Air, Water and Land Pollution
Chapter 8:
UV-Visible and Infrared Spectroscopic
Methods in Environmental Analysis
Copyright © 2010 by DBS
Contents
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Introduction to the Principles of Spectroscopy
UV-Visible Spectroscopy
Infrared Spectroscopy
Practical Aspects of UV-Visible and Infrared Spectroscopy
UV-Visible + IR Spectroscopic Methods
Introduction to the Principles of Spectroscopy
UV-Visible + IR Spectroscopic Methods
Introduction to the Principles of Spectroscopy
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Spectroscopy – interaction of electromagnetic radiation with matter
Spectrometry – spectrometric technique used to assess concentration of chemical
species
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Radiation is defined by Planck’s law:
E = hν = hc / λ
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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)
UV-Visible + IR Spectroscopic Methods
Introduction to the Principles of Spectroscopy
Ionization
occurs under
high energy
UV radiation
MW least
energetic,
does not
vibrate
UV-Visible + IR Spectroscopic Methods
Introduction to the Principles of Spectroscopy
UV-Visible + IR Spectroscopic Methods
Introduction to the Principles of Spectroscopy
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EM spectrum
Energy per photon: radio < micro < IR < VIS < UV < X-ray
Naked eye detects only 300 – 780 nm
UV-Visible + IR Spectroscopic Methods
Introduction to the Principles of Spectroscopy
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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 eemission (ionization)
IR photons have much
less energy, vibrate
molecules
UV-Visible + IR Spectroscopic Methods
Introduction to the Principles of Spectroscopy
Absorption – moves the atom to a higher energy level
UV-VIS = Transitions between levels
IR = Transitions between vibrational / rotational states
UV-Visible + IR Spectroscopic Methods
Introduction to the Principles of Spectroscopy
Emission – energy at higher state may return to ground
state
UV-Visible + IR Spectroscopic Methods
Introduction to the Principles of Spectroscopy
Fluorescence – energy at higher state may lose
some energy as heat and return to ground state by
emitting new longer wavelength radiation
UV-Visible + IR Spectroscopic Methods
Introduction to the Principles of Spectroscopy
UV-Visible + IR Spectroscopic Methods
Introduction to the Principles of Spectroscopy
Origins of Absorption in Relation to Molecular Orbital Theories
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Explains why UV causes excitation and why IR causes vibration
UV-Visible + IR Spectroscopic Methods
Introduction to the Principles of Spectroscopy
Origins of Absorption in Relation to Molecular Orbital Theories
Basic Concepts of Electronic Structure
• e- exists as both a particle and a wave
• e- occupies major shell, subshell, and orbital surrounding the nucleus
UV-Visible + IR Spectroscopic Methods
Introduction to the Principles of Spectroscopy
Origins of Absorption in Relation to Molecular Orbital Theories
Basic Concepts of Electronic Structure
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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
UV-Visible + IR Spectroscopic Methods
Introduction to the Principles of Spectroscopy
Origins of Absorption in Relation to Molecular Orbital Theories
UV-Visible + IR Spectroscopic Methods
Introduction to the Principles of Spectroscopy
Origins of Absorption in Relation to Molecular Orbital Theories
Types of Absorbing Electrons: σ, σ*, π, π* and n
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s electron has spherical shape, p orbital Is dumbbell shaped, 2px, 2py, 2pz where x, y,
and z indicate direction
UV-Visible + IR Spectroscopic Methods
Introduction to the Principles of Spectroscopy
Origins of Absorption in Relation to Molecular Orbital Theories
Types of Absorbing Electrons: σ, σ*, π, π* and n
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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
UV-Visible + IR Spectroscopic Methods
Introduction to the Principles of Spectroscopy
Origins of Absorption in Relation to Molecular Orbital Theories
Types of Absorbing Electrons: σ, σ*, π, π* and n
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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 epresent in each antibonding orbital are repulsive and and destabilize the molecule
Helps to think of e- as waves
UV-Visible + IR Spectroscopic Methods
Introduction to the Principles of Spectroscopy
Origins of Absorption in Relation to Molecular Orbital Theories
Types of Absorbing Electrons: σ, σ*, π, π* and n
UV-Visible + IR Spectroscopic Methods
Introduction to the Principles of Spectroscopy
Origins of Absorption in Relation to Molecular Orbital Theories
Types of Absorbing Electrons: σ, σ*, π, π* and n
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Fluorine (1s2, 2s2, 2p5)
σ bonding molecular orbital (σ bond) can also form via overlap of two 2p orbials endon (e.g. 2p orbitals of fluorine to form F2)
Fluorine, F2
UV-Visible + IR Spectroscopic Methods
Introduction to the Principles of Spectroscopy
Origins of Absorption in Relation to Molecular Orbital Theories
Types of Absorbing Electrons: σ, σ*, π, π* and n
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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-Visible + IR Spectroscopic Methods
Introduction to the Principles of Spectroscopy
Origins of Absorption in Relation to Molecular Orbital Theories
UV Absorption and Electronic Transitions
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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*
UV-Visible + IR Spectroscopic Methods
Introduction to the Principles of Spectroscopy
Origins of Absorption in Relation to Molecular Orbital Theories
UV Absorption and Electronic Transitions
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Certain molecules are UV absorbing whilst others are transparent to UV
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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
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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)
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n → π*, and π → π*: most organics absorb around 200-700 nm (UV-absorbing),
require unsaturated group (C=C) to provide π electrons
UV-Visible + IR Spectroscopic Methods
Introduction to the Principles of Spectroscopy
Origins of Absorption in Relation to Molecular Orbital Theories
UV Absorption and Electronic Transitions
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.
E = hν = hc
λ
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.
UV-Visible + IR Spectroscopic Methods
Introduction to the Principles of Spectroscopy
Origins of Absorption in Relation to Molecular Orbital Theories
UV Absorption and Electronic Transitions
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n → σ* transitions occur in a small number of molecules (with lone pairs)
UV-Visible + IR Spectroscopic Methods
Introduction to the Principles of Spectroscopy
Origins of Absorption in Relation to Molecular Orbital Theories
UV Absorption and Electronic Transitions
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Compounds with UV chromophores include alkenes (dienes and polyenes) (C=C),
carbonyl compounds (C=O), and benzene derivatives
UV-Visible + IR Spectroscopic Methods
Introduction to the Principles of Spectroscopy
Origins of Absorption in Relation to Molecular Orbital Theories
UV Absorption and Electronic Transitions
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e.g. buta-1,3-diene, possible transitions?
σ → σ*, π → π *, n → π *, n → σ*
UV-Visible + IR Spectroscopic Methods
Introduction to the Principles of Spectroscopy
Origins of Absorption in Relation to Molecular Orbital Theories
UV Absorption and Electronic Transitions
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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*
UV-Visible + IR Spectroscopic Methods
Introduction to the Principles of Spectroscopy
Origins of Absorption in Relation to Molecular Orbital Theories
IR Absorption and Vibrational and Rotational Transitions
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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
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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)
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UV-Visible + IR Spectroscopic Methods
Introduction to the Principles of Spectroscopy
Origins of Absorption in Relation to Molecular Orbital Theories
IR Absorption and Vibrational and Rotational Transitions
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Number of vibrational modes:
– Linear molecule = 3N-5
– Non-linear molecule = 3N-6 (where N = no. atoms)
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CO2 has 3N-5 = 4 vibrational modes which are responsible for the greenhouse effect
Movie
UV-Visible + IR Spectroscopic Methods
Introduction to the Principles of Spectroscopy
Origins of Absorption in Relation to Molecular Orbital Theories
IR Absorption and Vibrational and Rotational Transitions
UV-Visible + IR Spectroscopic Methods
Introduction to the Principles of Spectroscopy
Origins of Absorption in Relation to Molecular Orbital Theories
IR Absorption and Vibrational and Rotational Transitions
Both C=O bonds lengthen and
contract together (in-phase) – no
change in dipole moment
Symmetric stretching
Assymmetric stretching
One bond shortens while the
other lengthens – change in
dipole moment
Vertical bending
Horizontal bending
UV-Visible + IR Spectroscopic Methods
Introduction to the Principles of Spectroscopy
Origins of Absorption in Relation to Molecular Orbital Theories
IR Absorption and Vibrational and Rotational Transitions
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CO2 asymmetric stretch excited by IR at 2347 cm-1 (4.26 µm)
Two bending vibrations excited at 667 cm-1 (15 µm)
UV-Visible + IR Spectroscopic Methods
Introduction to the Principles of Spectroscopy
Origins of Absorption in Relation to Molecular Orbital Theories
IR Absorption and Vibrational and Rotational Transitions
UV-Visible + IR Spectroscopic Methods
Introduction to the Principles of Spectroscopy
Origins of Absorption in Relation to Molecular Orbital Theories
IR Absorption and Vibrational and Rotational Transitions
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Vibrational modes:
– Stretching
– Rocking
– Twisting
– Scissoring
– Wagging
UV-Visible + IR Spectroscopic Methods
Introduction to the Principles of Spectroscopy
Origins of Absorption in Relation to Molecular Orbital Theories
IR Absorption and Vibrational and Rotational Transitions
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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
UV-Visible + IR Spectroscopic Methods
Introduction to the Principles of Spectroscopy
Molecular Structure and UV-Visible/Infrared Spectra
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Spectrum – plot of absorption vs. wavelength
Used to deduce structures of unknown chemicals
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Comparing UV spectrum to IR spectrum of
benzene, IR yields more structural information
UV-Visible + IR Spectroscopic Methods
Introduction to the Principles of Spectroscopy
Molecular Structure and UV-Visible/Infrared Spectra
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Wavenumber
e.g. 4.26 µm:
ν = 10,000/ (4.26 µm)= 2347 cm-1
Wavenumber ~ frequency ~ Energy
UV-Visible + IR Spectroscopic Methods
Introduction to the Principles of Spectroscopy
Molecular Structure and UV-Visible/Infrared Spectra
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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)
UV-Visible + IR Spectroscopic Methods
Introduction to the Principles of Spectroscopy
Molecular Structure and UV-Visible/Infrared Spectra
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Near infrared (NIR): 12,800-4,000 cm-1
Mid-infrared (MIR): 4,000-200 cm-1
Far-infrared (FIR): 200-10 cm-1
UV-Visible + IR Spectroscopic Methods
Introduction to the Principles of Spectroscopy
Molecular Structure and UV-Visible/Infrared Spectra
UV-Visible + IR Spectroscopic Methods
Introduction to the Principles of Spectroscopy
Molecular Structure and UV-Visible/Infrared Spectra
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)
UV-Visible + IR Spectroscopic Methods
Introduction to the Principles of Spectroscopy
Molecular Structure and UV-Visible/Infrared Spectra
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A
Add spectrum
from BSc Van
Gogh experiment
here:
UV-Visible + IR Spectroscopic Methods
Introduction to the Principles of Spectroscopy
Quantitative Analysis with Beer-Lambert’s Law
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Beer-Lambert law relates absorption of light to concentration of a chemical
A=εlC
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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),
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A is linearly related to concentration for a fixed a, b and c
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Let l = 1 cm, slope of graph will be ε
UV-Visible + IR Spectroscopic
Methods
Introduction to the Principles of Spectroscopy
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
UV-Visible + IR Spectroscopic Methods
Introduction to the Principles of Spectroscopy
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 Methods
UV-Visible Spectroscopy
UV-Visible Instrumentation
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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 + IR Spectroscopic Methods
UV-Visible Spectroscopy
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-Visible + IR Spectroscopic Methods
UV-Visible Spectroscopy
UV-VIS as a Workhorse in Environmental Analysis
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US EPA uses a number of spectrophotometric methods for air and water
UV-Visible + IR Spectroscopic Methods
UV-Visible Spectroscopy
UV-Visible + IR Spectroscopic Methods
UV-Visible Spectroscopy
UV-VIS as a Workhorse in Environmental Analysis
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+
SO2 + CH2O + H2O → HOCH2SO3H
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(stable complex)
Red-purple pararosaniline methylsulfonic aicd forms in the presence of pararosaniline and formaldehyde
West and Gaeke (1956)
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UV-Visible + IR Spectroscopic Methods
UV-Visible Spectroscopy
UV-VIS as a Workhorse in Environmental Analysis
UV-VIS Methods for Atmospheric Pollutants
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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)
UV-Visible + IR Spectroscopic Methods
UV-Visible Spectroscopy
UV-VIS as a Workhorse in Environmental Analysis
UV-VIS Methods for Atmospheric Pollutants
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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-Visible + IR Spectroscopic Methods
UV-Visible Spectroscopy
UV-VIS as a Workhorse in Environmental Analysis
UV-VIS Methods for Pollutants
in Water
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Colorimetric methods
UV-Visible + IR Spectroscopic Methods
UV-Visible Spectroscopy
UV-VIS as a Workhorse in Environmental Analysis
UV-VIS Methods for Pollutants in Water
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Colorimetric methods, e.g. Fe via UV-VIS in Env Chem lab
UV-Visible + IR Spectroscopic Methods
IR Spectroscopy
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3 types:
– Fourier transform infrared spectrometers (FTIR)
– Dispersive infrared spectrometers (DIR)
– Nondispersive infrared spectrometers (NDIR)
UV-Visible + IR Spectroscopic Methods
IR Spectroscopy
Fourier Transform IR Spectrometers (FTIR)
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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
UV-Visible + IR Spectroscopic Methods
IR Spectroscopy
Fourier Transform IR Spectrometers (FTIR)
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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
UV-Visible + IR Spectroscopic Methods
IR Spectroscopy
Dispersive Infrared Instruments (DIR)
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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.
UV-Visible + IR Spectroscopic Methods
IR Spectroscopy
UV-Visible + IR Spectroscopic Methods
IR Spectroscopy
Nondispersive Infrared Instruments (NDIR)
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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
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4 major components:
– (1) source of IR
– (2) sample chamber
– (3) wavelength filter
– (4) IR detector
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Gas concentration is measured by its absorption of a specific wavelength of IR
CO2
Ultramat 3 gas
analyzer
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.
UV-Visible + IR Spectroscopic Methods
IR Spectroscopy
Applications in Industrial Hygiene and Air Pollution Monitoring
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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 Methods
Practical Aspects of UV-Visible and Infrared Spectroscopy
Common Tips for UV-Visible Spectroscopic Analysis
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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 Methods
Practical Aspects of UV-Visible and Infrared Spectroscopy
Sample Preparation for IR Spectroscopic Analysis
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Can analyze gas, liquid or solids
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Gas: long-path cell is required to
maximize sensitivity (Beer’s law)
10 cm (high concentration) and
20-100 m (low concentration)
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Pure liquids: sandwich between two
halide crystal disks (NaCl/KBr)
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Dilute solutions: use cells (NaCl/KBr)
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Solid samples: grind with mineral oil or
pellet with KBr
References
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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?