Lecture 1 Spectroscopy

8/11/2019 Lecture 1 Spectroscopy

1/29

What is Spectroscopy?

Spectroscopy is the study of the interaction between matter and radiated

energy. Historically, spectroscopy originated through the study of visible light

dispersed according to its wavelength, e.g., by a prism. Later the concept was

expanded greatly to comprise any interaction with radiative energy as a

function of its wavelength or frequency.

Spectrometry is the spectroscopic technique used to assess the

concentration or amount of a given chemical (atomic, molecular, or ionic)

species. In this case, the instrument that performs such measurements is a

spectrometer, spectrophotometer, or spectrograph.

Spectroscopy/spectrometryis often used in physical and analytical chemistry

for the identification of substances through the spectrum emitted from or

absorbed by them.
http://www.news-medical.net/health/Spectroscopy-What-is-Spectroscopy.aspxhttp://en.wikipedia.org/wiki/Matterhttp://en.wikipedia.org/wiki/Radiationhttp://en.wikipedia.org/wiki/Radiationhttp://en.wikipedia.org/wiki/Visible_spectrumhttp://en.wikipedia.org/wiki/Wavelengthhttp://en.wikipedia.org/wiki/Prism_(optics)http://en.wikipedia.org/wiki/Frequencyhttp://en.wikipedia.org/wiki/Frequencyhttp://en.wikipedia.org/wiki/Prism_(optics)http://en.wikipedia.org/wiki/Wavelengthhttp://en.wikipedia.org/wiki/Visible_spectrumhttp://en.wikipedia.org/wiki/Visible_spectrumhttp://en.wikipedia.org/wiki/Visible_spectrumhttp://en.wikipedia.org/wiki/Radiationhttp://en.wikipedia.org/wiki/Radiationhttp://en.wikipedia.org/wiki/Radiationhttp://en.wikipedia.org/wiki/Matterhttp://www.news-medical.net/health/Spectroscopy-What-is-Spectroscopy.aspx


2/29

Electromagnetic waves are typically described by any of the following

three physical properties: the frequencyf, wavelength, or photonenergy

E.

What are electromagnetic waves?

Electromagnetic waves are formed when an electric field

(shown as blue arrows) couples with a magnetic field (shown

as red arrows). The magnetic and electric fields of an

electromagnetic wave are perpendicular to each other and to

the direction of the wave. James Clerk Maxwell and Heinrich

Hertz are two scientists who studied how electromagneticwaves are formed and how fast they travel.

Electricity can be static, like what holds a balloon to the wall or

makes your hair stand on end.

Magnetism can also be static like a refrigerator magnet. But

when they change or move together, they make waves -

electromagnetic waves.
http://en.wikipedia.org/wiki/Frequencyhttp://en.wikipedia.org/wiki/Wavelengthhttp://en.wikipedia.org/wiki/Lambdahttp://en.wikipedia.org/wiki/Photonhttp://en.wikipedia.org/wiki/Energyhttp://science.hq.nasa.gov/kids/imagers/ems/consider.htmlhttp://science.hq.nasa.gov/kids/imagers/ems/consider.htmlhttp://science.hq.nasa.gov/kids/imagers/ems/consider.htmlhttp://science.hq.nasa.gov/kids/imagers/ems/consider.htmlhttp://en.wikipedia.org/wiki/Energyhttp://en.wikipedia.org/wiki/Photonhttp://en.wikipedia.org/wiki/Lambdahttp://en.wikipedia.org/wiki/Wavelengthhttp://en.wikipedia.org/wiki/Frequency


3/29

(a) and (b) represent two waves that are traveling at thesame speed.

In (a) the wave has long wavelength and low frequency

In (b) the wave has shorter wavelength and higher frequency

Frequency


4/29

The electromagnetic spectrum is the range of all possible frequencies of

electromagnetic radiation. The "electromagnetic spectrum" of an object is the

characteristic distribution of electromagnetic radiation emitted or absorbed

by that particular object.

Electromagnetic spectrum

Electromagnetic radiation(EM radiationor EMR) is a form of energyemitted

and absorbed by charged particles, which exhibits wave-like behavior as it

travels through space. EMR has both electricand magnetic fieldcomponents,which oscillate in phase perpendicular to each other and perpendicular to

the direction of energy and wave propagation. In vacuum, electromagnetic

radiation propagates at a characteristic speed, the speed of light.

Electromagnetic radiation
http://en.wikipedia.org/wiki/Spectrumhttp://en.wikipedia.org/wiki/Electromagnetic_radiationhttp://en.wikipedia.org/wiki/Energyhttp://en.wikipedia.org/wiki/Electric_fieldhttp://en.wikipedia.org/wiki/Magnetic_fieldhttp://en.wikipedia.org/wiki/Oscillatehttp://en.wikipedia.org/wiki/Wave_propagationhttp://en.wikipedia.org/wiki/Vacuumhttp://en.wikipedia.org/wiki/Speed_of_lighthttp://en.wikipedia.org/wiki/Speed_of_lighthttp://en.wikipedia.org/wiki/Speed_of_lighthttp://en.wikipedia.org/wiki/Speed_of_lighthttp://en.wikipedia.org/wiki/Speed_of_lighthttp://en.wikipedia.org/wiki/Speed_of_lighthttp://en.wikipedia.org/wiki/Vacuumhttp://en.wikipedia.org/wiki/Wave_propagationhttp://en.wikipedia.org/wiki/Wave_propagationhttp://en.wikipedia.org/wiki/Wave_propagationhttp://en.wikipedia.org/wiki/Oscillatehttp://en.wikipedia.org/wiki/Magnetic_fieldhttp://en.wikipedia.org/wiki/Magnetic_fieldhttp://en.wikipedia.org/wiki/Magnetic_fieldhttp://en.wikipedia.org/wiki/Electric_fieldhttp://en.wikipedia.org/wiki/Energyhttp://en.wikipedia.org/wiki/Electromagnetic_radiationhttp://en.wikipedia.org/wiki/Electromagnetic_radiationhttp://en.wikipedia.org/wiki/Electromagnetic_radiationhttp://en.wikipedia.org/wiki/Spectrum


5/29


6/29


7/29


8/29


9/29


10/29

What happens to absorbed radiationIt is possible for the packet of photonic energy to be absorbed, resulting in the

promotion of one or more electrons to higher energy levels. That is, electromagnetic

radiation is absorbed by the atom, which is converted from its ground to one of manypossible excited states.

Since the energy of electrons in orbital is fixed, it should be clear that when an electron is

promoted, a very specific amount of energy is requiredcorresponding to

the energy difference between the initial orbital and the final orbital.

Note that, if the photonic energy is very high, such as might be the case with x-rays, the

electron may be totally removed from the atom, leaving an ion in its place.

Atoms generally do not stay in an excited state and they tend to relax to their ground

states as quickly as possible. In doing so they must emit their excess energy as the electron

falls to a lower orbital.

The end result is that the atoms emit their excess energy once again as light. And the

light they radiate will correspond to a very specific set of photon energies.


11/29


12/29

Sigma and Pi orbitals


13/29

Electron transitions


14/29

Electron transitions


15/29

Roles of Chromophore and Auxochrome on absorption

A chromophoreis the part of a molecule responsible for its color. The color arises

when a molecule absorbs certain wavelengths of visible light and transmits or

reflects others. Example- nitro, azo group etc.

A molecule containing a chromophore is called chomogen.

An auxochromeis a group of atoms attached to a chromophore which modifies

the ability of that chromophore to absorb light or intensify the color.

There are mainly two types of auxochromes:

Acidic-COOH, -OH, -SO3H

Basic-NHR, -NR2, -NH2
http://en.wikipedia.org/wiki/Acidichttp://en.wikipedia.org/wiki/Carboxylhttp://en.wikipedia.org/wiki/Sulfonatehttp://en.wikipedia.org/wiki/Sulfonatehttp://en.wikipedia.org/wiki/Sulfonatehttp://en.wikipedia.org/wiki/Base_%28chemistry%29http://en.wikipedia.org/wiki/Base_%28chemistry%29http://en.wikipedia.org/wiki/Sulfonatehttp://en.wikipedia.org/wiki/Sulfonatehttp://en.wikipedia.org/wiki/Sulfonatehttp://en.wikipedia.org/wiki/Sulfonatehttp://en.wikipedia.org/wiki/Carboxylhttp://en.wikipedia.org/wiki/Carboxylhttp://en.wikipedia.org/wiki/Acidic


16/29

Group Structure nm

Carbonyl > C = O 280

Azo -N = N- 262

Nitro -N=O 270

Thioketone -C =S 330

Nitrite -NO2 230

Conjugated Diene -C=C-C=C- 233

Conjugated Triene -C=C-C=C-C=C- 268

Conjugated Tetraene -C=C-C=C-C=C-C=C- 315

Benzene 261

Chromophoric Structure


17/29

Beer Lambert Law

b

c

When


18/29

Exponential functions look somewhat similar to functions you have seen before, in that

they involve exponents, but there is a big difference, in that the variable is now the

power, rather than the base. Previously, you have dealt with such functions asf(x) = x2

,where the variable xwas the base and the number 2was the power.

In the case of exponentials, however, you will be dealing with functions such as g(x) =

2x, where the base is the fixed number, and the power is the variable.

Exponential functions

Let's look more closely at the function g(x) = 2x. To evaluate this

function, we operate as usual, picking values of x, plugging themin, and simplifying for the answers. But to evaluate 2x, we need to

remember how exponents work. In particular, we need to

remember that negative exponentsmean "put the base on the

other side of the fraction line".

So, while positivex-values give us values

like these:
http://www.purplemath.com/modules/exponent2.htmhttp://www.purplemath.com/modules/exponent2.htmhttp://www.purplemath.com/modules/exponent2.htmhttp://www.purplemath.com/modules/exponent2.htm


19/29

Beer Lambert Law


20/29

Transmittance, T = P / P0

% Transmittance, %T = 100 T

Beer Lambert Law

a=absorptivity,

= molar absorptivity

A= b c


21/29

The relationship between absorbance and transmittance is illustrated in the

following diagram:

Relationship between absorbance and transmittance

So, if all the light passes through a solution withoutany absorption, then

absorbance is zero, and percent transmittance is 100%. If all the light isabsorbed, then percent transmittance is zero, and absorption is infinite.


22/29


23/29

Why do we prefer to express the Beer-Lambert law using absorbance as a

measure of the absorption rather than %T ?

Answer :To begin, let's think about the equations...

A=abc%T = 100 P/P0= e

-abc

Now, suppose we have a solution of copper sulphate (which appears blue because

it has an absorption maximum at 600 nm). We look at the way in which the

intensity of the light (radiant power) changes as it passes through the solution in

a 1 cm cuvette. We will look at the reduction every 0.2 cm as shown in the diagram

below. The Law says that the fraction of the light absorbed by each layer ofsolution is the same. For our illustration, we will suppose that this fraction is 0.5 for

each 0.2 cm "layer" and calculate the following data:

Path length /cm 0 0.2 0.4 0.6 0.8 1.0

%T 100 50 25 12.5 6.25 3.125

Absorbance 0 0.3 0.6 0.9 1.2 1.5


24/29

0

20

40

60

80

100

120

0 0.2 0.4 0.6 0.8 1 1.2

%T

Path length, cm

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 0.2 0.4 0.6 0.8 1 1.2

Absorb

ance

Path length, cm

A = abctells us that absorbance depends on the total quantity of the

absorbing compound in the light path through the cuvette. If we plot

absorbance against concentration, we get a straight line passing through

the origin (0,0).The linear relationship between concentration and

absorbance is both simple and straightforward, which is

why we prefer to express the Beer-Lambert law using

absorbance as a measure of the absorption rather than

%T.


25/29

Problems

P bl


26/29

Problems


27/29

The linearity of the Beer-Lambert law is limited by chemical and

instrumental factors. Causes of nonlinearity include:

-deviations in absorptivity coefficients at high concentrations

(>0.01M) due to electrostatic interactions between molecules in

close proximity

-scattering of light due to particulates in the sample

-fluoresecence or phosphorescence of the sample

-changes in refractive index at high analyte concentration

-shifts in chemical equilibria as a function of concentration

-non-monochromatic radiation, deviations can be minimized by

using a relatively flat part of the absorption spectrum such as the

maximum of an absorption band

-stray light

Limitations of the Beer-Lambert law


28/29

(1) Chemical effects - analyte associates, dissociates or reacts to give

molecule with different

(2) Physical effects - stray light, polychromatic radiation or noise

non-linear calibration curve


29/29

Documents

Lecture 1 Spectroscopy