Ionic Liquids as Mobile Phase Additives

The popularity of ionic liquids has grown in several analytical separation techniques. Thus, the reports concerning the applications of ionic liquids are stiU increasing. The use of ionic liquids, mainly im-idazoUum-based, associated with chloride and tetrafluoroborate as mobile phase additives in high performance liquid chromatography (HPLC) has been reviewed (1). 

Mostly, ionic liquids just function as salts, but keep several kinds of intermolecular interactions, which are useful for chromatographic separations. Both cation and anion can be adsorbed on the stationary phase, creating a bilayer. This gives rise to hydrophobic, electrostatic and other specific interactions with the stationary phase and solutes, which modify the retention behavior and peak shape (1). 

The beneficial effects of several ionic liquids as mobile phase additives in HPLC using an electrochemical detection for the determination of heterocyclic aromatic amines have been evaluated (2). The tested ionic liquids were l-butyl-3-methyhmidazolium tetrafluoroborate, l-hexyl-3-methylimidazolium tetrafluoroborate, and 

Best resolution, lower peak-widths, and lower retention factors were obtained when using ionic liquids rather than ammonium 

The retention time of the target acid compounds was shortened with the increase of the alkyl chain length and the concentrations of ionic liquids, probably due to the delocalization of the positive charge on the imidazolium cation, the repulsion between chlorine ions of ionic liquids and the acid compounds, as well as the stereohindrance effect (3). 

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Ionic liquids as mobile-phase additives in liquid... 

Flieger, J. Czajkowska-Zelazko, A. (2011b). Ionic liquids as mobile phase additives in reversed phase high performance liquid, J.Liquid Chromatogr. Rel. Technol. In press. 

Flieger, J. (2009). Effect of ionic liquids as mobile phase additives on chromatographic parameters of neuroleptic drugs in reversed-phase high-performance liquid chromatography, Afjfl/..Lett. Vol. 42 (No. 11) 1632-1649. 

Ruiz-Angel, M.J., Carda-Broch, S., and Berthod A. Ionic liquids versus triethylamine as mobile phase additives in the analysis of P-blockers. J. Chromatogr A. 2006, 1119, 202-208. 

Salt modifiers and chromatographic application of ionic liquids in the last few years has generated significant interest [42, 43]. In HPLC these compounds are used as mobile-phase additives for the enhancement of basic analyte reten-... 

Fluoroquinolone Antibiotics. Ionic liquids differing in the length of the alkyl chain were tested as mobile phase additives for the separation using HPLC of fluoroquinolone antibiotics (15). The materials are listed in Table 1.4. Fluoroquinolone antibiotics are shown in Figure 1.9... 

It could be demonstrated that the ionic liquids are useful as mobile phase additives in reversed phase chromatography of phenothiaz-ine derivatives. A very important feature of these additives is their ability to decrease the peak width. In the absence of such strong ion-ion interaction reagents, wide peaks for cationic analytes are usually observed (28). 

Separation science focuses on room temperature ionic liquids (RTlLs), salts that are liquid at ambient temperature. They have been studied as extracting solvents, stationary and mobile phases, mobile phase additives, and other uses. Common RTILs consist of a bulky nitrogen- or phosphorus-containing organic cation (pyridinium or pyrrolidinium, alkyl-imidazolium, ammonium or phosphonium) and a variety of organic and inorganic anions (triflate, dicyanamide, trifluoroacetate, acetate trifluo-romethylsulfate, nitrate, perchlorate, bromide, chloride, chloroaluminate, tetrafluo-roborate, hexafluorophosphate). 

Several chromatographic parameters have been evaluated in the presence or absence of ionic liquids, or using ammonium acetate as the most common mobile phase additive, with three different Cis stationary phases. The effect of the acetonitrile content was also studied. Acetonitrile is shown in Figure 1.2. 

The length of the alkyl on the imidazolium ring and its counterion can also affect the resolution, because part of the ionic liquids coated on the surface of the stationary phase could suppress the free silanols of the surface. The comparison of the ionic liquids with standard mobile phase additives, such as ammonium formate, showed that the ionic liquids have advantages as silanol suppressors in HPLC (17). 

FIGURE 17.1 (iJ)-Af,Af,Af-trimethyl-2-aminobutanol-bis(trifluoromethane-sulfon)imidate as tbe cbiral additive. (A) CE BGE Na-pbospbate buffer pH 6.0 and lOmM ionic liquid, analyte propranolol. (B) HPLC, mobile phase H2O acetonitrile (AcN) (6 4) with 10 mM ionic liquid, analyte 2,2 -diamino-l,l -binafthalene. (C) GC capillary column coated with the chiral ionic liquid, analyte citronella. (Reprinted with permission from Anal. Lett., 39, 1447, 2006. Copyright 2006, Taylor Francis.)... 

Neutral and charged species may be separated by using IEC. In addition, the introduction of an organic modifier to the mobile phase may influence a separation, as do the ionic strength of the solute, the pH, the temperature, the buffer concentration, and the liquid flow rate. 

In liquid chromatography and electrophoretic methods, ILs are mostly used in diluted form in aqueous solutions. If its concentration is lowered to the millimolar range, an IL may be used as a mobile phase ionic additive. Their breakthrough for use in RP-HPLC was due to their ability to suppress deleterious effects of silanophilic interactions that represent the main drawback of silica-based stationary phases they also exhibit many other favorable physical attributes [116]. 

In addition to the three classical separation methods mentioned above, reversed-phase liquid chromatography (RPLC) is becoming increasingly popular for the separation of highly polar and ionic species, respectively. Long-chain fatty acids, for example, are separated on a chemically bonded octadecyl phase after protonation in the mobile phase with a suitable aqueous buffer solution. This separation mode is known as ion suppression [18]. 

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