Time-of-Flight Mass Spectrometry

Electrospray or electron impact produces positive ions. Electron impact removes an electron and may fragment molecules; electrospray commonly produces a protonated molecular species. An electric field accelerates ions. For singly charged ions accelerated through the same potential difference, kinetic energy is the same. The drift region is field-free, so ions travel at a constant speed before reaching the detector and gaining an electron. The measured current reflects how many ions arrive.

At fixed kinetic energy, a smaller mass gives a greater speed and a shorter flight time. For the same flight distance, mass is proportional to the square of flight time. The horizontal axis of a mass spectrum is mass-to-charge ratio, not neutron number. Peak height or area represents relative abundance. A 2+ ion has half the mass-to-charge ratio of the corresponding 1+ ion; check the assumed charge before assigning an isotope.

Combine $E_k=mv^2/2$ and $v=d/t$ to obtain $m=2E_kt^2/d^2$. This gives mass in kg for one ion when the other quantities are in SI units. Multiply by the Avogadro constant and convert kg to g if a molar mass is requested. A molecular-ion peak can identify relative molecular mass, but the most intense fragment need not be the molecular ion. Check ionisation method before interpreting a peak one mass unit above the neutral molecule.

Attempt these problems on paper before opening the worked reasoning. Explain your steps as well as giving a final answer.