Which of the following techniques is primarily used for sequencing proteins?

Mass Spectrometry is the primary technique used for sequencing proteins due to its ability to accurately determine the mass-to-charge ratio of ions. This provides quantitative information that can be used to deduce the amino acid sequence of a protein. In practice, proteins are first digested into smaller peptides, often using enzymes like trypsin. These peptides are then ionized and introduced into the mass spectrometer, where they are analyzed based on their mass and charge. The resulting mass spectra can provide information about the peptide sequences, which can be matched against known protein databases to identify the proteins.

In contrast, 2D Gel Electrophoresis is primarily used for separating proteins based on their isoelectric point and molecular weight but does not directly sequence proteins. Isoelectric Focusing is a technique specifically used to separate proteins based on their isoelectric point, which can identify proteins but again does not provide sequencing information. Protein Quantification methods, while crucial for determining the concentration of proteins in a sample, do not offer sequencing capabilities either. Therefore, Mass Spectrometry stands out as the technique best suited for protein sequencing.