5.1: Electromagnetic Radiation

• Electromagnetic Radiation – Refers to the various forms of energy in the form of waves.

• Wavelength – the distance between corresponding points on two adjacent waves.

• Frequency – the number of waves that passes a given point in 1s.

  • It is measured in cycles per second (Hertz).

  • Frequency can also be expressed in other units – wavenumbers. It is the inverse of the wavelength measured in centimeters.

• Photons - tiny packets of energy.

Electromagnetic Waves

• Gamma rays– are very energetic and can pass through matter. They can be dangerous to live in and they can damage or destroy cells.

• X-Rays– can also pass through most matter but are %%deflected by dense matter such as bones. The X-rays reflect off the bone and other dense tissue and are detected while the others pass through soft tissue.

• Ultraviolet – contains ultraviolet radiation and visible light. These two areas are lumped together because both UV and visible light have the same effects on matter.

• Light – is not energetic enough to pass through matter. Instead, when a molecule absorbs this light, electrons are shifted from one orbital to another.

  • Orbital –  an energy level where an electron resides.

• Infrared – when absorbed by matter, this type of light causes bonds between atoms in a molecule to vibrate like two weights on either end of a spring. This makes it useful as a tool for the identification of a pure substance.

  • The IR region is also very important in the analysis of chemical evidence in forensic science.

• Microwave –  These light waves cause molecules to rotate or spin. The practical effect of this is that, when adjacent molecules absorb microwave radiation and spin, they rub against each other and cause friction. This friction, in turn, generates heat. This is the principle behind microwave ovens.

• Radio waves – These have very long wavelengths and thus very low frequencies and relatively little energy. They carry radio and TV signals.

  • Modulation – radio waves are transported through the air to the radio receiver by a carrier wave.

    • Modulation can be accomplished using either amplitude (AM) or frequency (FM).

  • Demodulation – radio waves and carrier waves are separated.

5.2: Interaction of Matter with Specific Regions of Electromagnetic Radiation

• Forensic and analytical chemists are interested in the wavelengths and/or frequencies of radiation absorbed by matter.

• When a substance is exposed to electromagnetic radiation, it undergoes changes that may or may not be reversible and which depend upon the energy of the radiation.

• Forensic science, is more concerned with what happens to a substance when it is exposed to the light of much lower frequencies and energies.

• The behavior of matter, when exposed to light, is a very important chemical property and is greatly exploited by forensic scientists.

UV/Visible Spectrophotometry

• A photon that causes electron promotion in atoms and molecules is in the UV/ visible region of the electromagnetic spectrum.

  • When a substance is exposed to UV/ visible radiation, it will absorb certain photons of a particular energy.

• If the amount of each wavelength of light that is absorbed by a substance throughout the UV/visible region is plotted, a spectrum is generated.

• Many organic substances will also have a UV/visible spectrum because they usually possess several conjugated carbon/carbon double bonds – the alternating single and double (or triple) bonds in the molecule.

  • Any compound that is based on the benzene ring, for example, will absorb strongly in the UV/visible region.

  • It is conceivable that several substances could have the same chromophore and thus the same UV/visible spectrum. This is one reason why UV/visible spectra cannot be used for unequivocal identification of a substance.

• The UV/visible spectrum of a substance is obtained by using a UV/visible spectrophotometer.

• Monochromator – a device that selects one particular wavelength to be exposed to the sample.

  • It is a prism or grating that is rotated thus exposing the sample to steadily increasing or decreasing wavelengths of light throughout the entire spectrum.

• Detector – must be sensitive to changes in the intensity of UV/visible light that reaches it.

  • The most commonly used UV/visible detector is the photocell – a device that converts UV or visible light into an electric current.

• Transmission Spectrum– The computer stores the wavelength and corresponding electric current and then, when the entire spectrum has been obtained, will construct a graph of wavelength versus intensity of transmitted light.

• Absorption Spectrum – The computer can also convert this to the wavelength versus the amount of light absorbed by the sample.

• UV/visible spectrophotometry can also be used to determine the amount of a substance in a mixture.

  • This is because it adheres to Beer’s Law – the amount of an absorbing substance present to the quantity of absorbed light.

Molecular Fluorescence

• Fluorescence– this occurs when a substance absorbs energy and then emits it in the form of visible light.
  

• Most substances do not fluoresce. They absorb light and then emit the same wavelength back. Those substances that fluoresce will always emit light of a longer wavelength (lower energy) than they absorb.

• Most substances of forensic interest that fluoresce do so when the light absorbed is in the visible or ultraviolet range. Some substances such as certain inks undergo IR fluorescence, where the light absorbed and emitted is in the IR region.

• Fluorescence Spectroscopy – the detector is at a right angle to the source, with the sample at the apex, so that the detector does not see any light that leaves the source and is directly transmitted by the sample. The detector sees only light that is fluorescent by the sample.

  • It is used for studying structural changes in conjugated systems, aromatic molecules, and rigid, planar compounds.

IR Spectroscopy

• IR Spectroscopy - It is the measurement of the interaction of infrared radiation with the matter by absorption, emission, or reflection. It is used to study and identify chemical substances or functional groups in solid, liquid, or gaseous forms.

  • The IR spectrum of a substance is unique and can be used to unequivocally identify that substance.

• Fourier Transform – the wavelengths of light that are absorbed by the sample are sorted out and displayed as either a function of absorbance or transmittance.

  • Sampling for IR spectrophotometry is more flexible than for UV/visible spectrophotometry.

• Reflectance spectra – the visible region of the electromagnetic spectrum holds information that defines product color. There are two common methods for obtaining reflectance spectra:

  1. Diffuse Reflectance – uses a set of mirrors that direct the IR source light at the sample at an oblique angle to the surface of the material, which then absorbs some of the light.

  2. Attenuated Total Reflectance (ATR) – uses a special crystal that is brought into direct contact with the sample. The source radiation is directed at the sample through the crystal. Because of the nature of the crystal, the radiation bounces off the sample several times before reaching the detector.

• IR detectors are usually some type of thermocouple — a device that converts heat into electricity.

• IR Microspectrophotometry – enables an evaluation of colors to be made objectively using UV–visible transmittance profiles.

  • Used for evidence types include single fibers, paint chips including cross sections, drugs, inks, copier toners, polymers, and dyes and pigments.

• IR spectrophotometry is a practically universal technique for the analysis of evidence.

  • IR analysis relies upon the presence of a pure substance.

  • IR spectra of mixtures are difficult to interpret and are not suitable for identification.

  • For very small amounts of impure material, purification can be impractical and IR is not used.

5.3: Raman Spectroscopy

• Raman Spectroscopy - a molecular spectroscopic technique that utilizes the interaction of light with matter to gain insight into a material's makeup or characteristics, like FTIR.

• Elastic energy - the energy of the scattered photon is the same as that of the incident or absorbed photon

• Inelastic energy - the energy of the scattered photon can be greater or less than that of the incident one.

The Raman spectrum is measured as the chemical shifts of the emitted photons— which is the difference in energy between the incident photon and the inelastically scattered emitted photon.

• Raman active vibrations are IR inactive and vice versa.

• Raman spectroscopy is routinely used in the analysis of drugs, paints, inks and dyes, and fibers.

5.4: Mass Spectrometry

The resultant array of ions is called the mass spectrum and the techniques used to create the mass spectrum are collectively called mass spectrometry.

Mass spectrometry has two very important properties that make it a valuable tool in analytical chemistry and thus in forensic chemistry.

1. If the energy of the source is carefully controlled then the fragmentation pattern for a given substance will be very reproducible.

2. The fragmentation pattern for a given substance is unique. The mass spectrum of a pure substance is a reliable way of identifying it.

A mass spectrometer can be designed as a detector for a gas chromatograph or liquid chromatography.

• In the case of liquid chromatography, the mobile phase liquids are stripped off before the analyte is ionized.

• In Inductively Coupled Plasma Mass Spectrometry (IPMS), the glass is digested and transformed into an aerosol by a nebulizer that breaks up the sample into very small droplets.

Ionization

• Electron Impact – an ionization method in which energetic electrons interact with solid or gas phase atoms or molecules to produce ions.

• Chemical Ionization – relies on gas-phase chemical reactions that take place between the analyte of interest and ions generated from a reagent gas.

• Laser Desorption mass spectrometry – uses a laser to ionize the analyte.

  • Matrix-Assisted Laser Desorption Ionization (MALDI) – The matrix absorbs the laser energy and transfers it to the analyte.

Separation of Ions

• Magnetic Sector Mass Spectrometry: The ions are accelerated through a curved magnetic field toward the detector. Smaller ions are deflected to a greater extent as they pass through the field.

• Quadrupole Mass Spectrometry: Separate ions by their mass and pass them to a detector, where they are counted. Gas molecules are ionized before entering the quadrupole zone so that they have a charge and can be isolated in the tuned quadrupole zone.

• Ion Trap Mass Spectrometry: Ions are focused by a quadrupole into an ion trap where they are collected. They are then ejected toward a conventional detector.

• Time-of-Flight Mass Spectrometry: The ions are accelerated by a magnetic field of known strength. The time it takes for a given ion to reach the detector is then used to determine the mass/charge ratio

5.5: Atomic Spectroscopy

• Atomic Absorption Spectroscopy (AAS) – substances are analyzed in the vapor phase. The elements that are to be analyzed must be known in advance. The material is dissolved in a suitable solvent and then introduced into a flame or furnace so that it can be vaporized.

  • AA spectroscopy is very sensitive but the analyte must be vaporized and a separate experiment must be done for each element.

• Atomic Emission Spectroscopy (AES)– this is not as sensitive as AA and it is an analytical technique used for the quantification of metal atoms by measuring the intensity of light emitted by the atoms in excited states.

  • AE is used when the material being analyzed has a large number of elements that are being analyzed such as an unknown metallic material or sometimes an automotive paint chip.