Investigation of retention behavior for herbicides in icro-HPLC with temperature changes

The purpose of this study is to improve the detection limit for trace herbicides, and to separate all samples within a short time with the combination of temperature and flow programming with micro-reversed-phase liquid chromatography (RPLC). The optimum condition for separation was ACN: 0.05 M NaH2PO4 (46: 54) (v/v), pH = 4.3, 40degreesC. The mass sensitivity is in the range between 0.086 x 10(-13) g and 3.34 x 10(-13) g in micro-HPLC (HPLC). Micro-HPLC exhibits higher mass sensitivity and better heat transfer than conventional HPLC. Thus, in the case of micro-HPLC, the retention behavior of herbicides is easily determined according to the change of temperature. Especially, retention of triazines is decreased to a lesser extent with the increase of column temperature than that of other herbicides. This tendency has been accounted for the thermodynamic parameters DeltaHdegrees and DeltaSdegrees. The DeltaSdegrees for triazines is less negative than that for phenoxy acids, esters, and others. The chromatograms of herbicides were obtained within a short time with using both temperature and flow gradient, than with isocratic, isothermal, and isorheic conditions. When the mobile phase flow rate and the column temperature were changed simultaneously during the separation run, the analysis time was shortened up to 50%, while the separation efficiency was preserved.