Mass spectrometer

The mass spectrometer is a scientific instrument used to identify the elements and isotopes present in a gas. Inside the mass spectrometer, magnetic forces on moving charged particles are used to measure the masses of individual atoms. How does it work? Read the text aloud
How a mass spectrometer works
In the ionizing chamber, atoms are ionized (into positive ions) by removing one (or more) of their outermost electrons. Ions of a particular velocity are isolated in the velocity selector and then emerge in a narrow beam as a result of focusing slits. In the mass analyzer, these moving charges pass through a strong magnetic field. The atoms experience a magnetic force that causes them to move in circles; the radius of the circle depends on the mass of the ion. Ions of different masses will strike different parts of the detector—allowing you to determine their masses. Read the text aloud Show Velocity selector in a mass spectrometer
Cyclotron radius when a charged particle moves in a uniform magnetic fieldAccording to the right-hand rule, the magnetic field exerts a force on the ions that is always perpendicular to their velocity. As you learned on page 416, a constant force applied perpendicular to the object’s velocity will result in circular motion! The radius of circular motion for a charged particle in a uniform magnetic field is referred to as the cyclotron radius (or gyroradius). This is the explanation behind the circular tracks in the bubble chamber on the previous page. Read the text aloud
The cyclotron radius can be calculated by using equation (19.4) and the equation for the centripetal force. Set the centripetal force Fc = mv2/r equal to the magnetic force on the ions, FB = qvB (where sin θ = 1, because the field is perpendicular to the velocity), then solve for the mass of the ions to obtain equation (19.5).
(19.5) m= rqB v
r  = radius of trajectory (m)
q  = charge of particle (C)
B  = magnetic field strength (T)
v  = particle speed (m/s)
m  = particle mass (kg)
Mass of an ion
given its gyroradius

The mass of the ion is therefore proportional to the radius of curvature of its trajectory through the mass analyzer. That’s how the mass spectrometer works! Read the text aloud Show Applications of mass spectrometry
Why do more massive ions move along trajectories with larger radii than lower mass ions? Show

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