84.314 84.314 Analytical Chemistry II Analytical Chemistry II - - PowerPoint PPT Presentation

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84.314 84.314 Analytical Chemistry II Analytical Chemistry II (Instrumental Analysis) (Instrumental Analysis)

• Dr. David K. Ryan
• Dr. David K. Ryan

Department of Chemistry Department of Chemistry University of Massachusetts Low ell University of Massachusetts Low ell

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List Price $162 UML Bookstore $162 Internet as low as $85 ? Fifth edition 1998 Sixth ed. just out 2007 Excellent reference book

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Website Website

http://faculty.uml.edu/David_Ryan/84.314/ http://faculty.uml.edu/David_Ryan/84.314/

• Syllabus = course description

Syllabus = course description

• Schedule

Schedule

• Materials = Lecture Slides,

Materials = Lecture Slides, Handouts, Scanned Chapters Handouts, Scanned Chapters

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Skoog Skoog – – Chapter 1 Chapter 1 Introduction Introduction

• Basics of Instrumental Analysis

Basics of Instrumental Analysis

• Properties Employed in Instrumental

Properties Employed in Instrumental Methods Methods

• Numerical Criteria

Numerical Criteria

• Figures of Merit

Figures of Merit

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(LOD)

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Hypothetical Calibration Curve

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Skip the follow ing Skip the follow ing chapters chapters

• Chapter 2

Chapter 2 – – Electrical Electrical Components and Circuits Components and Circuits

• Chapter 3

Chapter 3 – – Operational Amplifiers Operational Amplifiers in Chemical Instrumentation in Chemical Instrumentation

• Chapter 4

Chapter 4 – – Digital Electronics and Digital Electronics and Microcomputers Microcomputers

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Skoog Skoog – – Chapter 5 Chapter 5 Signals and Noise Signals and Noise

• Signal to Noise Ratio

Signal to Noise Ratio All instrumental measurements All instrumental measurements involve a signal involve a signal Unfortunately all signals have Unfortunately all signals have noise present noise present Sometimes the noise is large Sometimes the noise is large Sometimes it is so small you Sometimes it is so small you can’t see it can’t see it

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Noise is often constant and independent of signal

Current measurements Current measurements (a) w ith noise, (a) w ith noise, (b) w ith noise averaged out (b) w ith noise averaged out

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Signal to Noise Signal to Noise Ratio (S/N) Ratio (S/N)

• Parameter describing quality of

Parameter describing quality of data data

• Often referred to as “figure of

Often referred to as “figure of merit” merit” S mean of signal x 1 S mean of signal x 1

• --- =

= --------------------------------

• ------------------------------- =

= ----

• --- =

= -------

• N standard deviation s RSD

N standard deviation s RSD RSD = relative standard deviation RSD = relative standard deviation

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NMR spectra for Progesterone A) S/N = 4.3 B) S/N = 43 Very little confidence in ability to determine peaks at low er S/N Detection Limit

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Sources of Noise Sources of Noise

• Chemical noise

Chemical noise – – temp, pressure, temp, pressure, humidity, etc. fluctuations = humidity, etc. fluctuations = uncontrolled variables uncontrolled variables

• Instrumental noise

Instrumental noise – – noise from noise from instrumental components instrumental components

• Thermal noise (Johnson noise)

Thermal noise (Johnson noise) – – thermal motion of electrons in load thermal motion of electrons in load resistor resistor v vrms

rms = 4 k T R

= 4 k T R ∆ ∆f f

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Thermal Thermal noise noise v vrms

rms = 4 k T R

= 4 k T R ∆ ∆f f v vrms

rms = root mean square noise voltage

= root mean square noise voltage k = k = Boltzmann Boltzmann constant 1.38 x 10 constant 1.38 x 10 -

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23 J/K

J/K T = temperature T = temperature R = resistance R = resistance ∆ ∆f = frequency bandw idth of noise f = frequency bandw idth of noise

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• Instrumental noise

Instrumental noise

• Shot noise

Shot noise – – movement of movement of electrons across a junction electrons across a junction i irms

rms = 2 i e

= 2 i e ∆ ∆f f i irms

rms = root

= root-

• mean square current

mean square current fluctuation fluctuation i = average current i = average current e = charge on electron e = charge on electron ∆ ∆f = frequency bandw idth f = frequency bandw idth

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• Instrumental noise

Instrumental noise

• Flicker noise

Flicker noise – – any noise any noise that is inversely that is inversely proportional to signal proportional to signal 1/f 1/f Significant at low frequency (<100 Hz) Significant at low frequency (<100 Hz)

• Environmental noise

Environmental noise – – composite of composite of many noise sources many noise sources e.g. any electrical device gives off e.g. any electrical device gives off EM (electromagnetic radiation) EM (electromagnetic radiation) ELF radiation = health controversy ELF radiation = health controversy instruments may pick up signals instruments may pick up signals

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Environmental noise sources Environmental noise sources (note frequency dependence) (note frequency dependence)

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Improving S/N Improving S/N

hardw are & softw are hardw are & softw are

• Hardw are

Hardw are

• Grounding & shielding

Grounding & shielding – – Faraday cage Faraday cage

• Analog filtering

Analog filtering – – RC filtering RC filtering

• Modulation

Modulation – – convert DC signal to convert DC signal to high frequency AC then demodulate high frequency AC then demodulate

• Signal chopping

Signal chopping – – rotating w heel to rotating w heel to differentiate e.g. IR source from heat differentiate e.g. IR source from heat

• Lock

Lock-

• in amplifiers

in amplifiers

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Primitive Faraday Cage for shielding instruments from EM Radiation – must be grounded

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Analog Filtering

• r RC Filtering

Noisy data RC filter

R C

Filtered data

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Modulation Modulation

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Signal chopping in an Signal chopping in an IR spectrophotometer IR spectrophotometer

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Chopper amplifier Chopper amplifier

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Improving S/N Improving S/N

hardw are & softw are hardw are & softw are

• Softw are

Softw are

• Ensemble averaging

Ensemble averaging – – adding spectra adding spectra

• Boxcar averaging

Boxcar averaging – –

• Digital filtering

Digital filtering – – moving w indow , moving w indow , sliding average sliding average

• Correlation methods

Correlation methods

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Ensemble Ensemble averaging averaging i.e. adding i.e. adding

• r
• r

averaging averaging signal signal

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Boxcar Boxcar averaging averaging