Provided are methods for the detection or quantitation of amyloid beta. In a particular aspect, provided herein are methods for detecting amyloid beta or fragments thereof by mass spectrometry. In another aspect, provided herein are methods for determining the ratio of amyloid beta 42 (Aβ42) to amyloid beta 40 (Aβ40). In another aspect, provided herein are methods for diagnosis or prognosis of Alzheimer's disease or dementia.