"Inverse" cloud point extraction coupled with large volume injection ion-pair chromatography: A green route integrating extraction, challenging sample cleanup and on-column concentration into fast simple operation.

The efficient isolation of trace hydrophilic compounds from complicated aqueous-rich samples remains a daunting challenge. Herein, to address the analytical bottleneck, a novel inverse cloud-point extraction (ICPE) strategy was proposed based on the extraction principle opposite to that used in traditional CPE. Then, an original large-volume injection ion-pair chromatography (LVI-IPC) method was developed and systematically investigated based on the retention mechanism of the dynamic ion-exchange model. The combined ICPE-LVI-IPC method integrated extraction, challenging sample cleanup, and (on-column) concentration in a fast, simple phase-separation and injection operation. This method was successfully applied to the determination of melamine in dairy samples, including milk, yogurt and milk powder. The organic-solvent-free and sorbent-free sample preparation process could be accomplished within 11 min, requiring only 1.2 mL of water solution per sample. As an efficient on-column concentration strategy, LVI significantly improved the sensitivity. The obtained limit of detection of 0.0028 mg kg was far below the limits established by the Codex Alimentarius Commission. Good recoveries were obtained from samples spiked at four concentration levels (92.6-95.8%), with satisfactory intraday and interday precisions of 1.9-3.9% and 1.6-5.2%, respectively. Further evaluation of the accuracy, by analyzing a certified milk reference material gave a small relative error of 2.2%. Comparisons with a variety of efficient methods showed the superiority of the proposed method in terms of sensitivity, speed, sample and solvent consumption, practicability, and throughput.