general properties of electromagnetic.pdf

7/27/2019 General Properties of Electromagnetic.pdf

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Instrumental Methods ofChemical Analysis

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CLASSIFICATION OF ANALYTICAL METHODS

Classical vs Instrumental

Qualitative instrumental analysis is that measured property

that indicates presence of analyte in matrix

Quantitative instrumental analysis is that magnitude of

measured property that is proportional to concentration of

analyte in matrix

Species of interest

All constituents including analyte.

Often need pretreatment - chemical extraction, distillation,

separation, precipitation


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INTRODUCTION

CLASSICAL:Qualitative- identification by color, indicators, boiling points,odors

Quantitative- mass or volume (e.g. gravimetric, volumetric)

INSTRUMENTAL:Qualitative- chromatography, electrophoresis and identification by measuring

physical property (e.g. spectroscopy, electrode potential)

Quantitative- measuring property and determining relationship to concentration

(e.g. spectrophotometry, mass spectrometry). Often, same instrumentalmethod used for qualitative and quantitative analysis.

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TYPES OF INSTRUMENTAL METHODS

PROPERTY EXAMPLE METHOD

Radiation Emission Emissionspectroscopy - fluorescence,phosphorescence, luminescence

RadiationAbsorption Absorptionspectroscopy -

spectrophotometry, photometry, nuclearmagnetic resonance, electron spin

resonance

Radiation Scaterring Turbidity,Raman

Radiation Refraction Refractometry, interferometry

Radiation Diffraction X-ray, electron

Radiation Rotation Polarimetry, circulardichroism


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TYPES OF INSTRUMENTAL METHODS

PROPERTY EXAMPLE METHOD

ElectricalPotential Potentiometry

Electrical Charge Coulometry

ElectricCurrent Voltammetry- amperometry,polarography

Electrical Resistance Conductometry

Mass Gravimetry

Mass-to-chargeRatio Mass spectrometry

Rate of Reaction Stopped flow, flow injectionanalysis

Thermal Characteristics Thermal gravimetry, calorimetry

Radioactivity Activation, isotope dilution

Often combined with chromatographic or electrophoretic methods

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INSTRUMENTS FOR ANALYSIS

Block diagram for the overall process of instrumental measurement.


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General Properties of

Electromagnetic

Radiation

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The electromagnetic radiation is looked at as

sinusoidal waves which are composed of a

combination of two fields.

An electric field (which we will use, in this course,

to explain absorption and emission of radiation

by analytes)

magnetic field at right angle to the electric field

(which will be used to explain phenomena like

nuclear magnetic resonance in the course of

special topics in analytical chemistry offered toChemistry students only).

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The classical wave model

The classical wave model describes

electromagnetic radiation as waves that have a

wavelength, frequency, velocity, and amplitude.

These properties of electromagnetic radiation can

explain classical characteristics of

electromagnetic radiation like reflection,

refraction, diffraction, interference, etc.

However, the wave model can not explain the

phenomena of absorption and emission of

radiation.


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We will only deal with the electric field of the

electromagnetic radiation and will thus

refer to an electromagnetic wave as an

electric field having the shape of a

sinusoidal wave.

The arrows in the figure below represent few

electric vectors while the yellow solid

sinusoidal wave is the magnetic fieldassociated with the electric field of the

wave.

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Wave Properties of

Electromagnetic Radiation


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Wave Parameters

1. Wavelength ()

The wavelength of a wave is the distance

between two consecutive maxima or two

consecutive minima on the wave. It can

also be defined as the distance between

two equivalent points on two successive

maxima or minima

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2. Ampli tude (A)

The amplitude of the wave is represented by

the length of the electrical vector at a

maximum or minimum in the wave.

In the figure above, the amplitude is the

length of any of the vertical arrows

perpendicular to the direction of

propagation of the wave.

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3. Frequency

The frequency of the wave is directly proportional to the

energy of the wave and is defined as the number of

wavelengths passing a fixed point in space in one

second.

4. Period (p)

The period of the wave is the time in seconds required

for one wavelength to pass a fixed point in space.


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5. Velocity (v)

The velocity of a wave is defined as the

multiplication of the frequency times the

wavelength. This means:

V =

The velocity of light in vacuum is greater

than its velocity in any other medium

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Since the frequency of the wave is a

constant and is a property of the source

The decrease in velocity of electromagnetic

radiation in media other than vacuum

should thus be attributed to a decrease in

the wavelength of radiation upon passage

through that medium.


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6. Wavenumber ()

The reciprocal of wavelength in centimeters

is called the wavenumber. This is an

important property especially in the study

of infrared spectroscopy.

= k


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Electromagnetic Spectrum

The electromagnetic radiation covers a vast

spectrum of frequencies and wavelengths. This

includes the very energetic gamma-rays

radiation with a wavelength range from 0.005

1.4 Ao to radiowaves in the wavelength range up

to meters (exceedingly low energy). However,

the region of interest to us in this course is rather

a very limited range from 180-780 nm. This

limited range covers both ultraviolet and visibleradiation.

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Mathematical Description of a

WaveA sine wave can be mathematically represented by theequation:

Y = A sin (t + )

Where y is the electric vector at time t, A is the amplitude ofthe wave, is the angular frequency, and is the phaseangle of the wave.

The angular frequency is related to the frequency ofradiation by the relation:

= 2

This makes the wave equation become:

Y = A sin (2t+ )

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Molecular Energy Levels

Molecules can have the following types of energyKinetic (due to motion)

Electronic (PE and KE of electrons)

Vibrational(oscillation of atoms in bonds)

Rotational

All except the KE are quantized

Emolecule

= Erotational

+ Evibrational

+ Eelectronic


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Molecular Energy Levels

Rotational

Energy Levels

Vibrational

Energy Levels

Ground

Electronic

State

Excited

ElectronicState

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Rotational Energy of a Diatomc Molecule

This type of Energy is associated with the overall rotation of the molecules with theatoms considered as fixed point mass

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A molecular vibration occurs when atoms in

a molecule are in periodic motion while the molecule as

a whole has constant translational and rotational motion.

The frequency of the periodic motion is known as a

vibration frequency, and the typical frequencies of

molecular vibrations range from less than 1012 to

approximately 1014 Hz.

Vibrational Energy

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