Quantitative aspects in on-line comprehensive two-dimensional liquid chromatography for pharmaceutical applications.

On-line comprehensive two-dimensional liquid chromatography (on-line LCxLC) is expected to offer impressive effective peak capacity. That makes it an attractive technique for the analysis of complex samples such as pharmaceuticals, for which impurities are often unknown both in number and in structure, and can be either totally different or similar to the active substance. A study on comprehensive on-line RPLCxRPLC (reversed phase liquid chromatography in both dimensions) with respect to quantitative aspect at low concentration levels was carried out with the objective of finding conditions able to meet regulatory requirements for the control of pharmaceutical impurities. Preliminary studies showed that the best approach for quantitative purpose was to relate the impurity concentration to the sum of the second dimension peak areas. RPLCxRPLC conditions were optimized according to a Pareto-optimality approach with a focus on sensitivity, thereby favouring low dilution factor. The resulting RPLCxRPLC separation led to a detection limit below 0.05% for impurities (compared with API). Furthermore, the properties of this analytical method was found to be satisfying in terms of linearity (R2 > 0.999), repeatability (RSD < 1% for second dimension retention times) and precision (RSD < 1.5% for the sum of second dimension peak areas). Finally, this on-line RPLCxRPLC approach was applied to the analysis of a batch of drug substance in the early stages of drug development. The method was able to detect impurities at concentrations as low as 0.04% (signal-to-noise ratio of 3). All these results clearly demonstrate the power of on-line RPLCxRPLC for the quantitative analysis of pharmaceutical samples.