Mass Spectrometry PDF

Mass Spectrometry  Background Definition: it is analytical technique that identify the types of molecules by measuring the mass to charge ratio and the abundance of gaseous ions. Its principle is based on that the molecule is vaporized and ionized by bombardment with a bea...

Mass Spectrometry  Background Definition: it is analytical technique that identify the types of molecules by measuring the mass to charge ratio and the abundance of gaseous ions. Its principle is based on that the molecule is vaporized and ionized by bombardment with a beam of high-energy electrons. This beam of electrons is used to displace an electron from the organic molecule to form a radical cation known as the molecular ion Mass spectrometry may also involve breaking molecules into fragments (small ions)- thus enabling its structure to be determined. What information can be determined? Molecular weight (mass) Molecular formula Structure (from fragmentation fingerprint) Isotopic incorporation / distribution  How it works Stage 1: Ionisation The atom is ionised by knocking one or more electrons off to give a positive ion. When a molecule loses one electron, it then has a positive charge and one unpaired electron. This ion is therefore called a radical cation  So, radical cation is : a species with a positive charge and one unpaired electron,& symbolized as M+. H H - - H C H + e H C H + 2e H H Molecular ion (M+ ) m/z = 16 Molecular ion (parent ion):The radical cation corresponding to the mass of the original molecule H H H H C H H C C H H H H  The impact of the stream of high energy electrons can also break the molecule or the radical cation into fragments. H H + H C C H molecular ion (M ) m/z = 30 H H H H H H - H C C H + e + H H C C H H H H m/z = 29 H H H C + C H (not detected by MS) m/z = 15 H H Stage 2: Acceleration The ions are accelerated so that they all have the same kinetic energy Stage 3: Deflection The ions are then deflected by a magnetic field according to their masses. 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. Stage 4: Detection The beam of ions passing through the machine is detected electrically in proportion to their abundance. A mass spectrum of the molecule is thus produced. The detector amplifies the signals which are then transmitted to the data system to be represented as peaks on a mass spectrum  Mass Spectrometry Terms The ion obtained by the loss of an electron from the Molecular molecule (consisting of essentially the whole ion molecule) The most intense peak in the MS, assigned 100% Base peak intensity M+ Symbol often given to the molecular ion Radical +ve charged species with an odd number of electrons cation Lighter cations formed by the decomposition of the Fragment molecular ion. ions These often correspond to stable carbcations.  Instrument : Mass Spectrometer The mass spectrometer is an instrument which can measure the masses and relative concentrations of atoms and molecules. It makes use of the basic magnetic force on a moving charged particle.  Mass Spectrum It displays the result in the form of a plot of ion abundance versus Mass-to-charge ratio (m/z): the ratio of the mass of an ion (m) to its charge (z) Base peak, is the most intense peak in the spectrum (100% abundance). The base peak is not necessarily the same as the molecular ion peak  Isotopes Most elements occur naturally as a mixture of isotopes. – The presence of significant amounts of heavier isotopes leads to small peaks that have masses that are higher than the parent ion peak. M+1 = a peak that is one mass unit higher than M+ M+2 = a peak that is two mass units higher than M+  Isotopes  Bromine: M+ ~ M+2 (50.5% 79Br & 49.5% 81Br) 2-bromopropane M+ ~ M+2 SDBSWeb : http://riodb01.ibase.aist.go.jp/sdbs/ (National Institute of Advanced Industrial Science and Technology, 11/1/09)  Mass Spectrometry of CH4 The mass spectrum of CH4 consists of more peaks than just the M peak. Since the molecular ion is unstable, it fragments into other cations and radical cations containing one, two, three, or four fewer hydrogen atoms than methane itself. Thus, the peaks at m/z 15, 14, 13 and 12 are due to these lower molecular weight fragments. 14  Mass Spectrometry of CH4 13CH 4= 17 The tallest peak in the mass spectrum is the base peak and here is also the M peak, although this may not always be the case. Most C atoms have an atomic mass of 12 However, 1.1% of C have a mass of 13 ( higher by 1 a.t.m). This is called the M + 1 peak(peak at m/z = 17) refers to 13CH4 15  Fragmentation Patterns Alkanes – Fragmentation often splits off simple alkyl groups: Loss of methyl M+ - 15 Loss of ethyl M+ - 29 Loss of propyl M+ - 43 Loss of butyl M+ - 57 – Branched alkanes tend to fragment forming the most stable carbocations.  Fragmentation Patterns Mass spectrum of 2-methylpentane  Helpful Points 1-Nitrogen rule :If a compound contains an even number of nitrogen atoms (including 0), its M will appear at an even mass number. If, however, a compound contains an odd number of nitrogen atoms, then its molecular ion will appear at an odd mass value. This rule is very useful for determining the nitrogen content of an unknown compound. Example1 : M for(ethylamine) [CH3CH2NH2]+, is m/z=45 amu, an odd number. We find that the number of nitrogen is one, also an odd number. Example 2 : M for 1,2-diaminoethane, [NH2CH2CH2NH2]+, is m/z= 60 amu, an even number;We find the number of nitrogens are two, also an even number.  Exact Masses of Some Common Elements and Their Isotopes: Element Symbol Exact Mass (u) Rel. Abundance % Hydrogen 1H 1.007825037 100.0 Deuterium 2H or D 2.014101787 0.015 Carbon 12 12C 12.00000 100.0 Carbon 13 13C 13.003354 1.11223 Nitrogen 14 14N 14.003074 100.0 Nitrogen 15 15N 15.00011 0.36734 Oxygen 16 16O 15.99491464 100.0 Oxygen 17 17O 16.9991306 0.03809 Oxygen 18 18O 17.99915939 0.20048 Fluorine 19F 18.998405 100.0 Sodium 23Na 22.9897697 100.0 Sulfur 32 32S 31.972074 100.0 Sulfur 33 33S 32.9707 0.78931 Sulfur 34 34S 33.96938 4.43065 Sulfur 36 36S 35.96676 0.02105 Chlorine 35 35Cl 34.968854 100.0 Chlorine 37 37Cl 36.965896 31.97836