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CHEM10812: Introduction to Forensic & Analytical Chemistry

Lecture VI

�Pyrolysis as an analytical technique�Define pyrolysis�Define pyrolysis� Interface with GC and MS�Some examples…

Roy Goodacre, 2.027 MIB, roy.goodacre@manchester.ac. uk

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Why pyrolysis?

�Not all analytes are amendable to GC or LC�Samples of low volatility�Polymers - Man made fibres - Paints�Polymers - Man made fibres - Paints

�Also not amenable to MS�Too big - especially polymers�cf. MALDI and ESI

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Pyrolysis is:

�The thermal degradation of complex material in an inert atmosphere or a vacuum.

� It causes molecules to cleave at their weakest � It causes molecules to cleave at their weakest points to produce smaller, volatile fragments called pyrolysate.

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Considerations

� Sensitive technique� Clean preparation

� Sample has to be dried

� Need an inert atmosphere� O2 → oxidation rather

than pyrolysis� No volatile or low

volatile analytes� No H2O → hyrolysis

rather than pyrolysis

than pyrolysis

� Pyrolysate is sticky� Need to limit

condensation during analysis

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Is it possible to control temperature?

�Needs to be accurately controlled�Different temp: different bonds break

‘tune’ into analyte class‘tune’ into analyte class

�Ratio monomer/dimerin the pyrolysis ofnatural rubber (polyisoprene) 0

0.2

0.4

0.6

0.8

1

300 350 400 450 500

Temperature (°C)

Mon

omer

/Dim

er

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How?

�Resistive heated filament�Pt coil, resistive conductor�Electric current generates heat

wood char coal

tarsH2

CH4H2O

�Furnace� Introduce sample into a hot zone

�Radiative heating�Laser

�Curie -point pyrolysis

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Ferromagnetic alloy conductors

� High frequency (RF) electromagnetic field induces an oscillating current

� Eddy currents (induction of

-

+

� Eddy currents (induction of electric current) and hysteresis cause the temperature to increase+

NS

-

� Curie-point of alloy:Ferromagnetic →→→→ Paramagnetic

� Thermostatic switch

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Curie points of someferromagnetic alloys

Fe % Ni % Co % Curie point temp. °C

0 100 0 3580 100 0 358

61.7 0 38.3 400

55 45 0 400

50 50 0 530100 0 0 770

0 0 100 1128

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Temperature profile of pyrolysis

400

500

600

Tem

pera

ture

(°C

)

Total heating time (THT)

Temperature equilibrium (Teq)530

0 1 2 3 4 50

100

200

300

Time (s)

Tem

pera

ture

( Total heating time (THT)

Temperature rise time (TRT)

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Pyrograms

� A plot of GC retention time vs. FID or total ion count� Would be matched against reference materials.

� Plastics for example:� Chromatographic retention times � MS spectra of monomers

� For complex samples; e.g., paints� Pattern recognition algorithms often necessary

11Chemical composition of natural and man-made fibers

Motor vehicle crimeAutomotive paint binders can

be identified from µg of topcoat

Wampler, T.P. (1995) Applied Pyrolysis Handbook. CRC Press.

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Pyrolysis-methylation

� In pyrolysis get conventional free radical pyrolysis processes. You can conduct chemical reactions:

� Thermally assisted hydrolysis and methylation (THM)� Uses quaternary alkyl ammonium compounds� Uses quaternary alkyl ammonium compounds� Reacts with (macro)molecules susceptible to hydrolysis to

give alkyl derivatives.

� Tetramethylammonium hydroxide (TMAH)� Very useful reagent because the methyl derivatives

are sufficiently volatile to be analysed by GC.� Often aids identification of parent product

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Typical base used in lipsticks

Py-GC

Surface coat natural resins and pigments. TMH-GC used to

establish the authenticity of oil paintings and works of art

THM-GC

TMH-GC

Wampler, T.P. (1995) Applied Pyrolysis Handbook. CRC Press.

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Application areas

� Forensics� Man-made fibres� (automotive) Paints�

� Break down a polymeric chain to monomers, oligomers, and other fragments� Adhesives

� Plastics and rubbers� Bacteria� Foodstuffs

fragments� Recycling

� Nylon carpets → monomer � Plastics� Tyres → pyrolysis oil

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Pyrolysis-Mass Spectrometry

Sample

Volatile organic fragments

Singly positively charged fragments

Curie-point pyrolysis

Low-energy ionizationpyro

lysi

s

Singly positively charged fragments

Ions of specific mass

Detector

Spectrum

Data storage

Ion separation(quadrupole)

Electron multiplier

Computer

Screen or printer

Mas

s Sp

ectr

omet

er

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A pyrolysis mass spectrometer

�Speed < 2mins sample�Automated - 300 samples/day�Quick sample preparation

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Sampling bacteria

Sample tube

�Cultures need to be:pure and cultured in the same way�Phenotypic typing method, ∴ physiology and

biochemistry of the sample important.

Foil

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PyMS spectrum of a bacterium

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2.5

3

3.5

4P

erce

ntag

e to

tal i

on c

ount

Library MatchingIdentify bacteria using

pattern recognition on a computer

60 80 100 120 140 160 180 2000

0.5

1

1.5

2

Per

cent

age

tota

l ion

cou

nt

Mass (m/z)

�Quantitative profile of 150 m/z intensities

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Field portable instrumentation

McClennen, W.H., Arnold, N.S. and Meuzelaar, H.L.C. (1994) Field-portable hyphenated instrumentation -the birth of the tricorder. TRAC-Trends in Analytical Chemistry 13, 286-293.

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Py-GC-DMS of bacteria

Cheung, W. et al. (2009) Analyst 134, 557-563

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QC of extra virgin olive oil8

6

4

2

rcen

tage

of

tota

l ion

cou

nt

Virgin olive oil - Dritta cultivar,adulterated with 5% soya oil

8

6

4

2

rcen

tage

of

tota

l ion

cou

nt

Virgin olive oil- Dritta cultivar

�Adulteration�Provenance

�Cultivar�Geographic distribution

20018016014012010080600

Mass range (m/z)

Per

20018016014012010080600

Mass range (m/z)P

er

Goodacre, R. et al. (1992) Nature 359, 594.

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Lecture VI: Take Homes

�Pyrolysis used to get non-volatile (usually complex) materials into the gas phase

�Temperature can be highly controlled

Roy Goodacre, 2.027 MIB, roy.goodacre@manchester.ac. uk

�Temperature can be highly controlled�Analysis by GC, MS, GC-MS, IMS/DMS�Many examples for forensics

�Man-made fibres, paints, adhesives,�Plastics and rubbers, bacteria, foodstuffs