Sample Preparation: Substrates
Substrate samples are prepared using a sample size of approximately 16 cm2 or 1.5 grams. Each substrate is burned by a modified destructive distillation method (MDDM) for 2 minutes.
Substrates are placed into an unlined quart metal paint can with the substrate surface most commonly adjacent to the heat source facing downward. For example, a carpet sample is positioned so the tufted or woven textile side is in contact with the bottom of the can. Nine holes of approximately 1 cm in diameter are punched into the can lid. Heat is applied with a propane torch so the tip of the torch is 4 cm from the bottom of the can and the flame is centered. Once smoke appears from the top of the can, the time interval of 2 minutes begins. At the end of the time interval, the propane torch is removed. The punctured lid is replaced with an intact lid and the can is cooled to room temperature.
Interfering products are collected by passive headspace adsorption onto activated carbon following the ASTM E1412 standard method. An activated carbon strip (10 mm x 22 mm) is suspended into the headspace of the can by a paperclip and un-waxed dental floss. The sealed can is heated for 16 - 18 hours at 66ºC. The carbon strip is removed from the can and placed in a vial with 500 microliters of carbon disulfide.
The substrate sample is analyzed by gas chromatography - mass spectrometry. The peak height (abundance) of the substrate sample’s most intense peak within the total ion chromatogram is recorded for future concentration calculations.
Sample Preparation: Weathered Ignitable Liquids
Ignitable liquids are evaporated to 50%, 75%, or 90% by reducing the volume by the respective fraction of the original volume. Heat is applied for samples of a heavier carbon range utilizing a dry bath. Twenty microliters of weathered ignitable liquid is diluted with one milliliter of carbon disulfide and then analyzed by gas chromatography - mass spectrometry. The peak height (abundance) of the substrate sample’s most intense peak within the total ion chromatogram is recorded for future concentration calculations.
Sample Preparation: Fire Debris
No Ignitable Liquid Residue (FD-SUB)
Fire debris samples consisting of a single substrate are created following the method described above and analyzed by gas chromatography – mass spectrometry (Substrate Database). Dilute single substrate samples where the peak height of the most intense peak had a minimum peak height of 100,000 (abundance) were accepted as fire debris samples. Fire debris samples consisting of multiple substrates are created by mixing equal volumes of the single substrate samples and analyzed by gas chromatography – mass spectrometry.
Containing Ignitable Liquid Residue (FD-ILR)
Fire debris samples containing an ignitable liquid residue are created by mixing a volume of a fire debris containing only substrates (FD-SUB) with a volume of a dilute weathered ignitable liquid (ILRC Database). Previously recorded peak heights (abundance) from the dilute weathered ignitable liquid (IL) and the fire debris samples containing only substrates (FD-SUB) are used to calculate a desired relative ignitable liquid to substrate ratio.
Agilent 7890A gas chromatograph and 5977E mass spectrometer
Agilent ALS autosampler7650
Column: HP-1 or equivalent
Length: 25 meters
Inner diameter: 0.20 mm
Film thickness: 0.50 microns
Carrier gas: Helium (ultra high purity)
Flow rate: 0.80 ml/min.
Linear velocity: approximately 36 cm/sec
Split ratio: 50/1
Injection volume: 1 microliter
Pre injection washes of sample: 2
Post injection wash of pentane: 10
Post injection wash of pentane: 10
Injection temperature: 50 degrees C
Initial hold time: 3 min.
Rate: 10 degrees C/min.
Final temperature: 280 degrees C
Final hold time: 4 min.
Total run time: 30 min.
Transfer line temperature: 280 degrees C
Acquisition mode: scan
Scan range: 30 – 350
Scan rate: 2-3 scans/sec; equivalent to 6 -10 scans per peak
Detector is off between 1.54 and 2.00 minutes