Lecture 4_Introduction to Spectroscopy

8/3/2019 Lecture 4_Introduction to Spectroscopy

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An Introduction to Spectroscopy

Highlights

Remote sensing: light interactions with matter (e.g.NASA Terra).

What is spectroscopy?How is spectroscopy used in Analytical Chemistry?

The properties of light

Analysis based on emission by analyte

Analysis based on absorption by analyte


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Spectroscopy

Using Remote sensing to obtain plant life info on earththrough light interactions with matter (e.g. NASATerra).

What is spectroscopy? =Measurement & interpretation

of light absorbed or emitted by a sample.

Spectrum/Spectra: Separation of light into variousbands/wavelengths

Spectrometer: measures the amount of light that occursin a spectrum at a particular ban or group of bands.


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Interactionsof light withmatter toproduceuseful

informatione.g.

vegetationcover on

earth


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Decreased intensity b/c of uptake oflight by water vapor, CO2, and ozonein the air.

Spectra of light emitted by the sun and for sunlightthat has passed through the atmosphere


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How is spectroscopy used in Analytical Chemistry?

Different types of spectroscopy instruments (next slide)Qualitative & Quantitative chemical analysis

Spectroscopy subdivided based on the type of analytesexamined

Molecular spectroscopyAtomic spectroscopy

Spectroscopy subdivided based on the type of lights orradiation they employ

See Table (next 2 slides)


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TMS: Trimethylsilane (reference sample)

Other techniques:13

C,15

N,13

P,19

F

Typical NMR spectrum of methanol in D2O


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Sir Isaac Newton (1642-1727)Drawing in notebook: Separation of sunlight into

distinct colors (ROYGBIV) with a glass prism

Type of light emitted is related to the chemical composition of material


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What is light?

Modern scientists define light as electromagneticradiation

i.e. a wave of energy that propagates through space

with both electrical and magnetic components2 ways to look at light (wave-particle duality)

Light=Properties of a wave

Light=distinct particles of energy


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Wave can pass through a vacuum or other transmitting media such as air, water, glass

The wave nature of light

Wave ofenergy thatmoves throughspace

Oscillating

electric fieldperpendicularto anoscillatingmagnetic field

Low intensity

High intensity


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Properties of light as a wave

1: Velocity, 2: frequency, 3: wavelength

Velocity (c)that the light is traveling (c =299,792,458 m/s~ 3.00 x 108 m/s in true vacuum or air)

The velocity of light other than vacuum = vThe ratio of c and v = refractive index, n

Refractive index, n: Is a ratio of the speed of light invacuum relative to that in the considered medium.

n = c/v


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Refractiveindex ischaracteristic

of a givenmaterial;

n = no unit


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Calculation CornerQuestion: Sir Isaac Newton separated sunlight into various colors by using a

glass prism. The material used in one type of glass prism has a refractiveindex of approximately 1.61 for red light and 1.65 for blue light. What isthe velocity for each of these types of light as they pass through this prism?

n =c/v; then v = c/n

Red light:

v =(3.00 x 108 m/s)/(1.61) = 1.86 x 108 m/s

Blue light:

v =(3.00 x 108 m/s)/(1.65) = 1.82 x 108 m/s


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Properties of light as a wave

2: frequency, 3: wavelength

Wavelength and frequency are related thus:

Frequency, v = c/(in a vacuum)Or

v = v/(in any medium)


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The Particle Nature of Light

A different view first suggested by Isaac NewtonHe proposed that light was composed of small particles

that moved at great speeds.

Photoelectric effect by Albert Einstein in 1905: Electronswere ejected when particles of light hit the surface ofcertain materials

Photon is used to describe these individual particles oflight.

The energy of a single photon of light (EPhoton) to its

frequency (v) using Max Plancks equation:

Ephoton= hvh = Plancks constant = 6.626 x 10-34JS

For true vacuum: Ephoton

= hc/


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Uptake and Release of Light by Matter

Light can be released or taken up by matter

Interactions ofsunlight withearth during

remote sensing


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Light can be released or taken up by matter

Emission of light: The release of light by matter (atom,ion, or molecule goes from an excited state to a lower

energy state)

Light emitted is characteristic of the element and related to intensity

and concentration


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Absorption of light: Transfer of energy from anelectromagnetic field to a chemical entity (atom ormolecule).

The total amount of light= transmitted + absorbed


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Absorption Spectra of Chlorophyll a and b


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Lecture 4_Introduction to Spectroscopy

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