Application of ionic liquids as mobile phase modifiers in liquid chromatorgaphy

Ionic liquids are molten salts composed of large, asymmetric, organic cations (e.g. imidazolium or piperidine) and inorganic anions such as e.g chloride or fluoroborate. These compounds are characterized by low melting point, below 1000C, however, they also have other interesting properties including high thermal stability, minor vapor pressure or negligible volatility. Moreover, by the appropriate selection of the cation and anion building the ionic liquid, it is possible to obtain the desired physicochemical properties of these salts. For this reason, ionic liquids are applied to the synthesis, catalysis, electrochemical methods, extraction methods, etc. Application of these compounds in separation techniques merits special attention, especially considering liquid chromatography. Ionic liquids are commonly used in this technique as free silanols suppressors, especially regarding the analysis of basic compounds. Moreover, the excess amount of ionic liquids ions may adsorb on the hydrophobic ligands present at the stationary phase surface, which also plays a significant role in the retention of analytes. Besides their application as silanols suppressors, these compounds were also used as ion pair reagents in the analysis of antisense oligonucleotides, which are short nucleic acid fragments with therapeutic potential due to the ability to bind with complementary sequences of ribonucleic acid. For this reason, antisense oligonucleotides are used in the treatment of several diseases. This article briefly presents structures, properties and the application of ionic liquids as mobile phase modifiers for the analysis of the wide range of different analytes using liquid chromatography. Moreover, a part of this paper was devoted to the analysis of antisense oligonucleotides with the use of the different chromatographic techniques, including the application of ionic liquids as mobile phase additives in ion pair chromatography