In MALDI-TOF mass spectrometry, how are biomolecules separated?

• A. Mass
• B. Mass to charge ratio
• C. Number of charges on the molecules
• D. Time it takes for vaporization by a laser

Answer: (B)

The idea being tested is that separation in MALDI-TOF mass spectrometry is based on how long ions take to travel to the detector, which depends on their mass-to-charge ratio (m/z). In TOF analysis, ions are accelerated by a fixed electric potential, giving them kinetic energy qV. Their velocity is v = sqrt(2 q V / m), so lighter ions or ions with higher charge reach the detector faster. The time of flight t is the flight path length divided by velocity, t = L / v ≈ L sqrt(m / (2 q V)). Since t increases with sqrt(m/z), ions are separated along the time axis according to their m/z. In practice, biomolecules often appear as singly charged ions with MALDI, so m/z roughly mirrors mass, but the fundamental separation is still by mass-to-charge ratio.

Why the other options don’t fit: separation by mass alone would ignore the role of charge states; the key measurement is how fast ions travel, not how quickly they vaporize by the laser; and the number of charges on a molecule affects m/z but is not the direct separation parameter by itself—the instrument resolves ions by time, which reflects m/z.