Ir spectroscopy a promising technique for glass analysis in forensic identification
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IR Spectroscopy: A Promising Technique for Glass Analysis in Forensic Identification . By : Brian Vlaisavich, Biology, Junior, Lewis University Pradip Patel, Biology, Junior, Lewis University. Sponsor: Dr. Salim M. Diab, University of St. Francis and Lewis University . Objective .

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IR Spectroscopy: A Promising Technique for Glass Analysis in Forensic Identification

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IR Spectroscopy: A Promising Technique for Glass Analysis in Forensic Identification

By :

Brian Vlaisavich, Biology, Junior, Lewis University

Pradip Patel, Biology, Junior, Lewis University

Sponsor: Dr. Salim M. Diab, University of St. Francis and Lewis University


  • To create an automotive glass database that will allow scientist to identify unknown glass samples from a crime scenes.

Why IR Spectroscopy?

  • There is no current forensic database for glass analysis using IR techniques.

  • Refractive index is the only instrumental technique used for way glass analysis in forensic laboratories today..

Refractive Index

  • Outdated instrumental method

  • Method is neither accurate nor precise.

  • Variation over time of these databases causes for improper statistical calculations.

Benefits of using FTIR

  • Method is quick and reliable.

  • Provides:

    • reliable accuracy

    • desirable speed and efficiency of data analysis and collection.

History of IR

  • Developed in the 1960's

    • Only intended for advanced research

  • High cost made it impossible to be used for commercial use

  • Advancements in technology - FTIR

    • reduced cost

    • enhanced capabilities making it a standard instrument in modern labs.

1. consists of a source, beamsplitter, two mirrors, laser, and detector.

2. beamsplitter splits the beam into two parts.

3.  One part is transmitted to a moving mirror; other part is reflected to a fixed mirror.

4.  The moving mirror moves back and forth at a constant velocity.  This velocity is timed according to the very precise laser wavelength in the system which also acts as an internal wavelength calibration.

5.  The two beams are reflected from the mirrors and recombined at the beamsplitter.

How does IR work?

6.  The beam from the moving mirror has traveled a different distance than the beam from the fixed mirror.

7.  When the beams are combined an interference pattern is created, since some of the wavelengths recombine constructively and some destructively.

8. This interference pattern is called an interferogram.

9. This interferogram then goes from the beamsplitter to the sample, where some energy is absorbed and some is transmitted.

10. The transmitted portion reaches the detector.

11. The detector reads information about the wavelengths in the infrared range simultaneously.

Continued . . .

Further explanation of IR…

  • IR spectroscopy is the use of light in the mid-infrared range to excite the bonds in a compound in order to learn about it by seeing which frequencies are absorbed.

Two main uses of IR spectroscopy in Forensic Science:

  • Confirmation of Functional Groups

  • Confirmation of the identity of a substance

Theory of IR Absorption

  • IR absorption is restricted to compounds with small energy differences in the likely vibrational and rotational states.

  • For a molecule to absorb in IR, the vibrations or rotations must cause a net change in dipole moment of the molecule.

  • If the frequency of radiation matches the vibrational frequency of the molecule, then radiation will be absorb in the form of a peak, also known as a ‘fingerprint’

Interpretation of IR spectrum?

  • IR peaks shows where the infrared radiation was absorbed in the molecule.

  • Collective analysis of peak shapes and frequencies, define the functionality/identity of the compound.

Why do IR peaks have different intensities and/or different frequencies?

  • The intensity of an IR absorption is proportional to:

    • The strength of the dipole moment change associated with the vibration

    • The number of bonds in the molecule

Types of Stretching Vibrations

  • Symmetrical stretching

    • the bonds stretch with the same strength at the same time.

  • Asymmetrical stretching

    • The bonds stretch at different times causing a change in the dipole moment.


  • Bench Status

    • Must have a Green Check

  • Collect

    • Experiment setup

      • Collect Tab

        • # of scans 32

        • Resolution 4

        • Final Format: % transmittance

        • Collect background after (Large #)

        • 32 scans for background



H2O & CO2

Large #


  • Bench Tab

    • Gain: autogain

    • Min Range 600 nm

      • Select OK

Library Setup

  • Analyze

    • Library Setup

      • Select Glass Library

        • Select Add

        • Select OK




  • Collect background

    • Click Col Bkg

      • Select OK

      • Add to window 1: Select NO

Prepare Glass

  • Place glass into cardboard

    • Place into middle slit

      • Cover the top

Glass Sample

  • Collect Sample

    • Click Col Smp

      • Add title of sample with date

      • Select ok

      • Add to window 1: Select Yes

  • Add to Library


  • Created a library for glass analysis:

    • Entered known samples previously collected from University of St. Francis students

    • Entered known samples collected at Canal St. Junkyard in Lockport, IL

    • Various other samples also entered

Confirmation of Samples

  • Each sample was entered into the database multiple times

  • Spectra that had the largest % match was then saved to the library

Test: 1985 Ford Bronco II Rear Back Window

Experimental Problems

  • Samples similar to each other produced indistinguishable spectra

  • Samples that are

    • Curved in shape

    • Bubbled in shape

    • Cracked in shape

    • Too small in size

      All produced distorted spectra.


  • Car manufactures may use similar compositions of glass in production

  • A larger range of different makes and models of cars needs to be collected

    • Most of the samples collected:

      • American cars

        • Ford

        • Chevrolet

Curved/Bubbled Samples

  • Produced different spectra depending on:

    • Angle and direction of light on sample

    • Example:

      • tail lights, head lights, etc.

Example: 1989 Pontiac Grand Prix Tail Light

Tail Light Spectra: Example 1

Tail Light Spectra Example 2

Identification of Unknown Samples

  • Three unknown glass samples were analyzed

  • Goal:

    • To determine the source of glass based on the Automobile Library that we created

Unknown 1

Unknown 2

Unknown 3

Future Research

  • More samples can be collect to build the library

  • Spectra collected can still use continued updating to be more accurate

Unknown Data

  • Unknown 2 & 3

    • appear to not match well with anything in the library.

  • Unknown 1

    • 70% match to 1995 Dodge Intrepid

    • Possible relationship?


  • From the initial research it appears that FTIR is a reliable method for glass analysis

  • Glass samples matched with corresponding cars

    • However:

      • Large % match was not always obtained

      • Future research necessary


  • FT-IR Dispersive Infrared. Theory of Infrared Spectroscopy Instrumentation.

  • Infrared Spectroscopy.

  • Introduction to IR Spectra.

  • Koons, Robert D. Distribution of Refractive Index Values in Sheet Glasses. Forensic Science Research Unit.

  • Lesney, Mark S. A Study in Infrared. Today’s Chemist at Work. October 2003.


  • Lewis University

  • University of St. Francis

  • Professor S. M. Diab

  • Br. Pierre St. Raymond

  • Canal St. Junkyard (Lockport, IL)

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