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  Principles of Mass Spectrometry, Chemistry tutorial

Introduction:

Mass spectrometry employs an instrument termed as a mass spectrometer and as the name recommends, it is mostly employed to find out the relative molecular masses. It is as well employed to investigate the structures of molecules.

Principles of Mass Spectrometry:

The Mass Spectrometry (MS) is an analytical method whereby materials are ionized and dissociated into fragments characteristic of the molecule(s) or elements present in the sample that is employed to get information for qualitative and quantitative analysis. Mass spectrometry utilizes an instrument termed as the mass spectrometer. The ions formed from the molecules or atoms of a sample are separated in space and detected according to their mass-charge ratio, m/z. The numbers of ions of each and every mass detected comprises a mass spectrum, which might be represented graphically or tabulated. Peak intensities are deduced as a percentage of that of the most plentiful ion which is designated the base peak. The spectrum gives structural information and often an accurate relative molecular mass from which an unknown compound can be recognized or structure confirmed. Quantitative analysis is mainly based on measuring the numbers of a specific ion present under closely controlled conditions.

The Mass Spectrometer:

There are various kinds of mass spectrometers available; however they all operate under the similar principle. Mass spectrometers encompass FOUR basic parts, namely the sample inlet system, the ion source, the mass analyzer and the detector (figure shown below). The spectrometer is operated under high vacuum of 10-4 to 10-7 Nm-2 to give ions a reasonable chance of travelling from one end of the instrument to the other devoid of any hindrance from air molecules. The whole operation of the mass spectrometer, and frequently the sample introduction process as well is under entire data system control on the modern mass spectrometers.

1984_Block Diagram of a Mass spectrometer.jpg

Fig: Block Diagram of a Mass spectrometer

The sample under investigation is introduced into the ionization source or chamber of the instrument. Once inside the ionization source, the sample molecules are ionized, as ions are simpler to manipulate than neutral molecules. Different kinds of ionization methods are employed in mass spectrometry like electron impact (EI), chemical ionization (CI), fast atom bombardment (FAB) and electrospray (ESI) ionization methods. Ions formed in the ionization chamber are accelerated all along a curved tube and via a strong magnetic field into the analyzer region of the mass spectrometer where they are separated according to their mass (m)-charge (z) ratios (m/z).  The separated ions are detected and this signal sent to a data system where the m/z ratios are stored altogether by their relative abundance for presentation in the format of a mass spectrum.

The Molecular Ion:

The molecular ion is frequently given the symbol M+ or M+•- the dot in this second version symbolizes the fact that somewhere in the ion there will be a single unpaired electron. It is generally the one half of what was originally a pair of electrons; the other half is the electron which was eliminated in the ionization procedure. 

The molecular ions tend to be unstable and a few of them break into smaller fragments. In the mass spectrum, the heaviest ion (that is, the one having the greatest m/z value) is probable to be the molecular ion. Some compounds encompass mass spectra which do not have a molecular ion peak, as all the molecular ions break into fragments.

The Mass Spectrum:

The richest ion in a mass spectrum is randomly given an abundance of 100% and is termed as the base peak. The abundance of the other ions is assessed relative to the intensity of the base peak. This is not always the heaviest ion in the spectrum. The peaks are usually very sharp and are often simply represented via vertical lines. The fragmentation patterns of (EI) and (CI) mass spectra are generally significantly different.

The ion made by the loss of one electron from the molecule in EI spectroscopy is termed as the molecular ion. It is generally the richest peak on the far right-hand side of the spectrum and its mass is similar as the relative molecular mass (RMM) of the compound. Though, it must be noted that a few EI spectra of compounds having RMM of less than 300 don't exhibit a molecular ion peak. Moreover, recognition can be complicated via the presence of isotopes in some of the molecular ions collected and counted via the instrument example: bromododecane [CH3(CH2)11Br] have 2 molecular ions having RMM values at 248 and 250 marked as M+. Such peaks are due to the presence of bromine isotopes 79Br and 81Br in the molecules of bromododecane.  It must be noted that not all size will based on the relative abundance of the isotope in the compound (figure shown below).

2277_EI Mass Spectrum of Bromododecane.jpg

Fig: EI Mass Spectrum of Bromododecane

In CI spectra the peak in the corresponding position to the molecular ion peak of EI spectra is that due to the MH+ ion whose mass is one unit higher than the RMM of the compound. In rarer situations, it might be due to the presence of the [M-H]+ ion whose RMM is one unit less than the RMM of the molecular ion. One of such molecular ions always takes place even in compounds having RMM of less than 300 and so CI spectra are very helpful for finding out the RMM of such compounds.

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