as a counter-ion in the back-ground electrolyte also offers the potential of improvedresolution between these ions when performing capillaryzone electrophoretic separations.

Acknowledgements The authors would like to thank the UnitedKingdom Home Office for funding this research programme andthe Forensic Explosives Laboratory, of the Defence Science andTechnology Laboratory, Fort Halstead, for their support.

References

1. Meissner T, Eisenbeiss F, Jastorff B (1999) J Chromatogr A838:81–88

2. Lide DR (ed) (1993) CRC handbook of chemistry and physics.Chemical Rubber Publishing Co., Boca Raton

3. Bocek P, Miedziak I, Deml M, Janak J (1977) J Chromatogr137:83–91

Fig. 3 Isotachopherogram of a separation of a sample containing7.5 mg L�1 chloride, 15 mg L�1 bromide and 40 mg L�1 iodide.Leading electrolyte 1.5 mmol L�1 HNO3, 3.5 mmol L�1 In(NO3)3,0.1% w/v HEC, pH 3.0 (glycylglycine). Terminating electrolyte20 mmol L�1 cyanoacetic acid. LE leading electrolyte, TEterminating electrolyte

1341

4. Fukushi K, Hiiro K (1990) J Chromatogr 518:189–1985. Madajova V, Turcelova E, Kaniansky D (1992) J Chromatogr

589:329–3326. Fukushi K, Hiiro K (1997) J Chromatogr A 760:253–2587. Gangolli S (ed) (1999) The dictionary of substances and their

effects. Royal Society of Chemistry, Cambridge, UK8. Beckers JL, Everaerts FM (1970) J Chromatogr 51:339–3429. HirokawaT,Tsuyoshi T,KisoY (1987) JChromatogr 408:27–41

10. Prest JE, Baldock SJ, Fielden PR, Goddard NJ, Treves BrownBJ (2003) Anal Bioanal Chem 376:78–84

11. Prest JE, Baldock SJ, Fielden PR, Goddard NJ, Kalimeri K,Treves Brown BJ, Zgraggen M (2004) J Chromatogr A1047:289–298

12. Tuck DG (1983) Pure Appl Chem 55:1477–152813. Dean JA (ed) (1999) Lange’s handbook of chemistry.

McGraw–Hill, London14. Prest JE, Baldock SJ, Fielden PR, Goddard NJ, Treves Brown

BJ (2003) Analyst 128:1131–1136

1342