Spectroscopy 1

8/18/2019 Spectroscopy 1

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Optical Electronic Spectroscopy 1

Lecture Date: January 23 rd , 2008


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The Electroma netic Spectrum

• UV-Visible• X-ray


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!hat is Electronic Spectroscopy"

• Spectroscopy of the electrons surrounding an atom or amolecule: electron energy-level transitions

Atoms: electrons are inhydrogen-like orbitals

(s, p, d, f)

Molecules: electrons are inmolecular orbitals (HOMO,

L MO, !)

("he L MO of ben#ene)("he $ohr model for nitrogen)

Fromhttp://education.jlab.org


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Optical Electronic Spectroscopy

• Definition: Spectroscopy in the optical (UV-Visible) rangeinvolving electronic energy levels excited byelectromagnetic radiation (often valence electrons )

• !his lecture is related to the "high-energy# ("non-optical#)electron spectroscopy covered in the X-ray lecture

• $ethods: % &tomic absorption % &tomic emission (e g ' -*+S) % $olecular UV-Visible absorption % ,uminescence .luorescence hosphorescence


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De#initions o# Electronic $rocesses

• +mission: radiation produced by excited molecules ionsor atoms as they relax to lo/er energy levels• &bsorption: radiation selectively absorbed by moleculesions or atoms accompanied by their excitation (orpromotion) to a more energetic state

• ,uminescence: radiation produced by a chemical reactionor internal electronic process possibly follo/ingabsorption


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%ore Electronic $rocesses

• .luorescence: absorption of radiation to an excited statefollo/ed by emission of radiation to a lo/er state of thesame multiplicity % *ccurs about 01 -2 to 01-3 seconds after photon absorption

• hosphorescence: absorption of radiation to an excitedstate follo/ed by emission of radiation to a lo/er state ofdifferent multiplicity % *ccurs about 01 to 01 -2 seconds after photon absorption


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!hat is Emission"

• &toms4molecules are driven to excited states (in this caseelectronic states) /hich can relax by emission ofradiation

$ 5 heat $6

• *ther process can be active such as "non-radiative#relaxation (e g transfer of energy by random collisions)

$6 $ 5 heat

∆E 7 h ν

&i her ener y

Lo'er ener y

• *+S 7 *ptical +mission Spectroscopy


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!hat is ()sorption"

• +lectromagnetic radiation travels fastest in a vacuum % 8hen +$ radiation travels through a substance it can be slo/edby propagation "interactions# that do not cause fre9uency

(energy) changes:

• &bsorption does involve fre9uency4energy changes sincethe energy of +$ radiation is transferred to a substanceusually at specific fre9uencies corresponding to naturalatomic or molecular energies % &bsorption occurring at optical fre9uencies involves lo/ to mid-

energy electronic transitions

i

i

c n ν =

c 7 the speed of light ( ; 11 x 013 m4s)ν

i 7 the velocity of the radiation in the medium in m4sn i 7 the refractive index at the fre9uency i


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()sorption and Transmission

• !ransmittance:! 7 4

1

b

1

• &bsorbance: & 7 -log01 ! 7 log 01 14

A is linear vs. b!(A preferred over T)

Graphs from http://teaching.shu.ac.u /h b/chemistr"/tutorials/molspec/beers#.htm


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The *eer+Lam)ert La'

• !he


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De iations -rom the *eer+Lam)ert La'

• Deviations from s la/ (i e deviations from thelinearity of absorbance vs concentration): % 'ntermolecular interactions at higher concentrations % hemical reactions (species having different spectra) % ea= /idth4polychromatic radiation

• s la/ is only strictly valid /ith single-fre9uency radiation• @ot significant if the band/idth of the monochromator is less

than 0401 of the half-/idth of the absorption pea= at half-height

%or an alternati&e &ie', see: $are, illiam * A More +edagogically ound "reatment of $eer s La': A eri&ation $ased on a .orpuscular-+robability Model, J. Chem. Educ. $%%%& 77, /0/*


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De iations #rom the *eer+Lam)ert La'

• Deviations caused by use of polychromatic light on aspectrum in /hich changes a lot over the band/idth ofthe light

• onsider t/o /avelengths a and b /ith εa and εb ε 6 7155, 155

ε 6 7815, 015

'oncentration ( )

A b s o r

b a n c e

( A )

++

= −−+ bcbbcaba

ba ba P P

P P A ε ε 7575log 55

55

ε 6 7555, 7555


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*asic .nstrument Layout #orOptical Spectroscopy

• &bsorption:/adiation

Source Sample!a elen th

Selector Detector

photoelectric transducer

• .luorescence hosphorescence and Scattering:Sample !a elen thSelector Detector photoelectric transducer

/adiationsource

• +mission and chemi-luminescence

Samplesource

!a elen thSelector

Detector photoelectric transducer

(A1B angle)


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(tomi ation: The Di idin Line #or (tomic %olecular

• Samples used in opticalatomic (elemental)spectroscopy are usuallyatomiCed

• !his destroys molecules (ifpresent) and leaves theatoms

• !he UV-visible spectrum ofthe atoms is of interest notthe molecular spectrum


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Elemental (nalysis

• Elemental analysis % 9ualitative or 9uantitativedetermination of the elemental composition of a sample

• *ptical electronic methods are heavily used in elementalanalysis

• *ther elemental analysis methods not discussed here: % $ass spectrometry ($S) e g ' -$S % X-ray methods % *ther methods (radiochemical) % lassical


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(tomic Electronic Ener y Le els

• +lectronic energy leveltransitions in hydrogen % the simplest of all

•


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• Used to denoteenergy levels andlabel term (Erotrian)diagrams for thehydrogen atom

%igure from the apphire 4lectronic pectroscopy oft'are +ackage, .a&endish 2nstruments Limited*

• !erm symbols and electronicstates: used to precisely definethe state of electrons

$*/$

s,p,d,f,g(l value)

$*/$

+#/$

jm j

s l 70 +spinmultiplicity

2j+1

s 7 total spin 9uantum number j 7 total angular momentum 9uantum number l 7 orbital 9uantum number (s p d fF)m j 7 state

$Term:

,evel:

-tate:


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(tomic Electronic Ener y Le els• !he population of energy levels partly determines the

intensity of an emission pea=

• !he alues from .a#es pg 8/, "able 7)


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1 2111 01111 02111 ?1111

1

?1111

I1111

H1111

31111

011111

8avelength 4 nm

' n t e n s

i t y

4 & r b

i t r a r y

U n

i t s

The simulated spectrum for the sodium atom


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(tomic Emission

• !/o types of emission spectra: % ontinuum % ,ine spectra

• +xamples: % ' -*+S (inductively-coupled

plasma optical emissionspectroscopy) also =no/n as ' - &+S

% ,'


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Torches and (tomic Emission• Listory: +mission came first (study of sunlight by .raunhofer in

030J identification of spectral "lines#) studied throughout the

0311>s and early 0A11>s &tomiCer4

+mission Source!emperature

(B )

.lame 0J11-;021

lasma (e g' ) I111-3111

+lectric arc I111-2111

+lectric spar= M01111

•


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$lasma Torches• lasma: a lo/-density gas

containing ions and electronscontrolled by +$ forces


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$lasma Torches• 'n the inductively-coupled

plasma (' ) torch the

sample /ill reside forseveral milliseconds atI111-3111G

• *ther torches % directcurrent plasma

• $icro/ave induced plasma

+hoto by Ste e ! ech , http:99'''*cee*&t*edu9program?areas9en&ironmental9teach9smprimer9icpms9icpms*htm@Argon 05+lasma9 ample 052oni#ation

• &n argon ' torch in action:


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%ore on $lasma Torches

iagram from Lagalante, Appl* pect* ;e&ie's* 3B, 7/7 (7///)

• &nother vie/ of an argon ' torch:


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(rc and Spar4 Sources #or (tomic Emission

• &rc and spar= sources % used for 9ualitative analysis oforganic and geological samples % *nly semi-9uantitative because of source instability % Spar= sources achieve higher energies

• Several mg of solid sample is pac=ed bet/eenelectrodes 0-;1 & of current is passed achievingseveral hundred volts potential

• &pplications include metals analysis or cases /heresolids must be analyCed


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(tomic Emission: %ono+ and $olychromators

• Diffraction gratings are usedto select /avelengths (incombination /ith collimatinglens and slits)

• +chelle (ladder) gratings:high dispersion and highresolution

% 0111-0211 grooves4mmtypical for UV-Vis /or= % Ne9uire filters to isolate

"orders# (i e n70)

m λ 7 d (sin i 5 sin r )


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(tomic Emission: Detectors

• &t the end of the spectrometer photons are detected

• ommonly used detectors: % hotomultiplier tubes ( $!) % dynamic range 01A

%Solid-state detectors:• harge-coupled devices ( D) % 0D or ?D arrays

(charge readout or "transfer# devices)• Silicon photodiodes /ith thousands of individualelements

• Very sensitive very /ell-suited to echelle grating

polychromators very fast


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%odern .5$+OES Spectrometers

• +xample system:Varian Vista N*

• .eatures:0 &xial flame vie/

? +chelle gratingpolychromator ; D detector

•D chips are

often made of sub-arrays matched toemission lines

%igure from >arian >ista +;O sales literature*


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Detection Limits o# .5$+OES

• !ypical detection limits(Varian Vista $ X):

• onsiderations includethe number of emissionlines spectral overlap

• ,inearity can spanseveral orders ofmagnitude

• See also .igure 01-0; inS=oog et al

Element !a elen th nmDetection Limit

a6ial u LDetection limit

radial u L &g ;?3 1H3 1 2 0 &l ;AH 02? 1 A I &s 033 A3 ; 0? &s 0A; HAH I 00


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(tomic ()sorption 7 Early &istory

• 'n the beginning % atomic emission /as the only /ay todo elemental analysis via optical spectroscopy

•


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(tomic ()sorption: Sources• Lollo/ cathode lamps % sputtering of an element of

interest generating a line emission spectrum:

• !ypical line/idths of 1 11? nm (1 1?P)• *ther && Sources: electrode-less discharge lamp (+D,) %

see S=oog h A


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(tomic ()sorption: %onochromators• !he monochromator filters out undesired light in &&

(typical band/idths are 0 angstrom41 0 nm)

• Unli=e ' -*+S /here the mono- or polychromatoractually analyCes the fre9uency % 'n other /ords % there is no need to scan the grating Oust set

(aimed through a slit) and run

• +chelle (ladder) gratings are popular:

Figure from T. ang& in 0. 'a1es& ed& 23 ing4s Anal"tical nstrumentation 5andboo 6


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Other -eatures o# (tomic ()sorption Systems

• Sample nebuliCers: roduces aerosols of samples tointroduce into the flame (oxyacetylene is the hottest)

• Detectors: ommon examples are photomultiplier tubesD (charge-coupled devices) and many more

• $onochromator: removes emissions from the flame(flame is often =ept cool Oust to avoid emission)• $odulated source (chopper): also removes the remainingemissions from the flame !he signal of interest is givenan & modulation and passed through a high-pass filter

• Spectral interferences: % &bsorption from other things (besides the element of interest) %other flame components particulates etcF Scattering cancause similar problems

%


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Detection Limits o# (tomic ()sorption Systems


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&o' (re Elements (ctually (naly ed"

• .or && and ' -*+S samples are dissolved or digestedinto solution• Samples are flo/ed into the flame4plasma and analyCed

• !/o methods for 9uantitative analysis: % Standard calibration: the un=no/n sample>s

absorbance4emission is compared /ith several references /hich"brac=et# the expected concentration (,inear relationship) % Standard addition: the un=no/n sample is divided into several

portions *ne portion is directly analyCed the others have thereference material added in varying amounts !he linear

relationship is determined and the intercept is used to calculatethe real concentration of the un=no/n

• &t the end: the results yield elements in ppm ppbmg4m, etcF


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(tomic -luorescence• 'nstrumentation

Sample !a elen thSelector Detector

photoelectric transducer

/adiationsource

(A1B angle)

• Sources include hollo/-cathode lamps

electrodeless dischargetubes (brighter) and lasers(brightest)

+icture from +erkin-4lmer

L + d d * 4d ' S t L *S


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Laser+.nduced *rea4do'n Spectroscopy L.*S• Qust li=e ' -*+S except a focussed laser creates the

plasma:

Figure from 7- Arm"/Ames

Fiber optic

El t l ( l i 'ith O ti l S t


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Elemental (nalysis 'ith Optical Spectroscopy• & comparison of the techni9ues % the choice is not al/ays clear

lasma +mission(' -*+S)

&& (.lame) &tomic.luorescence

Dynamic Nange 8ide ,imited 8ideRualitative &nalysis Eood oor oor $ultielement Scan Eood oor oor

!race &nalysis Eood Eood Eood

Small samples Eood Eood Eood$atrix interferences ,o/ Ligh ,o/

Spectralinterferences

Ligh ,o/ ,o/

ost $oderate ,o/ $oderate

• Speciated analysis: !he analysis of atomic "species# elements inchemically distinguishable environments • +xamples of hyphenation to add "speciation#:

% ' -*+S coupled to a L ,

% && coupled to a E


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&ome'or4 $ro)lemsOptical Electronic Spectroscopy

hapter 3:roblem 3-A

hapter 01:roblem 01-?


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-urther /eadin

Nevie/ S=oog et al hapters H-01Nevie/ aCes hapters ;-I

*ptical +lectronic SpectroscopyL & Strobel and 8 N Leineman " hemical

'nstrumentation: & Systematic &pproach# ;rd +d8iley (0A3A)

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