Reverse phase ion-pairing chromatography at pressures up to 345 MPa

The effects of pressure up to 345 MPa (50,000 psi) on a reverse phase ion-pairing system were studied in 50% methanol-water. Retention increased by a factor of 3.2 for methyl orange when the pressure was increased to 241 MPa (30,000 psi) and still larger (not measurable) at 345 MPa (50,000 psi). The retention of methyl red was increased by a factor of 2.2 by pressures of 241 MPa (30,000 psi) and by a factor of 2.5 by 345 MPa (50,000 psi). When the methanol concentration was increased to 75%, retention of methyl orange increased by a factor of 2.6 between 68.9 MPa (10,000 psi) and 345 MPa (50,000 psi). Other variables studied were concentration of counter ion and temperature. For methyl orange, retention decreased with pH, whereas for methyl red it increased. The effect of tetrahexylammonium ion concentration of retention was linear in the working range 0.5 mM to 2.0 mM. All samples eluted faster with increased temperature