Download Chapter 5 – Organic Analysis

5.3 Notes
Light & Spectrometry
Pg. 136-149
Theory of Light
 Color is a visual indication of the fact that
objects absorb certain portions of visible
light and transmit or reflect others.
 Different chemical substances absorb
certain types and certain amounts of light.
Two Models for
Explaining Light
1. Light described as a
continuous wave
2. Light described as a
stream of discrete
energy particles
Light as a continuous wave
c= λxf
 Wavelength (λ - lambda)
 Unit of measure = nanometer
 Frequency (f) refers to the speed at
which wavelengths pass a given point in
a unit of time
 Cycles per second
 Speed of light is 300 million meters per
second (c)
Light
 Visible light is identified by the
electromagnetic spectrum
 Distinguishable by wavelength and
frequencies.
 Can be described as continuous as long
as it travels space
LASER
 Light Amplification by the
Simulated Emission of Radiation
 Coherent light
 Light that has all its waves pulsating in
unison
Light as a stream
of discrete particles
 Occurs when radiation is absorbed by a
substance
 Discrete particles are known as photons
each with a definite amount of energy
 E = hf (E energy, f is frequency)
 h = Planck’s constant  6.6262 x 10-34 J/s
Spectrophotometry
 Measure the quantity of radiation that a
particular material absorbs as a function of
wavelength or frequency
 The invisible radiations of the electromagnetic
spectrum.
 This absorption by chemical substances is
selective because different materials have
different energy requirements and
therefore absorb at different frequencies
Spectrophotometer
 An instrument that measure absorption
spectrum of a chemical substance
 Produces a graph that shows the
absorption of UV, visible, and IR radiations
 Some radiation reflected so not all is
absorbed
 Just how much is absorbed depends on
the concentration of the absorbing
substance
 Used to obtain qualitative data.
Beer’s Law
 The relationship exists between absorption
and concentration
A = kc
 Where A is the absorption or quantity of light,
c is the concentration of the absorbing
material and k is a proportionality constant
FT-IR
 Fourier transport infrared spectrometer
using a Michelson interferometer
 The current laboratory approach.
 It uses a prism and two mirrors to direct
light to a sample
 As light passes through the sample it is
detected by an instrument that measures all
wavelengths simultaneously
 Then uses a mathematical operation (FT) to
decode the signals and record the
wavelength.
FT-IR
 It is calculated by a computer and
prepares a printout.
 Samples have been dissolved in a
solvent.
 Glass cells used to measure the visible
region
 Quartz to measure the UV region
 Sodium chloride or potassium bromide
used to measure the rest
UV & Visible
Spectrophotometry
 measures the absorbance of UV and
visible light based on wavelength or
frequency.
 Heroin has a wavelength of 278 nm.
 Sugar and starch are often the dilutants for
heroin and do not absorb UV light.
Absorption in IR Region
 is more specific and can be the equivalent
of a fingerprint based on the spectra.
 Thousands of organic compounds have been
indexed and catalogued.
GC-Mass Spec
 Use of gas chromatography and mass
spectrometry can now be used identify
specifically
 Substance is passed through a GC then
flows into a Mass Spec
 Material is ionized and decomposes
 Smaller fragments are separated by their
masses
 NO TWO SUBSTANCES PRODUCE THE
SAME FRAGMENTED PATTERN.
In-Class Assignment/Homework