A mass spectrometer is an instrument that identifies atoms or molecules by turning them into ions and separating them by mass-to-charge ratio, written as m/z. It is used in physics, chemistry, medicine, forensics, and space science because it can measure tiny amounts of material very precisely. The central idea is that charged particles can be accelerated by electric fields and bent by magnetic fields.
How much an ion bends reveals information about its mass and charge.
Key Facts
- Mass-to-charge ratio is m/z, often treated as m/q in physics calculations.
- Electric potential energy becomes kinetic energy: qV = 1/2 mv^2.
- Magnetic force on a moving ion is F = qvB when v is perpendicular to B.
- Circular motion in a magnetic field satisfies qvB = mv^2/r.
- Combining acceleration and magnetic bending gives r = mv/(qB).
- For ions accelerated through the same voltage, m/q = B^2r^2/(2V).
Vocabulary
- Ion
- An ion is an atom or molecule with a net electric charge because it has gained or lost electrons.
- Mass-to-charge ratio
- Mass-to-charge ratio is the ion mass divided by its charge, usually written m/z or m/q.
- Ionization chamber
- An ionization chamber is the part of a mass spectrometer where neutral particles are converted into charged ions.
- Accelerating voltage
- Accelerating voltage is the electric potential difference that gives kinetic energy to ions.
- Magnetic sector
- A magnetic sector is a region with a magnetic field that bends ion paths according to their mass-to-charge ratio.
Common Mistakes to Avoid
- Using mass alone to predict separation is wrong because the spectrometer separates by mass-to-charge ratio, not just mass.
- Forgetting the ion charge q is wrong because a doubly charged ion bends differently from a singly charged ion of the same mass.
- Assuming all ions follow the same radius is wrong because ions with larger m/q bend less in the same magnetic field.
- Mixing up electric and magnetic field roles is wrong because the electric field usually accelerates ions while the magnetic field curves their paths.
Practice Questions
- 1 A singly charged ion with charge q = 1.60 x 10^-19 C is accelerated through 1200 V. What kinetic energy does it gain in joules?
- 2 An ion has mass 3.32 x 10^-26 kg, charge 1.60 x 10^-19 C, speed 2.0 x 10^5 m/s, and enters a 0.50 T magnetic field perpendicular to its velocity. What is the radius of its circular path?
- 3 Two ions enter the same magnetic field at the same speed and have the same charge, but one has twice the mass of the other. Explain which ion follows the larger-radius path and why.