CEE 772: Instrumental Methods in Environmental Analysis Lecture #6 - - PowerPoint PPT Presentation

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CEE 772: Instrumental Methods in Environmental Analysis Lecture #6 - - PowerPoint PPT Presentation

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Updated: 21 September 2014 Print version CEE 772: Instrumental Methods in Environmental Analysis Lecture #6 Atomic Spectroscopy: Instrument Design (Skoog, Chapts. 8 & 9; pp.192-203, 206-227 ) (Harris, Chapt. 22) (pp.615-635) 1 CEE 772

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SLIDE 1

Lecture #6

Atomic Spectroscopy: Instrument Design

(Skoog, Chapts. 8 & 9; pp.192-203, 206-227)

David Reckhow CEE 772 #6 1

CEE 772: Instrumental Methods in Environmental Analysis

Updated: 21 September 2014

Print version

(Harris, Chapt. 22) (pp.615-635)

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SLIDE 2

Atomic Spectrophotometry

David Reckhow CEE 772 #6 2

 Use

 Analysis of metals  Very sensitive

 Three types

 Absorption (AAS)

 Flame and electrothermal (furnace)

 Emission (AES)

 Often used with plasma

 Fluorescence

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SLIDE 3

David Reckhow CEE 772 #6 3

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SLIDE 4

Atomic Absorption Spectrophotometers

David Reckhow CEE 772 #6 4

 Sample holder is replaced with an atomizer

Light Source Wavelength Selector Detector

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SLIDE 5

Atomic Absorption

David Reckhow CEE 772 #6 5

 General  Flame

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SLIDE 6

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Possible transitions

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SLIDE 7

Light Source

David Reckhow CEE 772 #6 7

 Hollow Cathode Lamps

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SLIDE 8

Hollow Cathode Lamps

David Reckhow CEE 772 #6 8

 Components

 Quartz Window  Cathode (negative)

 Contains element of interest

 Low pressure chamber

 With some Ar or Ne

 (become ionized)

 Three steps

 Sputtering

 Metal atoms are dislodged

 Excitation

 Through contact with fill gas ions

 Emission

Ar+ Mo M*

ν h

Mo Mo M* Ar+

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SLIDE 9

Hollow Cathode Lamps

David Reckhow CEE 772 #6 9

 Most are single element

 Some multi-element lamps are available

 More than one metal in the cathode

 Currents are optimized  Short life

 Moderate cost ($180-$250)

 Less suited for volatile elements

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SLIDE 10

Electrodeless discharge lamps (EDL)

David Reckhow CEE 772 #6 10

 Features

 Ratio frequency is applied to a coil

 Excites elements or its salts inside quartz bulb

 Requires a special power supply

 Comparison with hollow cathode lamps

 EDLs are brighter, more intense

 Give lower MDLs for As, Se, P

 EDLs have a longer life  EDLs have some problems with drift in intensity

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SLIDE 11

Flame

David Reckhow CEE 772 #6 11

 Burner design

Temperatures of some common flames

Fuel Oxidant Temperature (K) H2 Air 2000-2100 C2H

2

Air 2100-2400 H2 O2 2600-2700 C2H

2

N2O 2600-2800

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SLIDE 12

Flame AA sample treatment

David Reckhow CEE 772 #6 12

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SLIDE 13

Temperature

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SLIDE 14

Impact of flow and position

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SLIDE 15

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SLIDE 16

Instrument Design

David Reckhow CEE 772 #6 16

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SLIDE 17

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Background Correction

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SLIDE 18

Bandwidth

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 Slit widths are normally

recommended with method

 Narrow slit widths

 May increase linearity  May also decrease signal to noise

ratio

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SLIDE 19

Matrix Effects 1

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 Viscosity

 Phosphoric acid

example

 Sulfuric acid vs

MeOH

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SLIDE 20

Matrix Effects 2

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 Chemical Interference

 Formation of Ca3(PO4)2

 Ionization Interferences

 Ba ionizes readily  K ionizes even more easily &

elevates electron density in flame

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SLIDE 21

David Reckhow CEE 772 #6 21

 To next lecture