Mass Spectrometry (MS) — Introduction & Applications

What is Mass Spectrometry (MS)?

It is a technique used in analytical chemistry to identify and characterize chemical compounds and substances. MS involves instruments that create ions which are then separated by charge and mass.

Types of MS

There are many types of MS which are categorized by the way ions are created.

Uses for MS

MS is very useful when identifying substances by mass. One type of MS that is well known is GC-MS. GC-MS is often mentioned in television shows with forensic science which is used to solve crimes. GC-MS can be used to determine the identification of unknown substances found at crime scenes. There are different databases that contain the MS data which can be compared to unknown samples to identify them.

MS instruments are helpful for research scientists as well. Many research scientists make new compounds through chemical synthesis. The identity of the intended chemical product can be confirmed by using MS instruments. For example, if a newly synthesized compound should have an atomic mass of 804 grams per mole, it can be run on MS instruments to confirm the presence of a species with that mass. The MS data will also show if there remains any reactants which were used to make the product.

Isotopes and MS

All chemical elements have an average mass which can be found on the periodic table. When different elements are combined, there is an average mass to every chemical compound. The mass is an average because every atom has isotopes. Isotopes of atoms have have the same protons and electrons characteristic of each atom, but they differ in the number of neutrons. Changes to the neutrons do not change the identity of the atom, but they change the mass. Therefore, the average mass of an element takes into account different isotopes and their availability.

MS instruments take into account isotopes and usually have an isotope distribution for each chemical signal. If a pure substance is analyzed on MS with an exact mass of 88 g/mol, there will be more than one single signal at 88g/mol. There may be smaller signals around the main signal and this is due to isotopes. Compounds with lots of isotopes can have a characteristic isotope distribution pattern.

(Video) Spectrometry | Chemical Tests | Chemistry | FuseSchool

by FuseSchool - Global Education

This video will give you a quick introduction to mass spectrometry. This is a technique that scientists use to figure out the molecular mass of a sample. It can even be used to determine what an unknown sample is, like from a crime scene. Finally, it can also help us determine the relative abundance of isotopes in a sample.

How Does a Mass Spectrometer Work?

We know what a mass spectrometer does. It measures the mass-to-charge ratio of a sample. But, how does it actually work? Let's start at the beginning. The first thing we do is place our sample into a vacuum chamber. Why? We don't want our sample interacting with the air. So, we create a vacuum. The reduction in pressure causes a solid or liquid to vaporize. We could have started with a gas sample. Most instruments have a special injection valve for gases. After our sample is vaporized, we need to ionize it. How? The most common method is to shoot a beam of electrons. The beam of electrons collides with the sample. The collisions eject the sample's electrons. It's similar to the photoelectric effect. Sometimes, the electrons collide with bonding valence electrons. The bond is broken and our sample gets fragmented.

What happens after our sample is ionized? We use a magnetic or electric field to accelerate the ions. The field causes the ions to curve as they move toward the detector. But, the degree of curving depends on the mass and charge. An ion with a lot of mass will curve less than a smaller ion. An ion with a greater charge will be more influenced by the field. So, it will curve more. The separated ion then hit the detector. And the instrument records the abundance of the different ions. The computer then uses the data to determine the identity of the sample.