5.3 Notes5.3 NotesLight & SpectrometryLight & Spectrometry

Pg. 136-149Pg. 136-149

Theory of LightTheory of Light Color is a visual indication of the fact that Color is a visual indication of the fact that

objects absorb certain portions of visible objects absorb certain portions of visible light and transmit or reflect others. light and transmit or reflect others.

Different chemical substances absorb Different chemical substances absorb certain types and certain amounts of light.certain types and certain amounts of light.

Two Models for Two Models for Explaining LightExplaining Light

1.1. Light described as a Light described as a continuous wavecontinuous wave

2.2. Light described as a Light described as a stream of discrete stream of discrete energy particlesenergy particles

Light as a continuous waveLight as a continuous wavec = c = λ λ x f x f

Wavelength (Wavelength (λλ - - lambdalambda))Unit of measure = nanometerUnit of measure = nanometer

Frequency (Frequency (ff) refers to the speed at ) refers to the speed at which wavelengths pass a given point in which wavelengths pass a given point in a unit of time a unit of time Cycles per secondCycles per second

Speed of light is 300 million meters per Speed of light is 300 million meters per second (second (cc))

LightLight Visible light is identified by the Visible light is identified by the

electromagnetic spectrumelectromagnetic spectrum Distinguishable by wavelength and Distinguishable by wavelength and

frequencies.frequencies. Can be described as continuous as long Can be described as continuous as long

as it travels spaceas it travels space

LASERLASER

LLight ight AAmplification by the mplification by the SSimulated imulated EEmission of mission of RRadiationadiation

Coherent light Coherent light Light that has all its waves pulsating in Light that has all its waves pulsating in

unisonunison

Light as a stream Light as a stream of discrete particlesof discrete particles

Occurs when radiation is absorbed by a Occurs when radiation is absorbed by a substancesubstance

Discrete particles are known as photons Discrete particles are known as photons each with a definite amount of energyeach with a definite amount of energy

E = hfE = hf ( (EE energy, energy, f f is frequency) is frequency) h = Planckh = Planck’’s constants constant 6.6262 x 106.6262 x 10-34-34 J/s J/s

SpectrophotometrySpectrophotometry

Measure the quantity of radiation that a Measure the quantity of radiation that a particular material absorbs as a function of particular material absorbs as a function of wavelength or frequencywavelength or frequency The invisible radiations of the electromagnetic The invisible radiations of the electromagnetic

spectrum.spectrum. This absorption by chemical substances is This absorption by chemical substances is

selective because different materials have selective because different materials have different energy requirements and therefore different energy requirements and therefore absorb at different frequenciesabsorb at different frequencies

SpectrophotometerSpectrophotometer

An instrument that measure absorption An instrument that measure absorption spectrum of a chemical substancespectrum of a chemical substance

Produces a graph that shows the Produces a graph that shows the absorption of UV, visible, and IR radiations absorption of UV, visible, and IR radiations

Some radiation reflected so not all is Some radiation reflected so not all is absorbedabsorbed

Just how much is absorbed depends on Just how much is absorbed depends on the concentration of the absorbing the concentration of the absorbing substancesubstance

Used to obtain qualitative data.Used to obtain qualitative data.

BeerBeer’’s Laws Law The relationship exists between absorption The relationship exists between absorption

and concentrationand concentrationA = kcA = kc

Where Where AA is the absorption or quantity of light, is the absorption or quantity of light, cc is the concentration of the absorbing is the concentration of the absorbing material and material and kk is a proportionality constant is a proportionality constant

FT-IRFT-IR Fourier transport infrared spectrometer Fourier transport infrared spectrometer

using a Michelson interferometerusing a Michelson interferometer The current laboratory approach. The current laboratory approach.

It uses a prism and two mirrors to direct It uses a prism and two mirrors to direct light to a samplelight to a sample As light passes through the sample it is As light passes through the sample it is

detected by an instrument that measures all detected by an instrument that measures all wavelengths simultaneouslywavelengths simultaneously

Then uses a mathematical operation (FT) to Then uses a mathematical operation (FT) to decode the signals and record the wavelength. decode the signals and record the wavelength.

FT-IRFT-IR It is calculated by a computer and It is calculated by a computer and

prepares a printout. prepares a printout. Samples have been dissolved in a Samples have been dissolved in a

solvent.solvent. Glass cells used to measure the visible Glass cells used to measure the visible

regionregion Quartz to measure the UV regionQuartz to measure the UV region Sodium chloride or potassium bromide Sodium chloride or potassium bromide

used to measure the restused to measure the rest

UV & Visible UV & Visible SpectrophotometrySpectrophotometry

measures the absorbance of UV and measures the absorbance of UV and visible light based on wavelength or visible light based on wavelength or frequency. frequency. Heroin has a wavelength of 278 nm. Heroin has a wavelength of 278 nm. Sugar and starch are often the dilutants for Sugar and starch are often the dilutants for

heroin and do not absorb UV light.heroin and do not absorb UV light.

Absorption in IR RegionAbsorption in IR Region is more specific and can be the equivalent is more specific and can be the equivalent

of a fingerprint based on the spectra. of a fingerprint based on the spectra. Thousands of organic compounds have been Thousands of organic compounds have been

indexed and catalogued.indexed and catalogued.

GC-Mass SpecGC-Mass Spec Use of gas chromatography and mass Use of gas chromatography and mass

spectrometry can now be used identify spectrometry can now be used identify specificallyspecifically

Substance is passed through a GC then Substance is passed through a GC then flows into a Mass Specflows into a Mass Spec Material is ionized and decomposesMaterial is ionized and decomposes Smaller fragments are separated by their massesSmaller fragments are separated by their masses NO TWO SUBSTANCES PRODUCE THE SAME NO TWO SUBSTANCES PRODUCE THE SAME

FRAGMENTED PATTERN.FRAGMENTED PATTERN.

In-Class Assignment/HomeworkIn-Class Assignment/Homework