Mass Spectrometry Questions and Answers

Answer: C

A magnetic field bends each ion's path, and heavier ions curve less for the same accelerating voltage, so scanning the field brings each mass in turn onto the detector. The other three describe the quadrupole, the time-of-flight and the ion trap. Sector instruments give the highest resolution but are large and slow, which is why they have largely given way to the others outside specialist work. Naming the cost of a choice alongside its benefit is what turns a fact into a design decision.

Answer: D

Combined direct and radio-frequency voltages on four parallel rods give a stable trajectory to only one mass-to-charge ratio at a time; every other ion strikes a rod. It is a filter rather than a disperser. Its compactness, speed and modest cost made it the standard detector for gas and liquid chromatography, at a resolution well short of a sector instrument's.

Answer: A

All ions receive the same kinetic energy, so lighter ones travel faster and arrive first, and the flight time gives the mass directly. The other options describe sector, ion-cyclotron and trap instruments. Because every ion is measured rather than filtered, the technique wastes nothing and has no upper mass limit in principle, which is why it pairs so well with laser desorption.

Answer: B

Nothing about the arrangement excludes a heavy ion; it merely arrives later, whereas a quadrupole's stability region and a sector's field strength both impose real ceilings. The voltage is fixed, the tube is a practical length, and detector response actually falls for very heavy ions. A technique whose only cost for a heavier analyte is patience is unusual, and it is why proteins are weighed this way. Working one numerical case through both cases side by side fixes the distinction better than any rule of thumb.

Answer: C

Ions of the same mass but slightly different energies penetrate an opposing field to different depths and are turned around at different points, so they arrive together and the peak narrows. The path is longer as a side effect, sensitivity falls a little, and speed is unchanged. Correcting a spread by arranging for the faster ions to travel further is an elegant solution, and it turns a low-resolution analyser into a respectable one. Most errors here come from carrying a result across from a neighbouring case where it does not hold.

Answer: D

A trap holds ions in a small volume with radio-frequency fields and then ejects them in order of mass-to-charge ratio. The other options describe the sector, quadrupole and time-of-flight instruments. Because ions can be held and manipulated before ejection, a trap can isolate one mass, fragment it and analyse the products, all in one device - which is why tandem experiments are so natural there. The shared properties explain why these belong to one family, and the differences explain why more than one exists.

Answer: A

Selecting a precursor ion, breaking it by collision with a gas, and scanning the fragments gives structural information about that one component even in a complex mixture. It does not run two samples, repeat a measurement or combine sources. Isolating one species before fragmenting it is what makes the technique so powerful for mixtures, where an ordinary spectrum would be an uninterpretable superposition. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

Answer: B

Collision-induced dissociation accelerates the selected ion into argon or nitrogen, converting kinetic energy into internal energy until the ion breaks. Electron and photon methods exist for particular purposes, and a magnetic field only deflects. Converting motion into internal energy through collisions is a controllable way to supply exactly as much as is needed, which is why the collision energy is a tuned parameter of the method. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

Answer: C

Watching one precursor and one specific fragment means a signal is recorded only when both match, which rejects almost all interference and gives exceptional selectivity. It records no full spectrum and cannot identify an unknown, since both masses must be chosen in advance. Trading the ability to see everything for near-immunity to interference is the right trade in routine quantitation and the wrong one in discovery. Working one numerical case through both cases side by side fixes the distinction better than any rule of thumb.

Answer: D

The molecular ion is the whole molecule with one electron removed, so its mass gives the molecular weight directly. The heaviest fragment may or may not be it, the most abundant is the base peak, and the protonated species belongs to chemical ionisation. Identifying which peak is the molecular ion is the first task in reading any spectrum, and getting it wrong invalidates everything that follows. A single measurement chosen to respond to the one asymmetry beats several that respond to shared behaviour.