Chapter 20 · PDF file5 • When the electron beam ionizes the molecule, the species that...

Mass Spectroscopy

Chapter 20

Mass spectrometry is a technique used for measuring the molecular weight and determining the molecular formula of an

organic compound.

Mass Spectrometry (MS)

Mass Spectrometry (MS) In a mass spectrometer, a molecule is vaporized and ionized by bombardment

with a beam of high-energy electrons.

The energy of the electrons is ~ 1600 kcal (or 70 eV).

Since it takes ~100 kcal of energy to cleave a typical s bond, 1600 kcal is an enormous amount of energy to come into contact with a molecule.

The electron beam ionizes the molecule by causing it to eject an electron.

The ions formed may remain intact (as molecular ions, M+), or they may fragment

The resulting mixture of ions is sorted by mass/charge (m/z) ratio, and detected

Molecular weight and chemical formula may be derived from the M+ and M+1 ions

Molecular structure may be deduced from the distribution of fragment ions

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• When the electron beam ionizes the molecule, the species that isformed is called a radical cation, and symbolized as M+•.

• The radical cation M+• is called the molecular ion or parent ion.• The mass of M+• represents the molecular weight of M.• Because M is unstable, it decomposes to form fragments of

radicals and cations that have a lower molecular weight than M+•.• The mass spectrometer analyzes the masses of cations.• A mass spectrum is a plot of the amount of each cation (its

relative abundance) versus its mass to charge ratio (m/z, wherem is mass, and z is charge).

• Since z is almost always +1, m/z actually measures the mass (m)of the individual ions.

Introduction

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Mass Spectrometry

Consider the mass spectrum of CH4 below:

• The tallest peak in the mass spectrum is called the base peak.• The base peak is also the M peak, although this may not always

be the case.• Though most C atoms have an atomic mass of 12, 1.1% have a

mass of 13. Thus, 13CH4 is responsible for the peak at m/z = 17.This is called the M + 1 peak.

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Mass Spectrometry

• Since the molecular ion is unstable, it fragments intoother 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 theselower molecular weight fragments.

SDBSWeb : http://riodb01.ibase.aist.go.jp/sdbs/ (National Institute of Advanced Industrial Science and Technology, 11/1/09)

M+base peak

The mass spectrum of ethanol

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Mass Spectrometry (MS) A mass spectrometer produces a spectrum of masses based on the

structure of a molecule.

A mass spectrum is a plot of the distribution of ion masses corresponding to the formula weight of a molecule and/or fragments derived from it

The x-axis of a mass spectrum represents the masses of ions produced

The y-axis represents the relative abundance of each ion produced

The pattern of ions obtained and their abundance is characteristic of the structure of a particular molecule

Components of a mass spectrometer

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Mass Spectrometer

Schematic of a massspectrometer

The mass spectrometer records a mass spectrum

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ANALYZERS

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The Mass Spectrum Data from a mass spectrometer can be represented as a graph or table

The most abundant (intense) peak in the spectrum is called the base peak and is assigned a normalized intensity of 100%

The masses are based on rounding of atom masses to the nearest whole number (in low resolution mass spectroscopy)

The data and fragmentation patterns for ammonia are as follows

The base peak for ammonia is the molecular ion, but this is often not the case

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Isotopic Abundance

Easily Recognized Elements in MS Nitrogen: Odd number of N = odd MW

CH3CNM+ = 41


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Mass SpectrometryThe Nitrogen Rule

• Hydrocarbons like methane (CH4) and hexane (C6H14), as well ascompounds that contain only C, H, and O atoms, always have amolecular ion with an even mass.

• An odd molecular ion indicates that a compound has an oddnumber of nitrogen atoms.

• The effect of N atoms on the mass of the molecular ion in a massspectrum is called the nitrogen rule: A compound that containsan odd number of N atoms gives an odd molecular ion. Acompound that contains an even number of N atoms (includingzero) gives an even molecular ion.

• Two “street” drugs that mimic the effect of heroin illustrate thisprinciple.

Easily Recognized Elements in MS• Bromine:

• M+ ~ M+2 (50.5% 79Br/49.5% 81Br)

2-bromopropane

M+ ~ M+2


Easily Recognized Elements in MS Chlorine: M+2 is ~ 1/3 as large as M+

Cl


M+2

M+

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Mass SpectrometryAlkyl Halides and the M + 2 Peak• Most elements have one major isotope.• Chlorine has two common isotopes, 35Cl and 37Cl, which

occur naturally in a 3:1 ratio.Thus, there are two peaks in a 3:1 ratio for the molecularion of an alkyl chloride.The larger peak, the M peak, corresponds to thecompound containing the 35Cl. The smaller peak, the M + 2peak, corresponds to the compound containing 37Cl.Thus, when the molecular ion consists of two peaks (Mand M + 2) in a 3:1 ratio, a Cl atom is present.

• Br has two isotopes—79Br and 81Br, in a ratio of ~1:1.Thus, when the molecular ion consists of two peaks (Mand M + 2) in a 1:1 ratio, a Br atom is present.

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Mass Spectrometry

Alkyl Halides and the M + 2 PeakMass spectrum of

2-chloropropane [(CH3)2CHCI]

• The compound contains a chlorine, because M + 2 peak is 1/3 the height of the molecular ion peak

• The base peak at m/z = 43 results from heterolyticcleavage of the C–Cl bond

• The peaks at m/z = 63 and m/z = 65 have a 3:1 ratio, indicating the presence of a chlorine atom

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cleavage results from the homolytic cleavage of aC–C bond at the carbon

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Mass Spectrometry

Alkyl Halides and the M + 2 PeakMass spectrum of

2-bromopropane [(CH3)2CHBr]

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The weakest bond is the C–Br bond

The base peak is at m/z = 43 [M – 79, or (M + 2) – 81]

The propyl cation has the same fragmentation patternit exhibited when it was formed in the cleavage of pentane

Sulfur: M+2 larger than usual (4% of M+)

Easily Recognized Elements in MS

M+

Unusually large M+2

S


Easily Recognized Elements in MS Iodine I+ at 127 Large gap

Large gap

I+

M+


ICH2CN

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.

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.

Fragmentation Patterns Mass spectrum of 2-methylpentane

Fragmentation PatternsAromatics have a peak at m/z = 77 for the benzene ring.

NO2

77M+ = 123

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Rule of Thirteen The “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

Rule of Thirteen Example: The formula for a hydrocarbon with M+ =106 can be

found: CnHm

Step 1: n = 106/13 = 8 (R = 2)

Step 2: m = 8 + 2 = 10

Formula: C8H10

Rule of Thirteen If a heteroatom is present, Subtract the mass of each heteroatom from the MW Calculate the formula for the corresponding hydrocarbon Add the heteroatoms to the formula

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.

Problem 1

Mass – 59Odd-NIR-NH2

59-16=43

C3H9N

Problem 2

Mass – 72IR-1700 cm-1C=O

72-28=44

C4H8O

Problem 3

Mass- 120Br present (M+2)

120-79=41

C3H5Br

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