Mass spectrometry

Mass spectrometry (MS) is a special tool scientists use to study tiny parts of matter. It works by measuring the mass-to-charge ratio of ions, which are tiny particles with a charge. The results are shown in a graph called a mass spectrum, where scientists can see how strong the signal is for each mass-to-charge ratio.

This method helps scientists figure out what elements or molecules are in a sample, even if the sample is a mix of many different things. Whether the sample is a solid, liquid, or gas, it is first turned into ions. This can happen by shooting the sample with a beam of electrons, which may break the molecules into smaller pieces or simply give them a charge.

Once the ions are made, they are sorted by their mass-to-charge ratio using electric or magnetic fields. Ions with the same ratio move together. Finally, a special detector, like an electron multiplier, finds these ions and counts them. By looking at the pattern of masses, scientists can identify the atoms or molecules in the sample, using known masses or special patterns of fragments.

History of the mass spectrometer

Further information: History of mass spectrometry

In 1886, a scientist named Eugen Goldstein noticed special rays in gas discharges that moved in a unique way. These rays were called "canal rays" because they traveled through small openings. Later, another scientist, Wilhelm Wien, created a tool that could sort these rays by their charge-to-mass ratio. This helped show how different gases behaved.

The word spectrograph was used by 1884. Early tools that measured mass-to-charge ratios were called mass spectrographs. These tools recorded patterns of mass on a special plate. Over time, scientists improved these tools, and new methods were developed. In the 1940s, special tools called sector mass spectrometers or calutrons were used to separate isotopes of uranium for important scientific work.

Later, important advances earned Nobel Prizes. In 1989, the prize went to two scientists for creating a method called the ion trap technique. In 2002, the prize was awarded for new ways to study large molecules, like proteins.

Parts of a mass spectrometer

A mass spectrometer has three main parts: an ion source, a mass analyzer, and a detector. The ionizer changes a bit of the sample into ions. An extraction system takes these ions and sends them through the mass analyzer to the detector. The mass analyzer sorts the ions by their mass-to-charge ratio, and the detector measures how many of each type of ion there are.

For example, imagine a sample of sodium chloride (table salt). In the ion source, the sample is turned into a gas and changed into electrically charged particles, creating sodium (Na⁺) and chloride (Cl⁻) ions. The analyzer uses electric and magnetic fields to sort these ions. Lighter ions are bent more by the magnetic field than heavier ones. The detector then records how many of each ion there are, helping scientists learn what elements and isotopes are in the sample.

Creating ions

The ion source is the part of a mass spectrometer that changes the material being studied into ions. There are different ways to do this, which helps scientists analyze many kinds of samples. Two common methods are electron ionization and chemical ionization, used for gases and vapors.

Chemical ionization uses reactions between molecules to create ions. For liquid and solid samples, methods like electrospray ionization and matrix-assisted laser desorption/ionization (MALDI) are often used. These techniques help scientists study tiny particles and molecules in detail.

Mass selection

Mass analyzers separate ions based on their mass-to-charge ratio. This separation happens because charged particles move differently in electric and magnetic fields.

There are many types of mass analyzers, each with its own way of working. Some use steady fields, while others use changing fields. All of them follow the same basic rules about how ions move.

Important features of mass analyzers include how well they can tell apart ions of similar mass, how accurately they measure mass, and how many different types of ions they can study at once.

Detectors

The detector is the last part of a mass spectrometer. It records the charge or current when an ion passes by or hits a surface. In a scanning instrument, this creates a mass spectrum, showing ions based on their mass-to-charge ratio.

Common detectors include electron multipliers, but others like Faraday cups and ion-to-photon detectors are also used. Microchannel plate detectors are often found in modern instruments. In FTMS and Orbitraps, the detector uses metal surfaces that ions pass near, producing a weak alternating current.

Tandem mass spectrometry

A tandem mass spectrometer can do mass spectrometry more than once, often by breaking molecules into smaller pieces. For example, it can pick out a specific peptide from many others entering the machine. The peptide ions then hit a gas, which causes them to break apart. Another part of the machine then sorts these smaller pieces.

Tandem mass spectrometry can be used in different ways. Some common methods include watching for specific pieces after breaking apart, looking for all possible pieces that can come from a certain starting point, or searching for molecules that lose a specific part when broken apart. One special use of this technology is in radiocarbon dating with accelerator mass spectrometry.

The METLIN Metabolite and Chemical Entity Database holds a large collection of data from tandem mass spectrometry tests on known molecules. This helps scientists identify both known and unknown chemicals by comparing test results.

Common mass spectrometer configurations and techniques

Sometimes, scientists use a special mix of parts in their tools, and they give these mixes short names to make talking about them easier. For example, MALDI-TOF is a name for a tool that uses a special way to get particles ready and a timing method to measure their weight.

Other short names include inductively coupled plasma-mass spectrometry (ICP-MS), accelerator mass spectrometry (AMS), thermal ionization-mass spectrometry (TIMS), and spark source mass spectrometry (SSMS). There is also isotope-ratio mass spectrometry, which usually means using certain types of tools to study tiny differences in particle weights.

Separation techniques combined with mass spectrometry

An important way to improve mass spectrometry is to use it together with chromatographic and other separation methods.

Gas chromatography

Main article: Gas chromatography–mass spectrometry

A common way to combine these methods is gas chromatography-mass spectrometry (GC/MS or GC-MS). In this method, a gas chromatograph is used to separate different compounds. These separated compounds are then sent into an ion source, which is a metallic filament with voltage applied. This filament gives off electrons that turn the compounds into ions. These ions can break into pieces in predictable ways. Both the whole ions and the pieces go into the mass spectrometer's analyzer and are finally detected. However, the high temperatures (300 °C) used in GC-MS can sometimes break down the molecules before they are measured, so we might detect the broken pieces instead of the original molecules.

Liquid chromatography

Main article: Liquid chromatography–mass spectrometry

In a similar way to GC-MS, liquid chromatography-mass spectrometry (LC/MS or LC-MS) separates compounds before they go into the ion source and mass spectrometer. The difference is that LC-MS uses a liquid, usually a mix of water and organic solvents, instead of gas. Most often, an electrospray ionization source is used in LC-MS. Other common sources include atmospheric pressure chemical ionization and atmospheric pressure photoionization. There are also newer techniques like laser spray.

Capillary electrophoresis–mass spectrometry

Main article: Capillary electrophoresis–mass spectrometry

Capillary electrophoresis–mass spectrometry (CE-MS) combines the liquid separation process of capillary electrophoresis with mass spectrometry. CE-MS is usually connected to electrospray ionization.

Ion mobility

Main article: Ion-mobility spectrometry–mass spectrometry

Ion mobility spectrometry-mass spectrometry (IMS/MS or IMMS) is a method where ions are first separated by how long they take to drift through a neutral gas under an electric field before they go into a mass spectrometer. The time it takes to drift gives information about the size of the ions compared to their charge. The duty cycle of IMS is longer than most mass spectrometry methods, so the mass spectrometer can take samples during the IMS separation. This gives data about both the IMS separation and the mass-to-charge ratio of the ions, similar to LC-MS.

The duty cycle of IMS is shorter than liquid chromatography or gas chromatography separations, so it can be combined with these methods, creating triple combinations like LC/IMS/MS.

Data and analysis

Data representations

See also: Mass spectrometry data format

Mass spectrometry creates different kinds of data. The most common way to show this data is through a mass spectrum.

Some types of mass spectrometry data are better shown as a mass chromatogram. There are different kinds of chromatograms, like selected ion monitoring (SIM), total ion current (TIC), and selected reaction monitoring (SRM), among others.

Other data can be shown as a three-dimensional contour map. Here, the mass-to-charge ratio, m/z, is on the x-axis, the intensity is on the y-axis, and another experimental detail, like time, is shown on the z-axis.

Data analysis

Analyzing mass spectrometry data depends on the type of experiment done. It is important to know if the ions observed are negatively or positively charged, as this helps understand the molecules.

Different ways of creating ions from the original molecules can produce different pieces of the molecules. Some methods create many small pieces, while others create larger pieces that can carry more than one charge. Special techniques in mass spectrometry can create these pieces after the ions are made, changing the type of data collected.

Knowing where the sample came from can give clues about what molecules are in it and how they break apart. A sample from a manufacturing process might have small amounts of unwanted chemicals related to the main product. A sample from a living thing might have salt, which can mix with the molecules being studied.

How the sample is prepared can also affect the results. For example, the material used in certain methods can change how the molecules are turned into ions.

Mass spectrometry can help find the weight of molecules, their structure, and how pure a sample is. Each of these needs a different experiment, so it is important to know the goal before collecting data.

Interpretation of mass spectra

Main article: Mass spectrum analysis

To figure out the exact structure or sequence of a molecule, scientists look at how the molecule breaks apart. They often start by comparing the experimental data to a library of known patterns. If this doesn’t work, they may need to interpret the data by hand or use special software.

Computers can simulate how molecules break apart and compare these simulations to the observed data. This helps scientists determine the structure of the molecule.

Analyzing mass spectra can also use very precise mass measurements. When the mass-to-charge ratio is known very accurately, it helps narrow down the possible molecular formulas. A computer program called a formula generator can calculate all possible formulas that match the measured mass.

A newer method called precursor ion fingerprinting looks at small pieces of the molecule’s structure by comparing the data to a library of known patterns.

Applications

Mass spectrometry is a tool used to measure how heavy ions are compared to their charge. It can help identify unknown substances, study the makeup of elements in molecules, and look at the structure of compounds by seeing how they break apart. It is also used to measure how much of a substance is in a sample or to study the basic chemistry of ions.

This method has many benefits, such as being very sensitive and giving specific information about molecules. It can also tell us about the weight of molecules and the types of elements in them. However, it can sometimes struggle to tell apart very similar molecules or those that break apart in the same way.