بسم الله الرحمن الرحيم
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Marine Analytical Chemistry MC 361 (Chem 312)
Contents • Introduction to Spectroscopy • Concept of Spectroscopy • Infrared Absorption • Ultraviolet Molecular Absorption Spectroscopy • Colorimetry (Spectrophotometry) • Emission Spectrography (Flame Photometery) • Total Organic Carbon (TOC)
Chapter 11-Introduction to Spectroscopy • The Interaction between energy and matter: • In case of visible light
What is Radiant Energy? • Wave?? Or Small Particles?? • Properties: Wave like character • λ = wavelength γ = frequency • Energy E = hγ • h=Planck constant (6.6256 x 10-27 erg/sec) • γ= C/λ C= speed of light • E = h C/λ
Both chemical structures and the arrangement of the molecules affect the way in which any given material interacts with energy.
How does radiant energy interact with matter? A given molecule can absorb only radiation of a certain definite frequency. Red, blue and yellow light not just a single wavelength or frequency. Given molecule can exist only in certain well-defined energy state.
Energy levels are not continuous. • Quantized? The energy difference between two defined energy levels is fixed. • The molecules or atoms of same chemical species absorbsame frequency. • The molecules or atoms of different chemical species absorbdifferent frequency • The uniqueness of the frequencies at which a given molecular species absorbs is the basis of absorption spectroscopy.
Absorption Caused by Electronic Transistors in Molecules • The vibrational and rotational energies of the molecules are added to the electronic energies. • The difference in vibrational or rotational energy leads to the absorption of energy over widefrequency range.
In the case of molecules • Not all molecules fluorescence, and very few phosphorescence. • Fluorescence intensity is very high. • Can detect very small conc. of certain compound
Methods of Excitation of Atoms • 1-Electronic Discharge • Putting the sample in an electric discharge between two electrodes. • The sample breaks down into excited atoms • All different atoms emit their emission spectrum.
2-Flame Excitation • This is the basis of flame photometry. • Flame energy is lower than that of an electrical discharge • Fewer transitions are possible-Fewer spectrum lines.
3-Excitation by Radiation • Sample absorb uv and reemit fluorescence. • This the bases of Molecular Fluorescence (RF). • Phosphorescence: • If the e changes its direction of spin before returning to the G.S. emission will be difficult (forbidden). • The radiation takes place over a long period of time.
1-Lambert׳s Law • A = a b • A = absorbance • a = absorptivity of the liquid • b = optical path length • a b = log I0/I1 • I1 = I0 10 -ab • I0 = I1 10 ab • Relationship between absorbance and optical path length
2- Beer΄s Law • A = a c • c = concentration of solution. • a c = log I0/I1 • I1 = I0 10 -ac • I0 = I1 10 ac • Relationship between the absorbance and the concentration of the solution
3-Beer-Lambert Law • A = a b c • I0 = I1 10 abc • Linear relationship between A and c if b is constant and the radiation wavelength constant. • By measuring I1/I0 we can measure A, and we can calculate c. • Valid for low conc. But deviations are common at higher concentrations.
4-Deviation from Beer׳s Law • 1-Impurities. • 2-Chemical equilibrium. • 3-Optical slit. • 4-Dimerization. • 5- Interaction with solvent • Calibration curve will eliminate all the deviations.
Calibration Curves • Series of solutions with known concentration. • When a, b, I0 are constant • c ά –logI1 • Base line disturbed by an interfering compounds. • Can solve this problem : • 1-Mathmatically • 2-Double beam instrument (reference cell)
2-Standard Addition Method • This method used if: • 1-No suitable calibration curve. • 2-Time delay. • 3-No sufficient information on the solvent in the sample. • 4-Very low concentration. • Known conc. are added to the sample
Chapter 2Concepts of Spectroscopy It is important to know 1- The wavelength at which the sample emits or absorbs radiation. 2- The intensity of radiation or the degree of absorption. The basic design of all instruments: 1- Source of radiation (absorption) or excitation (emission). 2- Monochromatic (selecting the wavelength). 3- Sample holder. 4- Detector (measure the intensity of radiation).
a- Radiation Source • 1-Emite radiation of wavelength in the range to be studied (x-ray, infrared, ultraviolet). • 2-The Intensity in all range should be high. • 3-The intensity should not vary significantly at different wavelengths. • 4-The intensity should not fluctuate over long time intervals.
b-Monochromator • Function: disperse the radiation according to the wavelength. • 1-Prism Monochromators: • The prism bendslonger-wavelengths (red end) radiation less than it does shorter-wavelength radiation (blue end)?. • The refractive index of prism is greater for short-wavelength light than it is for long-wavelength light.
2-Grating Monochromator: • More popular than prism. • A series of parallel lines cut into a plane surface. • From 15000-30000 groves per square inch. • The more lines per square inch, the shorter λ of radiation that the grating can disperse and the greater the dispersing power. • Separation of light occurs because light of different λ is dispersed at different angles.
3-Resolution of a Monochromator • The ability to disperse radiation is called resolving power (dispersive power). • The resolving power of a prismincrease with the thickness of the prism. • The resolving power of a prism increase when the material used is improved. • The resolving power of a gratingincrease with increase the line groves.
c-Slits • Used to select the light beam after it has been dispersed by the monochromator. • The entrance slit selects a beam of light from the source. • The exit slit allow radiation from the monochromator to proceed to the sample and detector. • Only a selected λ range is permitted through the slit. • Other radiation is blocked and prevented from passing further. • The slits are kept as narrow as possible to ensure optimum resolution.
d-Detector • Measure the intensity of the radiation that falls on it. • Radiation energy is turned into electrical energy. • The amount of energy produced usually low and must be amplified. • Amplifying the signal from the detector increase its sensitivity. • If the signal is amplified too much, it becomes erratic and unsteady (noisy).
e- Uses for Single-Beam Optics • Single-beam optics are used for all spectroscopic emission methods. • The method allows the emission intensity and wavelength to be measured accurately and rapidly. • In spectroscopic absorption studies the intensity before and after passing the sample must be measured. • Any variation in the intensity lead to analytical error. (that is why double beam developed)
2-The Double-Beam System. • Used for spectroscopic absorption studies. • One very important difference from single beam: • The radiation from the source is split into two beams with equal intensity (reference beam and sample beam). • Any variation in the intensity of source I0 simultaneously decreases I1 but does not change the ratio I1/I0.
Chapter 3Infrared Absorption. • The wavelength (λ) of infrared (ir) radiation falls in the range 750 nm- 4500 nm. • The frequency (u) range: 2.2x1014-7.5x1015 cps. • Infrared radiation has less energy than visible radiation but more than radio waves.
A-Requirements for Infrared Absorption • 1-Correct Wavelength of Radiation: • Molecules absorb radiation when some part of the molecule (atom, or group) vibrates at the same frequency as the incident radiant energy. • After absorbing radiation, the molecule vibrate at an increased rate. • Atoms can vibrate in several ways. • The rate of vibration is quantized and can take place only at well defined frequencies that are characteristic of the atom.
2-Electric Dipole • For a molecule to be able to absorb ir, it must have a changeable electric dipole. • The dipole must change as a result of the vibrational transition resulting from ir absorption. • If the rate of change of the dipole during vibration is fast, the absorption of radiation is intense (and vice versa). Electric dipole: a slight positive and a slight negative electric charge on the atoms
B-Movements of Molecules: • The total radiant energy absorbed by molecules = (the molecule's vibrational energy + the molecule's rotational energy) • Rotational energy levels are very small compared to vibrational energy levels.
1- Vibrational Movement • u : frequency of vibration • K : constant. • f : binding strength of the spring • m : reduced mass.
Modes of vibration of C and H in methane molecule including: • a-symmetrical stretching. • b- asymmetrical stretching. • c- scissoring. • d- rocking. • e- wagging. • f- twisting. • Each of the modes vibration absorb radiation at different wavelength.
2- Rotational Movement • At the same time that the parts of a molecule vibrate toward each other, the molecule as a whole may rotate (spin). • The energy involved in spinning a molecule is very small compared to the energy required to cause it to vibrate.
C-Equipment (double beam system) • 1- Radiation Source: Both sources fulfill important requirements: 1-steady intensity. 2-intensity constant over long periods of time. 3-wide wavelength range. But the intensity of the radiation from them is not the same at all frequencies.
2- Monochromators • Select desired frequency from source, and eliminate the radiation at other frequencies. • a- Prism Monochromator: • Material must be: • 1- Transparent to IR radiation (not glass, not quartz). • 2-Smooth (prevent random scattering). • 3-High quality crystal. • 4-Dry all time (use heater to keep dry). • 5-The machine must be in air condition room. • Material used can be single large crystal metal salt : NaCl, (KBr,CsBr), or CaF2.
B-Grating Monochromators: • Recently it is more popular in IR spectroscopy. • Material : Aluminum. • Advantages of grating: • 1- Stable in the atmosphere and are not attacked by moisture. • 2- Can be used over considerable wavelength range.
3- Slit Systems: Allow small section of radiation beam to pass through exit slit to the detector. 4- Detectors:
