- Noise and S/N Calibration methods Digital and analog signals Chem 420 Update on problem set Problem 2c has been removed Point values have been

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Noise and S/N, Calibration methods, Digital and analog signals Chem 420 September 03, 2008
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Update on problem set z Problem 2c has been removed. z Point values have been assigned 1. 10 points 2. 10 points 3. 6 points 4. 7 points 5. 10 points 6. 10 points 7. 10 points 8. 7 points 9. 10 points
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Systematic errors z Present in all measurements made in the same way and introduce bias. z Instrumental errors z Wacky instrument behavior, bad calibrations, poor conditions for use z Electronic drift, temperature effects, 60Hz line noise, batteries dying, problems with calibration equipment. z Personal errors z Originate from judgment calls z Reading a scale or graduated pipette, titration end points z Method errors z Non-ideal chemical or physical behavior z Evaporation, adsorption to surfaces, reagent degradation, chemical interferences
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Noise z A signal is only of analytical value if it can be definitively attributed to the species/system of interest in the presence of noise. 0.5 1 1.5 2 2.5 0 2 04 06 08 0 1 0 0 data point signal 0.5 1 1.5 2 2.5 02 0 4 0 6 0 8 0 1 0 0 data point
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What is signal and noise?
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Signal-to-noise ratio z Signal-to-noise ratio is a measure of the quality of a method or of instrument performance z Ratio of the mean and the standard deviation of the signal z Higher value of S/N means that it’s easier to resolve signal. 1 x S N s S N RSD = = Rev. Sci. Inst. , 1966 , 37 , 93-102.
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Where does noise come from? z Chemical noise z Temperature, pressure, humidity, fumes, etc. z Instrumental noise
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Detector and post-detector noise z Thermal (Johnson) noise z Shot noise z Flicker (1/f) noise z Environmental noise z Popcorn (burst) noise z Microphonic noise
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Thermal (Johnson) noise z Random motions of charge carriers (electrons or holes) that accompany thermal motions of solid lattice of atoms. z Lead to thermal current fluctuations that create voltage fluctuations in the presence of a resistive element z Resistor, capacitor, etc. 4 rms kTR f ν = Δ rms = root-mean-square noise voltage k = Boltzman’s constant T = temperature R = resistance of element ( Ω ) Δ f = bandwith (Hz) = 1/3 t r t r = rise time
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Thermal (Johnson) noise continued z Dependent upon bandwidth ( Δ f ) but not f itself z white noise z Can be reduced by narrowing bandwidth z Slows instrument response time z More time required for measurement z
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This note was uploaded on 04/08/2010 for the course CHEM 420 taught by Professor Bailey during the Spring '08 term at University of Illinois at Urbana–Champaign.

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- Noise and S/N Calibration methods Digital and analog signals Chem 420 Update on problem set Problem 2c has been removed Point values have been

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