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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
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