Mass Spectrometry

Lab Technique - Mass Spectrometry Introduction Organic molecules have complex behavior. Their structure is complicated and need to be clearly understood. Chemists have been developing various techniques to see inside the organic molecules. These techniques can be broadly classified as spectrometry, separation and spectroscopy. Mass Spectrometry Mass spectrometry is a technique and an analytical tool in the hand of chemists which is used for measuring the molecular mass or atomic mass. The molecular masses of biomolecules, can be measured up to an accuracy of 0.01%. For smaller organic molecules, the molecular mass can be measured within an accuracy of 5 ppm or less. This accuracy is sufficient to confirm the molecular formula of a compound Principal of Mass Spectrometry Ions are electrically charged particles and they can be deflected by magnetic fields as electrically charged particles are affected by a magnetic field. Higher the mass less is deflection and lower the mass more is the deflection. Process of Mass Spectrometry The molecules of the sample are bombarded with electron. The atoms of molecules are ionized by losing one or more electrons to give a positive ion. This is true even for particles which

normally produce negative ions (chlorine, for example) or never form ions at all (argon, for example). Mass spectrometers always work with positive ion. The largest ion is the size of original molecule less one electron. It is a cation. For example, methane will produce a cation of the type CH4+. This cation is called molecular ion. The ions are accelerated through a magnetic field so that they all have the same kinetic energy. A magnetic force is experienced by the charged ion causing the deflection from their original path. The ions are deflected by the magnetic field according to their masses. Magnetic field can be varied to allow ions of different mass to pass through the spectrometer. The lighter they are, the more they are deflected. The amount of deflection also depends on the number of positive charges on the ion. In other words, on how many electrons were knocked off in the first stage. The more the ion is charged, the more it gets deflected. The deflection is along a curved path and radius of deflected curved path depends upon mass to charge ratio of the ion. The beam of ions passing through the machine is detected electrically. A schematic diagram of mass spectrometer is shown

Most of ions are having charge of 1+ after loosing one electron. Most of the ions have positive one charge because it very difficult to knock off 2nd electron after one electron is removed. Most of the ions passing through the mass spectrometer will have a charge of 1+, so that the mass/charge ratio will be the same as the mass of the ion.

The ions which have the smaller value of m/z will need less deflection to bring them on to the detector, and less deflection is possible by using a weaker magnetic field (a smaller sideways force). For heavier ion, stronger magnetic field will be needed. In other words all ions heavier or lighter can be brought to detector by controlling the magnetic field. The detector produces a current which is proportional to the number of ions arriving. The mass of each ion being detected is related to the size of the magnetic field used to bring it on to the detector. The output from the chart recorder is usually processed by a computer and is represented as a mass spectrum. This spectrum shows the relative current produced by ions of varying mass/charge ratio. Mass Spectrum Mass spectrum will usually be presented as a vertical bar graph, in which each bar represents an ion having a specific mass-tocharge ratio (m/z) and the length of the bar indicates the relative abundance of the ion. The most intense ion is assigned an abundance of 100, and it is referred to as the base peak. Most of the ions formed in a mass spectrometer have a single charge, so the m/z value is equivalent to mass itself. Modern mass spectrometers easily distinguish (resolve) ions differing by only a single atomic mass unit (amu), and thus provide completely accurate values for the molecular mass of a compound. The highest-mass ion in a spectrum is normally considered to be the molecular ion, and lower-mass ions are fragments from the molecular ion, assuming the sample is a single pure compound. simple

The following diagram displays the mass spectra of three gaseous compounds, carbon dioxide, propane and

cyclopropane. The molecules of these compounds are similar in size. CO2 and C3H8 both have a nominal mass of 44 amu, and C3H6 has a mass of 42 amu. The molecular ion is the strongest in the spectra of CO2 and C3H6, and it is moderately strong in propane. The unit mass resolution is readily apparent in these spectra (note the separation of ions having m/z=39, 40, 41 and 42 in the cyclopropane spectrum). Even though these compounds are very similar in size, it is a simple matter to identify them from their individual mass spectra.

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