instrumental analysis chem 4811 chapter 2 dr. augustine ofori agyeman assistant professor of...

INSTRUMENTAL ANALYSIS CHEM 4811

CHAPTER 2

DR. AUGUSTINE OFORI AGYEMANAssistant professor of chemistryDepartment of natural sciences

Clayton state university

CHAPTER 2

INTRODUCTION TO SPECTROSCOPY

DEFINITIONS

Spectroscopy- The study of the interactions of electromagnetic radiation

(radiant energy) and matter (molecules, atoms, or ions)

Spectrometry- Quantitative measurement of the intensity of one or more

wavelengths of radiant energy

Spectrophotometry- The use of electromagnetic radiation to measure

chemical concentrations(used for absorption measurements)

Spectrophotometer - Instrument used for absorption measurements

Optical Spectrometer- Instrument that consists of prism or grating dispersion devise,

slits, and a photoelectric detector

Photometer- Instrument that uses a filter for wavelength selection instead

of a dispersion device

DEFINITIONS

ELECTROMAGNETIC RADIATION

- Also known as radiant heat or radiant energy

- One of the ways by which energy travels through space

- Consists of perpendicular electric and magnetic fields that are also perpendicular to direction of propagation

Examplesheat energy in microwaves

light from the sunX-ray

radio waves

Gamma rays

X rays Ultr-violet

Infrared Microwaves Radio frequency FM Shortwave AM

Vis

ible

Visible Light: VIBGYORViolet, Indigo, Blue, Green, Yellow, Orange, Red

400 – 750 nm

- White light is a blend of all visible wavelengths

- Can be separated using a prism

Wavelength (m)

Frequency (s-1)

10-11 103

1020104


one second

λ1

λ3

λ2

ν1 = 4 cycles/second



amplitude

peak

trough


node

Wavelength (λ) - Distance for a wave to go through a complete cycle

(distance between two consecutive peaks or troughs in a wave)

Frequency (ν)- The number of waves (cycles) passing a given point

in space per second

Cycle- Crest-to-crest or trough-to-trough

Speed (c)- All waves travel at the speed of light in vacuum (3.00 x 108 m/s)



Plane Polarized Light- Light wave propagating along only one axis (confined to one plane)

Monochromatic Light- Light of only one wavelength

Polychromatic Light- Consists of more than one wavelength (white light)

Visible light- The small portion of electromagnetic radiation to which

the human eye responds

- Inverse relationship between wavelength and frequency

λ α 1/ν

c = λ ν

λ = wavelength (m)

ν = frequency (cycles/second = 1/s = s-1 = hertz = Hz)

c = speed of light (3.00 x 108 m/s)



- Light appears to behave as waves and also considered as stream of particles (the dual nature of light)

- Is sinusoidal in shape

- Light is quantized

Photons- Particles of light

h = Planck’s constant (6.626 x 10-34 J-s)

ν = frequency of the radiation

λ = wavelength of the radiation

E is proportional to ν and inversely proportional to λ

)(mwavenumberλ

1ν~ 1

ν~hcλ

hchν)(E photon one ofEnergy photon


- Takes place in many ways

- Takes place over a wide range of radiant energies

- Is not visible to the human eye

- Light is absorbed or emitted

- Follows well-ordered rules

- Can be measured with suitable instruments

INTERACTIONS WITH MATTER


- Atoms, molecules, and ions are in constant motion

Solids- Atoms or molecules are arranged in a highly ordered array (crystals)

orarranged randomly (amorphous)

Liquids- Atoms or molecules are not as closely packed as in solids

Gases- Atoms or molecules are widely separated from each other


Molecules

Many types of motion are involved- Rotation - Vibration

- Translation (move from place to place)

- These motions are affected when molecules interact with radiant energy

- Molecules vibrate with greater energy amplitude when they absorb radiant energy


Molecules

- Bonding electrons move to higher energy levels when molecules interact with visible or UV light

- Changes in motion or electron energy levels result inchanges in energy of molecules

Transition- Change in energy of molecules

(vibrational transitions, rotational transitions, electronic transitions)


Atoms or Ions

- Move between energy levels or in space but cannot rotate or vibrate

The type of interactions of materials with radiant energy are affected by- Physical state

- Composition (chemical nature)- Arrangement of atoms or molecules


Light striking a sample of matter may be- Absorbed by the sample

- Transmitted through the sample- Reflected off the surface of the sample

- Scattered by the sample

- Samples can also emit light after absorption (luminescence)

- Species (atoms, ions, or molecules) can exist in certain discrete states with specific energies

Transmission- Light passes through matter without interaction

Absorption- Matter absorbs light energy and moves to a higher energy state

Emission- Matter releases energy and moves to a lower energy state

Luminescence- Emission following excitation of molecules or atoms by

absorption of electromagnetic radiation


Ene

rgy

Absorption Emission

Excitedstate

Groundstate


Ground State: The lowest energy state

Excited state: higher energy state (usually short-lived)


- Change in state requires the absorption or emission of energy

λ

hchνE)(energyinChange

- Matter can only absorb specific wavelengths or frequencies

- These correspond to the exact differences in energy between the two states involved

Absorption: Energy of species increases (ΔE is positive)

Emission: Energy of species decreases (ΔE is negative)


- Frequencies and the extent of absorption or emission of species are unique

- Specific atoms or molecules absorb or emit specific frequencies

- This is the basis of identification of species by spectroscopy

Relative energy of transition in a moleculeRotational < vibrational < electronic

- The are many associated rotational and vibrational sublevels for any electronic state (absorption occurs in

closely spaced range of wavelenghts)


Absorption Spectrum- A graph of intensity of light absorbed versus frequency

or wavelength

- Emission spectrum is obtained when molecules emit energy by returning to the ground state after excitation

Excitation may include- Absorption of radiant energy

- Transfer of energy due to collisions between atoms or molecules- Addition of thermal energy

- Addition of energy from electrical charges

ATOMS AND ATOMIC SPECTROSCOPY

- The electronic state of atoms are quantized

- Elements have unique atomic numbers (numbers of protons and electrons)

- Electrons in orbitals are associated with various energy levels

- An atom absorbs energy of specific magnitude and a valence electron moves to the excited state

- The electron returns spontaneously to the ground state and emits energy


- Emitted energy is equivalent to the absorbed energy (ΔE)

- Each atom has a unique set of permitted electronic energy levels(due to unique electronic structure)

- The wavelength of light absorbed or emitted are characteristic of a specific element

- The absorption wavelength range is narrow due to the absence of rotational and vibrational energies

- The wavelength range falls within the ultraviolet and visible regions of the spectrum (UV-VIS)


- Wavelengths of absorption or emission are used for qualitative identification of elements in a sample

- The intensity of light absorbed or emitted at a given wavelength is used for the quantitative analysis

Atomic Spectroscopy Methods- Absortion spectroscopy- Emission spectroscopy

- Fluorescence spectroscopy- X-ray spectroscopy (makes use of core electrons)

Gamma rays X rays Ultr-

violetInfrared Microwaves Radio frequency

FM Shortwave AMV

isib

le

10-11 103

1020104

Bon

d br

eaki

ngan

d io

niza

tion

Ele

ctro

nic

exci

tati

on

vibr

atio

n

rota

tion

Molecular Processes Occurring in Each Region

MOLECULES AND MOLECULAR SPECTROSCOPY


- Energy states are quantized

Rotational Transitions- Molecules rotate in space and rotational energy is associated

- Absorption of the correct energy causes transition to a higher energy rotational state

- Molecules rotate faster in a higher energy rotational state

- Rotational spectra are usually complex


Rotational Transitions

- Rotational energy of a molecule depends on shape,angular velocity, and weight distribution

- Shape and weight distribution change with bond angle

- Molecules with more than two atoms have many possible shapes

- Change in shape is therefore restricted to diatomic molecules

- Associated energies are in the radio and microwave regions


Vibrational Transitions

- Atoms in a molecule can vibrate toward or away from each other at different angles to each other

- Each vibration has characteristic energy associated with it

- Vibrational energy is associated with absorption in the infrared (IR region)

Increase in rotational energy usually accompanies increase in vibrational energy


Vibrational Transitions

- IR absorption corresponds to changes in both rotational and vibrational energies in molecules

- IR absorption spectroscopy is used to deduce the structure of molecules

- Used for both qualitative and quantitative analysis


Electronic Transitions

- Molecular orbitals are formed when atomic orbitals combine to form molecules

- Absorption of the correct radiant energy causes an outer electron to move to an excited state

- Excited electron spontaneously returns to the ground state (relax) emitting UV or visible energy

- Excitation in molecules causes changes in the rotational and vibrational energies


Electronic Transitions

- The total energy is the sum of all rotational, vibrational, and electronic energy changes

- Associated with wide range of wavelengths (called absorption band)

- UV-VIS absorption bands are simpler than IR spectra


Molecular Spectroscopy Methods

- Molecular absorption spectroscopy- Molecular emission spectroscopy

- Nuclear Magnetic Resonance (NMR)- UV-VIS

- IR- MS

- Molecular Fluorescence Spectroscopy

ABSORPTION LAWS

Radiant Power (P)- Energy per second per unit area of a beam of light

- Decreases when light transmits through a sample(due to absorption of light by the sample)

Intensity (I)- Power per unit solid angle

- Light intensity decreases as light passes through an absorbing material

Transmittance (T)

- The fraction of incident light that passes through a sample

Io I

oI

IT

0 < T < 1

Io = light intensity striking a sampleI = light intensity emerging from sample

ABSORPTION LAWS

Transmittance (T)

- T is independent of Io

- No light absorbed: I = Io and T = 1

- All light absorbed: I = 0 and T = 0

Percent Transmitance (%T)

0% < %T < 100%

100%xI

I%T

o

ABSORPTION LAWS

Absorbance (A)

- No light absorbed: I = Io and A = 0

Percent Absorbance (%A) = 100 - %T

- 1% light absorbed implies 99% light transmitted

- Higher absorbance implies less light transmitted

logTI

Ilog

I

IlogA

o

o

ABSORPTION LAWS

Beer’s Law

A = abc

A = absorbance

a = absorptivitya = ε [molar absorptivity (M-1cm-1) if C is in units of M (mol/L)]

b = pathlength or length of cell (cm)

c = concentration

ABSORPTION LAWS

Beer’s Law

- I or T decreases exponentially with increasing pathlength

- A increases linearly with increasing pathlength

- A increases linearly with increasing concentration

- More intense color implies greater absorbance

- Basis of quantitative measurements (UV-VIS, IR, AAS etc.)

ABSORPTION LAWS

Absorption Spectrum of 0.10 mM Ru(bpy)32+

λmax = 452 nm

ABSORPTION LAWS

λmax = 540 nm

Absorption Spectrum of 3.0 mM Cr3+ complex

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

350 400 450 500 550 600

Wavelength (nm)

Abs

orba

nce

ABSORPTION LAWS

Maximum Response (λmax)

- Wavelength at which the highest absorbance is observed for a given concentration

- Gives the greatest sensitivity

ABSORPTION LAWS

Deviations from Beer’s Law

- Deviations from linearity at high concentrations

- Usually used for concentrations below 0.01 M

- Deviations occur if sample scatters incident radiation

- Error increases as A increases (law generally obeyed when A ≤ 1.0

ABSORPTION LAWS

Calibration

- The relationship between the measured signal (absorbance in this case) and known concentrations of analyte

- Concentration of an unknown analyte can then be calculated using the established relationship and

its measured signal

CALIBRATION METHODS

Calibration with External Standards

- Solutions containing known concentrations of analyte are called standard solutions

- Standard solutions containing appropriate concentration range are carefully prepared and measured

- Reagent blank is used for instrumental baseline

- A plot of absorbance (y-axis) vs concentration (x-axis) is made

CALIBRATION METHODS


CALIBRATION METHODS


- Equation of a straight line in the form y = mx + z is established

m = slope = ab

z = intercept on the absorbance axis

- Concentration of unknown analyte should be within working range (do not extrapolate)

- Must measure at least three replicates and report uncertainty

CALIBRATION METHODS

Method of Standard Additions (MSA)

- Known amounts of analyte are added directly to the unknown sample

- The increase in signal due to the added analyte is used to establish the concentration of unknown

- Relationship between signal and concentration of analyte must be linear

- Analytes are added such that change in volume is negligible

CALIBRATION METHODS


- Different concentrations of analyte are added to different aliquots of sample

- Nothing is added to the first aliquot (untreated)

- Concentrations in increments of 1.00 is usually used for simplicity

- Plot of signal vs concentration of analyte is made

CALIBRATION METHODS

curvencalibratioofslope

sampleuntreatedtoduesignalsampleunknownofionConcentrat


Useful

- In emergency situations

- When information about the sample matrix is unknown

- For elimination of certain interferences in the matrix

CALIBRATION METHODS

Internal Standard Calibration

- Signal from internal standard is used to correct for interferences in an analyte

- The selected internal standard must not be already present in all samples, blanks, and standard solutions

- Internal standard must not interact with analyte

Internal Standard- Known amount of a nonanalyte species that is added to all

samples, blanks, and standards

CALIBRATION METHODS


- For an analyte (A) and internal standard (S)

Signal ratio (A/S) is plotted against concentration ration (A/S)

Concentration ratio (A/S) of unknown is obtained from the linear equation

CALIBRATION METHODS

standard of (A/S) ratio signal

sampleunknownof(A/S)ratiosignal

standardof(A/S)ratioionConcentrat

sampleunknownof(A/S)ratioionConcentrat


Corrects errors due to

- Voltage fluctuations

- Loss of analyte during sample preparation

- Change in volume due to evaporation

- Interferences

CALIBRATION METHODS

- Indeterminate (random) errors are associated with all spectroscopic methods

Examples- Noise due to instability of light source

- Detector instability- Variation in placement of cell in light path

- Finger prints on cells

ERRORS ASSOCIATED WITH BEER’S LAW

EVALUATION OF ERRORS

TlogT

T0.434

c

ΔcionconcentratinerrorRelative

- ΔT is the error in transmittance measurement

- The relative error is high when T is very high or very low

- For greatest accuracy, measurements should be within 15% - 65% T or 0.19 - 0.82 A

- Samples with high concentration (A > 0.82) should be diluted and those with low concentrations (A < 0.19) should

be concentrated

EVALUATION OF ERRORS

Ringbom Method

(100 – %T) is plotted against log(c)

- The result is an s-shaped curve (Ringbom plot)

- The nearly linear portion of the curve (the steepest portion) is the working range where error is minimized

(100-%T)

Log(c)

OPTICAL SYSTEMS IN SPECTROSCOPY

Fundamental Concepts of Optical Measurements

- Measurement of absorption or emission of radiation

- Providing information about the wavelength of absorption or emission

- Providing information about the intensity or absorbance at the wavelength


Main Components of Spectrometers

- Radiation source

- Wavelength selection device

- Sample holder (transparent to radiation)

- Detector


- FT spectrometers do not require wavelength selector

- Radiation source is the sample if emission is being measured

- External radiation source is required if absorption is being measured

- Sample holder is placed after wavelength selector for UV-VIS absorption spectrometry so that monochromatic light falls

on the sample

- Sample holder is placed before the wavelength selector for IR, fluorescence, and AA spectroscopy

COMPONENTS OF THE SPECTROMETER

Po PLightsource

monochromator

(λ selector) sample readout detector

b

Absorption (UV-Vis)

Lightsource

monochromator

(λ selector) sample readout detector

Absorption (IR)


Source& sample

monochromator

(λ selector)

readout detector

Emission

- Sample is an integral portion of the source

- Used to produce the EM radiation that will be measured


Sourceλ selector

sample

monochromator

(λ selector)readout detector

Fluorescence


- Must emit radiation over the entire wavelength range being studied

- Intensity of radiation of the wavelength range should be high

- A reliable and steady power supply is essential to provide constant signal

- Intensity should not fluctuate over long time intervals

- Intensity should not fluctuate over short time intervals

Flicker: short time fluctuation in source intensity

RADIATION SOURCE

Two types of radiation sources

Continuum Sources and

Line Sources

RADIATION SOURCE

Continuum Sources- Emit radiation over a wide range of wavelengths

- Intensity of emission varies slowly as a function of wavelength- Used for most molecular absorption and fluorescence

spectrometric instruments

Examples- Tungsten filament lamp (visible radiation)

- Deuterium lamp (UV radiation)- High pressure Hg lamp (UV radiation)

- Xenon arc lamp (UV-VIS region)- Heated solid ceramics (IR region)

- Heated wires (IR region)

RADIATION SOURCE

Line Sources- Emit only a few discrete wavelengths of light

- Intensity is a function of wavelength- Used for molecular, atomic, and Raman spectroscopy

Examples- Hollow cathode lamp (UV-VIS region)

- Electrodeless discharge lamp (UV-VIS region)- Sodium vapor lamp (UV-VIS region)- Mercury vapor lamp (UV-VIS region)

- Lasers (UV-VIS and IR regions)

RADIATION SOURCE

RADIATION SOURCE

Tungsten Filament Lamp- Glows at a temperature near 3000 K

- Produces radiation at wavelengths from 320 to 2500 nm- Visible and near IR regions

Dueterium (D2) Arc Lamp- D2 molecules are electrically dissociated

- Produces radiation at wavelengths from 200 to 400 nm- UV region

Mercury and Xenon Arc Lamps- Electric discharge lamps

- Produce radiation at wavelengths from 200 to 800 nm- UV and Visible regions

Silicon Carbide (SiC) Rod - Also called globar

- Electrically heated to about 1500 K- Produces radiation at wavelengths from 1200 to 40000 nm

- IR region

RADIATION SOURCE

Also for IR Region

- NiChrome wire (750 nm to 20000 nm)

- ZrO2 (400 nm to 20000 nm)

RADIATION SOURCE

Laser

- Produce specific spectral lines- Used when high intensity line source is required

Can be used forUV

Visible FTIR

RADIATION SOURCE

WAVELENGTH SELECTION DEVICES

Two types

Filters and

Monochromators

FILTERS

- The simplest and most inexpensive

Two major types

Absorption Filters and

Interference Filters

FILTERS

Absorption Filters

- A piece of colored glass

- Stable, simple and cheap

- Suitable for spectrometers designed to be carried to the field

Disadvantage- Range of wavelengths transmitted is very broad (50 – 300 nm)

FILTERS

Interference Filters

- Made up of multiple layers of materials

- The thickness and the refractive index of the center layer of the material control the wavelengths transmitted

- Range of wavelengths transmitted are much smaller (1 – 10 nm)

- Amount of light transmitted is generally higher

- Transmits light in the IR, VIS, and UV regions

MONOCHROMATORS

- Disperse a beam of light into its component wavelengths- Allow only a narrow band of wavelengths to pass

- Block all other wavelengths

Components- Dispersion element

- Two slits (entrance and exit)- Lenses and concave mirrors

MONOCHROMATORS

Dispersion Element

- Disperses (spreads out) the radiation falling on it according to wavelength

Two main TypesPrisms

andGratings

MONOCHROMATORS

Prisms

- Used to disperse IR, VIS, and UV radiations

- Widely used is the Cornu prism (60o-60o-60o triangle)

ExamplesQuartz (UV)

Silicate glass (VIS or near IR)NaCl or KBr (IR)

MONOCHROMATORS

Prisms

- Refraction or bending of incident light occurs when a polychromatic light hits the surface of the prism

- Refractive index of prism material varies with wavelength

- Various wavelengths are separated spatially as they are bent at different degrees

- Shorter wavelengths (higher energy) are bent more than longer wavelengths (lower energy)

MONOCHROMATORS

Diffraction Gratings

- Consists of a series of closely spaced parallel grooves cut(or ruled) into a hard glass, metallic or ceramic surface

- The surface may be flat or concave

- Reflective coating (e.g. Al) is usually on the ruled surface

- Used for UV-VIS radiation (500 – 5000 grooves/mm) and IR radiation (50 – 200 grooves/mm)

MONOCHROMATORS

d

Top view Side view


MONOCHROMATORS


- Size ranges between 25 x 25 mm to 110 x 110 mm

- Light is dispersed by diffraction due to constructive interference between reflected light waves

- Separation of light occurs due to different wavelengths being dispersed (diffracted) at different angles

MONOCHROMATORS


- Constructive interference occurs when

nλ = d(sini ± sinθ)

n = order of diffraction (integer: 1, 2, 3, …)λ = wavelength of radiation

d = distance between groovesi = incident angle of a beam of light

θ = angle of dispersion of light

MONOCHROMATORS

Dispersive Resolution

Resolving Power (R): - Ability to disperse radiation

- Ability to separate adjacent wavelengths from each other

δλ

λR

λ = average of the wavelengths of the two lines to be resolved

δλ = difference between the two wavelengths

MONOCHROMATORS

Resolution of a Prism

dλ

dηtR

t = thickness of the base of the prism

dη/dλ = rate of change of the refractive index (η) with λ

- Resolving power increases with thickness of the prism and decreases at longer wavelengths

- Resolution depends on the prism material

MONOCHROMATORS

Resolution of a Grating

R = nN

n = the order

N = total number of grooves in the grating that are illuminated by light from the entrance slit (whole number)

Increased resolution results from - Longer gratings

- Smaller groove spacing- Higher order

MONOCHROMATORS

Dispersion of a Grating

dy

dλ)(D dispersion Reciprocal 1-

dλ = change in wavelength

dy = change in distance separating the λs along the dispersion axis

Units: nm/mm

MONOCHROMATORS

nF

dD 1-

Dispersion of a Grating

Spectral bandwidth (bandpass) = sD-1

s = slit width of monochromator

d = distance between two adjacent groovesn = diffraction order

F = focal length of the monochromator system

- D-1 is constant with respect to wavelength

ECHELLE MONOCHROMATOR

Echellette Grating

- Grooved or blazed such that it has relatively broad faces from which reflection occurs

- Has narrow unused faces

- Provides highly efficient diffraction grating

ECHELLE MONOCHROMATOR

- Contains two dispersion elements arranged in series

- The first is known as echelle grating

- The second (called cross-dispersion) is a low-dispersion prism or a grating

Echelle grating- Greater blaze angle

- The short side of the blaze is used rather than the long side- Relatively coarse grating

- Angle of dispersion (θ) is higher- Results in 10-fold resolution

OPTICAL SLITS

- Slits are used to select radiation from the light source both before and after dispersion by the λ selector

- Made of metal in the shape of two knife edges

- Movable to set the desired mechanical width

OPTICAL SLITS

Entrance Slit

- Allows a beam of light (polychromatic) from source to fall on the dispersion element

- Radiation is collimated into a parallel beam with lenses or front-faced mirrors

- One (selected) wavelength of light (monochromatic) is focused on the exit slit after dispersion

OPTICAL SLITS

Exit Slit

- Allows only a very narrow band of light to pass through sample and detector

- The dispersed light falls on the exit slit

- The light is redirected and focused onto the detector for intensity measurements

- Slits are kept as close as possible to ensure resolution

CUVET (SAMPLE CELL)

- Cell used for spectrometry

Identical or Optically Matched Cells - Cells that are identical in their absorbance or transmittance of light

Fused silica Cells (SiO2)-Transmits visible and UV radiation

Plastic and Glass Cells- Only good for visible wavelengths

NaCl and KBr Crystals- IR wavelengths

DETECTORS

- Used to measure the intensity of radiation coming out of the exit slit

- Produces an electric signal proportional to the radiation intensity

- Signal is amplified and made available for direct display

- A sensitivity control amplifies the signal

- Noisy signal is observed when amplification is too much

- May be controlled manually or by a microprocessor(the use of dynodes)

DETECTORS

Examples- Phototube (UV)

- Photomultiplier tube (UV-VIS)- Thermocouple (IR)

- Thermister (IR)- Silicon photodiode- Photovoltaic cell

- Charge Transfer Devices (UV-VIS and IR)Charge-coupled devices (CCDs)Charge injector devices (CIDs)

SINGLE-BEAM OPTICS

- Usually used for all emission methods where sample is at the location of the source

Drift- Slow variation in signal with time

- Can cause errors in single-beam methods

Sources of Drift- Changes in Voltage which changes source intensity

- Warming up of source with time- Deterioration of source or detector with time

Single-Beam Spectrometer

- Only one beam of light

- First measure reference or blank (only solvent) as Io

Io ILightsource

monochromator(selects λ) sample computer detector

b

SINGLE-BEAM OPTICS

DOUBLE-BEAM OPTICS

- Widely used

- Beam splitter is used to split radiation into two approximately equal beams (reference and sample beams)

- Radiation may also alternate between sample and referencewith the aid of mirrors (rotating beam chopper)

- Other variations are available

- The reference cell may be empty or containing the blank

- More accurate since it eliminates drift errors

Double-Beam Spectrometer

- Houses both sample cuvet and reference cuvet

Po

PLightsource

monochromator(selects λ) sample computer detector

reference

b

DOUBLE-BEAM OPTICS

SPECTROPHOTOMETERS

Photodiode Array Spectrophotometers- Records the entire spectrum (all wavelengths) at once

- Makes use of a polychromator

- The polychromator disperses light into component wavelengths

Dispersive Spectrophotometers- Records one wavelength at a time

- Makes use monochromator to select wavelength

FOURIER TRANFORM SPECTROPHOTOMETERS

- Have no slits and fewer optical elements

Multiplex- Instrument that uses mathematical methods to interpret and

present spectrum without dispersion devices

- Wavelengths of interest are collected at a time without dispersion

- The wavelengths and their corresponding intensities overlap

- The overlapping information is sorted out in order to plot a spectrum


- Sorting out or deconvoluting the overlapping signals of varying wavelengths (or frequencies) is a mathematical procedure

called Fourier Analysis

- Fourier Analysis expresses complex spectrum as a sum of sine and cosine waves varying with time

- Data acquired is Fourier Transformed into the spectrum curve

- The process is computerized and the instruments employing this approach are called FT spectrometers


Advantages of FT Systems

- Produce better S/N ratios (throughput or Jacquinot advantage)

- Time for measurement is drastically reduced (all λs are measured simultaneously)

- Accurate and reproducible wavelength measurements

instrumental analysis chem 4811 chapter 2 dr. augustine ofori agyeman assistant professor of...

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