# How does tandem mass spectrometry work?

Sep 24, 2015

One benefit of multiple mass spectrometers in a row allows for more precise identification of molecules with very similar masses, such as $\text{135.220 g/mol}$ vs. $\text{135.212 g/mol}$.

It also helps distinguish between molecules of similar masses based on their structure, since it improves the amount of fragmentation that occurs, and hence, extends the fragmentation pattern. Extend it enough, and you can notice small differences that may tell you the differences between two very similar structures.

The steps to tandem mass spec (MS/MS) are:

• MS 1: Load sample
• MS 1: Ionize sample using such techniques as electron "impact" (interaction; EI), electrospray ionization (ESI), and matrix-absorption laser desorption ionization (MALDI).

• EI --- Fire an electron beam to knock off an electron, ionizing the sample. This results in a hard ionization, which may make it difficult to spot the original parent peak fragments into daughter peaks.
• ESI --- Electrically charged droplets of sample progressively get overloaded with such charge and then they become an aerosol. This results in soft ionization, which minimizes unwanted fragmentation and makes it easier to spot the original parent peak.
• MALDI --- A suitable matrix material shields the sample from a pulsed laser, taking the brunt of the ionization, and then ionizes the sample, causing a soft ionization.
• MS 1: Separate ions of specific mass-to-charge ratios, with for example, flight chambers, a quadrupole ion trap, quadrupole filter, etc.

These essentially use electric and/or magnetic fields to induce motion, separating samples based on mass or charge or both.

• MS 1: Fragment sample. Tends to be done using collisions with neutral molecules for MS/MS.
• MS 2: Separate ions of specific mass-to-charge ratios, with for example, flight chambers, a quadrupole ion trap, quadrupole filter, etc.

These essentially use electric and/or magnetic fields to induce motion, separating samples based on mass or charge or both.

• MS 2: Detection of sample. You may see detectors like Faraday plates, Photomultiplier tubes, etc.

These essentially just count the number of ion-collisions onto their surface. Then some sort of transducer converts the number into an electrical signal that tells the computer what the abundance of ions is. The mass spectrum is plotted based on that, with abundance vs. m/z ratio.