Mass spectrometry

Mass spectrometry, often abbreviated as MS or mass-spec, is a powerful analytical technique. It is used to measure the mass-to-charge ratio of ions. An ion is a particle that carries an electric charge. By measuring this ratio, scientists can identify the chemical identity of a substance. They can also determine the structure of molecules or find isotopic signatures. This method works on pure samples and very complex mixtures. The final data is presented as a mass spectrum. This is a plot that shows signal intensity as a function of the mass-to-charge ratio.

A typical mass spectrometer consists of three main components. The first part is the ion source. This component converts a portion of the sample into ions. The sample may be a solid, a liquid, or a gas. After ionization, an extraction system moves the ions toward the mass analyzer. The mass analyzer is the part that sorts the ions. It uses electric or magnetic fields to separate them. The separation happens because ions with different mass-to-charge ratios react differently to these fields. Finally, a detector measures the ions. The detector provides data to calculate how many of each ion are present.

The process of ionization is a critical step in the mechanism. There are different ways to create ions depending on the sample type. Hard ionization techniques, such as electron ionization (EI), involve high energy. This process can cause molecules to break into many smaller fragments. These fragments provide a detailed pattern that helps identify unknown compounds. In contrast, soft ionization techniques impart very little residual energy. Examples include electrospray ionization (ESI) and matrix-assisted laser desorption/ionization (MALDI). Soft ionization results in much less fragmentation. This is very useful for studying large biological macromolecules like proteins.

To understand how the mass analyzer works, we can look at a sector-type analyzer. Imagine a sample of sodium chloride, which is common table salt. In the ion source, the salt is vaporized and ionized. This produces sodium ions (Na+) and chloride ions (Cl−). Sodium atoms are monoisotopic with a mass of about 23 daltons (Da). However, chloride is more complex because it has two stable isotopes. One type of chloride has a mass of about 35 u, making up about 75 percent of natural chloride. The other type has a mass of about 37 u, making up about 25 percent.

When these ions enter the analyzer, electric and magnetic fields exert forces on them. An electric field can change the speed of the charged particles. A magnetic field can alter their direction. The amount the ions bend, or their deflection, depends on their mass-to-charge ratio. According to Newton's second law of motion, lighter ions are deflected more by the magnetic force than heavier ions. As the ions are sorted, they pass into the detector. The detector records the relative abundance of each ion type. This allows scientists to see the exact ratio of the different isotopes present in the sample.

The history of this science is filled with important discoveries. In 1886, Eugen Goldstein observed positive rays in gas discharges. He called these "Kanalstrahlen," or canal rays. In 1899, Wilhelm Wien built a device using perpendicular electric and magnetic fields. This device could separate these rays by their charge-to-mass ratio. J. J. Thomson later improved this by reducing the pressure to create the mass spectrograph. Modern techniques were further developed by Arthur Jeffrey Dempster and F.W. Aston in 1918 and 1919.

Many specialized tools were created for specific scientific needs. Ernest O. Lawrence developed sector mass spectrometers known as calutrons. These were used during the Manhattan Project at the Oak Ridge Y-12 plant. They were essential for the enrichment of uranium isotopes. In the 1950s and 1960s, Hans Dehmelt and Wolfgang Paul developed the ion trap technique. This work earned them half of the 1989 Nobel Prize in Physics. Later, in 2002, John Bennett Fenn and Koichi Tanaka won the Nobel Prize in Chemistry. Their work with electrospray ionization and soft laser desorption revolutionized the study of proteins.

Today, mass spectrometry is connected to many different scientific fields. Inductively coupled plasma (ICP) sources are used to analyze cations in many sample types. These sources use high temperatures to strip electrons from atoms. Photoionization can also be used to study chemical kinetics. This involves using high energy photons, like X-rays or UV light, to break apart molecules. There are even ambient ionization techniques that allow sampling without much preparation. This makes mass spectrometry a vital tool for environmental and clinical science.