Method and apparatus for improved sensitivity in a mass spectrometer

A mass spectrometer comprising:

• a mass analyzer;
• an ion source for generating ions to be analyzed by the mass analyzer;
• a first vacuum chamber comprising an inlet aperture for receiving the ions and an exit aperture for transporting the ions from the first vacuum chamber;
• an ion guide having a predetermined cross-section, the ion guide positioned in the first vacuum chamber between the inlet and exit apertures;
• a power supply connected to the ion guide to provide an RF voltage thereto;
• wherein the size of the inlet aperture and the differential pressure between the ion source and the first vacuum chamber produces a supersonic free jet expansion in the first vacuum chamber;
• wherein the cross-section of the ion guide is sized to be at least 50% of the predetermined diameter of the barrel shock of the supersonic free jet expansion;
• and
• wherein ions within the supersonic free jet expansion are radially confined as the ions traverse the ion guide.

A method for performing mass analysis comprising:

• generating ions in a high pressure region;
• passing the ions into a vacuum chamber comprising an inlet aperture for passing the ions from the high-pressure region into the vacuum chamber, and an exit aperture for passing ions from the vacuum chamber;
• providing an ion guide between the inlet and exit apertures, the ion guide having a predetermined cross-section defining an internal volume;
• applying an RF voltage to the ion guide for radially confining the ions within the internal volume of the ion guide;
• wherein the configuration of the inlet aperture and the pressure difference between the high pressure region and the vacuum chamber provides a supersonic free jet expansion downstream of the inlet aperture, the supersonic free jet expansion comprising a barrel shock of predetermined diameter;
• and
• wherein the cross-section of the ion guide is sized to be at least 50% of the predetermined diameter of the barrel shock of the supersonic free jet expansion.