Air, Water and Land Pollution Chapter 8: UV-Visible and Infrared Spectroscopic Methods in Environmental Analysis Copyright © 2010 by DBS

Air, Water and Land Pollution

Chapter 8:UV-Visible and Infrared Spectroscopic

Methods in Environmental Analysis

Copyright © 2010 by DBS

Contents

• Introduction to the Principles of Spectroscopy• UV-Visible Spectroscopy• Infrared Spectroscopy• Practical Aspects of UV-Visible and Infrared Spectroscopy

UV-Visible + IR Spectroscopic MethodsIntroduction to the Principles of Spectroscopy


• Spectroscopy – interaction of electromagnetic radiation with matter• Spectrometry – spectrometric technique used to assess concentration of chemical

species

• Radiation is defined by Planck’s law:

E = hν = hc / λ

• Where E = energy (J), ν = frequency (s-1), λ = wavelength (m), h = planck’s constant (6.62 x 10-34 Js) and c = speed of light (3 x 108 m s-1)


Ionization occurs under high energy UV radiation

MW least energetic, does not vibrate



• EM spectrum• Energy per photon: radio < micro < IR < VIS < UV < X-ray• Naked eye detects only 300 – 780 nm


• Chemicals absorb EM radiation at certain wavelengths• Ground state is most stable electronic configuration• Photons in UV and VIS spectrum can excite ground level e-

Arrows indicate possible transitions

High energy UV, X-ray photons may cause e- emission (ionization)

IR photons have much less energy, vibrate molecules


Absorption – moves the atom to a higher energy level

UV-VIS = Transitions between levels

IR = Transitions between vibrational / rotational states


Emission – energy at higher state may return to ground state


Fluorescence – energy at higher state may lose some energy as heat and return to ground state by emitting new longer wavelength radiation



Origins of Absorption in Relation to Molecular Orbital Theories

• Explains why UV causes excitation and why IR causes vibration



Basic Concepts of Electronic Structure• e- exists as both a particle and a wave• e- occupies major shell, subshell, and orbital surrounding the nucleus



Basic Concepts of Electronic Structure

• Major shell – primary energy level, n = 1, 2, 3, 4• Subshells – s, p, d, f. Number of subshells within a major shell is equal to n (n = 1

has one subshell (s), n = 2 has 2 subshells (s and p), n = 3 has 3 subshells (s, p and d) etc.

• Orbitals – subshells are further divided into. 1 for s, 3 for p, 5 for d, 7 for f• Each orbital has 2 e- (Pauli Exclusion Principle). Max. number of e- is 2n2 or 2, 8, 18

and 32





Types of Absorbing Electrons: σ, σ*, π, π* and n

• s electron has spherical shape, p orbital Is dumbbell shaped, 2px, 2py, 2pz where x, y, and z indicate direction




• s electron has spherical shape, p orbital Is dumbell shaped, 2px, 2py, 2pz where x, y, and z indicate direction

• When 2 s orbitals overlap form σ bond (e.g. hydrogen)• Energy is released as two orbitals overlap up to a point of maximum stability when

two nuclei are a certain distance apart = bonding molecular orbital or bond




• σ* is a sigma antibonding molecular orbital• This is a detraction from formation of a bond between two atoms• Located outside the region of two distinct nuclei• The overlap of the constituent orbitals said to be 'out of phase' and as such the e-

present in each antibonding orbital are repulsive and and destabilize the molecule• Helps to think of e- as waves







• Fluorine (1s2, 2s2, 2p5) • σ bonding molecular orbital (σ bond) can also form via overlap of two 2p orbials end-

on (e.g. 2p orbitals of fluorine to form F2)

Fluorine, F2




• π orbital formed by lateral overlap of p orbitals, called a π bond• Overlap of two out-of-phase p orbitals forms a π* antibonding molecular orbital



UV Absorption and Electronic Transitions

• Excitation of valence e- leads to:

(1) transitions involving σ, σ*, π, π* , and n electrons (n = nonbonding orbital)

(2) transitions involving charge transfer electrons (inorganic species)

(3) transitions involving d and f electrons

For simplicity easier to deal with (1) σ, π, and n

σ → σ*, π → π *, n → π *, n → σ*




• Certain molecules are UV absorbing whilst others are transparent to UV

• σ → σ*: large energy requirement (short wavelength), e.g. CH4 has max. absorbance at 125 nm (UV-absorbing), not seen in typical UV-VIS spectra (200-700 nm)e.g. hexane and water only contain σ bonds and is transparent in UV range, good solvents for UV-VIS spectroscopy

• n → σ*: occur in saturated compounds containing atoms with spare pairs of e-, require less energy than above, absorb around 150-250 nm (UV-absorbing)

• n → π*, and π → π*: most organics absorb around 200-700 nm (UV-absorbing), require unsaturated group (C=C) to provide π electrons




Remember that bigger jumps need more energy and so absorb light with a shorter wavelength. The jumps shown with grey dotted arrows absorb UV light of wavelength less that 200 nm.

That means that in order to absorb light in the region from 200 - 800 nm (which is where the spectra are measured), the molecule must contain either π bonds or atoms with non-bonding orbitals. Remember that a non-bonding orbital is a lone pair on, say, oxygen, nitrogen or a halogen.

E = hν = hcλ




• n → σ* transitions occur in a small number of molecules (with lone pairs)




• Compounds with UV chromophores include alkenes (dienes and polyenes) (C=C), carbonyl compounds (C=O), and benzene derivatives




• e.g. buta-1,3-diene, possible transitions?

σ → σ*, π → π *, n → π *, n → σ*




• e.g. buta-1,3-diene• Absorption peaks at a value of 217 nm. This is in the ultra-violet and so there would

be no visible sign of any light being absorbed (colorless)

• In buta-1,3-diene, CH2=CH-CH=CH2, there are no non-bonding electrons. That means that the only (measurable) electron jumps taking place are π → π*



IR Absorption and Vibrational and Rotational Transitions

• Molecular vibrations – diatomic molecule represented by two spheres and a spring• Vibration causes the atoms to move toward and away from each other at a certain

frequency

• Where v = frequency, k = constant, m1 and m2 = masses on the spring

• The lighter the masses (atoms) or the tighter the spring (chemical bond), the higher the frequency

• Vibrational frequencies are quantized (only certain energies are allowed)




• Number of vibrational modes:– Linear molecule = 3N-5– Non-linear molecule = 3N-6 (where N = no. atoms)

• CO2 has 3N-5 = 4 vibrational modes which are responsible for the greenhouse effect

Movie







Symmetric stretching

Assymmetric stretching

Vertical bending

Horizontal bending

Both C=O bonds lengthen and contract together (in-phase) – no change in dipole moment

One bond shortens while the other lengthens – change in dipole moment




• CO2 asymmetric stretch excited by IR at 2347 cm-1 (4.26 µm)

• Two bending vibrations excited at 667 cm-1 (15 µm)







• Vibrational modes:– Stretching– Rocking– Twisting– Scissoring– Wagging




• Trends:– Stronger the bond the more energy is required to excite the stretching vibration

(E ~ ν)– Triple bonds occur at higher frequencies than double bonds, double bond

stretches occur at higher frequencies than single bonds– The heavier an atom the lower the frequency of vibration


Molecular Structure and UV-Visible/Infrared Spectra

• Spectrum – plot of absorption vs. wavelength• Used to deduce structures of unknown chemicals

• Comparing UV spectrum to IR spectrum of benzene, IR yields more structural information



• Wavenumber

e.g. 4.26 µm:

ν = 10,000/ (4.26 µm)= 2347 cm-1

Wavenumber ~ frequency ~ Energy



• Two regions on a IR spectra:

(1) group frequency region > 2000 cm-1 (3 – 8 µm)

(2) fingerprint region < 2000 cm-1 (8 – 14 µm)



• Near infrared (NIR): 12,800-4,000 cm-1

• Mid-infrared (MIR): 4,000-200 cm-1

• Far-infrared (FIR): 200-10 cm-1





Aromatic C-H stretch at 3100-3000 cm-1

Aromatic C-C at 1200 cm-1

Bending C-H at 1000 cm-1 (in-plane) 675 cm-1 (out-of-plane)



• A

Add spectrum from BSc Van Gogh experiment here:


Quantitative Analysis with Beer-Lambert’s Law

• Beer-Lambert law relates absorption of light to concentration of a chemical

A = ε l C

• Where A = absorbance of radiation at a particular wavelength (=log(I0/I)), ε = proportionality constant (molar absoptivity (L mol-1 cm-1)), l = thickness of substance or pathlength of the light-beam (cm),and C = concentration of absorbing species (mol L-1),

• A is linearly related to concentration for a fixed a, b and c

• Let l = 1 cm, slope of graph will be ε


Diagram of Beer–Lambert absorption of a beam of light as it travels through a cuvette of width b. I = transmitted light, Io = incident light

ε


Quantitative Analysis with Beer-Lambert’s Law

Note: conversion factor: mol/L x 46 g/mol x 1 L/1000 mL x 106 µg/g = 46 x 1000

UV-Visible + IR Spectroscopic MethodsUV-Visible Spectroscopy

UV-Visible Instrumentation

• 5 major components:– (1) source of continuous radiation– (2) monochromator for wavelength selection– (3) sample cell– (4) detector (radiant energy converts to electrical signal)– (5) read out


UV-Visible Instrumentation

(1) source of continuous radiation:hydrogen or deuterium lamp for UV and tungsten lamp for visible

(2) monochromator for wavelength selection:

entrance slit narrows bandwidth of radiation, collimator makes the radiation parallel, grating disperses unwanted radiation, exit slit isolates desired wavelength

(3) sample cell:Quartz is transparent to UV and VIS radiation, plastic also used

(4) detector (photodiode, photoemissive tube, photomultipliers):

changes radiation into a current or voltage (photoelectric effect)


UV-VIS as a Workhorse in Environmental Analysis

• US EPA uses a number of spectrophotometric methods for air and water




UV-VIS Methods for Atmospheric Pollutants

• Sulfur Dioxide (SO2): uses an impinger to scrub air samples through a solution of tetrachloromercurate (TCM) and formaldehyde

[HgCl4]2- + 2SO2 + 2H2O → [Hg(SO3)2]2- + 4Cl- + H+ (stable complex)

SO2 + CH2O + H2O → HOCH2SO3H

• Red-purple pararosaniline methylsulfonic aicd forms in the presence of para-rosaniline and formaldehyde

Wes

t and

Gae

ke (

1956

)




• Ozone (O3): 1 % Potassium iodie (KI) is used to scrub O3 from the air

• I2 measured spectrometrically (buffered at pH 6.8

2KI + O3 + H2O → 2KOH + O2 + I2

I2 + starch blue/purple complex (352 nm)




• Nitrogen Dioxide:

• Saltzman method (1954) – NO2 is bubbled through water to form HNO2

• HNO2 reacts with a base (sulfanilic acid) to form nitrosamine

• Reaction proceeds to form diazonium ion• Diazonium ions contain triple bonded N

atoms which couple with N-(1-napthyl)ethylenediamine to form a strong colored compound



UV-VIS Methods for Pollutants in Water

• Colorimetric methods



UV-VIS Methods for Pollutants in Water

• Colorimetric methods, e.g. Fe via UV-VIS in Env Chem lab

UV-Visible + IR Spectroscopic MethodsIR Spectroscopy

• 3 types:

– Fourier transform infrared spectrometers (FTIR)– Dispersive infrared spectrometers (DIR) – Nondispersive infrared spectrometers (NDIR)


Fourier Transform IR Spectrometers (FTIR)

• Uses Michelson interferometer (an arrangement of mirrors) to produce interference signals of the sample source light

• Source light is split into 2 beams and reflected back by each mirror• Moveable mirror and static mirrror• Movable mirror causes phase-lags and an interference pattern composed of all

frequency signals of the sample in a time-domain• Signal is Fourier-transformed into a frequency-domain giving the FTIR spectrum


Fourier Transform IR Spectrometers (FTIR)

• 5 major components:

(1) Light source: IR source e.g. Nernst glower (rare earths heated to 1500-2000 ºC)

(2) Michelson interferometer: semitransparent beam splitter, mirrors (1 fixed, 1 movable)

(3) Detector: pyroelectric bolometer (temperature based radiation detector) with fast response time


Dispersive Infrared Instruments (DIR)

• Similar to UV-VIS design except sample and reference cell is located between IR source and monochromator, not after

• IR source same as FTIR• No Michelson interferometer as used in FTIR• Monochromator (diffraction grating and slit) is used to disperse IR wavelengths (made

from IR transparent crystals, not glass)


Pavia, D.L., Lampman, G.M., and Kriz, G.S. (2009) Introduction to Spectroscopy. Brooks/Cole.


Nondispersive Infrared Instruments (NDIR)

• Designed for a specific compound – gas detectors• NDIR use filters to isolate particular wavelengths for measurement• Do not record a spectrum

e.g. analysis of CO and CO2 in car exhaust

• 4 major components:– (1) source of IR– (2) sample chamber– (3) wavelength filter– (4) IR detector

• Gas concentration is measured by its absorption of a specific wavelength of IR

CO2

Keeling, C.D. and T.P. Whorf (2005) Atmospheric CO2 records from sites in the SIO air sampling network. In Trends: A Compendium of Data on Global Change. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy, Oak Ridge, Tenn., U.S.A.

Ultramat 3 gas analyzer


Applications in Industrial Hygiene and Air Pollution Monitoring

• Out of 189 hazardous air pollutants• Listed by EPA more than 100 absorb IR• Major disadvantage of IR is limitation

of measuring low concentrations• Requires long pathlength• Limited to auto exhausts, industrial

occupational exposure etc.

UV-Visible + IR Spectroscopic MethodsPractical Aspects of UV-Visible and Infrared Spectroscopy

Common Tips for UV-Visible Spectroscopic Analysis

• Use only quartz cuvettes for UV range (200-280 nm), plastic is ok for the visible range (380-800 nm) but will melt in hexane

• Do not hold or scratch the smooth sides of the cuvette

• Always select λmax for maximum sensitivity

• Unlike IR, only liquid samples are allowed for UV-VIS• Linnear analytical range is important, if A > 0.9 sample must be diluted, if A < 0.05 a

larger sample is required

UV-Visible + IR Spectroscopic MethodsPractical Aspects of UV-Visible and Infrared Spectroscopy

Sample Preparation for IR Spectroscopic Analysis

• Can analyze gas, liquid or solids

• Gas: long-path cell is required to maximize sensitivity (Beer’s law)

10 cm (high concentration) and 20-100 m (low concentration)

• Pure liquids: sandwich between two halide crystal disks (NaCl/KBr)

• Dilute solutions: use cells (NaCl/KBr)

• Solid samples: grind with mineral oil or pellet with KBr

References

• Beer, R. (1992) Remote Sensing by Fourier Transform Spectroscopy, in the Chemical Analysis Series, Vol. 120, John Wiley & Sons, New York.

• Eubanks, L.P., Middlecamp, C., Pienta, N., Heltzel, C., and Weaver, G. (2006) Chemistry in Context: Applying Chemistry to Society, 5th Edition, ACS, Washington, D.C.

• Field, L.D., Sternell, S., Kalman, J.R. (2003) Organic Structures from Spectra, 3rd Edition, John Wiley & Sons, West Sussex, England, pp. 1-19.

• Guicherit, R., Jeltes, R. and Lindqvist, F. (1970) Determination of ozone concentration in outdoor air near Delft The Netherlands. Environmental Pollution, Vol. 3, No. 2, pp. 91-110.

• Kebbekus, B.B. and Mitra, S. (1998) Environmental Chemical Analysis. Blackie Academic & Professional, London, UK, pp. 54-102.

• Manahan, S.E. (2005) Environmental Chemistry, 8th Edition, CRC Press, pp. 681-747.• Saltzman, B.E. (1954) Colorimetric microdetermination of nitrogen dioxide in the

atmosphere. Analytical Chemistry, Vol. 26, No. 12, pp. 1949-1955.• West, P.W. and Gaeke, G.C. (1956) Fixation of sulfur dioxide as disulfitomercurate (II)

and subsequent colorimetric estimation. Analytical Chemistry, Vol. 28, No. 12, pp. 1816-1819.

Questions

15. Which one of the following is true regarding Beer’s law:(a) Absorbance is proportional to both path length and concentration of absorbing species(b) Absorbance is proportional to the log of the concentration of absorbing species

(c) Absorbance is equal to P0/P?

18. The absorbance of a 2-cm sample cell of a 10 ppm solution is 0.43, what would be the absorbance of a 1-cm cell of 15 ppm solution of the same chemical.

21. Draw a schematic diagram of the following: (a) UV-VIS spectrometer, (b) FTIR spectrometer.

24. Explain: (a) Why in situ atmospheric CO2 can be monitored by IR? (b) Why in situ atmospheric (stratospheric) O3 can be measured by UV?

25. Explain: (a) Why O2 and N2 will not effect the monitoring of CO in auto emission using IR? (b) Why a long cell is needed to monitor trace organic compounds using IR?

Documents

Air, Water and Land Pollution Chapter 8: UV-Visible and Infrared Spectroscopic Methods in Environmental Analysis Copyright © 2010 by DBS