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.
Understanding Physics: The Mass Spectrometer
A real instrument has several linked stages, and each stage can affect the final reading. First, the sample must enter as a gas, vapour, or tiny spray of droplets. An ion source then removes electrons, adds protons, or attaches another charged particle to the sample.
Different sources suit different materials. Electron impact is useful for small stable molecules, but it can break them into pieces. Electrospray is gentler and is often used for proteins or medicines dissolved in liquid.
A large molecule may gain more than one charge. This makes it appear at a lower mass-to-charge position than its actual mass might suggest.
The ions travel through a near vacuum because collisions with air would scatter them and spoil the sorting. Electric fields give ions a controlled amount of energy before they enter the analysing region. The instrument must keep the voltage and magnetic field steady.
Small changes shift the positions of peaks. Slits can make the beam narrower, which improves precision, though fewer ions then reach the detector. This trade-off is called resolution versus sensitivity.
High resolution matters when two ions have almost the same mass. It can separate substances that would otherwise form one broad peak.
The detector does not usually report a list of atoms by name. It counts ions arriving at different mass-to-charge positions and produces a spectrum. Each peak has a position and an intensity.
The position helps identify an ion. The intensity shows its relative amount, but it is not always a direct measure of how much material was in the original sample. Some substances ionise more easily than others.
A useful pattern comes from isotopes. Chlorine, for example, gives two main peak positions separated by about two mass units, with a characteristic height pattern. Fragment peaks can reveal parts of a molecule, since broken pieces have masses linked to its structure.
Students should separate three ideas when reading spectra. Mass is the amount of matter in one particle. Charge is the number of elementary charges carried by that particle.
Mass-to-charge position is what the machine measures directly. A peak at a particular position may come from a light ion with one charge or a heavier ion with several charges. Units need careful handling too, especially when converting atomic mass units to kilograms in physics calculations.
Calibration is equally important. Scientists run ions with known masses to check that a measured position corresponds to the correct value. This careful checking is why mass spectrometry can be trusted in areas such as checking water pollution, identifying drugs in blood samples, studying meteorites, and finding molecules from distant planets.
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.