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

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 1 1

List Price $162 UML Bookstore $162 Internet as low as $85 ? Fifth edition 1998 Sixth ed. just out 2007 Excellent reference book 2 2

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 3 3

Skoog – – Chapter 1 Chapter 1 Skoog 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 � 4 4

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(LOD) 6 6

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Hypothetical Calibration Curve 9 9

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 10 10

Skoog – – Chapter 5 Chapter 5 Skoog 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 11 11

Current measurements Current measurements (a) w ith noise, (a) w ith noise, (b) w ith noise averaged out (b) w ith noise averaged out Noise is often constant and independent of signal 12 12

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 13 13

NMR spectra for Progesterone Very little A) S/N = 4.3 confidence B) S/N = 43 in ability to determine peaks at low er S/N Detection Limit 14 14

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 ∆ f = 4 k T R ∆ v rms f v rms = 4 k T R 15 15

Thermal noise noise Thermal ∆ f = 4 k T R ∆ v rms f v rms = 4 k T R v rms = root mean square noise voltage v rms = root mean square noise voltage 23 J/K -23 k = Boltzmann Boltzmann constant 1.38 x 10 constant 1.38 x 10 - J/K k = T = temperature T = temperature R = resistance R = resistance ∆ f = frequency bandw idth of noise ∆ f = frequency bandw idth of noise 16 16

� Instrumental noise Instrumental noise � � Shot noise Shot noise – – movement of movement of � electrons across a junction electrons across a junction ∆ f = 2 i e ∆ i rms f i rms = 2 i e i rms = root- -mean square current mean square current i rms = root fluctuation fluctuation i = average current i = average current e = charge on electron e = charge on electron ∆ f = frequency bandw idth ∆ f = frequency bandw idth 17 17

� 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 18 18

Environmental noise sources Environmental noise sources (note frequency dependence) (note frequency dependence) 19 19

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 � 20 20

Primitive Faraday Cage for shielding instruments from EM Radiation – must be grounded 21 21

Analog Filtering or RC Filtering Noisy data RC filter R Filtered data C 22 22

Modulation Modulation 23 23

Signal chopping in an Signal chopping in an IR spectrophotometer IR spectrophotometer 24 24

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Chopper amplifier Chopper amplifier 26 26

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 � 27 27

Ensemble Ensemble averaging averaging i.e. adding i.e. adding or or averaging averaging signal signal 28 28

Boxcar averaging averaging Boxcar 29 29