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84.314 Analytical Chemistry II (Instrumental Analysis)

Dr. David K. RyanDepartment of Chemistry

University of Massachusetts Lowell

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List Price $140

UML Bookstore $103

Internet as low as $52

Fifth edition 1998

Sixth ed. due out 2006

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Website

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

• Syllabus = course description• Schedule• Materials = Lecture Slides,

Handouts, Scanned Chapters

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Skoog – Chapter 1Introduction

• Basics of Instrumental Analysis• Properties Employed in

Instrumental Methods• Numerical Criteria• Figures of Merit

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(LOD)

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Hypothetical Calibration Curve

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Skip the following chapters• Chapter 2 – Electrical

Components and Circuits• Chapter 3 – Operational

Amplifiers in Chemical Instrumentation

• Chapter 4 – Digital Electronics and Microcomputers

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Skoog – Chapter 5Signals and Noise• Signal to Noise RatioAll instrumental measurements

involve a signalUnfortunately all signals have

noise presentSometimes the noise is largeSometimes it is so small you

can’t see it

12Noise is often constant and independent of signal

Current measurements(a) with noise, (b) with noise averaged out

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Signal to Noise Ratio (S/N)

• Parameter describing quality of data

• Often referred to as “figure of merit”

S mean of signal x 1---- = -------------------------------- = ---- = -------N standard deviation s RSD

RSD = relative standard deviation

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NMR spectra forProgesteroneA) S/N = 4.3B) S/N = 43

Very little confidencein ability todeterminepeaks atlower S/N

DetectionLimit

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Sources of Noise• Chemical noise – temp, pressure,

humidity, etc. fluctuations = uncontrolled variables

• Instrumental noise – noise from instrumental components• Thermal noise (Johnson noise) –

thermal motion of electrons in load resistor

vrms = 4 k T R ∆f

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Thermal noise

vrms = 4 k T R ∆f

vrms = root mean square noise voltagek = Boltzmann constant 1.38 x 10-23 J/KT = temperatureR = resistance∆f = frequency bandwidth of noise

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• Instrumental noise• Shot noise – movement of

electrons across a junction

irms = 2 i e ∆f

irms = root-mean square current fluctuation

i = average currente = charge on electron∆f = frequency bandwidth

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• Instrumental noise• Flicker noise – any noise

that is inversely proportional to signal

1/fSignificant at low frequency (<100 Hz)• Environmental noise – composite of

many noise sourcese.g. any electrical device gives offEM (electromagnetic radiation)ELF radiation = health controversyinstruments may pick up signals

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Environmental noise sources(note frequency dependence)

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Improving S/Nhardware & software• Hardware

• Grounding & shielding – Faraday cage• Analog filtering – RC filtering• Modulation – convert DC signal to

high frequency AC then demodulate• Signal chopping – rotating wheel to

differentiate e.g. IR source from heat• Lock-in amplifiers

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Analog Filteringor RC Filtering

Noisy data

RC filter

R

C

Filtered data

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Modulation

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Signal chopping in anIR spectrophotometer

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Chopper amplifier