Application of mass spectrometry to the analysis of mixtures

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A Thesis Submitted for the Degree of PhD at the University of Warwick

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APPLICATION

_{OF MASS SPECTROMETRY}

TO

THE ANALYSIS OF MIXTURES

A thesis submitted by

David Mitchell, B. Sc.

for the degree of

Doctor of Philosopy

BEST COPY

AVAILABLE

Poor text in the original

thesis.

Some text bound close to

the spine.

Contents

Table _{of Contents}

List _{of Figures}

List _of Tables

Acknowledgments

Declaration

Summary

Abbreviations

Chapter 1- _{Instrumental Techniques}

1.1. _{Mass Spectrometry}

1.1.1. Sample Introduction

1.1.2. _{Ion Production}

1.1.3. Ion Separation

1.1.4. Ion Detection _and Data Presentation

1.2. _Ion Production

1.2.1. Vapour

1.2.1.1.

1.2.1.2.

1.2.2.

Desorp

1.2.2.1.

1.2.2.2.

1.2.2.3.

1.2.2.4.

Phase Techniques

Electron Impact

Chemical Ionisation

Lion Techniques

Desorption Chemical Ionisation

Laser Desorption

Field Desorption

Fast Atom Bombardment

1.2.2.5.

Thermospray

1.3.

Mixture

Analysis

1.3.1. On-line Chromatographic Techniques

1.3.2. Soft Ionisation

1.3.3. High Resolution Mass Spectrometry

1 Page i v ix Xl

X110; %

ii

1.3.4.

_Tandem

_Mass

_Spectrometry

₃₃

1.3.5.

_Quantitative

_Mixture

_Analysis

₃₇

1.4. _{Ion Decomposition Pathways}

and Structure Elucidation 37

Chapter _{2- Analysis}

of Anionic _and Non-Ionic Surfactant

Mixtures _{by Fast Atom Bombardment Mass Spectrometry (FABMS) 39}

2.1. _{Introduction 40}

2.1.1.

_Detergents

₄₀

2.1.2. _{Surfactants 40}

2.1.3. _{Surfactant Analysis 44}

2.1.4. _{Mixture Analysis by FABMS 45}

2.2. _{Experimental 47}

2.3. _{Anionic Surfactant Results 48}

2.3.1. _{Negative Ion FAB 49}

2.3.2. _{Positive Ion FAB 54}

2.3.3. _{Counter Surfactant Addition 57}

2.4. _{Non-Ionic Surfactant Analysis 66}

2.4.1. _{Negative Ion FAB 66}

2.4.2. _{Positive Ion FAB 66}

2.5. _{Conclusions 70}

Chapter 3- Thermospray Analysis _of Cationic Surfactants

and Long Chain Amines 72

3.1. _{Introduction 73}

3.2. _{Experimental 74}

3.3. Analysis _of Commercial Mixtures ₇₆

3.4. Thermospray Ionisation _of Quaternary Ammonium Salts ₈₂

3.4.1. Experimental Investigation _of

iii

3.4.2.

_Hoffman

_Elimination

_{or Alkyl}

_Halide

_Loss

₉₃

3.5.

_Thermospray

_of

_Amines

₉₅

3.6.

_Conclusion

₉₇

Chapter _{4- Mass Spectrometric Analysis of Organo-Tin}

Stabilisers

₉₈

4.1. _{Introduction 99}

4.2. _{Experimental 101}

4.3. _{Results and Discussion 102}

4.3.1. _{Probe CI 102}

4.3.2.

_Field

_Desorption

₁₀₆

4.3.3. _{Fast Atom Bombardment 106}

4.3.3.1. _{Positive Ion FAB 106}

4.3.3.2.

_Negative

_Ion

_FAB

₁₂₁

4.3.4.119Sn _{NMR 123}

4.3.5. _{Analysis of an Extracted Tin Stabiliser 126}

4.4. Conclusion ₁₂₈

Chapter 5- _{Mass Spectra of} 3-nitrochromes, 3-nitrochromans

and corresponding chroman-3-one-oximes 129

5.1. _{Introduction 130}

5.2. _{Experimental 131}

5.3. _{Comparison of} 3-nitrochromenes _and 3-nitrochrora:. _{s 131}

5.3.1. _Electron Impact Mass Spectra ₁₃₂

iv

5.4.

_Electron

_Impact

_of

2-aryl-chroman-3-one-oximes

145

5.5.

_Conclusions

150

V

List _of Figures

1.1. _{- Forward geometry magnetic sector mass spectrometer}

1.2. _{- FD} ion _source

1.3.

_{- FAB experimental}

_arrangement

1.4.

_{- Thermospray}

_experimental

_arrangement

2.1. _{- Negative ion FAB spectrum for C12/C14 2EO}

2.2. _{- Positive ion FAB spectrum for C12/C14 2E0}

2.3. _{- Negative} ion _{FAB spectrum for C13/C15 3E0 alcohol}

ethoxylate

2.4. _{- Positive} ion _{FAB spectrum for C131C15} 3E0 _alcohol

ethoxylate

3.1.

_{- FAB spectrum}

_of

Softlan

JGE 930

3.2.

_{- Thermospray}

_spectrum

_of

Softlan

JGE 930

3.3. _{- FD spectrum of} Arquad NF

3.4. _- Plot _of I (R4N+) /I (R3NH+)

_against

_vaporiser

vi

3.5.

- Plot of I (R4N+) /I (R3NH+)

against

source

temperature

3.6. _{- Plot of I(R4N+)/I(R3NH+) against quaternary}

ammonium _{salt concentration}

3.7.

_{- Plot}

_of

_{I (R4N+) /I (R3NH+)}

_against

_alkyl

_group

_size

3.8. _{- Thermospray spectrum of benzyl triethyl ammonium}

chloride

3.9. _{- Thermospray spectra of (a) Syn} 35 (b) Syn 35M

(c)

_{Syn 35 DM}

4.1. _{- Ammonium} CI _{spectrum of} dibutyl tin _{stabiliser up}

to

_probe

temperature

_of

670 K

4.2. _- Ammonium CI _{spectrum of} dibutyl tin stabiliser at

probe

temperature

above

670 k.

4.3. _{- FD spectrum of} dimethyl tin stabiliser

4.4. _- Positive ion FAB spectrum of dimethyl tin

stabiliser

4.5. _- CAD daughter ion spectrum of the m/z 865 ion from

vii

4.6. _{- CAD daughter ion spectrum of the m/z 865} ion _from

mono-n-octyl _{at collision energy of} 24 eV

4.7. _{- CAD daughter} ion _{spectrum of the m/ z 865 ion from}

mono-n-octyl _{at collision energy of} 10 eV

4.8. _{- CAD daughter} ion _{spectrum of m/z 579 from dimethyl}

tin _{stabiliser at a collision energy of 10 eV}

4.9. _{- Fragmentation pathways for organo-tin stabilisers}

4.10. _{- Fragmentation pathways} for ion _m/z 1016

4.11. _{- Negative} ion FAB _{spectrum of mono-n-octyl} tin

stabiliser

4.12. _- 119Sn

stabiliser

NMR _{spectrum of} the di-n-octyl tin

4.13. _- Positive ion FAB _spectrum of extracted tin

stabiliser from PVC bottle

5.1.

_-

EI

_spectra

for

(a)

2-phenyl-3-nitrochromene

(b)

2-phenyl-3-nitrochroman

5.2. _{- Formation of} ArCH2

Viii

5.4.

_{- Electron}

impact

_spectrum

_of

2-phenyl-chroman-3-one

oxime

i.

List

_of

Tables

1.1.

_{- Proton}

_affinites

_of

_{some common CI reagent}

_gases

2.1.

_{- Standard}

_alcohol

_ether

_sulphates

2.2. _{- Relative peak intensities for ethoxy components in}

major

homologous

_series

2.3. _{- Relative positive ion peak intensities for ethoxy}

components _{of major} homologous _series

2.4. _{- Relative peak intensities after addition of}

synprolam

35TMQC or kenamine

2.5.

_-

_Relative

_peak

intensities

_after

_synprolam

_F3

addition

2.6. _{- Relative peak} intensities _{of ethoxy components}

after addition of synprolam DHQC

2.7. _{- Major} to _minor homologous _series ratios upon

addition of counter surfactant

2.8.

_-

Relative

_peak

intensities

of

ethoxy

components

of

major alkyl series

X

3.2.

_-

_Enthalpy

_values

_for

_reaction

_and

_formation

_of

tetramethyl

_and

_ethyl

_chloride

_and

_bromide,

_and

tetrapropyl

_chloride

4.1. _{- Standard organo-tin stabiliser}

5.1. _{- Relative peak intensities for EI mass spectra of}

eight 2-aryl-3-nitrochromenes

5.2.

_-

_Relative

_peak

intensitites

_for

_EI

_mass

_spectra

_of

ten

2-aryl-3-nitrochromans

5.3. _{- Relative peak} intensities for _{EI mass spectra of}

.. i

Acknowledgements

Firstly I _would like to thank _my two _supervisors

Prof Keith Jennings _{at Warwick and Dr Jim Scrivens at ICI}

for _their invaluable _{help and encouragement.}

I would also like to thank _{all members of} the _mass

spectrometry _{research group at} Warwick from 1984-1987 for

their intellectual _{and social support. Special mention}

goes to Dr Rod Mason, Dr David Newton _and Dr Richard

Bowen. I _am indebted to the _{support services and}

technicians _of the Department _of Chemistry _at Warwick,

especially the mechanical and electrical workshops. I

would also like to thank the people at ICI especially Mr

Gary Manton _and Dr Trevor Blease for their time _and

patience whilst I was there.

Specific _{acknowledgments go} to Dr David Catlow for

obtaining the Field Desorption spectra and Dr Trevor

Blease for the low energy collisionally activated

daughter ion _spectra, Dr Aiden Harrison for the NMR

spectra and Dr Gilberto de Nucci for his typing

expertise.

I _{would also} like to acknowledge the financial

XI (4q

Declaration

The

_work

_presented

_in

_this

_theses

_is

my own and original

_except

were

due

_reference

_or

_{acknowledgment}

_is

_given.

_{Some of this}

_work

has

_been

_previously

_presented

at

conferences

_or

_accepted

for

publication.

Mass

Spectra

_of

2-Aryl-3-Nitrochromans

_and

2-Aryl-3-

Nitrochromenes,

D. Mitchell,

R. D. Bowen,

K. R. Jennings,

R. S.

Varma,

G. W. Kabalka,

J.

Chem.

Soc.

Perkin

II,

1495,1987

Mass

Spectra

_of

2-Aryl-chroman-3-one

Oximes,

R. D. Bowen,

D. Mitchell,

R. S. Varma,

G. W. Kabalka,

Org.

Mass

Spectrom.,

22,231,1987.

Fast

Atom

Bombardment

_{Mass Spectra}

_of

_Surfactants

_{and Tin}

Stabilisers,

D. Mitchell-,

K. R. Jennings,

J. H. Scrivens,

10th

International

_nass

Spectrometry

Congress,

Swansea,

1985.

Mass

Spectra

_of

2-Aryl-3-Nitrochroman

_and

2-Aryla-3-

Nitrochromenes,

D, MitcheLL

R. D. Bowen,

K. R. Jennings,

R. S.

Varma,

G. W. Kabalka,

British

Mass Spectrometry

Society

Meeting,

Brighton,

1986,

Mass

Spectra

_of

Tin

Stabilisers,

D. Mitchell,

K. R. Jennings,

J. H. Scrivens,

Roval

Society

_of

Chemistry

_annual

Congress,

Swansea,

1987,

Thermospray

Mass

Spectra

_of

Quaternary

Ammonium

Salts,

D.

Mitchell,

K. R- J enni

_s,

J. H. Scrivens,

American

Society

for

Mass

SDecurometry,

35th

Annual

_meeting,

Denver,

1987,

Fast

Atom

Bombardment

Mass

Spectrometry

of

Anionic

Surfactants,

D, I1itchei

1,

K. R. Jennings,

J. H. Scrivens,

British

Mass

Spectrometry

Society

Meeting,

1987,

York,

X'_1

Summary

The _{work presented} in _{this thesis} involves _the

application _{of mass spectrometry} to the _{analysis of a}

number _of both _{commercial products and model organic}

compounds. Use _{of soft} ionisation techniques for

thermally labile _and ionic _{compounds (mainly fast atom}

bombardment, _{field desorption and thermospray) has} been

extensively _explored for the _provision of both

qualitative _{and quantitative} information. More

established ionisation techniques (electron impact and

chemical ionisation) have also been utilized in order to

obtain qualitative data. In many cases comparisons have

been _made between different ionisation _modes, and some

discussion _of the _{processes occuring} is given. Ion

structures and decomposition pathways have been

elucidated by both tandem mass spectrometry and high mass

resolution measurements.

Analysis _of both complex mixtures and pure

compounds has been performed. The systems studied were:

surfactants (anionic - sodium alcohol ether sulphates,

nonionic _- alcohol ethoxylates, cationics - mainly

quaternary ammonium salts), organotin PVC heat

stabilisers and organic heterocyclic compounds (some

x111

mmHg KV

KJmol-1

S

Vm-1

FTICR

secdec-1

mA

µl

K

NMR

MHz

Cµg"-1

Abbreviations

millimeters

_of

_mercury

kilovolts

kilojoules

_per

_mole

seconds

volts

per

meter

Fourier _transform ion _{cyclotron resonance}

seconds _per decade

milliamps

microliters

degrees _Kelvin

nuclear

magnetic

resonance

mega Hertz

Coulombs

_per

_microgram

Chapter I

INSTRUMENTAL

2

1.1 _{Mass Spectrometryl-3}

A mass _{spectrometer may} be defined _{as an} instrument

which _{separates gaseous} ions _{according to their mass to}

charge _ratio (m/z) by _{some physical means.}

Development _{of mass spectrometry began in the early}

part _of this _{century4-6. Mass spectrometers were first}

applied to the determination _{of the} different _elemental

isotopes, _{and their relative abundances. Application of}

mass _{spectrometers widened} to _usage in the _chemical

industry _{for mixture analysis of oils. This practice}

ceased _with the _{advent of gas chromatography7} in the

1960s, _which is _{considerably simpler and less expensive}

instrument _{for mixture analysis. Mass spectrometry}

retained analytical usage mainly due to its ability to

provide accurate mass and structural information.

Since the 1960s _extensive development in the field

of mass spectrometry has occured. Notable inovations

include: _{the coupling of chromatography with} mass

spectrometry, new ionisation techniques, fast scanning,

use of computers, high mass range options, and tandem

mass spectrometry. These, together with the inherent

sensitivity, specificity, and adaptability of mass

spectrometers, has led to their high profile in

analytical laboratories.

The

_operation

_of

a

mass

spectrometer

can

be

considered

as

four

sequential

parts:

(a)

Sample

3

1.1.1. _{Sample introduction}

There _{are a number of methods of admitting a sample}

into _{a mass spectrometer. Choice}

of method used for a

particular _sample depends _upon its: (a) _Volatility. (b)

Thermal _{stability. (c) Whether the sample is known to be}

pure _{or a mixture.}

The _only instrumental _{constraints are: the necessity}

of operating the _{mass spectrometer at} low _pressure (10-1-

10-8 _{Torr, where 1 Torr =1 mmHg), and often at high}

source _voltages (magnetic _sector instruments).

1.1.2. _{Ion Production}

Over the last two decades _{or so a comprehensive}

array of ionisation methods have been developed. Often an

analyst has a number of ionisation techniques that can be

applied to a sample. Which technique used depends upon:

(a) _{The physical state of} the sample. (b) Thermal

stability of the sample. (c) What information is

required.

The different methods of ion production will be

discussed in _some detail later.

1.1.3. Ion Separation

After _an ion has been produced in the ion source it

is transmitted into an analyser region. There are five

major types of analyzer: (a) Magnetic sector8'

L.

QuadrupolelO.

_(c)

_Ion

cycloton

_resonance

(ICR) 11.

(d)

Time

_of

_flight

_{(TOF. )12.}

_(e)

_Ion

_trapl3.

It

is

_possible

_to

_combine

magnetic

and

_quadrupole

sectors

into

_what

_are

_known

_as

_hybrid

mass

spectrometers14,15

The _{results presented and discussed}

within this

thesis _{have been obtained}

almost exclusively _using

magnetic _sector instruments, _{although some use has been}

made _{of a hybrid machine.}

There _{are two possible geometries for double}

focusing _{magnetic sector instruments: (a) Forward, or}

Nier-Johnson, _{where the ESA (electrostatic analyzer)}

preceeds the _magnet8. (b) Reverse, _where the _magnet

preceeds the ESA9.

Forward _geometry instruments have been _used

predominantly for this _work. Fig. 1.1 shows a schematic

diagram _of this type. Ions formed in the ion _{source are}

accelerated at a high potential (1 _- 10 KV) through a

series of focusing lenses, resolution slits, and first

field free _region into the ESA. Field free (or drift)

regions are sections of the mass spectrometer where the

ions _{are not under} the influence _{of electric} or magnetic

fields.

An ion _{of mass} (M), travelling with a velocity (v)

and which has a number (z) of electronic charges (e),

0 4 - r-I ýt 4J

(1)

PQ

Fig 1.1.

Forward

16

ion

_source,

_{a kinetic}

energy

_given

by

1 mv2 = ezV

₍₁₎

2

The initial _{spread of translational}

energies of gaseous

molecules _within the _{source results in a slight spread of}

kinetic _{energies of ions leaving the}

source. This places severe

restrictions _upon the _{mass resolution of the} instrument.

Alleviation _{of this problem by the use of an ESA is common. The}

ESA _{comprises of two quarter circular parallel plates (of}

radius R),, across _which is applied _{a potential} difference. Ions

passing between the _{plates are} forced to travel in a circular

path of radius Re dictated by

R=

2V

E

(2)

Where V is the _accelerating voltage, and E the electric

field _strength between the plates. Hence, the ESA is a kinetic

energy analyzer. Therefore, placing a slit between the ESA and

the _{magnet results} in ions of only a closely defined kinetic

energy passing through the second field free region and into

the

_magnetic

_analyzer.

experience _{a force of magnitude F}

F=

_Bevz

₍₃₎

7

towards _{the center}

of _{a circle of radius r. This force is}

equal to the _{centrifugal force on the ion. Hence, the following}

expression

_{can be obtained}

Bzev

_{= mv2}

r

which _can be _{combined with equation (1) to give}

(4)

B2r2e

₍₅₎

z

2V

Although it is possible to bring _all the ions to _strike the

detector _{simultaneously} 16

, it is normal to scan the instrument.

Scanning _enables ions _of differing _{m/z ratios} to _pass

sequentially

through

a

slit

and

onto

the

detector.

From

equation

(5)

it

is

obvious

that

one of

two parameters

may be

varied (magnetic field or accelerating voltage), whilst the

other

is maintained

at a constant

value.

Since

sensitivity

is

proportional

to

the

accelerating

voltage,

magnetic

field

scanning

is

the

preferred

option.

8

instrument

used in this _{work. Quadrupole mass}

analysers

consist _of four _{parallel hyperbolic}

rods. A dc (direct

current) _{potential, and a Rf (radio frequency)}

potential

are _{applied across opposite}

rods. The _electric field

created _{cause the ions to oscillate. The}

equations _of

motion _of the ions _{in a quadrupole}

are _complex. It is

sufficient to _{say that the m/z}

value _of the transmitted

ions _depends

upon the _{rod separation, the radio}

frequency, _{and the potentials}

applied. Ions _of differing

m/z _{rations can} be _{given stable trajectories sequentially}

by _{either sweeping the}

radio frequency _{at constant}

potentials, _{or scanning the potentials such that the}

ratio _of the dc to Rf _{potentials are constant. The latter}

method is _{preferred as} it _{provides a linear mass range.}

The _{practical differences between magnetic and}

quadrupole _{analysers are:} (a) The latter _requires

substantially lower _{source voltages} (several _{volts, as}

opposed to kilovolts). It is important, therefore, to

decelerate ions leaving _{the magnetic sector region of an}

hybrid _machine, before _entering the _{quadrupole section.}

(b) Quadrupoles _{are capable of} faster _{scan speeds.} (c)

But lower _{mass range, maximum of} 4000 amu (atomic mass

units) compared with over 20,000 amu on magnetic sector

instruments. (d) Also _{quadrupoles can only give nominal}

mass resolution, whereas with magnetic sectors, mass

resolving powers of 100,000 are possible. (e) Quadrupoles

simultaneously eject positive and negative ions. (f) They

are also much more compact, and less expensive than

9

1.1.4.

_Ion

_Detection,

and

Data

Presentation

There _{are basically two types}

of ion detection

systems _{used with magnetic sector instruments. As}

mentioned _{earlier an} instrument _{may, or may not be}

scanned. The former is _{rarely used, but offers the}

advantage _of high _{sensitivity since all the ions that}

pass through the _{analyzer are detected. A photosensitive}

plate is _{normally used as a detector.}

The _scanning instruments _{are less sensitive as only}

a fraction _{of scan} time is _{spent on each specific mass,}

but _{they offer the powerful advantages of fast data}

acquisition, _{and simple} interfacing to _computers. In this

type _of instrument _{either a photomultiplier, or more}

commonly an electron multiplier detector is used. The

detector _{produces an electric current which} is

proportional to the number of ions striking it (often a

gain of 104 _- 106 is obtainable). This signal is normally

then transmitted to _{either a uv} (ultra _violet) recorder

or

computer.

Sensitivity _{of a scanning} instrument can be improved

(up to _a thousand fold) by fixing the instrument

parameters to allow ions of one, or a small number of

masses to pass through the analyzer. Using this technique

femtogram detection is possible.

Recently _use has been made of array detectors17.

Here _a small portion (4%) of the mass range can be

10

All _{other instrument types}

normally _{use either}

photomultiplier _{or electron}

multiplier detection.

1.2. _{Ion Production}

Methods

_of

_ion

_production

_in

organic

_mass

spectrometry

_can

_be

_categorised

_into

_two

groups:

(a)

Vapour

_phase,

_ionisation

of

_vapours

_and

(b)

_Desorption,

ionisation

_of

_condensed

phases.

1.2.1. _{Vapour Phase Techniques}

These _{are generally the older,}

more _established

ionisation _{methods. They involve the}

presentation _of

sample _molecules, in _{the vapour phase, into the ion}

volume. Although the _{main two are electron impact (EI)}

and _chemical ionisation (CI), _{other methods of some}

importance _{include: photoionisation18,}

atmospheric

pressure ionisation19 _and field ionisation20.

1.2.1.1.

_Electron

_Impact

Electron impact is the _{oldest and still} the _most

commonly used form of ionisation. Electrons are produced

from _an heated filament _and then _{are accelerated} into the

ion _{volume by a potential} difference between the filament

and the. source. The magnitude of this potential

difference determines the _average energy of the

electrons. An electron beam with an average energy of 70

eV (electron volts, 1 eV = 96 KJmol-1) is typically used.

70 _{eV represents a section of} the ionisation efficiency

11

affect

the

ion

_beam

intensity

_to

_any

_appreciable

_extent,

so

enabling

_good

_{reproducibility.}

_These

_high

_energy

electrons

interact

_with

_the

vapourised

_sample

_molecules

(M)

_to

_form

excited

_radical

_cations

_(M+').

M+

_eýM++

_2e

₍₆₎

Most _{organic molecules have ionisation energies which lie}

in _{the range 7- 15 eV. Therefore it is}

possible for _{a radical}

cation to _{contain an excess of} internal _{energy, the magnitude}

of which depends _upon: (a) The strength _of interaction between

the ionising _{electron and the molecule. (b) Ionisation}

potential _of the _molecule. (c) Thermal _{energy of the molecule.}

Variation _{of parameters} (a) _and (c) for _{a specific sample}

result in a distribution of internal energies for the molecular

cations.

Since

excess

internal

energy

cannot

be lost

through

collisions (because of the low source pressure, approximately

10-6 _Torr)

, it is randomised throughout the ion in the form of

vibrational energy. Accumulation of sufficient energy within

a specific bond can result in a unimolecular decomposition.

This is the basis _{of the} Quasi Equilibrium Theory (QET) 21, which

following

_expression

12

k=v

_(E-E0/E)

_S-1

ýýý

where k is the _{rate constant for}

a _{specific unimolecular}

decomposition, _{E0 is the}

energy _required for the _{reaction, E}

is _{the internal}

energy _{contained within the} ion, _{v is a}

frequency _{factor, and s is the}

effective _{number of oscillators.}

s=

3n-6

₍₈₎

3

Where _{n is the number of atoms in the molecule.}

There

_are

basically

_two

_types

_of

_unimolecular

_reactions

that

_{can occur}

in

_{an EI source:}

_(a)

_Simple

_cleavage.

_For

_this

type

_of

_reaction

_{v tends}

_to

be high,

_{and constant.}

_{The only}

necessary constraint is the flow of sufficient energy to _a

specific

bond.

(b)

Intramolecular

rearrangement.

In

these

cases

v

is

generally

low.

For

a

rearrangement

to

occur

accumulation of sufficient energy in a specific bond is no

longer the _{only reaction requirement.} In addition, the ion _must

have

the

_correct

_spatial

_orientation.

This

_geometrical

constraint lowers the frequency factor, and hence the reaction

rate.

Since _an ion _resides in the _source for _about 10-6 _s,

1-3

not

lead

to

_{any observed}

_fragment

ions.

_Therefore,

_even

_if

_an

ion

_contains

_enough

_energy

_{to decompose,}

_{the rate}

_constant

_may

be too

_slow

_for

it

_to

_{do so. This}

_explains

_the

_predominance

_of

simple

_cleavage

_over

_{rearrangement}

_reactions

_{in the EI source,}

for _{although the former require more energy, they}

occur at a

faster _{rate than the latter.}

If _{a reaction has a rate constant between 104 - 106 s-1 then}

the ion _{may fragment after leaving the source, but before}

reaching the detector. These _are termed _metastable decomposi-

tions. _{These reactions can normally only be observed if they}

occur in the _second field free _region. Metastable _{peaks appear}

in _{the "normal" mass spectrum using a uv recorder, as being}

broad

_{(due to the energy}

_release

_during

_{fragmentation),}

_{and are}

of low intensity in _relation the _{normal mass peaks.} These

metastable peaks arise at apparent, rather than real masses.

It is possible to _calculate their _{actual masses} by the _equation

M* =M2

₂

M1

(9)

Where M1 is the _{mass of the} parent ion, M2 the daughter ion,

and M* the

apparent

mass observed

in the

spectrum.

Metastable

decompositions _occuring in the first field free region, and ESA

are filtered out by the ESA. Those decompositions in the

magnetic

region

are

deflected

out

by

the

magnetic

fields.

1

third

_field

_free

_region

are

_not

_normally

_seen,

_{and a special}

detector

_is

_required

_to

study

them22.

Thermochemistry

_determines

which

_{of several}

_simple

_cleav-

age

_reactions

_an

ion

_{may undergo}

_(since

v values

_are

_very

similar)

Generally

_{a radical}

_cation

_(M+')

can fragment

by a simple

cleavage

M+ ' ---ý A+ +B

(10)

to form _{an even electron ion (A+)}

and a radical (B-) Conversely

it _{may decompose via rearrangement to lose}

a neutral molecule

(D).

M+ --ýº C" +D

(11)

The even electron ion (A+) can now fragment only by the loss

of a neutral

species

(even

electron

rule)

_.

Inherent

stability

of even

electron

ions

as compared

to radical

ions

constitutes

the

_reason

for

this

_rule.

Exceptions

to

this

_rule

_are

_rarely

observed,

and only

occur

when the

products

are

exceptionally

stable

(e. g.

di-iodides,

which

may

lose

successive

iodide

radicals23)

_.

1.2.1.2. Chemical Ionisation

One of the major problems associated with EI is the of-7-en

15

the

_molecular

_cation.

_If

_the

activation

_energy

for

the

first

_unimolecular

_{decomposition}

_is

_low,

molecular

ions

of low _{abundance are the result. The obvious}

consequence

of this is difficulty _{in the determination}

or

confirmation _{of a sample's chemical identity. Molecular}

ion _{abundance can}

often be improved by _using low _energy

electrons (up to 20 _{eV)24. Wide application of this}

technique _{has been hampered by several}

problems, _mainly

lower _{total ion currents, and difficulties in}

maintaining

reproducibility _{of results.}

A series _{of soft} ionisation _{techniques have become}

available, _which have been developed _{to overcome the}

problem _{of excessive} fragmentation _{commonly seen with} EI.

Soft ionisation _{methods can be defined as those that}

provide enhanced molecular, or pseudo-molecular ion peaks

as compared to _results obtained from EI. The most

commonly _used is chemical ionisation (CI) 25.

Chemical ionisation involves the _{collision of a}

sample molecule with a reagent ion to form a sample

product

ion.

The _{reagent gas, or a mixture of} gases is admitted

into

_the

_source

_at

_{a pressure}

_of

_{approximately}

1 Torr.

Sample is introduced _{at a low} concentration (around 0.1%)

relative to the reagent gas (es) _. This is to ensure that

as little EI of the sample occurs as possible. A high

energy electron beam is accelerated into the source

(typically 100 _- 400 eV) to produce a plasma of ions.

The _major advantage of CI is that it enables the

16

molecules, hence _{controlling the extent of fragmentation.}

There

_are

_several

_{CI techniques}

_that

may be employed.

(a) _{Charge Transfer}

This _{simply involves the transfer}

of an electron from _an

ionised

_reagent

_gas

_species

_(normally

a monoatomic

gas)

to

a

sample _{molecule. A well} defined _{amount of energy is transferred}

to _{the sample molecule, which is equal to the}

recombination

energy

_of

the

_reagent

_gas.

_{The magnitude}

_of

internal

_energy

contained _within the _{resultant sample} ion is _{equal to the}

difference _{between the recombination energy of the reagent gas,}

and the

ionisation

_energy

_{of the}

_sample.

Thus,

it

is possible

to _{select the} level _{of sample} ion internal _{energy by choice of}

reagent

gas.

In _{general charge} transfer _{offers no practical advantage}

over EI for molecular weight determination because the residual

internal

_energies

_tend

_to

_{be sufficient}

_to

_cause

_comparable

fragmentation.

(b)

_Proton

Transfer

This

is

the

_most

_popular

form

of

CI

used

at

present.

Basically

it

involves

the transfer

of a proton

from a protonated

reagent

ion

(AH+)

to

a

sample

molecule

(M)

to

form

the

protonated

molecular

ion

(MH ')

_.

17

AH+

is

_formed

_in

_high

_{concentrations}

at

high

_reagent

_gas

pressures.

_For

_example,

_in

_methane

_at

_{a source}

pressure

of

1 Torr

_{90% of}

_the

_total

_ion

_current

_consists

of

CHS

and

C2H5

ions.

The _{exothermicity of reaction (12) determines the}

internal

_energy

_deposited

within

the

_protonated

_sample

molecular

ion.

Therefore,

_the

_degree

_of

_{fragmentation}

_can

be

influenced

_by

_judicious

_choice

of

the

_reagent

_gas.

A

second factor that improves _{the molecular ion region with}

proton transfer CI _{as compared to EI,} is _{the inherent}

stability

_of

the

_protonated

_molecular

ions.

_Such

ions

_are

even _{electron, and so} do _{not readily undergo simple}

cleavage decompositions to form _{radicals as} is _commonly

seen _with EI.

From table 1.1 it _can be _{seen that methane} is _a

relatively harsh CI reagent gas, whereas use of isobutane

will cause less fragmentation, with ammonia being the

"softest"

_popular

_{CI reagent}

_gas.

Reagent Gas Reagent Ion Proton Affinity

(KJ mo1-1)

Ammonia NH4+ 861.0

Isobutane

C4H9+

826.9

Methane CH5+ 548.1

Table

1.1

_{- Proton}

affinities

for

some common reagent

gases.

1a

This

involves

_the

_abstraction

of

an hydride

ion

from

_a

sample

_molecule

_{(M) to}

_{a reagent}

_ion

_(X+)

.

X+ +M

_---->

_{XH + M- H+}

_(i3)

This _{reaction is possible whenever the hydride ion}

affinity

of X+ is _greater than _{that of M-H. Hydride ion transfer has found}

little _{practical use.}

(d)

_Reagent

_Ion

_Capture

If the _{proton affinity of the reagent ion is greater than}

that _{of the sample proton transfer becomes slow. In which case}

other

_reactions,

_such

_{as condensation,}

_{may become}

important.

The products _of ion-molecule _{condensation reactions are often}

seen _when ammonia is _{used as reagent gas.} Both _proton transfer

and association _{reactions may occur.} This can be useful for

determination _{of the relative molecular weight of a sample.}

(e) _{Negative Ion} Chemical Ionisation

This _can take the form _of; (i) Reagent ion _reaction, (ii)

Proton _abstraction, (iii) Resonant electron capture.

(i) _Reagent ion _{reactions with sample molecules are}

often

observed

if

oxygen

is

present.

The reactive

species

is

L9

displacement

_from

_carbonyl

groups26.

R2CO + 0---r

_{RCO2- + R-}

₍₁₄₎

(ii)

_Proton

_abstraction

_CI

_is

possible

_when

_{a strong}

Bronstead

_base

_(B)

_is

_present

_in

_the

_source.

B- +M

_---ý

_{BH + M- H-}

(15)

The exothermicity _{of this reaction} is decided _{by the} difference

in _{proton affinities of the acid, and the sample. Since most}

of the _{excess energy resides} in the _newly formed bond (BH) _, the

sample deprotonated _{molecular anion} tends to have _a low

internal _{energy. Consequently spectra typically show} intense

deprotonated _{molecular anion peaks and} little fragmentation.

CH-CO is _{sometimes used as a reagent} ion by _{use of methyl} 12

nitrite

reagent

gas

(proton

affinity

1593 KJmol-)7.

(iii) _{Resonant electron capture} is _{not strictly} CI. It

involves _{the adhearence of an electron} to _{a sample molecule}

M*

_{c 16}

M+e

_--ap

Under _normal CI _conditions (70 eV electrons) the kinetic

20

sufficient _{molecule electron interaction to}

result in a

stable _{anion. This problem is overcome by}

using _{a CI gas}

as _{an electron moderator so producing the}

near thermal

energy _electrons (approximately _{0 ev) required. Resonance}

capture _{may now} take _{place, providing the sample molecule}

has _{a sufficiently high}

electron _affinity to _allow the

anion to have _{a sufficiently long lifetime to be}

stabilized by _collision.

Electron _{capture reactions (15) tend to have very}

fast _{reaction rates( 400 times the diffusion limit for an}

ion _{molecule reaction). Hence in favourable cases,}

improvements _{in sensitivity of several orders of}

magnitude _can be _{obtained over} EI _{or positive} ion CI.

Typical _{electron capture spectra show} intense _radical

molecular anion peaks, with little fragmentation.

An _{unfortunate problem with} this _{method of}

ionisation is _{the dependence of the results on}

experimental conditions (source pressure, temperature

etc) Marked differences in results are often observed

from day to day, _and from lab to lab.

1.2.2.

Desorption

Techniques

All _{vapour phase} ionisation techniques suffer from

one serious drawback; the need to volatilise the sample

prior to ionisation. Therefore it is necessary for the

analyte to be thermally stable, or to be derivatised

before _mass spectral analysis can be performed. The

exception to this; pyrolysis mass spectrometry28,

21

Pyrolysis _{mass spectrometry has}

mainly been _{applied to}

the _{fingerprinting}

of _{polymers, and} biological _systems.

This _{technique aside, the thermal}

stability _{of an analyte}

can _{constitute a serious}

problem, _{which excludes a}

significant _{number of potential}

systems from _mass

spectrometric _study.

Extensive _{research has been}

expended _on the

development _{of new methods}

of ionisation _{with a view} to

overcoming the thermal _{stability problem.}

This has _{led to the introduction of}

a wide range of

desorption _{methods which have found to be}

useful for

analysing thermally labile _{and ionic samples. The}

following _{sections discusses some of those techniques}

which have been _used for _{or are referenced to, the work}

presented in this thesis. The _{only major} desorption

technique _{not mentioned is Plasma Desorption Mass}

Spectrometry (PDMS)29.

1.2.2.1 _{Desorption Chemical Ionisation (DCI)30,31}

A probe extension is used to place the sample

directly into the ion _volume. The _sample is _rapidly

heated, _normally in _{a CI environment.}

Ionisation _of the _sample is thought to involve

thermal desorption _of ions _which have been _{produced on}

the _probe (surface ionisation), and/or evaporation of

neutrals from the probe that are subsequently ionised via

CI _processes in the source.

Evaporation _of intact ions _{and or neutrals} occurs