lecture 29 ch18&19

8/9/2019 Lecture 29 Ch18&19

1/18

CHM 320 Lecture 30 Chapt. 18, 20

Chapter 18 Fundamentals of

Spectrophotometry (contd)AND

Chapter 20 - Spectrophotometers

Homework for Chapter 20 Due Friday, April 19

Problems 20-1, 20-3, 20-4, 20-6, 20-26 Note: We will talk about some applications of spectrophotometryin class, but will not have any homework assigned from Chapt. 19.

8/9/2019 Lecture 29 Ch18&19

2/18


Absorption of Light

8/9/2019 Lecture 29 Ch18&19

3/18


T = P/P owhere T => transmittance

P => power of transmittedradiationPo => power of incident

radiation%T = (P/P o)*100

where %T => percent transmittance

Absorption Methods,Transmittance

8/9/2019 Lecture 29 Ch18&19

4/18


Absorbance => A = - log 10T = - log 10 (P/P o)

Absorbance and transmittance

8/9/2019 Lecture 29 Ch18&19

5/18


Absorption Methods,Beers Law

A = bc

b => path length (cm)c => concentration (M)

=> molar absorptivity(M -1cm -1)

A

Concentration (M)

8/9/2019 Lecture 29 Ch18&19

6/18

8/9/2019 Lecture 29 Ch18&19

7/18


Light sources

8/9/2019 Lecture 29 Ch18&19

8/18


Examples of Sources

Deuterium lamp - good source for UV continuumradiation - widely used in UV spectrophotometers

Tungsten filament lamp - Good source of continuumradiation in the 330- 2000 nm range; common invisible, near-IR colorimeters, spectrophotometers.

Stable light output with regulated power supply.Similar to regular light-bulb but run at 3000 K. Heated metal and ceramic filaments used for the

mid-IR, which is ~ 500-4000 cm -1 (A warm objectis an efficient IR source). Common source for IRspectroscopy is called a Nernst Glower, globar, or anichrome wire.

8/9/2019 Lecture 29 Ch18&19

9/18


Purpose of monochromator:

Separation of multi- light into individual s

8/9/2019 Lecture 29 Ch18&19

10/18


MonochromatorsComponents:

entrance slit, collimating element (lens or mirror),prism or grating as dispersing element, focusingelement (lens or mirror), exit slit

8/9/2019 Lecture 29 Ch18&19

11/18


monochromator

http://www.chem.hope.edu/~polik/labtour/monochromator-inside.html

An important use of diffraction isin produci ng monochromaticlight from white light sources

8/9/2019 Lecture 29 Ch18&19

12/18


How do gratings work?

The incident beam isdiffracted in a mannergiven by Equation (1)

The term m isreferred to as theorder of thediffraction

Dispersion increases

with order butintensity diminishes

http://www.optics.org/spectrogon/holograt/specprop.html

8/9/2019 Lecture 29 Ch18&19

13/18


Sample ContainersUltra-Violet

quartz

Visible quartz

glassInfrared

NaCl AgCl KBr

Criteria for Sample Containers Transparent to excitation light

Compatible with samples Rugged

8/9/2019 Lecture 29 Ch18&19

14/18


Types of DetectorsUV-Visible

Photon Detectors Vacuum Phototubes Photomultiplier Tubes

Photodiodes Linear Photodiode arrays

Charge-Transfer (Charge Coupled Device, CCD)Infrared

Heat Detectors

8/9/2019 Lecture 29 Ch18&19

15/18


Photoelectric effect based detectors:

phototubes

Phototubes are usedfor high light intensityapplications

Photons producephotoelectrons; thephotocurrent isproportional tointensity

8/9/2019 Lecture 29 Ch18&19

16/18


Photomultiplier tubesare used for low lightintensity and photoncounting applications

Photons strike primaryelectrode, emits e-;these electrons strike

successive secondarysurfaces, finallyproducing a largepulse of electrons

http://laxmi.nuc.ucla.edu:8248/M248_99/autorad/Scint/pmt.html

8/9/2019 Lecture 29 Ch18&19

17/18


Semiconductor-base Detectors (Photodiodes and CCDs)

8/9/2019 Lecture 29 Ch18&19

18/18


Detectors for IR

Heat detectors

thermocouple Ferroelectric

Semiconductor-based detectors (like HgCdTe)

Low bandgap semiconductors

Must be cooled for good quantum efficiency,low noise

lecture 29 ch18&19

Documents