department of organic chemistry, imm, radboud university

Department of Organic Chemistry, IMM, Radboud University Nijmegen

Instrumental Analysis in Molecular Chemistry 1

Mass Spectrometry

• Introduction• Ionization• Separation• Fragmentation• Isotope Effects• High Resolution



Introduction Mass SpectrometryMass Spectrometry allows the weight of

individual molecules to be determined accurately.

The molecules must bebrought into vapour phase (by working in

vacuo),charged (by bombarding them with fast

electrons)and separated (by making them follow different

paths in a magnet field).



Schematic Representation of a Mass Spectrometer

Magnetic Field(perpendicular

to page)

Recorder

dc-Amplifier

ElectrometertubeSample leak

Sample moleculesIonisation area

Anode

Vacuum

Ions with asmall mass

Ions with alarge mass

Acceleratingpotential

Filament forelectronbeam

Ion beam

Exit slit

Collector



Mass Spectrometry, special aspects

In the mass spectrum, differences in isotope composition are detected (as satellite peaks).

The structures of the weighted molecules can be determined by studying how the molecular ion fragments (fragmentation).

By very accurate measurements, and from the pattern of satellite peaks, the element composition of a molecular ion can be determined.



Example of Mass Spectrum- Peak at 136, nominal molecular weight of ester- Base (most intense) peak at 105, loss of methoxy group- Further fragments (phenyl, cyclobutadienyl cation)

due to loss of CO, CH2=CH2



Volatilization

Low pressure: 10-6 torr or lower is applied

Sample can be heated to 200 – 300 oCThis is enough to bring most small organic moleculesin the vapour phase:Polar organic molecules till MW 300Apolar organic molecules till MW 1000

In vapour phase, molecules can be ionized by‘electron impact’ (EI) or ‘chemical ionization’ (CI)

For larger and more polar molecules, more advancedtechniques are required for desorption/ionization



Electron Impact (EI)Ionization (1)

Sample is volatilized nearheated filament

Electrons are accelerated fromfilament to anode by apotential difference of 70 eV:70 eV electrons

Interaction (not strictly collision)of electrons with volatilizedmolecules: Ionization (7 – 10 eV required) to give molecular radicalcation M+.

No electrons taken up to giveradical anions



Electron Impact (EI) Ionization (2)

Impact of 70 eV electrons leads to ionization of 1 out of 104 moleculesLoss of electron from HOMO, typical: n > π > σ

M + e → M+. + 2eTypical ionization energy 7 – 13 eV (1 eV ≈ 23 kcal/mole ≈ 96 kJ/mole)

X = -H 11.07 X = -ONO2 11.07 X = -Cl 10.82-CH3 10.63 -OAc 10.54 -Br 10.18-C2H5 10.34 -OH 10.17 -I 9.26-CHO 9.86 -NH2 8.78-CH=CH2 9.5-COCH3 9.34

Ionization potential (eV) of some monofunctionalized aliphaticcompounds CH2CH2CH2X



Electron Impact (EI) Ionization (3)

Ion picks up 2-7 eV which can give fragmentation (typical bond 4 eV)

Fragmentation possibilities:a) Loss of radical M+. → A+ + B.

i.e. an odd electron (OE) ion givesan even electron ion (EE) + a radical

b) Loss of neutral fragment, even-electron molecule M+. → C+. + Di.e. OE ion gives other OE ion

Examples of energies of some common bonds inorganic molecules (kcal/mol per bond)

C-H 98.7 C-F 116.0 C=C 145.8 C=C 199.6C-C 82.6 C-Cl 81.0 C=N 147.0 C=N 212.6C-N 72.8 C-Br 68.0 C=O 179.0C-O 85.5 C-I 51.0 C=S 128.0C-S 65.0



MassaspectrometrieDependingon the energyof the electronbeam,fragmentationcan occur tovarious extentsupon electronimpact ionization.70 eV is standard !

OH

O

Benzoic acidC7H6O2, Mr 122

+.

O+

CO (28)

+

HC CH

C7H5O (105)

.OH (17)

(26)

C6H5 (77)

+C4H3 (51)



Drawbacks of EI (1)

a) No molecular ion observed for molecules that are proneto fragmentation. See the example of a branched alkane



Drawbacks of EI (2)

b) It is often difficult to distinguish between isomers.

c) Thermal lability:decomposition prior toionization, orfragmentation afterionization, because of thetemperatures requiredfor vaporization.

d) Many compounds notvolatile enough, e.g.polymers.

e) Peaks at mass > M (e.g.MH+) due to ion-moleculereactions.

Example of thermal lability:Boc-Leu-Ala-OMe



Chemical Ionization (1)

Chemical Ionization:ionization of a reactant gas(1 torr) by EI (300 eV),followed by protonationof the analyte M to give MH+

with peak at m/z M+1.

Less energy impacted, a softer ionization technique.

The possibility of fragmentation dependson the energytransferred with theprotonation, i.e.on the strength of the acidproduced from the reaction gas.




CH4 + e CH4+. + 2e

CH4+. CH3

+ + H.

CH4+. CH5

+ + CH3.

CH3+ + CH4 C2H5

+ + H2

CH5+ + M CH4 + MH+

C2H5+ + M C2H4 + MH+

Example of reactant gas: CH4, 1 torr, 300 eV, low analyte vapour pressure

48 % CH5+, 41 % C2H5

+, produced, strong Brønsted acidsIf analyte can accept protons: If not:

CH5+ + M CH4 + (M-H)+ + H2

C2H5+ + M C2H4 + (M-H)+ + H2

So either a (M + 1)+ or a (M – 1)+ peak appears in the mass spectrumAn even-electron (EE) Quasi-molecular ion




Fragmentation with CI dependent on energy of ion and exothermicityof H+-transfer, strength of Brønsted acid, i.e. choice of reactant gas:

CH4 > isobutane > NH3

Little exothermic H+ transfer with NH4+, little fragmentation.

Even electrophilic addition possible if analyte does not accept H+,Quasi-molecular ion at [M+NH4]+

Negative ion chemical ionisation, reactant gas CH4/N2O 1/1N2O + e N2 + O.-

O.- + CH4 OH- + CH3.

M + OH- [M-H]- + H2O

An EE [M-1]- peak appears in negative mode MS.

CH4 most commonly used !



EI and CI (CH4 reagent) of ephedrine



Ionization and Separation

Magnet

Ionsource

Tovacumpump

Ion collector

Acceleration in electric field:½.m.v2 = z.e.V

m = mass of ionv = velocityz = charge

e = unit charge (electron)V = potential of electrostatic field

Bending by Lorentz forcein magnet (centripetal force)

FL = B.z.e.v

In circular path also centrifugal force:FC = m.v2/r

FC = FL, som/z = B2.r2.e/V



Path of ion in Magnetic Analyser

With the detector slit at a distance 2r from the source slit, V can bevaried with a fixed B so that ions with a certain m/z reach the detector.

More common: Fix V andscan m/z by variation of B.



Accurate Isotope Weights

Element/ isotope Natural Abundance Isotope Weight

Hydrogen 1H 99.985 % 1.007 8252H (D) 0.015 % 2.014 102

Carbon 12C 98.9 % 12.000 00013C 1.1 % 13.003 354

Nitrogen 14N 99.64 % 14.003 07415N 0.36 % 15.000 108

Oxygen 16O 99.8 % 15.994 91517O 0.04 % 16.999 13318O 0.2 % 17.999 160



Element/ isotope Natural abundance Isotope weight

Hydrogen 1H 99.985 % 1.007 8252H (D) 0.015 % 2.014 102

Carbon 12C 98.9 % 12.000 00013C 1.1 % 13.003 354

Nitrogen 14N 99.64 % 14.003 07415N 0.36 % 15.000 108

Oxygen 16O 99.8 % 15.994 91517O 0.04 % 16.999 13318O 0.2 % 17.999 160

Fluorine 19F 100 % 18.998 40Silicon 28Si 92.21 % 27.976 93

29Si 4.70 % 28.976 4930Si 3.09 % 29.97 376

Phosphorus 31P 100 % 30.973 76Sulfur 32S 95.0 % 31.972 07

33S 0.76 % 32.971 4634S 4.22 % 33.967 8636S 0.014% 35.967 09

Chlorine 35Cl 75.8 % 34.968 85537Cl 24.2 % 36.965 896

Bromine 79Br 50.5 % 78.918 34881Br 49.5 % 80.916 344

Iodine 127I 100 % 126.900 4



Resolution

Element composition with a mass of approximately 29,possibilities:C2H5 29.03915HCO 29.01403N2H 29.00273

Resolutiondefinedas (M/ΔM)10%

ΔM 0.02512 (M/ΔM)10% 1154ΔM 0.01130 (M/ΔM)10% 2566



Double Sector Instrument

Electrostatic analyser with voltage ECentripetal force FE = z.e.E, centrifugal FC = m.v2/rRadius of path r = m.v2/z.e.E = ½.m.v2/z.e.Ei.e. selection by kinetic energy



Resolution, Peak Match

a) Accurate measurement of massVG 7070: m/Δm ± 3000 over whole spectrumaccurate to 2 decimals

b) Exact mass of a peak determined by peak match:relate to an exactly known mass in region of unknown massPFK (perfluorokerosene) reference compound CF3(CF2)nCF3

Resolution 5,000 – 15,000, accurate to 3-4 decimals

A peak match gives an element composition and is accepted as evidence for the proposed formula of a compound, butsupport it with a careful discussion of significance of difference with other possible formulas (σ etc.)

It is not a substitute for Elemental Analysis as it is not a measure ofthe purity of the compound !



Isotope EffectsSpectrum of HCl

Atom weights:H: 1; Cl: 35,5

Expected (naively): M+. at 36,5Cl+ fragment at 35,5

Observed:Strong peaks at 36 and 38Weak at 35 and 37

Reason: Cl occurs asa mixture of isotopes35Cl and 37Cl inapprox. 3 : 1 ratio



Natural Abundance and Distribution of IsotopesNominal (non-accurate) weights

A A+1 A+2 Type ofElement Weight % Weight % Weight % elementH 1 100 2 0.015 - - AC 12 100 13 1.1 - - A+1N 14 100 15 0.37 - - A+1O 16 100 17 0.04 18 0.2 A+2F 19 100 - - - - ASi 28 100 29 5.1 30 3.4 A+2P 31 100 - - - - AS 32 100 33 0.79 34 4.4 A+2Cl 35 100 - - 37 32.0 A+2Br 79 100 - - 81 97.3 A+2I 127 100 - - - - A



Isotopes, Satellite Peaks

The occurrence ofvarious isotopes withdifferent weights leadsto the observation ofso-called satellite peaksin the mass spectrum.

35Cl and 37Cl occur inapprox. 3 : 1 ratio → anion with 1 Cl has peaksat M and M+2, in 3:1 ratio.

Cf. 79Br and 81Br in approx.equal quantities → an ionwith 1 Br has 1:1 peaks atM and M+2.

See graph for mixed Cl-Brcompounds.



Calculation of Intensity of Satellite Peaks

nk

The multiplicity and intensities of the satellite peaks can be calculatedwith of Newton’s binomium:

For 3 Br atoms, a = b = 1 : 1(a + b)3 = a3 + 3a2b + 3ab2 + b3 = 1 + 3 + 3 + 1i.e. 4 peaks with intensity ratio 1 : 3 : 3 : 1

In general, for a combination of element x, in ratio a : b, combined with m atoms of element y, with ratio c : d, use the coefficients of

(a + b)n (c + d)m

(a + b)n = an + n.an-1.b + n(n-1)an-2.b2 + ....1! 2!



SummaryIn conventional MS techniques, small (apolar, < MW 1000; polar, < MW 300) molecules are volatilized, ionized, and the ions separated by a magnet.

Ionization is by Electron Impact (EI) and can lead to fragmentationwhich is dependent on the energy.

With Chemical Ionization (CI) the sample is indirectly ionized byway of a ‘superacid’ from a reaction gas, and the energy transfercan be tuned by choice of reaction gas.

Fragmentation can help to elucidate the structure.

A double (electrostatic + magnetic) sector instrument is used if highresolution is required.

The natural abundance of several isotopes for an element leads to satellite peaks.

department of organic chemistry, imm, radboud university

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