Ultra-fast gradient elution separations of a mixture of four peptides. Chromatographic conditions: Column, 30 mm × 2.1 mm i.d. Halo C18 (shell particles); flow rate, 3.0 mL/min; temperature, 100 °C; detection by UV absorption at 214 nm; gradient times were 6, 9, 12, and 18 s (top to bottom), and the system and column were reequilibrated for 3 s between injections; gradients were started at 5% acetonitrile and terminated at an acetonitrile level that positioned the last peak near the end of the gradient, and 0.1% trifluoroacetic acid in water was used as the aqueous component of the eluent (Dr. Xiaoping Li, unpublished work, University of Minnesota).