Fiber Guidelines, Chapter 5 (FSC, April 1999)
April 1999 - Volume 1 - Number 1
Chapter 5 of Forensic Fiber Examination Guidelines
Pyrolysis Gas Chromatography of Textile Fibers
These guidelines are intended to assist individuals and laboratories that conduct pyrolysis gas chromatography (PGC) in their selection, application, and evaluation of PGC as a method for forensic fiber case work. Some of the procedures referenced in these guidelines involve the use of hazardous chemicals, temperatures, or some combination of both. These guidelines do not address the possible safety hazards or precautions associated with their application. It is the responsibility of the user of this document to establish appropriate safety and health practices and to determine the applicability of regulatory limitations prior to use.
SWGMAT Quality Assurance Guidelines
SWGMAT Trace Evidence Handling Guidelines
ASTM E 1610-94 Standard Guide for Forensic Paint Analysis and Comparison
Chromatography: A method of analysis in which substances are separated by their differential migration in a mobile phase flowing through a porous and absorptive medium.
Gas Chromatogram: The visual display of the progress of a separation achieved by chromatography. A chromatogram shows the response of a chromatographic detector as a function of time.
Capillary Column: A columnar assembly of a thin film on the inner periphery and an unobstructed and open lumen running the entire length of the column, which acts as the stationary phase or plate of the chromatograph.
Mobile Phase: In gas chromatography, the mobile phase is the inert carrier gas that moves the volatile analytes through the length of the column.
Packed Column: A metal tube evenly filled with a solid support material that is coated with a liquid stationary phase of low vapor pressure.
Stationary Phase: In a packed column, the stationary phase is a low vapor pressure liquid that coats a solid support. Compounds are selectively retained on the basis of their solubility in this liquid. In a capillary (WCOT) column, the stationary phase is generally a modified or unmodified polysiloxane compound coating the walls of a fused silica column. Compounds are selectively retained on the basis of their interaction with the coating’s functional groups.
These guidelines are intended to advise and assist individuals and laboratories that conduct forensic fiber examinations and comparisons in their effective application of pyrolysis and pyrolysis gas chromatography (PGC) to the analysis of fiber evidence. The guidelines are concerned with the pyrolysis of single fibers and fibers from bulk material, classification of the generic class of polymer, and interpretation of the resulting pyrograms. The protocols and equipment mentioned in this document are not meant to be totally inclusive or exclusive.
Pyrolysis is a destructive analytical method; therefore, consideration must be given to the applicability of this procedure to each case depending on the sample size and the amount of sample consumption that can be tolerated (8). Pyrolysis of polymers is the breaking apart of larger polymer chains into smaller fragments by the application of heat in an inert atmosphere (8). When the heat energy applied to the polymer chains is greater than the energy of specific bonds in that polymer chain, these bonds will fragment in a predictable and reproducible way at a specific temperature. In PGC, the fragments generated by pyrolysis are introduced into a gas chromatograph (GC) for separation and characterization. PGC can be used to identify the generic type of an unknown fiber, and in some cases it can identify subclasses within a generic class (1).
Each laboratory should develop its own standard chromatograms of the different generic fibers if performing fiber identification analyses. These chromatograms demonstrate the analytical potential as well as the limitations of PGC performed on fibers by a particular system.
The potential of pyrolysis gas chromatography of fibers include:
5.1. Comparative analyses of two or more fibers;
5.2. Identification analyses of known or questioned fibers or both. With regard to some fiber types, such as acrylics, PGC can be used in conjunction with infrared spectroscopy to provide differentiation within generic classes (6); and
5.3. The technique is extremely sensitive and can be used to analyze a wide variety of materials (7).
As with any instrument, PGC has limitations to its application, the two most important of which are the number of parameters and the control of these parameters. First, PGC encompasses a great variety of parameters. Fluctuation in any one of these parameters will produce pyrolysis product changes. These parameters can be related to the sample and its preparation and can include sample homogeneity, sample size, sample shape, sample placement within and contact with the quartz tube, and sample weight. Other variables related to the pyrolysis instrument include the temperature of the pyrolysis, the rate of temperature rise, the time of the pyrolysis, and the pyrolysis chamber atmosphere. Second, the variables must be controlled to ensure reproducible results. For more complex samples, the reproducibility of replicate sample pyrograms becomes more involved. Therefore, users should establish their systems’ capability to discriminate various copolymer ratios.
Proper sample preparation and technique are prerequisites for obtaining reproducible results. Fibers being compared should be analyzed using the same parameters and approximately the same sample size and shape (2, 3).
Samples are prepared using low-power magnification, and clean tools must be used to handle samples and the quartz tube (4).
The user is required to maintain authenticated and traceable reference standards of fibers for comparison, identification analyses, or both. These known standards can include fibers obtained from testing services, manufacturers, or both. Control samples should be routinely run as established by laboratory procedures.
7.1. Sample Temperature
A pyrolysis instrument must be able to heat a sample to a preset temperature at a known rate for a specific amount of time. These conditions must be accurately reproducible and predictably varied (5).
The gas chromatograph used in fiber pyrolysis should (a) have a reproducible temperature profile and stable carrier gas flow rate; (b) have a capillary column capable of distinguishing different fiber types; and (c) have the capability to reproducibly separate and identify pyrolysis products.
After establishing standard methods and protocols, standard pyrograms should be run to check the temperature setting and resulting pyrogram pattern. A polymer material such as low-density polyethylene or polypropylene can be used for routine performance checks. The frequency of routine instrument performance checks should be established by each laboratory.
The instrument performance sample should be an easily obtainable material that yields reproducible chromatograms having peaks over the entire range of the pyrogram with major peaks near the start, in the middle, and near the end.
The pyrolysis unit must be checked in conjunction with the gas chromatograph at routine intervals as established by each laboratory. The pyrolyzer should be checked after the gas chromatograph has been checked.
7.2. Pyrolyzer Calibration
A pyrolyzer can be calibrated by observing the melting points (mp) of two inorganic compounds within a 7-mm band approximately in the center of the quartz tube probe. Possible compounds include potassium chloride (KCl) mp (approximately 770°C); sodium tungstate (Na2WO4) mp (approximately 700°C); potassium iodide (KI) mp (approximately 686°C); or potassium iodate (KIO3) mp (approximately 560°C). Any pyrolysis unit should be recalibrated by the manufacturer when shipped for necessary repairs.
An instrument performance sample should be introduced into the GC during routine performance checks as established by each laboratory. New instrument performance sample chromatograms must be compared with previous ones in order to establish relative sensitivity, resolving power, and baseline profiles. This ensures that the case samples and the reference library are still comparable. In some cases instrument performance changes sufficiently to require that new reference standards be generated. These instrument performance sample chromatograms should be kept in the instrument logbook for a predetermined length of time as established by laboratory protocol.
7.3. Analysis Procedures
The following series of procedures must be followed for an analysis:
7.3.1. Run controls and blanks as established by individual laboratory procedures. Allowable maximum peak heights in blank samples should be defined in laboratory procedures;
7.3.2. Run an instrument performance sample according to laboratory procedures and ensure the instrument is operating properly; and
7.3.3. Adjust column head pressure and split-flow rates in accordance with established procedures.
7.4. Sample Comparison: Known to Questioned
Run known and questioned samples under the same conditions and compare their chromatograms. Known samples should be run in duplicate to assess variations in the pyrograms and to ensure reproducibility.
7.5. Identification By Means of a Reference Library
Identification is accomplished by comparison of a known sample, questioned samples, or both to a reference library. To do so, the individual performing the analysis should run the known sample and questioned sample and compare their chromatograms to the reference library. The library chromatograms should originate from the same instrument and protocol used in the current analysis. All identifications must be confirmed by running an authenticated fiber reference standard at the time of analysis. Known samples, and questioned samples if necessary, should be run in duplicate to assess variations in the pyrograms and to ensure reproducibility.
7.6. Procedures Established by Laboratory Protocol
The following procedures should be performed as established by laboratory protocol:
7.6.1. Check to ensure even spacing along the platinum coil on a pyroprobe coil unit. This coil should be visually inspected before each use;
7.6.2. The pyrolysis quartz tube must be heat cleaned after each use. If the quartz tube is reused, each laboratory should develop, document, and use a cleaning procedure that can be demonstrated to be noncontaminating to subsequent runs; and
7.6.3. Check gas cylinders and change when the pressure drops to a predetermined level. If gas-line moisture traps, oxygen scrubbers, and so forth are being used, these should be changed when tanks are changed or as necessary to maintain system performance.
7.7. Scheduled Routine Maintenance Procedures
Scheduled routine maintenance procedures must be performed per individual laboratory procedures. Record performed maintenance in an instrument logbook. This must include cleaning the detector, reassembling the detector and checking flows, changing GC septa and pyroprobe O-ring seals, cleaning injection port, checking glass liners during routine maintenance, performing other cleaning as needed, and performing any additional scheduled routine maintenance.
Instrument performance must be checked whenever a new column is installed or whenever repairs are done to the pyrolyzer.
Documentation must include data obtained through the analytical process. Deviations from the written protocol (other than standard operating procedures) must be documented.
The following instrumental variables (parameters) must be recorded in the laboratory and be accessible for later reference or included in the case file:
8.1. Specific GC used;
8.2. Type of GC column, including
8.2.4. Coating thickness;
8.2.5. Type of carrier gas and detector;
8.2.6. Flow rates;
8.2.7. Split flow (if applicable); and
8.2.8. Chart speed (if applicable);
8.3. Oven temperature program, including
8.3.1. Initial temperature;
8.3.2. Ramp rates;
8.3.3. Final temperature-to-temperature holding durations;
8.3.4. Injector and detector temperatures; and
8.3.5. Specific pyrolyzer unit used;
8.4. Pyrolysis temperature, including
8.4.1. Interface temperature;
8.4.2. Ramp rates;
8.4.3. Final temperature; and
8.4.4. Temperature holding durations (interval).
The data generated by PGC is dependent upon various factors such as sample size, condition, and handling. Likewise, interpretation of PGC data is dependent upon the training and experience of the examiner. Awareness of the strengths and limitations of the technique must be considered. In addition, the examiner must assess the variability of the instrument and variations within the pyrograms. Therefore, the examiner must complete a formalized training program conducted under the supervision of an experienced examiner prior to casework. This training program must include analyzing and comparing pyrograms of various polymers in order to discriminate between polymer types.
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(2) Walker, J. G., Lackson, M. T., and Maynard, J. B. Chromatographic Systems: Maintenance and Troubleshooting. Academic Press, New York, 1973.
(3) Irwin, W. J. Analytical Troubleshooting: A Comprehensive Guide. Marcel Dekker, New York, 1982.
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(5) Pyroprobe 100 Series Solids Pyrolyzer Operating and Service Manual. Chemical Data Systems, Inc., Oxford, Pennsylvania, 1984.
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Takekoshi, Y., Kanno, S., Kawase, S., Kiho, T., and Ukai, S. Forensic chemical study on nylon fibers by acid-catalyzed pyrolysis gas chromatography, Japanese Journal of Toxicology and Environmental Health (1996) 42:28-31.