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April 1999 Volume 1
Number 1
Chapter 5 of Forensic
Fiber Examination Guidelines
1.0. Scope
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.
2.0.
Reference Documents
SWGMAT Quality Assurance
Guidelines
SWGMAT Trace Evidence Handling Guidelines
ASTM E 1610-94 Standard Guide for Forensic Paint Analysis and
Comparison
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3.0. Terminology
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.
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4.0.
Summary of Guidelines
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.
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5.0.
Significance and Use
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.
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6.0.
Sample Handling
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).
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7.0. Analysis
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.
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8.0.
Report Documentation
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
8.2.1. Length;
8.2.2. Diameter;
8.2.3. Coating;
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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9.0. References
(1) Skoog and West. Analytical
Pyrolysis GC: Getting Started Is Easy. Chemical Data Systems,
Inc., Oxford, Pennsylvania.
(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.
(4) Wampler, T. P. and Levy,
E. J. Reproducibility in pyrolysis: Recent developments, Journal
of Analytical and Applied Pyrolysis (1987) 12:75-82.
(5) Pyroprobe 100 Series
Solids Pyrolyzer Operating and Service Manual. Chemical Data
Systems, Inc., Oxford, Pennsylvania, 1984.
(6) Gaudette, B. G. The forensic
aspects of textile fiber examination. In: Forensic Science
Handbook (Vol. 2). Ed., R. Saferstein. Prentice Hall Inc.,
Englewood Cliffs, New Jersey, 1988.
(7) McClamroch, D. L. Pyrolysis
Gas Chromatography. Presented at FBI Trace Evidence in Transition
Symposium, San Antonio, Texas, 1996.
(8) Challinor, J. Fibre identification
by pyrolysis techniques. In: Forensic Examination of Fibres.
Ed., J. Robertson. Ellis Horwood, Chichester, United Kingdom,
1992.
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10.0.
Bibliography
Almer, J. Subclassification
of polyacrylonitrile fibres by pyrolysis capillary gas chromatography,
Journal of the Canadian Society of Forensic Science (1991)
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Blackledge, R. D. Application
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Challinor, J. M. Forensic
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Challinor, J. M., Collins,
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