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84.314Analytical Chemistry II(Instrumental Analysis)Dr. David K. RyanDepartment of ChemistryUniversity of Massachusetts Lowell1
List Price 162UML Bookstore 162Internet as low as 85 ?Fifth edition 1998Sixth ed. just out 2007Excellent reference book2
Websitehttp://faculty.uml.edu/David Ryan/84.314/ Syllabus course description Schedule Materials Lecture Slides,Handouts, Scanned Chapters3
Skoog – Chapter 1Introduction Basics of Instrumental AnalysisProperties Employed in InstrumentalMethodsNumerical CriteriaFigures of Merit4
5
(LOD)6
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Hypothetical Calibration Curve9
Skip the followingchapters Chapter2 – ElectricalComponents and Circuits Chapter 3 – Operational Amplifiersin Chemical Instrumentation Chapter 4 – Digital Electronics andMicrocomputers10
Skoog – Chapter 5Signals and Noise Signalto Noise RatioAll instrumental measurementsinvolve a signalUnfortunately all signals havenoise presentSometimes the noise is largeSometimes it is so small youcan’t see it11
Current measurements(a) with noise,(b) with noise averaged out12Noise is often constant and independent of signal
Signal to NoiseRatio (S/N) Parameterdescribing quality ofdata Often referred to as “figure ofmerit”Smean of signalx1---- -------------------------------- ---- ------Nstandard deviationsRSDRSD relative standard deviation13
NMR spectra forProgesteroneA) S/N 4.3B) S/N 43Very littleconfidencein ability todeterminepeaks atlower S/NDetectionLimit14
Sources of Noise Chemicalnoise – temp, pressure,humidity, etc. fluctuations uncontrolled variables Instrumental noise – noise frominstrumental components Thermal noise (Johnson noise) –thermal motion of electrons in loadresistorvrms 4 k T R f15
Thermal noisevrms 4 k T R fvrms root mean square noise voltagek Boltzmann constant 1.38 x 10-23 J/KT temperatureR resistance f frequency bandwidth of noise16
Instrumental noiseShot noise – movement ofelectrons across a junctionirms 2 i e firms root-mean square currentfluctuationi average currente charge on electron f frequency bandwidth17
InstrumentalnoiseFlicker noise – any noisethat is inverselyproportional to signal1/fSignificant at low frequency ( 100 Hz) Environmental noise – composite ofmany noise sourcese.g. any electrical device gives offEM (electromagnetic radiation)ELF radiation health controversyinstruments may pick up signals18
Environmental noise sources(note frequency dependence)19
Improving S/Nhardware & software Hardware Grounding & shielding – Faraday cageAnalog filtering – RC filteringModulation – convert DC signal tohigh frequency AC then demodulateSignal chopping – rotating wheel todifferentiate e.g. IR source from heatLock-in amplifiers20
PrimitiveFaradayCage forshieldinginstrumentsfrom EMRadiation –must begrounded21
Analog Filteringor RC FilteringNoisy dataRC filterRFiltered dataC22
Modulation23
Signal chopping in anIR spectrophotometer24
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Chopper amplifier26
Improving S/Nhardware & software Software Ensemble averaging – adding spectraBoxcar averaging –Digital filtering – moving window,sliding averageCorrelation methods27
Ensembleaveragingi.e. addingoraveragingsignal28
Boxcar averaging29
Analytical Chemistry II Analytical Chemistry II (Instrumental Analysis) Dr. David K. Ryan Department of Chemistry University of Massachusetts Lowell. 2 List Price 162 UML Bookstore 162 Internet as low as 85 ? Fifth edition 1998 Sixth ed. just out 2007 . Materials Lecture Slides,
