PHAN SPECTROPHOTOMETRY

Spectroscopy

Interaction of Radiation with a sample

Spectroscopy

The study of molecular or atomic structure of a substance by observation of

its interaction with electromagnetic radiation

Spectroscopy

measurement of spectra

QUANTITATIVELY

Spectroscopy For determining the amount of material in a sample

QUALITATIVELY

Spectroscopy For identifying the chemical structure of a sample

Spectrometry

Radiation reacts with matter àreflected, scattered, absorbed, fluoresce and cause photochemical reaction like bond breaking.

Spectrometry

• Position (wavelength) in the spectrum where interaction takes place

• Measurement of these interactions

Light

is electromagnetic radiation within a certain portion of the electromagnetic spectrum.

Light

The word usually refers to visible light, which is visible to the humaneye and is responsible for the sense of sight.

wavelength (UNITS: micrometer, nanometer, angstrom)

The length of a complete wave or cycle from peak of wave to the peak of the next

Wavenumber

number of waves per centimeter, equivalent to 1/wavelength(cm)

number of waves per centimeter, equivalent to 1/wavelength(cm)

Wavenumber is the

FREQUENCY

complete cycle which pass a given point per second (units: cps/ hertz)

cps/ hertz (1 hertz = 1cps)

units of FREQUENCY

Radiation Energy

The strength of the radiation energy will interact with the molecules in different ways

X-Ray, γ Rays,…

High energy sources produce breaking of bonds

UV / VISIBLE Spectroscopy

Medium energy sources excite electrons

Infrared Energy

Low energy sources produce vibrations in chemical bonds

Microwaves and Radio waves

Very low energy sources produce rotation of the chemical bonds

optical methods

are physicalchemical methods of analysis that are based on the optical range of the scale of electromagnetic waves

220 – 380 nm

range of wavelength of UV

380 – 780 nm

range of wavelength of Visible

780 – 3000 nm

range of wavelength of Near IR

3.0 – 15 um

range of wavelength of Medium

15 – 300 um

range of wavelength of Far IR

cm, µm (10-4 cm) , nm (10-7 cm), Å (10-8cm)

The ranges of the wavelength of radiant energy are measured in

Radio waves

Microwaves

Infrared rays

Visible light

Ultraviolet rays

X-rays

Gamma rays

Types of Radiation from Low to High frequency

Spectrophotometry

is a branch of spectrometry which embraces the measurement of the absorption by chemical species of radiant energy of definite and narrow wavelength approximating monochromatic radiation.

Colorimetry

is branch of spectrophotometry in which absorption takes place in the visible region

VISIBLE SPECTROSCOPY

Absorbance and reflectance

When a sample only absorbs light of a single wavelength the eye sees COMPLEMENTARY colours.

WHITE

Visible Spectroscopy
If the material completely REFLECTS all light it appears

GREY

Visible Spectroscopy

If the material absorbs a constant fraction of the light across the spectrum it appears

BLACK

Visible Spectroscopy

If the material completely ABSORBS all the light it appears

What is the fundamental principle regarding how substances interact with radiant energy?

Selective Absorption. Every substance absorbs or transmits certain wavelengths but not others, depending on its molecular structure.

Chromophore

functional group which absorbs radiant energy in the UV or Vis regions

Ethylene

Ketones

Aldehydes

Acetylene

Organic acids

Azomethines

Example of chromophore

concentration

Intensity of the transmitted radiant energy is a function of the

transverse wave

Light transmitted is in the form of ___ at an enormous velocity

wave

Energy maybe transmitted as

Amplitude

is the distance of half height of the wave

Wavenumber (ṽ)

is the number of waves per centimeter and is equal to 1/wavelength (cm).

photon

Energy maybe absorbed as

photons or quanta

Radiant energy absorbed is in the form of discrete parcels of energy known as

frequency of the radiation

The energy E of photon depends upon the

increases

As frequency (ν) increases, energy (E) of light

Electronic (UV, Vis)

Energy can be transferred to the ff forms
Associated with the overall motion of electrons around the nuclei

Vibrational (IR)

Energy can be transferred to the ff forms
Associated with the motion of atoms within the molecule

Rotational (IR, Microwave)

Energy can be transferred to the ff forms
Associated with the overall rotation of the molecule

Translational (not quantized)

Energy can be transferred to the ff forms
Associated with the motion of the molecule as a whole

Absorbed energy

A molecule may exist only in certain permitted energy states and cannot possess any arbitrary amount of energy

Electronic energy (UV, Vis)

Involves the transfer of electrons between different electronic state à ground state to the excited state

fluorescence

When UV is absorbed an electron is raised from the ground state to excited state, loses its vibrational energy, decays to ground state, emitting UV or visible light →

phosphorescence

When the transition back to the ground state is delayed it results to

Stretching

distance between two atoms increases or decreases

Bending (deformation)

position of the atom changes relative to the original bond axis

Transmittance

Defined as the ratio of the intensity of light emerging from the solution (I) to that of incident light entering (Io)

Absorbance

Also called optical density, absorbancy, extinction coefficient

Absorbance α concentration graph

Transmittance 1/ α to concentration and absorbance

Beer’ s Law

states that the power of a transmitted radiant beam decreases exponentially as the concentration of the solution containing the absorbing chemical species increases arithmetically.

Beer’ s Law

Absorbance is directly proportional to concentration

Lambert or Bouguer’s Law

states that the power of a transmitted radiant beam decreases exponentially as the thickness of the solution containing the absorbing chemical species increases arithmetically

Beer–Lambert or Beer–Bouguer’ s Law

is a combination of the above law and relates the power of the incident and the transmitted radiant beam to the thickness and concentration of the solution containing the absorbing chemical species.

Spectrophotometer

These instruments may either be manually operated or automatic recording instruments
Instruments have

1.) radiant power source

2.) radiant energy dispersing device

3.) a sample compartment

4.) the associated electronics which permit the measurement of radiant power transmitted by the sample.

SA curve

spectral absorbance curve, plot of absorbance values against wavelength

Beer’s plot

plot of absorbance values against a series of known solute concentration