Instrumental Week1 CH1&5-2010.ppt - uml.edu

Dr. Gang WangDr. David K. Ryan

Department of ChemistryUniversity of Massachusetts Lowell

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List Price $162UML Bookstore $231?Internet as low as $85 ?Fifth edition 1998Sixth ed. just out 2007Excellent reference book

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Website

http://faculty.uml.edu/David_Ryan/84.314/Syllabus = course descriptionScheduleMaterials = Lecture Slides,  Handouts, Videos from last year

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Skoog – Chapter 1Introduction

Basics of Instrumental AnalysisProperties Employed in Instrumental MethodsNumerical CriteriaFigures of Merit

Nobel PrizesWilliam H. Bragg analysis of crystal structure by means of X-rays(physics) 1915

Francis W. Aston for his discovery, by means of his mass spectrograph, of isotopes (Chemistry) 1922

Friderik Pregl invention of the method of micro-analysis of organic substances (Chemistry) 1923

Arne Tiseliushis research on electrophoresis and adsorption analysis, especially for his discoveries concerning the complex nature of the serum proteins (Chemistry)

1948

Felix BlochEdward M. Purcell

development of new methods for nuclear magnetic precision measurements and discoveries (Physics) 1952

Archer J P MartinRichard L M Synge Invention of partition chromatography (Chemistry) 1952

Jaroslav Heyrovsky discovery and development of the polarographic methods of analysis (Chemistry) 1959

Rosalyn Yalow development of radioimmunoassays of peptide hormones (Physiology or Medicine) 1977

Kai M. Siegbahn contribution to the development of high-resolution electron spectroscopy (physics) 1981

Gerd BinnigHeinrich Rohrer

design of the scanning tunneling microscope (physics) 1986

Analytical Methods

Classical

Instrumental

Precipitation

Extraction

Distillation

ColorsBoiling or Melting Points

Solubilities

Odors

Optical

GravimetricVolumetric

Physical PropertiesConductivityElectrode PotentialLight Absorption or EmissionMass-to –Charge ratioFluorescence

ChromatographicElectrophoretic

Instruments for Analysis

Performance CharacteristicsOfInstruments

Precision

The precision of a measurement system, also called reproducibility or repeatability, is the degree to which repeated measurements under unchanged conditions show the same results.

accuracy of a measurement system is the degree of closeness of measurements of aquantity to its actual (true) value.

BiasBias provides a measure of the systematic, or determinate, error of an analytical method.Δ = μ − τWhere μ is the population mean for the concentration of an analyteτ is the true value

SensitivitySensitivity of a method or instrument is a measure of its ability to discriminate between small differences.Two factors limit sensitivity

The slope of calibration curveReproducibility or precision of the measuring deviceS = mc + Sbl

Detection limitMinimum concentration or mass of analyte can be detected at a known confidence levelSm = Sbl + ksbl

Usually, k = 3

Dynamic range

LOQ limit of quantitationLOL limit of linearityDynamic range should be at least a few orders of magnitude

SelectivitySelectivity refers to the degree to which the method is free from interference by other species in sample

Calibration of instrumental methodCalibration determines the relationship between analytical response and analytical concentration.1. external‐standard calibration

No interference effectsObtain response signal as a function of known analyte concentration

Calibration of instrumental method2. Standard‐ additional methods

Adding one or more increments of standard solution to sample aliquotsSx = K1 CxST = K1 (Cx + Cs)

Cx = CsSx /(ST ‐ Sx)When ST = 0Cx = ‐Cs

Calibration of instrumental method3. internal‐standard methodAdd a constant amount of substance to all samples, blanks, and calibration standard.Plot ratio of analytesignal and internal‐standard as a function of analyteconcentration.

Homework: P23 1-10

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Skip the following chapters

Chapter 2 – Electrical Components and CircuitsChapter 3 – Operational Amplifiers in Chemical InstrumentationChapter 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 itNoise is constant and independent, small signal large noise

S/N is very important

20Noise 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 dataOften referred to as “figure of merit”

S        mean of signal               x          1‐‐‐‐ = ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐ = ‐‐‐‐ = ‐‐‐‐‐‐‐N     standard deviation          s        RSD

RSD = relative standard deviationImpossible to detect a signal when S/N less than 2 or 3

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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 NoiseChemical 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  Voltage fluctuation                   

vrms =   4 k T R Δf              Δf =1/3trNarrow bandwidth to decrease noise, but instrument will be slower 

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Instrumental noiseThermal noise

vrms =    4 k T R Δf 

vrms = root mean square noise voltagek = Boltzmann constant 1.38 x 10‐23 J/KT = temperatureR = resistanceΔf = frequency bandwidth of noise

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Instrumental noiseShot noise – movement ofelectrons across a junction

irms =    2 i e Δf 

irms = root‐mean square current fluctuationi = average currente = charge on electronΔf = frequency bandwidth

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Instrumental noiseFlicker noise – any noisethat is inversely proportional to signal

1/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 signals

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Environmental noise sources(note frequency dependence)

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Improving S/Nhardware & software

HardwareGrounding & shielding – Faraday cageAnalog filtering – RC filteringModulation – convert DC signal to high frequency AC then demodulateSignal chopping – rotating wheel to differentiate e.g. IR source from heatLock‐in amplifiers

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PrimitiveFaradayCage forshieldinginstrumentsfrom EMRadiation –must begrounded

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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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Improving S/Nhardware & software

SoftwareEnsemble averaging – adding spectraBoxcar averaging –Digital filtering – moving window, sliding averageCorrelation methods

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Ensembleaveragingi.e. addingor averagingsignal

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Boxcar averaging