Triethylamine (TEA) is an important additive used in chromatography, particularly in reverse-phase liquid chromatography RPLC), where it influences the retention of analytes. Its impact can be understood through several mechanisms. Like any technique, RPLC comes with its challenges, particularly when dealing with basic compounds, which constitute about 70% of drug substances.
✅The Role of Triethylamine in Mitigating Peak Tailing of Basic Compounds:
In RPLC, the stationary phase typically consists of nonpolar entities like C8 or C18 bonded to a silica base. Silica surfaces inherently have free silanol (SiOH) groups. Not all of these groups bind with C8 or C18, leaving about 50% or so as free silanol groups, especially in non-endcapped stationary phases.
Silanol groups are acidic and can interact with basic analytes, causing tailing or increased retention times. Adding TEA (which is again a base) to the mobile phase, it interacts with these acidic silanol groups, effectively reducing their ability to interact with basic analytes. This leads to better peak shapes of basic compounds.
✅How TEA Influences Retention Time:
By adding TEA to the mobile phase, it interacts with the ionized silanol groups, reducing the secondary interactions of the protonated basic compounds with the stationary phase. This leads to a decrease in the retention time for basic compounds.
✅The Ion Pairing Effect of TEA in Acidic pH:
TEA also influences retention times through ion pairing, particularly in acidic conditions. In its protonated form, TEA transforms into triethyl ammonium cations. These cations, through their nonpolar ethyl groups, interact with the nonpolar components of the stationary phase (C8, C18), following the principle of 'like attracts like.'
This interaction can lead to an increased retention of ionized acidic compounds (negatively charged compounds), as they are attracted to the positively charged triethyl ammonium cations present on the stationary phase.
However, the retention time of ionized bases decreases due to repulsion from the similarly charged stationary phase.
✅Implications for Non-Ionized Compounds:
It's essential to note that the retention time of non-ionized compounds remains unaffected by the presence of TEA, as they do not interact with the silanol groups.
✅Solvent Properties Modification: TEA can also alter the properties of the mobile phase. By changing the solvent strength or polarity, it can influence the overall retention behavior of analytes on the stationary phase. This is particularly important when separating compounds with varying polarities. ✅Competitive Interactions: TEA can compete with analytes for interaction sites on the stationary phase. By binding to potential interaction sites, TEA can prevent analytes from binding as strongly, which can reduce their retention time, especially for basic analytes.
✅Conclusion: The Significance of Triethylamine
In conclusion, TEA in the mobile phase plays a critical role in reverse-phase chromatography, particularly for basic compounds. It reduces the peak tailing and retention time of ionized bases and influences the separation efficiency. Understanding its function and application is crucial for anyone venturing into pharmaceutical analysis.