Which method is used to analyze molecules based on their mass?

Mass spectrometry is a powerful analytical technique used to measure the mass-to-charge ratio of ions. This method provides detailed information about the molecular weight and structure of compounds. When a sample is ionized, it's accelerated in an electric field and subsequently passes through a magnetic field, where its path is altered based on its mass and charge. The resulting data can be used to identify and quantify molecules, making mass spectrometry essential in various fields, including biochemistry, proteomics, and metabolomics.

Gel electrophoresis, Western blotting, and chromatography are all valuable techniques in molecular biology, but they serve different purposes. Gel electrophoresis separates molecules based on their size and charge, Western blotting is used to detect specific proteins after separation, and chromatography separates compounds based on their interaction with a stationary phase. While these methods can provide insights into the properties and quantities of molecular components, they do not provide direct mass analysis like mass spectrometry does. Hence, mass spectrometry is specifically designed for analyzing molecules based on their mass, establishing it as the correct choice for this question.