Introduction to Introduction to Spectrochemical Spectrochemical

MethodsMethods

Chapter 7Chapter 7

IntroductionIntroduction Spectrochemical methodsSpectrochemical methods

– Absorption or emission of lightAbsorption or emission of light More than half or all instrumental methods of analysisMore than half or all instrumental methods of analysis

– Spectroscopy or spectrometrySpectroscopy or spectrometry Science that deals with lightScience that deals with light

– Its absorption and emission by solutionsIts absorption and emission by solutions– Other material substancesOther material substances

Instrument usedInstrument used– SpectrometerSpectrometer

When using a light sensor/phototube = spectrophotometerWhen using a light sensor/phototube = spectrophotometer– Spectrochemical analysisSpectrochemical analysis

Degree which light absorbed or the primary light emittedDegree which light absorbed or the primary light emitted– Related to the amount of analyte present in the sampleRelated to the amount of analyte present in the sample– Critical measurements!Critical measurements!

Atomic spectroscopyAtomic spectroscopy– Spectral differences between atomsSpectral differences between atoms– Molecular spectroscopyMolecular spectroscopy

Characterizing LightCharacterizing Light Dual nature of lightDual nature of light

– ParticlesParticles Photons or quantaPhotons or quanta

– Particle theory of lightParticle theory of light

– WavesWaves Electromagnetic disturbances or electromagnetic Electromagnetic disturbances or electromagnetic

waveswaves– Wave theory of lightWave theory of light

– Dual nature not unlike modern description of Dual nature not unlike modern description of electronselectrons Described as particles Described as particles

– To explain aspects of their behaviorTo explain aspects of their behavior For more accurate descriptionFor more accurate description

– Must be described as entities of energy and NOT Must be described as entities of energy and NOT particlesparticles

Characterizing LightCharacterizing Light

Wave theory of lightWave theory of light– Light travels in a fashion similar to that Light travels in a fashion similar to that

of a series of repeating waves of waterof a series of repeating waves of water Wave pool at an amusement parkWave pool at an amusement park

– Electromagnetic wavesElectromagnetic waves They are wave disturbances that have an They are wave disturbances that have an

electrical component and a magnetic electrical component and a magnetic componentcomponent

– Do NOT require matter to existDo NOT require matter to exist Can travel through a vacuumCan travel through a vacuum

Characterizing LightCharacterizing Light

Wavelength, speed, frequency, Wavelength, speed, frequency, energy, and wavenumberenergy, and wavenumber– Wavelength (Wavelength (λλ) )

The physical distance from a point on one The physical distance from a point on one wave, to the same point on the next wavewave, to the same point on the next wave

– Measured in metric unitsMeasured in metric units

Characterizing LightCharacterizing Light

Wavelength, speed, frequency, Wavelength, speed, frequency, energy, and wavenumberenergy, and wavenumber– Speed of light (Speed of light (cc))

Speed in which electromagnetic waves Speed in which electromagnetic waves movemove

– Speed of light in a Vacuum Speed of light in a Vacuum ≈ 3.00 x 10≈ 3.00 x 101010 cm/sec cm/sec Accounts for instantaneous speed that light Accounts for instantaneous speed that light

fills a room when switch turned onfills a room when switch turned on ALLALL ELECTORMAGNETIC WAVES TRAVEL ELECTORMAGNETIC WAVES TRAVEL

AT THE SAME SPEED IN A VACUUM AT THE SAME SPEED IN A VACUUM REGARDLESS OF THEIR WAVELENGTHREGARDLESS OF THEIR WAVELENGTH

Characterizing LightCharacterizing Light Wavelength, speed, frequency, energy, Wavelength, speed, frequency, energy,

and wavenumberand wavenumber– Frequency (Frequency (νν = nu) = nu)

Number of moving electromagnetic waves past a Number of moving electromagnetic waves past a fixed point in 1 secondfixed point in 1 second

– Expressed in waves or cycles per secondExpressed in waves or cycles per second = hertz (Hz)= hertz (Hz) Units = secUnits = sec-1 -1

Wavelength, speed, frequency can be expressed Wavelength, speed, frequency can be expressed mathematicallymathematically

C = C = λνλν units are cm x secunits are cm x sec-1-1 or cm/sec or cm/sec– Wavelength and frequency are inversely Wavelength and frequency are inversely

proportionalproportional As one increases As one increases Other decreasesOther decreases

Characterizing LightCharacterizing Light Wavelength, speed, frequency, energy, and Wavelength, speed, frequency, energy, and

wavenumberwavenumber– EnergyEnergy

B/c light is a form of energyB/c light is a form of energy– Each wavelength or frequency has certain amount of Each wavelength or frequency has certain amount of

energyenergy– Considered to be the energy associated with a single Considered to be the energy associated with a single

photon of lightphoton of light .: particle theory and wave theory linked via energy.: particle theory and wave theory linked via energy

E = hE = hνν E = energy, h = proportionality constant called E = energy, h = proportionality constant called

Planck’s constantPlanck’s constant depends on units used-metric = 6.63 x 10depends on units used-metric = 6.63 x 10-34-34 J/sec J/sec

.: E.: E = hc/ = hc/λλ

Characterizing LightCharacterizing Light

Wavelength, speed, frequency, Wavelength, speed, frequency, energy, and wavenumberenergy, and wavenumber– Wavenumber (Wavenumber (νν))

Wavelength expressed in centimetersWavelength expressed in centimeters– Characterized by the reciprocal of this wavelengthCharacterized by the reciprocal of this wavelength

νν = 1/ = 1/ λλ (cm) (cm) Used in conjunction with infrared Used in conjunction with infrared light.light.

The Electromagnetic The Electromagnetic SpectrumSpectrum

The Electromagnetic The Electromagnetic SpectrumSpectrum

Electromagnetic Electromagnetic spectrumspectrum– So broad broken So broad broken

down into regionsdown into regions Visible lightVisible light

– That portion of the That portion of the spectrum we see spectrum we see with our eyeswith our eyes

≈ ≈ 350 nm to 350 nm to ≈750 nm≈750 nm

Very narrow Very narrow regionregion

UV, infrared, x-ray, UV, infrared, x-ray, radio, and televisionradio, and television

The Electromagnetic The Electromagnetic SpectrumSpectrum

Electromagnetic SpectrumElectromagnetic Spectrum– UV, visible, and infrared regionsUV, visible, and infrared regions

Mostly ones emphasizedMostly ones emphasized– Nanometer and micrometer units used for wavelengthNanometer and micrometer units used for wavelength– Something to rememberSomething to remember

Long wavelength = low energyLong wavelength = low energy Infrared regionInfrared region

– Wavelengths extremely shortWavelengths extremely short– Have higher energy than radio or televisionHave higher energy than radio or television– Cause no harmCause no harm

Remotes for TVs, VCRs, etcRemotes for TVs, VCRs, etc UV, x-rays, and gamma raysUV, x-rays, and gamma rays

– Very short wavelengthsVery short wavelengths– Very high energyVery high energy– Very dangerous!Very dangerous!

Interaction of Light With Interaction of Light With MatterMatter

Light striking matter causes different Light striking matter causes different eventsevents– TransmittedTransmitted

Pass without interaction through the materialPass without interaction through the material– Light passing through glassLight passing through glass

– ReflectedReflected Changes directionsChanges directions

– Light in a mirrorLight in a mirror

– ScatteredScattered Deflected into many different directionsDeflected into many different directions Occurs when light strikes a substance composed of Occurs when light strikes a substance composed of

many individual, small particlesmany individual, small particles– AbsorbedAbsorbed

Light fives up some or all of its energy to the material Light fives up some or all of its energy to the material

Absorption SpectraAbsorption Spectra

Instruments used to measure absorptionInstruments used to measure absorption– Some in the UV and visible regionsSome in the UV and visible regions– Others for the infrared region Others for the infrared region – Methods usedMethods used

Beam of light formedBeam of light formed Sample measured contained so that light passes Sample measured contained so that light passes

throughthrough Absorption of the wavelengths present in light Absorption of the wavelengths present in light

beam measured by a sensor and signal beam measured by a sensor and signal processorprocessor

Absorption SpectraAbsorption Spectra

An absorption spectrumAn absorption spectrum– Plot of the amount of light absorbed by a Plot of the amount of light absorbed by a

sample vs. the wavelength of the lightsample vs. the wavelength of the light Light absorbed called the absorbance (A)Light absorbed called the absorbance (A) Obtained by using a spectrometer toObtained by using a spectrometer to

– Scan a particular wavelength regionScan a particular wavelength region– To observe amount of light absorbed by the sample To observe amount of light absorbed by the sample

along the wayalong the way

– It’s a continuous spectrum (fig. 7.13, pg. It’s a continuous spectrum (fig. 7.13, pg. 189)189) The spectrum is an unbroken patternThe spectrum is an unbroken pattern

– Does not display breaks or sharp peaksDoes not display breaks or sharp peaks

Absorption SpectraAbsorption Spectra

Absorption vs. wavelength Absorption vs. wavelength – Can be displayed as a transmission spectrumCan be displayed as a transmission spectrum

Plotting the amount of light transmitted by a samplePlotting the amount of light transmitted by a sample Rather than the light absorbedRather than the light absorbed

– y-axis is transmittance (T) or percent transmittance y-axis is transmittance (T) or percent transmittance instead of the absorbance (fig. 7.16, pg. 191)instead of the absorbance (fig. 7.16, pg. 191)

High transmittance = low absorbance and vice High transmittance = low absorbance and vice versaversa

– Absorption patternAbsorption pattern Differs from compound to compoundDiffers from compound to compound

– ““molecular fingerprint”molecular fingerprint” Often useful for identificationOften useful for identification Detecting impuritiesDetecting impurities Other sample componentsOther sample components

Absorption SpectraAbsorption Spectra Light EmissionLight Emission

– Matter will emit lightMatter will emit light Molecular and ionic analytesMolecular and ionic analytes Useful for qualitative and quantitative analysisUseful for qualitative and quantitative analysis Called an emission spectrumCalled an emission spectrum

– Plot of emission intensity vs. wavelengthPlot of emission intensity vs. wavelength– Product of the change in the energy level of an electronProduct of the change in the energy level of an electron

From excited state to ground state (lowest energy From excited state to ground state (lowest energy level)level)

– FluorescenceFluorescence When molecules/complex ions emit light under certain When molecules/complex ions emit light under certain

conditionsconditions– When absorption of light in the UV region is followed by When absorption of light in the UV region is followed by

emission of light in the visible regionemission of light in the visible region– Involves the loss in energy from an excited state to a Involves the loss in energy from an excited state to a

lower statelower state

Absorbance, Transmittance, Absorbance, Transmittance, and Beer’s Lawand Beer’s Law

A = A = εεbcbc– Beer’s LawBeer’s Law

εε = extinction coefficient or absorptivity = extinction coefficient or absorptivity– Units depends on other parametersUnits depends on other parameters

Absorbance is dimensionless quantityAbsorbance is dimensionless quantity b = path lengthb = path length

– Distance the light travels through the measured solutionDistance the light travels through the measured solution Inside diameter of the sample containerInside diameter of the sample container Usually centimeters or millimetersUsually centimeters or millimeters

c = concentrationc = concentration– Expressed in any concentration unitExpressed in any concentration unit– Usually expressed in molarity, ppm, or grams/100ml.Usually expressed in molarity, ppm, or grams/100ml.

.: c= molarity, b = cm, then absorptivity = L mol.: c= molarity, b = cm, then absorptivity = L mol-1-1 cm cm-1-1

Absorbance, Transmittance, Absorbance, Transmittance, and Beer’s Lawand Beer’s Law

Container to be usedContainer to be used– Varies according to the methodVaries according to the method

UV-VISUV-VIS– Small test tube or square tube with an inside path Small test tube or square tube with an inside path

length of 1 cmlength of 1 cm Called a cuvetteCalled a cuvette

IRIR– Container called the IR liquid sampling cellContainer called the IR liquid sampling cell

Sample contained in a space between two salt Sample contained in a space between two salt platesplates

Created with a thin spacer between the platesCreated with a thin spacer between the plates Path length is the thickness of the spacerPath length is the thickness of the spacer

Absorbance, Transmittance, Absorbance, Transmittance, and Beer’s Lawand Beer’s Law

Quantitative analyses by Beer’s lawQuantitative analyses by Beer’s law– Prepared series of standard solutionsPrepared series of standard solutions

Measure absorbance of each in identical Measure absorbance of each in identical containerscontainers

Plotting the measured absorbance vs. Plotting the measured absorbance vs. concentrationconcentration

– Creates a standard curveCreates a standard curve

Absorbance of an unknown solution then Absorbance of an unknown solution then measured and concentration determined measured and concentration determined from the standard datafrom the standard data