Refers collectively to techniques  in which a column of smaller than  conventional inner diameter is used for  separation. The term micro-HPLC most often  is used for HPLC in columns with inner  diameters smaller than 0.5 mm; micro-HPLC  is used in high-sensitivity analysis when the  sample amount is limited and with certain  ionization techniques in LC–MS in which  the volume of solvent flowing into the ionization source must be minimized.