Suppression of deleterious effects of free silanols in liquid chromatography by imidazolium tetrafluoroborate ionic liquids

https://doi.org/10.1016/j.chroma.2003.09.020Get rights and content

Abstract

Silica-based stationary phases are commonly used in liquid chromatography, but their surface acidity causes known problems, especially when separating basic compounds. Deleterious effects of free silanols are not fully removed by standard prevention procedures consisting in adding alkylamines or other amino quenchers to the eluents. We found that ionic liquids of the imidazolium tetrafluoroborate class, added to mobile phases at concentrations of 0.5–1.5% (v/v), blocked silanols and provided excellent thin-layer chromatographic separations of strongly basic drugs which were otherwise not eluted, even with neat acetonitrile as the mobile phase. The silanol suppressing potency of imidazolium tetrafluoroborates was demonstrated to markedly exceed that of the standard mobile phase additives, like triethylamine, dimethyloctylamine and ammonia. The proposed new mobile phase additives were also demonstrated to provide reliable lipophilicity parameters of base drug analytes as determined by gradient mode of high-performance liquid chromatography. By applying the readily available and environmentally friendly imidazolium tetrafluoroborate ionic liquids, simple and efficient means of improvement of liquid chromatographic analysis of organic bases were elaborated.

Introduction

The favorable physical characteristics of silica makes silica-based stationary phases the most popular in liquid chromatography, both in HPLC and in TLC [1]. Silica is also used to make capillaries for CE. However, a serious undesirable property of silica is its surface acidity due to the free or isolated (non-hydrogen-bonded) silanols. Effects of free silanols on HPLC and TLC retention are difficult to control and are especially deleterious as regards the chromatographic behavior of basic analytes. Retention of acids can also be affected by the free silanols due to electrostatic exclusion phenomena [2]. The problem concerns even the most modern highly purified silica supports, including those considered to be the least acidic ones [3]. The reason is that the coverage of the silanol groups in the chemically bonded phases is less than 50–60% [4] and the physical deactivation of silica by adding adsorbable cations to the mobile phases provides coverage of only one third of all the silanol groups [5], [6], [7].

Numerous adsorbable amino quenchers have been tested to suppress free silanol effects in liquid chromatography and in capillary electrophoresis. An exhaustive list of these agents, with respective original references, has recently been provided by Righetti et al. [8]. Those authors mention: triethylamine (TEA), propylamine, morpholine, glucosamine, galactosamine, N,N-diethylethanolamine, N-ethyldiethanolamine, triethanolamine, ethanolamine, hydroxylamine, ethylamine, tetramethylammonium chloride, 1,3-diaminopropane, 1,4-diaminobutane (putrescine), 1,5-diaminopentane (cadaverine), ethylendiamine, N,N,N′,N′-tetramethyl-1,3-butanediamine, hexamethonium bromide, decamethonium bromide, diethylenetriamine, triethylenetetramine, N,N′-bis(3-aminopropyl)-1,4-butanediamine (spermine), 1,4,7,10-tetraazocyclodecane (cyclen), chitosan, polyethylenimine, polydimethylallyl ammonium chloride and the recently introduced quenchers for dynamic coating of silica walls in capillary electrophoresis, i.e. quaternary piperazine derivatives like N-(methyl-N-ω-iodo-butyl), N′-methylpiperazine [9], [10].

The mobile phase additives most often applied in liquid chromatography are triethylamine and dimethyloctylamine (DMOA) [11]. In TLC, ammonia is still commonly used. These additives are often ineffective in case of strongly basic analytes. Moreover, their presence causes slow equilibration of the chromatographic system when changing mobile phases [1]. Hence, their use should be avoided, especially if the gradient elution mode is to be applied.

In search for efficient suppressors of free silanols, we turned our attention to the imidazolium tetrafluoroborate derivatives possessing properties of ionic liquids. Ionic liquids (“green solvents”) call much attention as solvents for catalytic and organic reactions due to their unique interactions with the active species [12], [13].

Alkylammonium nitrate and thiocyanate liquid salts have already been studied as potential solvents for reversed-phase liquid chromatography [14]. These compounds have not called wider interest, however. Whereas the alkylammonium nitrate salts were claimed to be suitable for eluent strength modification in HPLC, the thiocyanates appeared of a little use due to their corrosive action on the chromatographic system. Instead, tetralkylammonium sulfonate ionic liquids were studied as potential stationary phases in gas chromatography [15]. Later on, also 1-butyl-3-methylimidazolium hexafluorophosphate and the analogous chloride salt [16] as well as 4-methyl-n-butylpiridinum tetrafluoroborate [17] were employed as stationary phases in GC. On the other hand, dialkylimidazolium-based liquid organic salts were used as buffer electrolytes in non-aqueous capillary electrophoresis [18], [19]. Room temperature ionic liquids were employed as running electrolytes in CE by Yanes et al. [20].

Previous attempts to exploit ionic liquids focused on the modification of interactions of analytes with the mobile phases. No actual advantages of ionic liquids over the conventional chromatographic eluents have been demonstrated, however. That does not mean that the strong proton–acceptor properties of new classes of ionic liquids cannot be utilized in chromatography, e.g. to suppress deleterious effects of free silanols on liquid chromatographic separations.

Dialkylimidazolium ionic liquids that contain such anions as [BF4] are water-stable compounds which dissolve in typical liquid chromatographic solvents, like acetonitrile. By attaching alkoxy group to imidazolium cations new ionic liquids have recently been obtained which display particularly strong antielectrostatic effect [21]. These properties attracted our attention.

A separate analytical problem caused by free silanols concerns lipophilicity determinations by liquid chromatography [22]. The first attempts to improve correlations between reference parameter of lipophilicity, log P, and the reversed-phase HPLC retention factors, log k, determined on chemically bonded alkylsilica columns included the reduction of free silanol sites in the column by the additional silylation. To further improve determination of lipophilicity of neutral and acidic compounds, Unger and Chiang [23] used phosphate buffer with added NaCl to which N,N-dimethyloctylamine at a concentration of 4 mmol/l was added. The lipophilic N,N-dimethyloctylamine was to swamp out the binding of analytes to residual silanol sites on the stationary phase material. Here, we propose ionic liquids as the residual free silanol blocking agents.

The following ionic liquids were subjected to the study: 1-ethyl-3-methylimidazolium tetrafluoroborate (IL 1) from Aldrich (Milwauke, WI, USA), 1-methyl-3-hexylimidazolium tetrafluoroborate (IL 2) and 1-hexyl-3-heptyloxymethylimidazolium tetrafluoroborate (IL 3), both synthesized by Pernak et al. [21]. IL 2 is at present also available commercially (Fluka, Buchs, Switzerland). As the test analytes served eight basic drugs which were found in preliminary experiments not to be moved from the application spot on neither the silica- nor octadecylsilica-covered TLC plates by 100% acetonitrile as the eluent. Among test analytes were four phenothiazine derivatives reported previously [24] to interact strongly with silica surface. Additional test compounds were two acids (acetylsalicylic acid and salicylic acid), phenol and 2,3-dimethoxytoluene. In the study of the effects of ionic liquid additive on chromatographic parameters of lipophilicity, a series of known basic drugs was used.

Section snippets

Materials

Ionic liquids IL 1, IL 2 and IL 3 were used as obtained, without any additional pretreatment. Test analytes were from the reference drug substance and reagent collection of the Department of Biopharmaceutics and Pharmacodynamics, Medical University of Gdańsk. Acetonitrile and methanol of chromatographic quality were from Merck (Darmstadt, Germany). Ammonia (NH4OH) was from POCh (Gliwice, Poland). Triethylamine and dimethyloctylamine were from Fluka (Buchs, Switzerland). Water was prepared with

Results and discussion

In Fig. 1, the structural formulae of the three imidazoline tetrafluoroborate ionic liquids tested are given. All the agents affected the TLC retention of basic drugs on both underivatized silica and on the octadecylsilica stationary phases when added to the eluent. Fig. 2 illustrates the effect of concentration of one of the ionic liquids, IL 1, added to neat acetonitrile as the eluent, on the retardation factor of eight basic drugs on an octadecyl-bound silica stationary phase. As evident

Conclusions

A simple and effective approach is proposed to reduce deleterious effects of free silanols on liquid chromatographic separation of basic and acidic analytes. Imidazolium tetrafluoroborate ionic liquids employed for that purpose are convenient to use, inexpensive, non-explosive, do not oxidize, and have no measurable vapor pressure [30]. The replacement of the commonly used alkylamine additives with the readily available imidazolium tetrafluoroborates improves chromatographic separations of the

References (30)

  • J.J. Gilroy et al.

    J. Chromatogr. A

    (2003)
  • E. Bayer et al.

    J. Chromatogr.

    (1987)
  • S.M. Hansen et al.

    J. Chromatogr.

    (1991)
  • J. Nawrocki et al.

    J. Chromatogr.

    (1988)
  • L.C. Tan et al.

    J. Chromatogr. A

    (1998)
  • R. Sebastiano et al.

    J. Chromatogr. A

    (2000)
  • E. Olivieri et al.

    J. Chromatogr. A

    (2000)
  • H.A. Claessens

    Trends Anal. Chem.

    (2001)
  • P.H. Shetty et al.

    J. Chromatogr.

    (1987)
  • S.K. Poole et al.

    Anal. Chim. Acta

    (1989)
  • M. Vaher et al.

    J. Chromatogr. A

    (2002)
  • T. Dzido et al.

    J. Chromatogr.

    (1990)
  • L.R. Snyder, J.J Kirkland, J.L. Glajch, Practical HPLC Method Development, second ed., Wiley, New York, 1977, p....
  • J.H. Knox et al.

    Faraday Discuss. R. Chem. Soc.

    (1980)
  • P.G. Righetti et al.

    Electrophoresis

    (2001)
  • Cited by (169)

    View all citing articles on Scopus
    View full text