2 BackgroundMass spectrometry (Mass Spec or MS) uses high energy electrons to break a molecule into fragments.Separation and analysis of the fragments provides information about:Molecular weightStructure
3 BackgroundThe impact of a stream of high energy electrons causes the molecule to lose an electron forming a radical cation.A species with a positive charge and one unpaired electronMolecular ion (M+)m/z = 16
4 BackgroundThe impact of the stream of high energy electrons can also break the molecule or the radical cation into fragments.m/z = 15
5 Background Molecular ion (parent ion): The radical cation corresponding to the mass of the original moleculeThe molecular ion is usually the highest mass in the spectrumSome exceptions w/specific isotopesSome molecular ion peaks are absent.
6 Background Mass spectrum of ethanol (MW = 46) M+ SDBSWeb : (National Institute of Advanced Industrial Science and Technology, 11/1/09)
7 BackgroundThe cations that are formed are separated by magnetic deflection.
8 Background Only cations are detected. Radicals are “invisible” in MS. The amount of deflection observed depends on the mass to charge ratio (m/z).Most cations formed have a charge of +1 so the amount of deflection observed is usually dependent on the mass of the ion.
9 BackgroundThe resulting mass spectrum is a graph of the mass of each cation vs. its relative abundance.The peaks are assigned an abundance as a percentage of the base peak.the most intense peak in the spectrumThe base peak is not necessarily the same as the parent ion peak.
10 Background The mass spectrum of ethanol base peak M+ SDBSWeb : (National Institute of Advanced Industrial Science and Technology, 11/1/09)
11 Background 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+
12 Easily Recognized Elements in MS Nitrogen:Odd number of N = odd MWSDBSWeb : (National Institute of Advanced Industrial Science and Technology, 11/2/09)
13 Easily Recognized Elements in MS Bromine:M+ ~ M+2 (50.5% 79Br/49.5% 81Br)2-bromopropaneM+ ~ M+2SDBSWeb : (National Institute of Advanced Industrial Science and Technology, 11/1/09)
14 Easily Recognized Elements in MS Chlorine:M+2 is ~ 1/3 as large as M+M+M+2SDBSWeb : (National Institute of Advanced Industrial Science and Technology, 11/2/09)
15 Easily Recognized Elements in MS Sulfur:M+2 larger than usual (4% of M+)M+Unusually large M+2SDBSWeb : (National Institute of Advanced Industrial Science and Technology, 11/1/09)
16 Easily Recognized Elements in MS IodineI+ at 127Large gapM+Large gapI+SDBSWeb : (National Institute of Advanced Industrial Science and Technology, 11/2/09)
17 Fragmentation Patterns The impact of the stream of high energy electrons often breaks the molecule into fragments, commonly a cation and a radical.Bonds break to give the most stable cation.Stability of the radical is less important.
18 Fragmentation Patterns AlkanesFragmentation often splits off simple alkyl groups:Loss of methyl M+ - 15Loss of ethyl M+ - 29Loss of propyl M+ - 43Loss of butyl M+ - 57Branched alkanes tend to fragment forming the most stable carbocations.
19 Fragmentation Patterns Mass spectrum of 2-methylpentane
20 Fragmentation Patterns Alkenes:Fragmentation typically forms resonance stabilized allylic carbocations
21 Fragmentation Patterns Aromatics:Fragment at the benzylic carbon, forming a resonance stabilized benzylic carbocation (which rearranges to the tropylium ion)M+
22 Fragmentation Patterns Aromatics may also have a peak at m/z = 77 for the benzene ring.77M+ = 123
23 Fragmentation Patterns AlcoholsFragment easily resulting in very small or missing parent ion peakMay lose hydroxyl radical or waterM or M+ - 18Commonly lose an alkyl group attached to the carbinol carbon forming an oxonium ion.1o alcohol usually has prominent peak at m/z = 31 corresponding to H2C=OH+
24 Fragmentation Patterns MS for 1-propanolM+-18M+SDBSWeb : (National Institute of Advanced Industrial Science and Technology, 11/28/09)
25 Fragmentation Patterns AminesOdd M+ (assuming an odd number of nitrogens are present)a-cleavage dominates forming an iminium ion
27 Fragmentation Patterns Ethersa-cleavage forming oxonium ionLoss of alkyl group forming oxonium ionLoss of alkyl group forming a carbocation
28 Fragmentation Patterns MS of diethylether (CH3CH2OCH2CH3)
29 Fragmentation Patterns Aldehydes (RCHO)Fragmentation may form acylium ionCommon fragments:M+ - 1 forM for
30 Fragmentation Patterns MS for hydrocinnamaldehyde91M+ = 134105SDBSWeb : (National Institute of Advanced Industrial Science and Technology, 11/28/09)
31 Fragmentation Patterns KetonesFragmentation leads to formation of acylium ion:Loss of R formingLoss of R’ forming
32 Fragmentation Patterns MS for 2-pentanoneM+SDBSWeb : (National Institute of Advanced Industrial Science and Technology, 11/28/09)
33 Fragmentation Patterns Esters (RCO2R’)Common fragmentation patterns include:Loss of OR’peak at M+ - OR’Loss of R’peak at M+ - R’
34 Frgamentation Patterns 10577M+ = 136SDBSWeb : (National Institute of Advanced Industrial Science and Technology, 11/28/09)
35 Rule of ThirteenThe “Rule of Thirteen” can be used to identify possible molecular formulas for an unknown hydrocarbon, CnHm.Step 1: n = M+/13 (integer only, use remainder in step 2)Step 2: m = n + remainder from step 1
36 Rule of ThirteenExample: The formula for a hydrocarbon with M+ =106 can be found:Step 1: n = 106/13 = 8 (R = 2)Step 2: m = = 10Formula: C8H10
37 Rule of Thirteen If a heteroatom is present, Subtract the mass of each heteroatom from the MWCalculate the formula for the corresponding hydrocarbonAdd the heteroatoms to the formula
38 Rule of ThirteenExample: A compound with a molecular ion peak at m/z = 102 has a strong peak at 1739 cm-1 in its IR spectrum. Determine its molecular formula.
39 GC-Mass Spec: Experiment 23 Mass Spec can be combined with gas chromatography to analyze mixtures of compounds.GC separates the components of the mixture.Each component is analyzed by the Mass Spectrometer.
40 GC-Mass Spec: Experiment 23 Assignment:Observe the GC-mass spec experimentRecord experimental conditionsAnalyze the mass spectrum of each component of your mixture:Parent ion peak?Heteroatoms apparent from spectrum?A minimum of 1 or two significant fragments and their structures
41 GC-Mass Spec: Experiment 23 Assignment (cont.):Using the Mass Spec data, retention times, and boiling points, identify the components of your mixture.Write three paragraphs (one per compound) summarizing and interpreting all data. See your data sheet for more details.