Post on 01-Sep-2019
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