Mass Spectrometry - What It Is and How It Works

Mass spectrometry (MS) is an analytical laboratory technique to separate the components of a sample by their mass and electrical charge. The instrument used in MS is called mass spectrometer. It produces a mass spectrum that plots the mass-to-charge (m/z) ratio of compounds in a mixture.

How a Mass Spectrometer Works

The three main parts of a mass spectrometer are the ion source, the mass analyzer, and the detector.

Step 1: Ionization

The initial sample may be a solid, liquid, or gas. The sample is vaporized into a gas and then ionized by the ion source, usually by losing an electron to become a cation. Even species that normally form anions or don't usually form ions are converted to cations (e.g., halogens like chlorine and noble gases like argon). The ionization chamber is kept in a vacuum so the ions that are produced can progress through the instrument without running into molecules from air. Ionization is from electrons that are produced by heating up a metal coil until it releases electrons. These electrons collide with sample molecules, knocking off one or more electrons. Since it takes more energy to remove more than one electron, most cations produced in the ionization chamber carry a +1 charge. A positive-charged metal plate pushes the sample ions to the next part of the machine. (Note: Many spectrometers work in either negative ion mode or positive ion mode, so it's important to know the setting in order to analyze the data.)

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Step 2: Acceleration

In the mass analyzer, the ions are then accelerated through a potential difference and focused into a beam. The purpose of acceleration is to give all species the same kinetic energy, like starting a race with all runners on the same line.

Step 3: Deflection

The ion beam passes through a magnetic field which bends the charged stream. Lighter components or components with more ionic charge will deflect in the field more than heavier or less charged components.

There are several different types of mass analyzers. A time-of-flight (TOF) analyzer accelerates ions to the same potential and then determines how long is needed for them to hit the detector. If the particles all start with the same charge, the velocity depends on the mass, with lighter components reaching the detector first. Other types of detectors measure not only how much time it takes for a particle to reach the detector, but how much it is deflected by an electric and/or magnetic field, yielding information besides just mass.

Step 4: Detection

A detector counts the number of ions at different deflections. The data is plotted as a graph or spectrum of different masses. Detectors work by recording the induced charge or current caused by an ion striking a surface or passing by. Because the signal is very small, an electron multiplier, Faraday cup, or ion-to-photon detector may be used. The signal is greatly amplified to produce a spectrum.

Mass Spectrometry Uses

MS is used for both qualitative and quantitative chemical analysis. It may be used to identify the elements and isotopes of a sample, to determine the masses of molecules, and as a tool to help identify chemical structures. It can measure sample purity and molar mass.

Pros and Cons

A big advantage of mass spec over many other techniques is that it is incredibly sensitive (parts per million). It is an excellent tool for identifying unknown components in a sample or confirming their presence. Disadvantages of mass spec are that it isn't very good at identifying hydrocarbons that produce similar ions and it's unable to tell optical and geometrical isomers apart. The disadvantages are compensated for by combining MS with other techniques, such as gas chromatography (GC-MS).