Pyrolysis : Instrumentation and Application

1. Pyrolysis: Instrumentation and Application By: Ben King

2. What is Pyrolysis? • A technique that is used in the analysis of natural and artificial polymers or macromolecules • A sample is heated up (mainly in a inert atmosphere or vacuum) to decomposition to produce smaller units which are carried by a gas such as helium to the next instrument for characterization. • Pyrolyzer is usually linked to a GC and a detector such as MS or FTIR. Reference 16, 2

3. Py-GC/MS Auto sampler Heated transfer line MS GC Pyrolysis controller pyrolyzer http://www.csam.montclair.edu/earth/eesweb/imageU90.JPG

4. How Does it Work? • Use either one of three pyrolysis designs: Isothermal furnace, Curie Point filament (inductively heated), and resistively heated filament. • Sample heated to a pyrolysis temperature slowly or rapidly and held for a few seconds. • Cleavage of chemical bonds within the macromolecular structure producing low molecular weight, more volatile chemical moieties that are specific units of a particular macromolecule. Reference 16,2

5. Sample Preparation • Normally no sample preparation is powdered or particulate materials • Some samples require an extraction with an organic solvent to remove any low molecular mass components. • Some solid samples need to be dissolved in solvents or ground up. • Amount of sample preparation depends on type of polymer and how homogeneous the sample is. • Methylating reagents, which increase the volatility of polar fragments, can be added to a sample before pyrolysis. • Tetramethylammonium hydroxide (TMAH) and trimethyl sulfonium hydroxide (TMSH) Reference 16, 2

6. The Three Pyrolyzers • Each type can give reproducible results for small samples • Furnace and resistively heated filament pyrolyzers can be used for slow heating or rapid heating. • Curie Point is used only in rapid heating mode • Selectivity depends on personal preference, experimental requirements, budget, or availability Reference 2

7. Furnace Pyrolyzer • Small mount on the inlet of GC • The metal or quartz sample tube is wrapped with heating wire and thermally insulated • The furnace pyrolyzer has a much larger sample chamber than the filament pyrolyzers as seen in the figure. Reference 2

8. Furnace Pyrolyzer Design • Carrier gas enters from top or front to sweep past sample inlet (carrying of the pyrolyzate) before moving then directly into injection port of chromatograph • Temperature is stabilized to within ±10 °C of the desired temperature setpoint. • Thermocouple or resistance thermometer used to indicate wall temperature Reference 2

9. Furnace Pyrolyzer http://www.sge.com/uploads/lh/_0/lh_0zRR1NSHibbVkFiPo4A/pyrojector.jpg

10. Furnace Pyrolyzer Sample Introduction • Can’ t usually admit air during sample introduction due to GC • Heat rate dependent on sample material and composition of sample introduction device • Liquid samples are injected by a syringe. • Solids are dissolved and injected, or injected using a solid injecting syringe • A cool chamber is used to load samples into a crucible which is lowered into hot zone. Reference 2

11. Furnace Pyrolyzer Temperature Control • Resistive heating element is around the central tube of furnace • Temperature is monitored by sensor with data feedback to the controller for adjustments of thermal energy. • Temperature control also depends on size and mass of sample, and residence time inside furnace. Reference 2

12. Furnace Pyrolyzer Advantages • Inexpensive and relatively easy to use • Isothermal heating, with no heating ramp rate or pyrolyis time unless that is the intention. • Liquid and gas pyrolysis is more easily achieved than with filament type. Reference 2

13. Furnace Pyrolyzer Disadvantages • Since the tube is considerably larger than sample, temperature control is more difficult to achieve • Large volume for sample to pass through to get to analytical device • Excessively low carrier gas flow may lead to secondary pyrolysis • Temperature stability depends on sample size, nature, and geometry Reference 2

14. Furnace Pyrolyzer Disadvantages • Metal systems, initial pyrolysis may produce smaller organic fragments which encounter hot surface of tube and undergo secondary rxns • Generally necessitating split capillary analysis • Has longer retention times, broad peak shapes, and interference peaks. Reference 2, 13

15. Heated Filament Pyrolyzer • Sample placed directly onto cold heater then rapidly heated to pyrolysis temperature • Two Methods: • resistance-controlled current is passed through heating filament • Inductive- current is induced into heating filament which is made of ferromagnetic metal • Sample size limited to an amount compatible with mass of filament. (low to high microgram range) • A sample must also be compatible for the analytical devices that are linked up to the pyrolyzers. • GC, FTIR, ICP, MS, etc. Reference 2

16. Filament Pyrolyzer Examples Fischer America Curie Point Pyrolyzer Analytix Ltd Resistively Heated Filament Pyrolyzer

17. Inductively Heated Filament: Curie-pt Pyrolyzer • Electrical current induced onto a wire made of ferromagnetic metal by use of magnet or high frequency coil • Continual induction of current wire will begin to heat until it reaches a temperature at which it is no longer ferromagnetic • Becomes paramagnetic, no further current may be induced in it. • Heated to pyrolysis temperature in milliseconds Reference 2

18. Inductive Heating Characteristics of Alloys Reference 13

19. Curie-pt Design • Insertion: • Pyrolysis chamber which is surrounded by coil, is opened and sample wire is dropped or place inside • Sample wire is attached to a probe which is inserted through a septum into the chamber which is surrounded by the coil Reference 2, 13

20. Curie-pt Pyrolyzer Design • Chamber can be attached directly to part of GC or isolated from GC by valve • Allows for autosampling and for loading wires into glass tubes for sampling and inserting into coil zone. • Controls for parameters of pyrolysis wire and also temp selection for interface chamber housing the wire. Reference 2, 13

21. Curie-pt Pyrolyzer Sample Introduction • Sample and wire kept to low mass • Samples either coated onto filament as very thin layer • Soluble materials dissolved in appropriate solvent and wire dipped into. • Solvent dries and leaves thin deposit • Non-soluble: • finely ground samples maybe deposited onto wire from a suspension which is dried to leave coating of particles • Applied as melt • Create a trough with wire • Bend or crimp wire around material • Encapsulate sample with foil of ferromagnetic material and dropped into high frequency cell chamber. Reference 2

22. Curie-pt Pyrolzer: Temperature Control • Pyrolysis temperature is determined by the composition of the ferromagnetic material • Reproducible and accurate temp control depends on accuracy of wire alloy, power of coil, and placement of wire into system • Use the same manufacturer, same sample loading, and placement to minimize variation of sample results Reference 2, 13

23. Curie-pt Advantages • Self-limiting temperature • Rapid heating • No temperature calibration to perform • Can prepare several samples and store • Can be automated b/c no connections to wire- simple insertion • Can either clean and reuse wire or discard • Gives sharper characteristic peaks than furnace type • Demonstrates constant pyrolysis product composition yield even with sample weight increases • Good heat transfer Reference 2, 13

24. Curie-pt Disadvantages • Limited temperatures to choose • Harder to optimize pyrolysis temperature • Concerns of catalytic effect of metals on very small samples. • Range of temps 350 - 1000°C (10 - 20 specific alloys ) • Can’t have linear heating Reference 2

25. Resistively Heated Filament Pyrolysis • Heat from ambient to pyrolysis temperature quickly also with small samples • Current supplied is connected directly to filament • A filament made of material with high electrical resistance and wide operating range. (Ex: Fe, platinum, and nichrome Reference 2

26. Resistively Heated Filament: Design • Sample placed onto pyrolysis filament which is then inserted into the interface housing and sealed to insure flow to column. • Flat strip, foil, wire, grooved strip, or coil. • Coil- tube or boat inserted into filament, like very small rapidly heating furnace • Must be connected to controller capable of supplying enough current to heat filament rapidly with some control or limit • Temperature measured by resistance of material or by external measure such as optical pyrometry or thermocouple. Reference 2

27. Resistively Heated Filament Diagram

28. Resistively Heated Filament: Sample Preparation • Solution applied to filament by syringe • Powder solids use small quartz tubes which is inserted into coiled filament • Place in tube, held in position using plugs of quartz wool, weighed, and inserted into coiled element. • Rise and final temp different then directly on filament • Not used for soils, ground rock, textiles, and small fragments of paint • Viscous liquid applied on surface of filament or suspended on surface of filler material. Reference 2

29. Resistively Heated Filament: Interfacing • Can be easily interfaced with other analytical devices as long the filament is positioned right and the probe is sealed off from air. • Need a heated interface between pyrolyzer and column • Interface has its own heater to prevent condensation of pyrolyzate compounds and should have minimal volume • Valve needed between pyrolyzer and column so insertion or removal of filament can be done. Reference 2

30. Resistively Heated Filament: Temperature Control • Temperature is related to current passing through it • Conditions have to be very similar for good reproducibility • Computers control and monitor filament temp, control voltage used and adjusted for changes in resistance • Use photodiode to read actual temp of filament • Can select any final pyrolysis temp and any desired rate • Can heat as slow as .01 °C/min and as rapidly as 30000 °C/sec Reference 2

31. Resistively Heated Filament: Advantages • Can measure how materials are affected by slow heating (TGA) • Permits interface of spectroscopic techniques with constant scanning for 3d, time-resolved thermal processing. • Can be inserted directly into ion source of MS or light path of FTIR • Products monitored in real time throughout heat process. Reference 2

32. Resistively Heated Filament: Disadvantages • Can’t automate process since multiple samples need same filament and multiple filaments need same instrument • Any damage or alteration to the resistance of part of the loop will have an effect on actual temp produced by controller. • Introduction of some samples into heated chamber before pyrolysis may produce volatilization or denaturation, altering nature of sample before degradation. • Not good heat transfer • Yields can decrease as sample weight increases Reference 2

33. Slow-rate Pyrolysis • Related to TGA, multiple step degradation • Gives time-resolved picture of production of specific products • Programmable furnace and resistively heated filament • 50-100 °C/min to extract organics Reference 2

34. Direct/Indirect Transfer of Pyrolyzate to Detectors • Direct • Collection directly onto GC, at ambient or subambient conditions • Direct to MS or FTIR • Pyrolyzer inserted into an expansion chamber, which flushed or leaked into spectrometer, or the pyrolyzer is inserted directly into instrument • Indirect • A trap is connected to pyrolyzer and is later connected to analytical device Reference 2

35. Reproducibility of Pyrolysis • Sources of error- size and shape, homogeneity, and contamination of sample • For polymers, need to make same size and shape samples • Overloading affects rate at which sample heats (thickness of material- thermal gradient) • 10-50 microgram samples desirable for direct pyrolysis to GC and twice that for FTIR Reference 2

36. Increasing Reproducibility by Homogeneity • Ground up material under cryogenic conditions • Chop sample finely using scalpel and then analyze small fragments together • Made into solution • Bigger samples of .1mg • Use a split mode GC injection with a large split ratio to avoid signal saturations • Pass pyrolyzate in carrier gas through small sample loop attached to a valve which is interfaced to analytical unit. (clean run to run) Reference 2

37. Accuracy of Pyrolysis • Study of compositional determination of styrene-methacrylate using Py-GC and H NMR • Standard deviation: 1-2% compared to 1% for NMR • Accuracy effected by pyrolysis temp rise time, sample size, sample surface area, and sample thickness • Small sample size, little sample prep, rapid turnaround time, relatively inexpensive, easily operable, and can be automated Reference 8

38. Accuracy of Pyrolysis • 550-650 °C yielded reproducible fragmentation • Difference between NMR and GC pyrolysis results are in the range of 0-4% and 0-4.8% for styrene/n-butyl methacrylate and styrene/methyl methacrylate • Standard deviation for py-GC was from 1.2 to 2.1 % Reference 8

39. Precision of Pyrolysis • Evaluating Emission of various materials for PAH’s released (Py-GC/MS) • Pyrolyzed at 1000 °C for 60 sec (resistively heated) • RSD from 7.5% (1-methyl naphthalene) to 18% (acenaphtene) • Most abundant species RSD less than or equal to 15% , less abundant much higher • Increase of precision and repeatability if using offline system • Shows good repeatability, limit of quantification, and linearity • Reasonably good for properly evaluating the quantity of PAHs emitted from different kinds of materials. Reference 9

40. Precision of Pyrolysis • Investigation of Food Stuffs (Py-Elemental Analysis) • 65 Foods analyzed • RSD from 1 to 13% for Carbohydrates in each one of the samples that also contained protein, fats, and dietary fibers Reference 7

41. Sample Amount and Selectivity • Sample amount • Milligrams or micrograms • Selectivity • Cellulose • Altering heating conditions improve selectivity • Sample vs Standards of PVC, PS, SB, PMMA, and PC mixture • All main marker compounds very similar • Naphthalene peak of polymer mixture 96% recovered relative to standards Reference 15

42. Sensitivity of Pyrolysis • Volatile elements • Slurries- high sensitivity for pyrolysis temp < 400 °C, decrease from 400-800 °C • Aqueous and digested standards sensitivity plateaus across temps • Digested better sensitivity than aqueous 15% (As) & 65% (Pb) • High sensitivity obtained for As is obviously related to the presence of carbon in the plasma and increase sensitivity at low pyrolysis temp is in agreement with above-discussed charge-transfer mechanism. • Using modifiers Pd/Mg or raising concentrations of organics raises sensitivity at low temps. • Sensitivity changes due to differences in analyte transport from the ETV to the ICP produced by carrier effects and/or changes in analyte ionization in the plasma. Reference 14

43. Detection Limit and Quantification Limit of Pyrolysis • Detection Limit is dependent on analytical device it is attached to • GC’ s detection limit • Can be as low as ng or pg • Analysis of polymer mixture Py - ETV - ICP - MS • Limit of Quantification • 500ng, 10 mg / kg dry mass • Limit of Detection • 150ng, • S / N = 3 • Linearity in a range from .5 to 100 microgram Reference 15

44. Application of Pyrolysis • Pyrolysis can be applied to the analysis of many natural and artificial macromolecules • Natural: lignin, cellulose, chitin, etc • Artificial: PVC, acrylics, varnishes, etc • Can be used for applications similar to TGA • Used in several specific areas as well

45. Presence of 5-hydroxyguaicyl as Unit Native in Lignin • Lignin content was estimated by the Klasan method • Curie-pt pyrolyzer, pyrolysis temp- 610 °C • Fibers were finely ground to sawdust • In samples of eucalypt, abaca, and kenaf, compounds 3-methoxycatechol, 5-vinyl-3-methoxycatechol, and 5-propenyl-3-methoxycatechol were detected. • Compounds arise from the pyrolysis of 5-hydroxyguaiacyl lignin moieties • Only the first one ever really detected, the other two rarely until using pyrolysis-GC/MS technique Reference 6

46. Determination of Abaca Fiber Composition for Paper Pulping • Nonwoody source for paper for developing countries • Curie-pt pyrolyzer, pyrolysis temp-610 °C • Pyrolysis in presence of tetramethylammonium hydroxide (prevents decarboxylation) • Abaca fiber is 13.2% lignin • Main compounds of lignin are p-hydroxyphenyl (H), guaiacyl (G),and syringyl (S) • Reference 4

47. Determination of Abaca Fiber Composition for Paper Pulping syringyl guaiacyl • S/G-4.9 • Efficiency of pulping directly proportional to amount of syringyl units in lignin due to easy delignification of S-lignin • S-lignin is mainly linked by a more labile ether bond • S-lignin is relatively unbranched • S-lignin is lower condensation degree than the G lignin Reference 4

48. Pyrogram of Abaca Reference 4

49. Composition of Abaca Fibers Reference 4

50. Composition of Abaca Fibers Reference 4