Quickly Understand Tandem Mass Spectrometry (MS/MS)

Tandem mass spectrometry or MS/MS basically combines two different types of MS separation techniques to separate the compounds in a sample twice based on their mass-to-charge ratio (m/z) for improved resolution.

In a nutshell, this is done by first ionizing the sample using ESI, MALDI, EI, etc., generating a mixture of ions from the sample. (All topics I have already covered btw) So called precursor ions of a specific mass-to-charge ratio are selected from the first mass spectrometry. These are then further fragmented to generate the product ions for detection. This process can then be repeated further to increase the resolution even more.

Moving on, let us take a closer look at how this process can be carried out. Since Tandem MS involves 3 distinct steps of selection-fragmentation-detection, the separation of these 3 steps can be realized in space or in time. Tandem MS in space typically include Triple Quadrupole (QqQ), Quadrupole Time-of-flight (QTOF) and hybrid ion trap/FTMS. We’ll use Quadrupole time-of-flight or QTOF as an example.

In QTOF, precursor ions are selected with the help of the quadrupole mass spectrometer and sent to the Collision Cell for fragmentation. Then the generated product ions are detected by time-of-flight (TOF) mass spectrometry.

In other words, the first quadrupole (Q1) operates as a mass filter, allowing for specific selection based on their mass-to-charge ratio. Then the second quadrupole (Q2) acts as a collision cell where ions are bombarded by neutral gas molecules such as nitrogen or argon, resulting in fragmentation of the ions by a process known as induced dissociation.

After the ions leave the quadrupole, they are analyzed using the time-of-flight mass analyzer. Here, all of the ions are accelerated with the same force, and the resulting differences in travel velocity through the free drift region of the TOF analyzer occurs as a consequence of the size of the ions. Subsequently, by measuring the time it takes them to reach the detector at the end of the drift region.

Since ions are separated multiple times based on their mass-to-charge ratio, the resolution is improved. This is also the main reason why tandem mass spectrometry is so useful!