Components of Optical Instruments - Instrumental Analysis

Components  of  Op,cal  Instruments  

CHEM  314  Skoog  n  Holler  Ch  7  

Objec,ves  •  State  the  components  and  phenomena  that  can  be  

probed  with  op,cal  instruments.  •  Recall  the  methods  of  wavelength  isola,on  •  Diagram,  label,  describe,  and  compare  prism-­‐  vs  

diffrac,on-­‐based  monochromators  •  State  and  be  able  to  perform  calcula,ons  related  

to  mono  performance  characteris,cs  and  λ  dispersion.  

•  Recall  UV-­‐Vis  detectors  •  Diagram,  label,  describe,  and  compare  the  

following  detectors:  PMT,  silicon  diode  

Op,cal  Instrumenta,on  Phenomena  probed  •  Absorp,on  •  Luminescence  •  Emission    •  ScaQering  

Components  1.  Stable  radia,on  source  2.  Transparent  sample  holder  3.  Wavelength  isola,on  4.  Detector  5.  Signal  processing  

Jablonski  Diagram  

0.0 0.1 0.2 0.3 0.4 0.50.0

0.2

0.4

0.6

0.8

1.0

Abs

orba

nce

(arb

)

[Kool-aid]

A=1.9 [Kool-aid]r2=0.97

0.0 0.1 0.2 0.3 0.4 0.50.0

0.2

0.4

0.6

0.8

1.0

Abs

orba

nce

(arb

)

[Kool-aid]

Beer’s  Law:  the  rela,onship  between  concentra,on  and  absorbance  

A=ε b  [Kool-­‐aid]  b=  path  length  

y=mx  +  b  

ε molar  absorb,vity    

Sources  of  Nonlinearity  of  Beer’s  law    

1.  Solu,on  factors  2.  Non-­‐monochroma,c  light  3.  Not  analyzing  at  lmax    4.  Stray  light  5.  Mismatched  cuveQes  6.  Instrument  noise  

0.0 0.5 1.0 1.5 2.0 2.5 3.00.0

0.5

1.0

1.5

2.0

2.5

3.0

Abs

orba

nce

(arb

)

[Kool-aid]

Components  of  Op,cal  Instruments  

CHEM  314  Skoog  n  Holler  Ch  7  

Building  a  spectroscopic  instrument  

Building  a  spectroscopic  instrument  

This  lecture  will  focus  on  common  components  of  instruments  for  atomic  and  molecular  spectroscopies    

Components  1.  Stable  radia,on  source  2.  Wavelength  isola,on  3.  Transparent  sample  holder/  op,cs  4.  Detector  5.  Signal  processing  

Source  

Why  does  this  chart  differen,ate  between  line  and  con,nuum  sources?  When  would  you  use  a  line  rather  than  con,nuum  source?  

Op,cs  

Sample  cuveQes  

0  

1  

2  

3  

4  

190   490   790   1090  

Absorban

ce    

Wavelength  (nm)  

Quartz  or  plas;c?  

Quartz   Plas,c  

Building  a  spectroscopic  instrument  

Components  1.  Stable  radia,on  source  2.   Wavelength  isola;on  3.  Transparent  sample  holder/  op,cs  4.   Detector  5.  Signal  processing  

Wavelength  selec,on  

Bandwidth  measurements  

Filters  

Filters  

Monochromators  1.  Entrance  slit-­‐  provides  rectangular  op,cal  image  2.  Collima,ng  lens  or  mirror-­‐  makes  light  beams  parallel  3.  Dispersive  element-­‐  disperses  light  into  component  

wavelengths  4.  Focusing  element-­‐  reforms  rectangular  op,cal  image  

focused  on  focal  plane  5.  Exit  slit-­‐  on  focal  plane,  selects  desired  bandwidth  

Monochromators  Prisms  vs  Gra,ngs  

Refrac,on  

Reflec,on  

λ1>λ2  

Consider  the  figures,  is  λ1  or  λ2  the  longer  λ  and  why.    

Monochromators:  Prisms  vs  Gra,ngs  

When  might  a  prism  be  beQer  than  a  diffrac,on  mono?  

Prisms  

Snell’s  law  

Refrac,ve  index  

Prisms   Bunsen  Prism  

Prisms  Cornu  Prism   LiQrow  Prism  

Reflec,on  gra,ng  Monochromator  

λ1>λ2  

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EchelleQe-­‐  diffrac,on  long  edge  

Monochromator  Performance  Characteris,cs  1.  Spectral  purity  2.  Dispersion  of  gra,ng  (D)        

 Reciprocal  linear  dispersion  (D-­‐1)  3.  Resolving  power  (R=  λ/Δλ)  4.  Effec,ve  bandwidth  (Δλeff)  5.  Light  gathering  power  (F)      Focal  length  (f)  

Effec,ve  bandwidth  

Monochromator  Performance  Characteris,cs  1.  Spectral  purity  2.  Dispersion  of  gra,ng  (D)        

 Reciprocal  linear  dispersion  (D-­‐1)  3.  Resolving  power  (R=  λ/Δλ)  4.  Effec,ve  bandwidth  (Δλeff)  5.  Light  gathering  power  (F)      Focal  length  (f)  

Echelle  Monochromator  

Echelle  Monochromator  

Building  a  spectroscopic  instrument  

Components  1.  Stable  radia,on  source  2.   Wavelength  isola;on  3.  Transparent  sample  holder/  op,cs  4.   Detector  5.  Signal  processing  

Detectors  

Detectors  

Lytle,  1974  

Ideal  detectors  1.  High  sensi,vity  2.  High  signal  to  noise  3.  Constant  detector  response  as  a  func,on  of  l  4.  Fast  response  ,me  5.  No  dark  current  6.  Signal  propor,onal  to  radiant  power  7.  Rugged,  cheap,  simple  

 S  =  kP  +  kd  

Photomul,plier  tube  (PMT)  

pn  junc,ons  

Silicon  photodiode  

Mul,channel  Si-­‐based  detectors  Photodiode  array  (PDA)  Charge  Injec,on  Device  (CID)  Charge  Coupled  Device  (CCD)    

Mul,channel  Si-­‐based  detectors  Photodiode  array  (PDA)  Charge  Injec,on  Device  (CID)  Charge  Coupled  Device  (CCD)    

Mul,channel  Si-­‐based  detectors  Photodiode  array  Charge  Injec,on  Device  (CID)  Charge  Coupled  Device  (CCD)    

Comparing  Detector  Sensi,vity  

detector   λ 1  s   10  s   100  s  PMT   UV   30   6.3   1.8  

Vis   122   26   7.3  PDA   UV   6000   671   112  

Vis   3300   363   62  CCD   UV   31   3.1   0.3  

Vis   17   1.7   0.2  

Harris,  Table  19-­‐2