In this thesis the power of electrophoretic on-line concentration procedures for peptidomics and metabolomics is demonstrated. Tow examples are given: isotachophoresis with spacer molecules and electroextraction. Electromigration is capable of enhancing selective analyte migration. Selective analyte migration can be used to increase the amount of analyte that is injected into an analytical system, while it leaves the sample volume with the contaminants behind. In consequence, the injection of contaminants and bulk components that may disturb sample pretreatment, separation and detection can be avoided and LOD values can be lowered without the problems associated with direct injection of larger sample volumes. The work that is presented in this thesis is focused on fast electrophoresis-based LOD improvement of the analysis of peptides and/or metabolites in plasma and urine.