Question

In mass spectrometry, a sample’s molecules are ionized in a vacuum and then exposed to a uniform electric field created by a high-voltage plate in the acceleration chamber. The electric field accelerates the ions until they arrive at the next section of the device, designated as the separation chamber. In this section, the drifting ions are sorted by their mass-to-charge ratio (m/q).

The separation chamber in a time-of-flight mass spectrometer (TOF-MS) is linear and has no electric or magnetic fields. The ions travel at a constant velocity through the chamber until they reach the detector. The time it takes for an ion to reach the detector depends on its m/q ratio.

A magnetic sector mass spectrometer (MS-MS) has a curved separation chamber where a magnetic field is generated. The magnetic field exerts a centripetal force on drifting ions, bending their trajectories into curved paths. The radius of the curvature depends on the ion’s m/q.

The centripetal force (F) acting on a particle can be determined from its mass (m) and velocity (v) and the radius ® of the curved path:

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Which of the following best describes the high voltage plate if it accelerates positive ions away from it?

1. It is positively charged, and its electric field lines point away from it.
2. It is positively charged, and its electric field lines point toward it.
3. It is negatively charged, and its electric field lines point away from it.
4. It is negatively charged, and its electric field lines point toward it.

Answer 1

The correct choice is

A. It is positively charged, and its electric field lines point away from it.

the ions are positively charged and they can be accelerated away only by a like charge. hence the plate must be positive. a positive charge emits electric field lines away from it. hence the electric field lines point away from the plate.