Introduction to mass spectrometry an overview
Mass spectrometry (MS) is an analytical laboratory technique used to separate the components of a sample by their mass and their electrical charge. The instrument used in MS is called a mass spectrometer. It produces a mass spectrum which traces the mass-charge ratio (m / z) of the 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 can be a solid, a liquid or a gas. The sample is vaporized in a gasthen ionized by the ion source, usually by losing an electron to become a cation. Even species that normally form anions or generally do not form ions are converted to cations (for example, halogens like chlorine and rare gases like argon). The ionization chamber is maintained under vacuum so that the ions produced can progress through the instrument without falling into the molecules of the air. Ionization comes from electrons which are produced by heating a metal coil until it releases electrons. These electrons collide with molecules in the sample, causing one or more electrons to fall. Since it takes more energy to remove more than one electron, most of the cations produced in the ionization carry a +1 charge. A positively charged metal plate pushes the ions from the sample to the next part of the machine. (Note: Many spectrometers operate in negative ion or positive ion mode, so it is important to know the setting to analyze the data!)
Step 2: Acceleration
In the mass analyzer, the ions are then accelerated by a potential difference and focused in a beam. The goal of acceleration is to give all species the same kinetic energy, like starting a race with all the runners on the same line.
Step 3: Deviation
The ion beam crosses a magnetic field which curves the charged current.
Lighter components or components with a higher ionic charge will deform more in the field than heavier or less charged components.
There are several types of mass analyzers. A time-of-flight analyzer (TOF) accelerates ions to the same potential and then determines the time it takes for them to reach the detector. If the particles all start with the same charge, the speed depends on the mass, with the lightest components reaching the detector first. Other types of detectors measure not only how long it takes for a particle to reach the detector, but also how much it is deflected by an electric and / or magnetic field, which gives information other than mass.
Step 4: Detection
A detector counts the number of ions at different deviations. The data are plotted in the form of a graph or a spectrum of different masses . Detectors work by recording the charge or current induced by an ion striking or passing through a surface. Because the signal is very small, an electron multiplier, a Faraday cup or an ion-to-photon detector can be used. The signal is greatly amplified to produce a spectrum.
Uses of mass spectrometry
MS is used for qualitative and quantitative chemical analysis. It can be used to identify the elements and isotopes of the sample, to determine the masses of molecules and as a tool to help identify chemical structures.
It can measure the purity of the sample and the molar mass.
Advantages and disadvantages
A great advantage of mass specification 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. The disadvantages of the mass specification are that it is not very good at identifying hydrocarbons that produce similar ions and it is unable to differentiate between optical and geometric isomers. The disadvantages are offset by combining MS with other techniques, such as gas chromatography (GC-MS).
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