Electrospray ionization (ESI) is a mainstay of LC/MS. It can be used to analyze large and small analytes. Agilent’s electrospray ion source uses our patented orthogonal nebulization and heated counterflow drying gas system to achieve excellent sensitivity and robust, reliable performance. It can generate both positive and negative ions, and ion polarity can be switched on a scan-by-scan basis to double the information acquired from a single run. A dual-nebulizer ESI source is available for TOF and Q-TOF instruments. The second nebulizer is dedicate to the continuous introduction of a reference mass to ensure the best possible mass accuracy.