FORS-2107EL Lecture Notes - Lecture 6: Molar Attenuation Coefficient, Electromagnetic Spectrum, Absorbance
27 Jun 2018
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Forensic Science - Day 6 2018.02.01.
Definitions and Terminology
-Analytical chemistry involves measurement of properties of a substance or system
such that information about the chemical make up of that system is obtained
-Qualitative analysis: some or all of the constituents of the system in question are
identified
-Quantitative analysis: the concentration (or mass) of some or all the constituents in a
system are measured
-Quantitative or Qualitative?
-Accelerants in fire debris: qualitative
-Drugs in an unidentified powder: both
-Blood alcohol concentration: quantitative
-Trace elements in glass: both
-Composition of a fibre: qualitative
-Particles recovered from the hand of a suspected shooter: both
-In analytical analysis, the most important part to forensics is the interpretation, which
is what you will be asked to testify in court
Method Selection
-No analytical method is capable of identifying/quantitating all analytes (the substance
you are testing for)
-The choice of method depends on whether the analytes of interest are
-Elemental or molecular in form
-Organic or inorganic
-Volatile or Non-volatile
-Low MW (eg. ethanol) vs medium MW (eg. morphine) vs high mW (eg. DNA)
-Ionic vs. Non-ionic
Definitions
-Sensitivity: the magnitude of the change in signal resulting from an incremental
change in analyte concentration (a parameter)
-How much detection the method is capable of, resulting from an change in analyte
concentration
-Ex. phentanol needs 3 ng/ml to kill you so high sensitivity is needed
-Limit of detection (LOD): the lowest concentration of a compound that can be
detected and differentiated from all the other components present in the compound
(background noise), this is an actual measurable amount
-The higher the sensitivity of a method, the lower its LOD
-Selectivity: the ability of the method to discriminate between two different atoms/ions/
compounds
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-Ex. in a clinical setting, the selectivity will be very low when they are trying to detect
overdose, but in forensic they have to be very high when detecting what kind of
drug killed someone
-Sensitivity and selectivity may be interdependent, therefore increasing the selectivity
of a method may lead to reduced background signal -> increased sensitivity
Principles of Spectroscopy
-Spectroscopy typically involves the measurement of the interaction between matter
and radiation
-The radiation may be the form of light, electrons or magnetic fields
-Most forensic application involve the interaction between matter and light or between
matter and electronic radiation
-A common form of spectroscopy examines the absorption of light by chemical
constituents in a mixture
Types of Optical Spectroscopy
-UV-Vis absorbance spectroscopy
-IR absorbance spectroscopy (IR)
-Atomic absorption spectroscopy (AAS)
-Atomic/molecular fluorescence spectroscopy
UV-Vis absorbance spectroscopy
Absorbance
-Absorbance: absorption of photons released, that are of a wavelength within the UV
and visible regions of EM spectrum, which will involve promotion of electrons to
higher energy levels
-When the electron is promoted, then goes back down to a lower level, it will release
energy which can be measured
-Since energy levels are quantized, only certain wavelengths of incident light have
significant probability of absorption
-Absorption may be quantitatively characterized by Beer’s Law:
-Transmittance (T) = I (λ) /Io (λ)
-Absorbance:
-A = -log (T) = log [ I (λ) /Io (λ) ] = ƐbC
-Where
-I (λ) = intensity of light after passing through sol’n being analyzed
-Io (λ) = intensity of incident light, therefore light before passing through sol’n
being analyzed
-Ɛ = molar absorptivity (L / mol*cm) at a given wavelength
-b = length of path that light takes through sol’n being analyzed
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Document Summary
Analytical chemistry involves measurement of properties of a substance or system such that information about the chemical make up of that system is obtained. Qualitative analysis: some or all of the constituents of the system in question are identi ed. Quantitative analysis: the concentration (or mass) of some or all the constituents in a system are measured. Particles recovered from the hand of a suspected shooter: both. In analytical analysis, the most important part to forensics is the interpretation, which is what you will be asked to testify in court. No analytical method is capable of identifying/quantitating all analytes (the substance you are testing for) The choice of method depends on whether the analytes of interest are. Low mw (eg. ethanol) vs medium mw (eg. Low mw (eg. ethanol) vs medium mw (eg. morphine) vs high mw (eg. Low mw (eg. ethanol) vs medium mw (eg. morphine) vs high mw (eg. dna)
