Time of Flight Mass Spectrometry (AQA A Level Chemistry): Video

Hi, I'm Eleanor with 3 years of experience teaching Chemistry, and this video is about time of flight mass spectrometry. Every stage inside the instrument exists to make the time an ion takes to travel along the flight tube depend on nothing but its mass.

Mass spectrometry is the most useful instrument we have for accurately determining the relative atomic mass of an element and the relative molecular mass of a molecule. It does that by turning the sample into positive ions, giving every ion the same kinetic energy, and timing how long each one takes to drift down a flight tube. With the kinetic energy fixed, mass is the only thing left that can change that time, so the time of flight is a measurement of mass.

We'll take the instrument in the order the sample takes it: what a mass spectrum shows, then ionisation, acceleration and ion drift, then detection. After that, the two equations the calculations are built on, and a worked example using them.

As a sample passes through the spectrometer, a spectrum is produced, plotting the abundance of ions against their mass-to-charge ratio, m over z. It can be used to accurately determine the A r of an element or the M r of a molecule. Two peaks on the spectrum have names: the peak with the highest m/z value is called the molecular ion peak, M plus, which represents the intact ionised molecule and the tallest peak is the base peak which represents the most abundant ion. The whole apparatus is kept under a high vacuum to prevent ions colliding with air molecules. That plot is what the instrument exists to produce, and the four stages that follow are how it gets there.

The first stage is ionisation: the sample is converted into positive ions, by one of two methods. Electron impact is used for elements and low-mass compounds. High-energy electrons from an electron gun bombard the vaporised sample and knock an electron off each particle, forming a 1+ ion, the molecular ion M plus. Being a high-energy process, it can also break that molecular ion into smaller pieces called fragments, and those fragments pass through the spectrometer and appear on the final spectrum too. Electrospray ionisation is used for higher-mass compounds like proteins, to prevent fragmentation. It is a soft technique: the sample is dissolved in a volatile solvent and injected through a high-voltage needle, and each particle gains a proton from the solvent ionising the particle and adding one to its mass. Both methods leave the instrument with a positive ion it can accelerate.

The positive ions are attracted towards a negatively charged plate and accelerated by an electric field, and the key principle is that all the ions are accelerated to the same kinetic energy. Since kinetic energy is a half m v squared, ions with a lower mass have a higher velocity, and ions with a higher mass have a lower velocity. The ions pass through a hole in that plate and enter the flight tube, a region with no electric field. The time an ion takes to travel from entry of the flight tube to the detector is its time of flight. Ions with a lower mass-to-charge ratio travel faster and have a shorter time of flight. This is the stage that makes the measurement work: with the kinetic energy fixed, mass is the only thing left that can change the time.

At the end of the flight tube the ions arrive at a detector, an electron multiplier. When an ion hits it, the ion gains an electron, and that generates a small electric current. The size of that current is directly proportional to the abundance of that specific ion. A computer records the time of flight and the relative abundance for each ion and produces the mass spectrum. On this boron spectrum, the peak heights show the relative abundances directly: boron-10 at 19.9 per cent, boron-11 at 80.1 per cent. The times the detector records are what the calculations turn back into mass.

The calculations rest on two equations. Kinetic energy is a half m v squared, with m the mass in kilograms and v the velocity in metres per second. Velocity is distance over time: d, the length of the flight tube in metres, divided by t, the time of flight in seconds. Rearranging the first gives v equals the square root of two KE over m, and rearranging the second gives t equals d over v. Substituting one into the other gives t equals d multiplied by the square root of m over two KE. The length of the flight tube and the kinetic energy are the same for every ion, so the time of flight is proportional to the square root of the mass of the ion. Unit conversions are crucial: the mass must be in kilograms and the tube length in metres. To get the mass of a single ion in kilograms, divide the relative isotopic mass by the Avogadro constant, six point zero two two times ten to the twenty-three, then divide by a thousand to convert grams into kilograms.

Three things to fix in your head. All the particles are accelerated to the same kinetic energy. The time of flight is proportional to the square root of the mass of the ions. And in electrospray ionisation, the mass of the detected ion is the relative molecular mass of the sample plus one.

Time of flight mass spectrometry gives accurate relative atomic and relative molecular masses. The sample is ionised, accelerated to the same kinetic energy as every other ion, allowed to drift down a field-free flight tube, and detected. The calculation is t equals d multiplied by the square root of m over two KE, with the mass in kilograms and the tube length in metres. And the reason any of it works: because every ion carries the same kinetic energy, the time it takes to travel down the tube depends only on its mass.