Interpretation of Static SIMS Spectra

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INTERPRETATION OF STATIC SIMS SPECTRA ALEX HENDERSON SURFACE ANALYSIS RESEARCH CENTRE UNIVERSITY OF MANCHESTER, UK Joint IAEA-SPIRIT-Japan Technical Meeting on Development and Utilization of MeV-SIMS Inter-University Centre, Dubrovnik, Croatia. 21-25 May 2012

description

This presentation covers factors that influence the form of a Static SIMS spectrum and various issues that may arise in its interpretation. Presented at the Joint IAEA-SPIRIT-Japan Technical Meeting on Development and Utilization of MeV-SIMS. Inter-University Centre, Dubrovnik, Croatia. 21-25 May 2012

Transcript of Interpretation of Static SIMS Spectra

Page 1: Interpretation of Static SIMS Spectra

INTERPRETATION OF

STATIC SIMS SPECTRA ALEX HENDERSON SURFACE ANALYSIS RESEARCH CENTRE

UNIVERSITY OF MANCHESTER, UK

Joint IAEA-SPIRIT-Japan Technical Meeting on Development and Utilization of MeV-SIMS Inter-University Centre, Dubrovnik, Croatia. 21-25 May 2012

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What’s the question?

Know the chemistry of the sample?

– an understanding of the experimental parameters

Don’t know the sample, but believe it to be pure?

– looking for identification

Believe the sample to be an unknown mixture?

– just looking for help in identifying the components

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Accurate mass of molecular ion

Is accurate mass of the molecular ion sufficient to

determine molecular structure?

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Accurate mass for molecular structure

Isomers have the same mass but

different structures

4-dimethylamino-benzaldehyde

2-methylacetanilide

Molecular formula = C9H11NO

Mass = 149.08 u

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Mass Spectrometry::Interpretation

Spectra are different

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4-dimethylamino-benzaldehyde

2-methylacetanilide

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Accurate mass of molecular ion

Is accurate mass of the molecular ion sufficient to

determine molecular formula?

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Accurate mass for molecular formula

Depends on

Mass resolution of the instrument

Mass accuracy of the instrument

Consider possible chemical structures containing

C, H, N, O, S, P†

How many structures have nominal mass of 149 u?

†BMC Bioinformatics (2006), 7:234

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Nominal m/z 149 u Formula Mass Formula Mass Formula Mass Formula Mass

C2NOPS2 148.9159 C3H5NO2P2 148.97955 C3H8N3PS 149.01765 C7H7N3O 149.05891

C2HNOP2S 148.92541 C2H4N3OPS 148.98127 CH3N5O4 149.0185 C5H12NO2P 149.06056

CN3PS2 148.92713 CH3N5S2 148.98299 C6H3N3O2 149.02253 C4H11N3OS 149.06228

C2NO3PS 148.93365 C3H4NO4P 148.9878 C4H8NO3P 149.02418 CH7N7O2 149.06612

C2H2NOP3 148.93493 C4N5P 148.98913 C3H7N3O2S 149.0259 C5H11NO4 149.06881

CHN3P2S 148.93665 C2H3N3O3S 148.98951 C11H3N 149.02655 C6H7N5 149.07014

C3H3NS3 148.94276 C2H5N3OP2 148.99079 C3H9N3P2 149.02717 C4H12N3OP 149.0718

C2HNO3P2 148.94317 CH4N5PS 148.9925 C8H7NS 149.02992 C3H11N5S 149.07351

CN3O2PS 148.94489 C7H3NOS 148.99354 C4H7NO5 149.03242 C4H11N3O3 149.08004

CH2N3P3 148.94616 C3H3NO6 148.99604 C5H11NS2 149.03329 C3H12N5P 149.08303

C6NPS 148.94891 C4H7NOS2 148.99691 C5H3N5O 149.03376 C9H11NO 149.08406

C2NO5P 148.95141 C2H4N3O3P 148.99903 C3H8N3O2P 149.03541 C6H15NOS 149.08743

C3H4NPS2 148.95228 CH3N5O2S 149.00075 C2H7N5OS 149.03713 C3H11N5O2 149.09127

CHN3O2P2 148.9544 CH5N5P2 149.00202 C8H8NP 149.03943 C8H11N3 149.09529

C6HNP2 148.95843 C7H4NOP 149.00305 C5H12NPS 149.0428 C6H16NOP 149.09695

C3H3NO2S2 148.96052 C6H3N3S 149.00477 C3H7N3O4 149.04365 C5H15N3S 149.09866

C3H5NP2S 148.9618 C4H8NOPS 149.00642 C4H3N7 149.04499 C2H11N7O 149.1025

CN3O4P 148.96265 C2H3N3O5 149.00727 C2H8N5OP 149.04665 C6H15NO3 149.10519

C6NO2P 148.96667 C3H7N3S2 149.00814 C8H7NO2 149.04768 C5H16N3P 149.10818

C3H4NO2PS 148.97004 CH4N5O2P 149.01026 CH7N7S 149.04836 CH11N9 149.11374

C3H6NP3 148.97131 C7H3NO3 149.01129 C5H11NO2S 149.05105 C5H15N3O2 149.11642

C2H3N3OS2 148.97176 C6H4N3P 149.01428 C5H13NP2 149.05232 C10H15N 149.12044

C5N3OP 148.9779 C4H7NO3S 149.01466 C2H7N5O3 149.05489 C4H15N5O 149.12765

C3H3NO4S 148.97828 C4H9NOP2 149.01594 CH8N7P 149.05788 C3H15N7 149.13889

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High mass resolution required

Nominally all at m/z 86

Lipid (DPPC)

m/z = 86.0969692

Unknown

Silicon substrate [Si3H2]+

m/z = 85.9464332

Separation = 0.15 u

m/z 86

Analytical chemistry 80 (2008) 9058-9064

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Enumeration of structures

Structures of natural

products with

mathematically possible

isomers

Not all are chemically

likely

Journal of Chemical Information and Modeling 46 (2006) 1643–1656

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Mass spectrometry literature

Can’t always rely on ‘traditional’ MS literature and

resources

Most MS assumes a separation step

GC-MS

LC-MS

Therefore doing identification of pure material

SIMS involves mixtures so spectra are overlapped

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Bond breaking – EI vs. SIMS

EI produces radical cations – odd electron ions (OE)

SIMS provides mostly even electron ions (EE)

Fragmentation

However – electrospray (ESI) data and MS/MS may

be helpful†

M Cation Radical

or

M RadicalCation Neutral

†Henderson et al., Surface and Interface Analysis (2012) accepted

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Nitrogen rule

Nitrogen atoms are even mass, but odd (3 or 5)

valent. Only element with this property.

In EI-MS ‘nitrogen rule’ states:

“If a compound contains zero (or an even number of)

nitrogen atoms, its molecular ion will be at an even

mass numberӠ

For SIMS this is the other way round – all peaks

will be at odd mass unless they contain an odd

number of nitrogen atoms

†‘Interpretation of Mass Spectra’, McLafferty and Tureček, University Science Books, 1993

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Nitrogen rule in action

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Stability

Potential for a peak to be intense in a spectrum is a

function of its concentration and its stability

Electron-withdrawing groups (F, Cl, Br, I, OH, NO2)

can destabilise a positive centre

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Inductive effect

Electron transfer toward a positive charge is called the

inductive effect

Alkyl groups can donate electron density in the following

order

(CH3)3C+ > (CH3)2CH+ > CH3CH2

+ >> CH3+

Fragmentation of hydrocarbons can give peaks at m/z 57

and 43 with higher abundance than m/z 29 and 15

‘Interpretation of Mass Spectra’, McLafferty and Tureček, University Science Books, 1993

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Stevenson’s rule

In principle the positive charge could go with either

product species

Criterion originally established for the

fragmentation of alkanes by Stevenson in 1951

Homolytic dissociation of a C–C bond always

produces product pairs, their relative abundances

being basically governed by Stevenson's rule

When a fragmentation takes place, the positive charge

remains on the fragment with the lowest ionization energy

Discuss. Faraday Soc. 10 (1951) 35-45

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Aromatic stability

Aromatic resonance structures are particularly

stable

These result in intense spectra features

Tropylium ion m/z 91

R

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Contamination

Hydrocarbons

Particularly in cities due to car exhaust gasses

Siloxanes

From any plastic material

Poly(dimethylsiloxane) used as a release agent

Phthalates

Common polymer additives

Salts

For example, sodium causes cationisation adducts

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Hydrocarbons

Poly(ethylene), high density (positive ion)

The Static SIMS Library, SurfaceSpectra Ltd

m/z

100959085807570656055504540353025201510

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nsity

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Poly(dimethyl siloxane) (PDMS)

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Phthalates

Phthalate structure is benzyl di-ester

R group defines type of ester

Phthalates have intense m/z 149

Also exhibit R group patterns

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Salts cause cationisation

Alkali metal (Li, Na, K, Rb, Cs) adducts to neutral

molecules and fragments

Pattern of peaks shifted by mass of adduct element

Can also see NaxCly in some samples

Historically, samples deposited on silver to improve

signal

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Isotope patterns

Very useful in identifying fragments from

organometallics

Chlorine and bromine very distinctive

Patterns can be used in archaeology to map trade

routes†

Non-terrestrial patterns help analysis of meteorites‡

†Applied Surface Science 252 (2006) 7124–7127 ‡ Science 314 (2006) 1724–1728

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Oligomers and unzipping

Long chain polymers and hydrocarbons ‘unzip’

Repeating patterns with in/de-creasing numbers of

units

Behenic acid (positive ion)

The Static SIMS Library, SurfaceSpectra Ltd

m/z

340330320310300290280270260250240230

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Poly(ethylene glycol), cationised

Poly(ethylene glycol) dimethacrylate MW=1000 (cationised) (positive ion)

The Static SIMS Library, SurfaceSpectra Ltd

m/z

1,8001,6001,4001,2001,0008006004002000

Inte

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Poly(ethylene glycol) dimethacrylate MW=1000 (cationised) (positive ion)

The Static SIMS Library, SurfaceSpectra Ltd

m/z

2,0001,8001,6001,4001,2001,0008006004002000

Inte

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Libraries and matching

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Spectral matching

Library data useful if in same field or with

contamination, but limited in scope

Use as a educational tool – similar molecules

fragment in similar ways. Extrapolate to find match

Vector matching indicates some data robust to ion

source and analyser type

Problems with mixtures

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Vector matching

†Henderson et al., Surface and Interface Analysis (2012) accepted

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Multivariate approaches

Downside - needs a large number of spectra

Principal Components Analysis (PCA) most common

and used for exploratory analysis

Is all data the same? – quality control

Is there a pattern we’d not expected?

Supervised analysis useful when we have known

classes of samples

Which spectral features separate classes A and B?

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PCA of bacteria

5 species of bacteria

No a priori

knowledge used

in PCA

Positive ion spectra

1-800 u, rebinned to

1 u steps

Square root of

intensity

Sum-normalised

Applied Surface Science 252 (2006) 6719-6722

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PC-CVA bacteria classification

5 species of bacteria

Class structure used

9 PCs selected using

PRESS test

Cross-validation

indicates percent

correctly classified:

Cf 75% CC

Ec 92% CC

En 100% CC

Kp 25% CC

Pm 50% CC

Applied Surface Science 252 (2006) 6869-6874

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Analysis of bacteria

Principal Components Analysis Canonical Variates Analysis (CVA)

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CVA interpretation

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Summary

SIMS interpretation is difficult, but even without

absolute answers the analysis is useful

What SIMS lacks in ultimate species identification it

makes up for in spatial location

New ionisation sources produce data nearer to

‘traditional’ MS, opening up resources

Tandem MS approaches are breaking through

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www.sarc.manchester.ac.uk

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