5.3 Notes

1. 5.3 Notes Light & Spectrometry Pg. 136-149

2. 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.

3. Two Models for Explaining Light • Light described as a continuous wave • Light described as a stream of discrete energy particles

4. 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)

5. 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

6. LASER • Light Amplification by the Simulated Emission of Radiation • Coherent light • Light that has all its waves pulsating in unison

7. 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

8. 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

9. Spectrophotometer • An instrument that measure absorption spectrum of a chemical substance • Produces a graph that shows the absorption of UV, visible, and IR radiations

10. 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.

12. 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

13. 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.

14. 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

15. 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.

16. 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.

17. 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.

18. In-Class Assignment/Homework