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Instrumental Slides 1-2008

Instrumental Slides 1-2008 - 84.314 Analytical Chemistry...

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Unformatted text preview: 84.314 Analytical Chemistry II (Instrumental Analysis) Dr. David K. Ryan Department of Chemistry University of Massachusetts Lowell 1 List Price $162 UML Bookstore $162 Internet as low as $85 ? Fifth edition 1998 Sixth ed. just out 2007 Excellent reference book 2 Website http://faculty.uml.edu/David_Ryan/84.314/ Syllabus = course description Schedule Materials = Lecture Slides, Handouts, Scanned Chapters 3 Skoog Chapter 1 Introduction Basics of Instrumental Analysis Properties Employed in Instrumental Methods Numerical Criteria Figures of Merit 4 5 (LOD) 6 7 8 Hypothetical Calibration Curve 9 Skip the following chapters Chapter 2 Electrical Components and Circuits Chapter 3 Operational Amplifiers in Chemical Instrumentation Chapter 4 Digital Electronics and Microcomputers 10 Skoog Chapter 5 Signals and Noise Signal to Noise Ratio All instrumental measurements involve a signal Unfortunately all signals have noise present Sometimes the noise is large Sometimes it is so small you can't see it 11 Current measurements (a) with noise, (b) with noise averaged out 12 Noise is often constant and independent of signal 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 13 NMR spectra for Progesterone A) S/N = 4.3 B) S/N = 43 Very little confidence in ability to determine peaks at lower S/N Detection Limit 14 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 15 Thermal noise vrms = 4 k T R f vrms = root mean square noise voltage k = Boltzmann constant 1.38 x 10-23 J/K T = temperature R = resistance f = frequency bandwidth of noise 16 Instrumental noise Shot noise movement of electrons across a junction irms = 2 i e f irms = root-mean square current fluctuation i = average current e = charge on electron f = frequency bandwidth 17 Instrumental noise Flicker noise any noise that is inversely proportional to signal 1/f Significant at low frequency (<100 Hz) Environmental noise composite of many noise sources e.g. any electrical device gives off EM (electromagnetic radiation) ELF radiation = health controversy instruments may pick up signals 18 Environmental noise sources (note frequency dependence) 19 Improving S/N Hardware hardware & software 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 20 Primitive Faraday Cage for shielding instruments from EM Radiation must be grounded 21 Analog Filtering or RC Filtering Noisy data RC filter R Filtered data C 22 Modulation 23 Signal chopping in an IR spectrophotometer 24 25 Chopper amplifier 26 Improving S/N Software hardware & software Ensemble averaging adding spectra Boxcar averaging Digital filtering moving window, sliding average Correlation methods 27 Ensemble averaging i.e. adding or averaging signal 28 Boxcar averaging 29 ...
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