VOLUME 01
October 2018
ISSUE 10
Fourier Transform Infrared (FTIR) Spectroscopy
Why is FTIR important to you?
Forensic evidence is invaluable for many
types of investigations. Finding the factors that may have led
to a fire, explosion, chemical spill, and chemical and/or
environmental incident is of utmost importance. In many cases,
evidence may be found at the scene that is of unknown composition
and potentially crucial in assessing any incident. Arming an
attorney with chemical evidence, documentation and analysis results
can be pivotal for a favorable decision in litigation.
Providing jurors with information that is clear and based on factual
analytical results can assist in the believability of your case.
Fourier Transform Infrared Spectroscopy (FTIR) is a laboratory
method that is commonly used as part of many incident
investigations.
What is FTIR?
FTIR is a non-destructive analytical method
that can give the investigator information about materials with
unknown chemical compositions. It has widespread uses and can
characterize polymers, organic materials and sometimes inorganic
materials in solid, liquid or gas form. In
forensics, chemicals from samples such as paints, fire debris,
synthetic materials, polymer coatings, drugs, accelerants, and
organic contaminants can be identified. Not only
is FTIR a well-established analytical method, but it is very
reliable. This makes FTIR a powerful method for
collecting chemical information on evidence from an incident scene.
Infrared spectroscopy is the analysis of
infrared light interacting with molecules. It measures the amount of
light the chemical bonds of a sample absorbs. All
molecules have specific frequencies that they vibrate at, caused by
specific energy levels that are associated with the bonds inside the
molecule. In infrared spectroscopy analysis,
infrared radiation is transmitted through a sample. Some of the
infrared radiation is absorbed by the sample, and some of the
infrared radiation is transmitted through the sample. The resulting
signal at the detector is a spectrum representing a molecular
“fingerprint” of the sample. The absorption peaks correspond to the
frequencies of vibrations associated with the bonds of the atoms
making up the material.
Like a human fingerprint, no two different molecular structures produce exactly the same infrared spectrum (although some can be very similar). This FTIR spectrum can be automatically compared to a library of compounds for a computer generated most-likely best fit, with an example seen in Figure 1. An experienced forensic chemist must apply knowledge of the chemicals and chemical interactions pertinent to the incident in order to definitively identify the compounds found in the spectra. Relying only on the computer generated best fit can leave the compound identification open to attack.
Medical Oxygen Cylinder Case
During the investigation of a flash fire in a
medical oxygen cylinder, FTIR was used to analyze a debris fragment
found inside of the regulator. The analysis raised
additional questions since the results seemed to point to a
non-combustible gasket material that is commonly found in regulators.
After further visual and microscopic analysis of the debris
fragment, additional FTIR analysis was needed to understand the
composition of the material. The outer surface of the
fragment was scraped off to analyze the chemical composition of the
interior material. The figures below show the FTIR
spectrum of that sample taken from inside the oxygen tank regulator.
The first spectra show the results of the sample in its original
state. The second spectra show the result after the
outer layer of the sample had been scraped in the laboratory.
The outer layer of residue masked crucial evidence.
The analysis of the scraped sample revealed a
cellulosic material which was found to be a contaminant inside of the
oxygen tank regulator and the fuel for the fire. In
this case, a high degree of understanding of FTIR analysis was
essential. This is important because materials often
volatize and re-condense during an incident. This can
leave evidence collected at the scene coated with a thin layer that does
not represent the underlying material. Removal of an
interfering coating can be critical in establishing cause, origin and
the responsible party. In this case, knowing that FTIR will only examine
the outermost layer of a material, having a comprehensive analytical
background and considering the key internal portion of the sample may
have been hidden by fire condensed chemical residue was crucial in
determining causation.
Definitive Evidence
Data interpretation is not straightforward.
Only an investigator with a strong analytical chemistry
background can discern if the results require further analysis.
However, when interpreted by an experienced chemist, FTIR has the
advantage of being considered definitive evidence.
When presented to a judge or jury, it can provide a non-ambiguous
insight into the nature and causation of an incident.
Conclusion
The chemical complexities of any incident scene can
offer a rich source of vital information, but can be confusing and
evidence is easily missed by an inexperienced forensic investigator.
The team at Gossman Forensics has the background and experience
to asses an incident site and uncover and collect potential chemical
evidence that can be imperative to a case.
Additionally, our team excels in understanding and evaluating laboratory
results, including FTIR spectrograms. We are able to
scrutinize results, identify masked or hidden evidence, and provide
expert witness testimony and guidance in litigation.