SPECTROSCOPY

Introduction of Spectrometric Analyses The study of how the chemical compound interacts with different wavelengths in a given region of electromagnetic radiation is called spectroscopy or spectrochemical analysis. The collection of measurements signals (absorbance) of the compound as a function of electromagnetic radiation is called a spectrum.

Energy Absorption The fundamental principle is the absorption of certain amount of energy. The energy required for the transition from a state of lower energy to a state of higher energy is directly related to the frequency of electromagnetic radiation that causes the transition.

THE NATURE OF LIGHT Light is composed of electric and magnetic fields, which are mutually perpendicular and which radiate out from a source It is therefore a form of electromagnetic radiation. Electric and magnetic fields are propagated through space as wave functions which may be characterized by wavelength, λ (the distance from one part of the wave to the corresponding position on the next wave) and frequency, ν (the number of times a wave passes through a fixed point in space every second).

This relationship can be rearranged ; λ = c / ν , this relationship means that the frequency and wave length are inversely related ( higher frequency means shorter wave length). thus a red light color of wave length of 650nm and a green color of 540nm wave length , the red light has a higher wave length thus it has a lower frequency) . As for energy: the light with the highest energy will be the one with the highest frequency - that will be the one with the smallest wavelength. Light of each color has a different wavelength - blue light has a shorter wavelength than red light. Blue light therefore has a larger number of peaks per unit of length thus it has higher frequency and larger energy.

The Electromagnetic Spectrum

Electronic Excitation The absorption of light energy by organic compounds in the visible and ultraviolet region involves the promotion of electrons in , , and n-orbitals from the ground state to higher energy states. This is also called energy transition. These higher energy states are molecular orbitals called antibonding.

Electronic Molecular Energy Levels The higher energy transitions ( *) occur a shorter wavelength and the low energy transitions (*, n *) occur at longer wavelength.

Chromophore is a functional group which absorbs a characteristic ultraviolet or visible region. UV 210 nm Double Bonds 233 nm Conjugated Diene 268 nm Conjugated Triene 315 nm Conjugated Tetraene

Spectrophotometer An instrument which can measure the absorbance of a sample at any wavelength.

The essential components of spectrophotometer Collimator Cuvette sample container Light’s band λ1 λ2 λ3 λ4 Photometer or detector Light source Monochromator Prism Photocell Slit Wavelength selector

Components of a Spectrophotometer 1-Light Source Deuterium Lamps－a truly continuous spectrum in the ultraviolet region (160nm~375nm) Tungsten Filament Lamps－the most common source of visible and near infrared radiation.

Components of a Spectrophotometer 2-Monochromator Used as a filter: the monochromator will select a narrow portion of the spectrum (the bandpass) of a given source Used in analysis: the monochromator will sequentially select for the detector to record the different components (spectrum) of any source or sample emitting light

Single and Double Beam Spectrometer Single-Beam: There is only one light beam or optical path from the source through to the detector. Double-Beam: The light from the source, after passing through the monochromator, is split into two separate beams-one for the sample and the other for the reference.```````````

Single Beam Spectrophotometer Detector Lamp Lens cuvette

Dual Beam Spectrophotometer

Sample Cells (Quvettes) UV Spectrophotometer Quartz (crystalline silica)  Visible Spectrophotometer Glass Plastic  

The Beer-Lambert law When a beam of radiation (light) passes through a substance or a solution, some of the light may be absorbed and the remainder transmitted through the sample. The ratio of the intensity of the light entering the sample (Io) to that exiting the sample (I1) at a particular wavelength is defined as the transmittance (T). z % T = (Io / I1 ) x 100 A = - log (T)

The Beer-Lambert law ε = molar absorptivity or extinction coefficient of the chromophore at wavelength λ (the optical density of a 1-cm thick sample of a 1 M solution). ε is a property of the material and the solvent. L = sample pathlength in centimeters c = concentration of the compound in the sample, in molarity (mol L-1) Absorbance A= -log(I1/I0)=ε·c·l

Transmittance The ratio of the intensities of the transmitted and incident light gives transmittance. T = I/I0 I0 is the intensity of incident radiation I is the intensity of transmitted radiation A 100% value of T represents a transparent substance whereas a zero value represents a totally opaque.

Deviation of the Beer’s law At high concentrations many molecules form dimers or higher polymers which have a spectrum that differs than their monomeric form , which could lead to either positive or negative deviation. Also at high concentrations aggregation can occur which frequently leads to scattering of light thus decreasing the amount of light transmitted and thus causing positive deviations. Deviations in absorptive coefficients at high concentrations (>0.01M) due to electrostatic interactions between molecules in close proximity. Stray light ; it is the quantity of light that reaches the detector that is of a wavelength other than that selected. Therefore, stray light causes the measured transmittance to be high , thus leading to negative deviation

Deviations

Factors affecting absorption properties of a chromophore; The spectrum of a chromophore is primarily determined by the chemical structure of the molecule. However a large number of environmental factors can cause detectable changes in λmax and ε these factors are ; pH ; The pH of the solvent determines the ionization state of the chromophore. The polarity of the solvent or or neighboring molecules. For polar chromophores the value of λmax is affected , for example the λmax For tyrosine is less in polar solvents. Orientation effects; Geometric features have strong effects on λmaxand ε The best example of such an effect is the hypochromism of nucleic acids . That is the absorption coefficient of a nucleotide decreases when the nucleotide is contained in a single-stranded polynucleotide in which the bases are in close proximity .There is a further decrease with a double stranded polynucleotide because the bases are arranged in an even more stacked array.

Steps in Developing a Spectrometric Analytical Method Run the sample for spectrum 2. Obtain a monochromatic wavelength for the maximum absorption wavelength. 3. Calculate the concentration of your sample using Beer Lambert Equation: A = ECL Wavelength (nm)

Practice Examples   1. Calculate the Molar Extinction Coefficient E at 351 nm for aquocobalamin in 0.1 M phosphate buffer. pH = 7.0 from the following data which were obtained in 1 Cm cell. Solution C x 105 M Io I A 2.23 100 27 B 1.90 100 32 2. The molar extinction coefficient (E) of compound riboflavin is 3 x 103 Liter/Cm x Mole. If the absorbance reading (A) at 350 nm is 0.9 using a cell of 1 Cm, what is the concentration of compound riboflavin in sample?  

3. The concentration of compound Y was 2 x 10-4 moles/liter and 3. The concentration of compound Y was 2 x 10-4 moles/liter and the absorption of the solution at 300 nm using 1 Cm quartz cell was 0.4. What is the molar extinction coefficient of compound Y?