Patent ID: 7256395

Claim:
A mass spectrometer comprising: an ion source for generating ions in a high-pressure region; 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; an ion guide between the inlet and exit apertures and having a predetermined cross-section defining an internal volume; a power supply for providing 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 ion source 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.