Inorganic Analysis

Introduction

  • Inorganic Evidence: Non-carbon-containing substances, often the physical evidence.  

  • Forensic scientists analyze inorganics such as tools, coins, weapons, explosives, poisons, metal scrapings, and trace components in paints and dyes. 

  • Many products contain small quantities of elements, known as trace elements, present in concentrations of less than 1 percent.

  • For the criminalist, the presence of trace elements is helpful because they provide “invisible” markers that may establish the source of a material

  • The forensic scientist must perform tests that will determine the specific chemical identity of the suspect material to the exclusion of all others.

Analytical Instrumentation
  • Techniques available to forensic scientists for determining the elemental composition of materials are:
  1. %%Emission Spectroscopy%%

   

  1. ==Atomic Emission Spectrophotometry (AES)→ Inductively coupled plasma==
  2. __**Atomic Absorption Spectrophotometry (AAS)**__
    1. ^^Neutron Activation Analysis (NAA)^^
    2. @@X-Ray Diffraction@@
Emission Spectrograph
  • Recall: molecules can be ABSORBED by UV, Vis or IR
  • Molecules can also EMIT radiation.
    • Forms the basis of two important analytical techniques
    • __**Emission spectroscopy**__
    • ^^Atomic absorption spectroscopy^^
\n Origin of Spectra
  • Recall…atoms are composed of a nucleus (protons and neutrons) with electrons orbiting the nucleus 
    • Energy Level: %%Orbits have a certain amount of energy%% 
  • Each element has its own number of energy levels→ an atom will absorb a certain amount of energy 
Emission Spectrograph
  • An emission spectrograph:
    • heats substance
    • gets “excited”
    • emits energy (light)
    • ^^based on what elements are in the substance, it will produce a unique line pattern^^
Atomic Emission Spectroscopy: Inductively Coupled Plasma
  • AES: convert samples to atoms 
    • By“exciting”them→bringingthemtohigherelectronicstatesBy “exciting” them→bringing them to higher electronic states
Atomic Emission Spectroscopy: Inductively Coupled Plasma
  • Inductively coupled plasma emission spectrometry (ICP):
    • The sample (aerosol) is heated (charged)
    • Charged particles emit light
    • Different wavelengths depend on what elements are present
  • ==How is it used in forensics??==
    • ==Identification and characterization of mutilated bullets==
    • ==Identification and characterization of glass fragments==

b. Atomic Absorption

  • Atomic Absorption Spectrophotometry:
    • ^^Heat substances (“excites” atoms)^^
    • ^^Expose it to radiation for a specific element^^

If the element is present, a portion will be absorbed

  • If want to detect a certain element can shine specific light patterns to see if the element present  
Radiochemical Methods: Neutron Activation

Radioactive Decay: Unstable atoms will decompose

  • ==Three types of radiation==

    • ^^Alpha rays^^
    • BetaraysBeta rays
    • %%Gamma rays%%

  • Neutrons produce radioactive isotopes

measure gamma-ray energy (specific to the element)

Neutron Activation
  • %%Very sensitive→identifies and quantifies 20 to 30 trace elements.%% 
  • How is it used in forensics?
    • Find trace elements in metals, drugs, paint, soil, gunpowder residue, and hair.
  • Since this technique requires access to a nuclear reactor, however, it has limited value to forensic analysis.
AES, AAS & NAA
  • @@Provide elements present but no info about how elements are combined in a compound@@
  • So, aim a beam of X-rays at a crystal to study how they interact with the atoms
    • X-ray Diffraction
    • ^^ONLY applied to the study of solid, crystalline materials.^^ 
    • ^^Fortunately, 95% of all inorganics are crystalline^^
X-ray Diffraction
  • X-rays penetrate the crystal→ some reflected→ combine to form light and dark bands 
    • known as diffraction patterns
  • ^^Every inorganic compound produces its own “fingerprint” of diffraction patterns^^