A-1. Protein Fractionation 1. Achieve highest resolution - The Agilent 3100 OFFGEL Fractionator provides pI-based fractionation of peptides or proteins with exceptional resolution. Samples can be fractionated into 12 or 24 fractions for standard or highest resolution. 2. Obtain greater reproducibility - Fractionation by OFFGEL electrophoresis is highly reproducible. Fractions from several runs may be pooled for enrichment of low abundance proteins or peptides. A-2. Peptide Fractionation 1. Gain sensitivity - Efficient reduction of sample complexity increases sensitivity of downstream analysis. This example compares the efficiency of fractionation of the tryptic peptides of immunodepleted plasma by strong cation exchange (SCX) with OFFGEL electrophoresis. 2. Increase efficiency - Whereas proteins are often represented by isoforms with different pI values, peptides have distinct, individual pI values. In this example, 90 % of all individual tryptic peptides focused efficiently into one or two OFFGEL fractions.