5.3 Notes Light & Spectrometry Pg

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 = λ x f  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  x 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