"Inverse" cloud point extraction coupled with large volume injection ion-pair chromatography: A green route...

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^-1 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.