Chem 551 :Instrumental Methods of Analysis

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Chem 551 Instrumental Methods of Analysis

• Ralph Allen

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Instrumental Analysis

• There is much more than the Instrument
• You are the analyst

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Why are you taking this class?

• What do you want to learn?
• What analytical techniques do you want to study?

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(http//www.people.virginia.edu/roa2s/chem_551/ho
me.html).
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Get help on the web

• http//www.virginia.edu/enhealth

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You dont need a course to tell you how to run an
instrument

• They are all different and change
• Most of you wont be analysts
• We will talk about experimental design
• Learn about the choices available and the basics
  of techniques

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Analytical Chemistry

• art of recognizing different substances
  determining their constituents, takes a prominent
  position among the applications of science, since
  the questions it enables us to answer arise
  wherever chemical processes are present.
  
• 1894 Wilhelm Ostwald

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Questions to ask???

• Why? Is sample representative
• What is host matrix?
• Impurities to be measured and approximate
  concentrations
• Range of quantities expected
• Precision accuracy required

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More things to ask.

• Where is analysis to be conducted
• How many samples (per day total)
• How soon are results needed
• Are there standards (analytical QC)
• Long term reliability
• Form of answer required
• Special facilities available

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The Analytical Approach

• Identify the problem.what do you want to know
• What instrumental methods can provide necessary
  results
• Which method is best
• What do the results mean

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What you want to learn

• analytical process and skills
• tools for research
• solve practical problems
• medical uses (including DNA)
• how instruments work and general concepts
• environmental and forensic applications
• new advances

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Techniques

• mass spectrometry
• NMR
• spectroscopy (UV, IR, AA)
• chromatography (GC, HPLC)
• measure radioactivity, crystallography, PCR, gas
  phase analysis

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Off flavor cake mix (10)

• Send it off for analysis
• Do simple extractions
• Separation and identification by GC/MS
• Over 100 peaks but problem was in a valley
  between peaks (compare)
• Iodocresol at ppt
• Eliminate iodized salt that reacted with food
  coloring (creosolmethyl phenol)

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Pan Am 10385 of the Maid of the
Searecovered16,000 pieces of
propertyrecovered
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Reason to understand how an instrument works

• What results can be obtained
• What kind of materials can be characterized
• Where can errors arise

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Design of instrumentation to probe a material

• Signal Generation-sample excitation
• Input transducer-detection of analytical signal
• Signal modifier-separation of signals or
  amplification
• Output transducer-translation interpretation

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Characterization of Properties

• chemical state
• structure
• orientation
• interactions
• general properties

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Molecular Methods

• macro Vs micro
• pure samples Vs mixtures
• qualitative Vs quantitative
• surface Vs bulk
• large molecules (polymers, biomolecules)

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Molecular SpectroscopyIR, UV-Vis, MS, NMR

• What are interactions with radiation
• Means of excitation (light sources)
• Separation of signals (dispersion)
• Detection (heat, excitation, ionization)
• Interpretation (qualitative easier than
  quantitative)

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Elemental Analysis

• bulk, micro, contamination (matrix)
• matrix effects
• qualitative Vs quantitative
• complete or specific element
• chemical state

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Extreme trace elemental analysis

• Direct instrumental determination - multi-element
  - direct excitation---should be least expensive
• These are relative physical methods requiring
  appropriate standards systematic errors like
  spectral interferences occur
• NAA, XRF, sputtered neutral MS

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Extreme trace elemental analysis

• Multi-stage procedures --- sample separation
  and preparation before quantitation
• Standards are less of a problem
• Time consuming subject to losses or
  contamination
• Chromatography coupled with analysis

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Comparing Methods

• Detection limits
• Dynamic range
• Interferences
• Generality
• Simplicity

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Your ideas

• cost
• sensitivity
• accuracy/precision
• time
• compatibility
• conditions
• availability

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Statisticsare no substitute for judgment

• Common sense put into a mathematical form
• Analysis of results - accuracy precision
• Elimination of errors
• Detection limits - signal to noise
• Chemometrics - what do the results mean

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There is a difference - you
need both
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Errors in Analytical Measurements

• Determinant - unidirectional errors ascribable to
  a definite cause
  
• Indeterminate - uncertainties from unknown or
  uncontrollable factors - generally random -
  noise

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Systematic errors - sources

• Inhomogeneity - handling storage
• Contamination - sampling to reagents
• Adsorption on surface or volatilization
• Unwanted or incomplete chemical reactions
• Matrix effects on generation of analytical signal
• Incorrect standards or calibration

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Recognition of systematic errors

• Reproducibility gives NO information on accuracy
  (high std. dev. hints at problems)
• Make comparisons with other methods
• Check standard reference materials (available
  from NIST)
• Run blanks (be sure background is small and
  reproducible)

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Errors in Analytical Measurements

• Determinant - unidirectional errors ascribable to
  a definite cause
• Indeterminate - uncertainties from unknown or
  uncontrollable factors - generally random -
  noise

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Gaussian Distribution

• Random fluctuations
• Bell shaped curve
• Mean and standard deviation
• 1sigma 68.3, 2sigma 95.5, 3sigma 99.7
• Absolute Vs Relative standard deviation
• Accuracy and its relationship to the measured mean

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Limit of detection

• signal - output measured as difference between
  sample and blank (averages)
• noise - std dev of the fluctuations of the
  instrument output with a blank
• S/N 3 for limit of detection
• S/N 10 for limit of quantitation

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Sources of Noise

• Environmental - 60 Hz electrical, vibrational
  (shield)
• Johnson (thermal) noise - random fluctuations in
  charge carriers (cool)
• Shot noise - pulses
• 1/f (flicker) noise - important at low frequencies

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Noise Reduction

• Avoid (cool, shield, etc.)
• Electronically filter
• Average
• Mathematical smoothing
• Fourier transform

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Single channel scanning

• 3 objects each measured 3 times (averaged to
  reduce noise)
• Balance requires 9 measurements
• Monochromator - broad band source to dispersive
  device and then wavelengths are selected one at a
  time
• Increase intensity by scanning slower or
  increasing bandpass

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Multidetector Spectrometer

• Get 3 balances and measure all 3 samples
  simultaneously on separate balances
• Can make measurements in 1/3 time or measure 3
  times as much (noise is random and proportional
  to square root of number of measurements)
• Use of diode arrays instead of slits

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Signal Transformation

• Double pan balance - mesure multiple objects
  simultaneously measure linear combinations
• y(1)X(1) X(2)
• y(2)X(1) X(3)
• y(3)X(2) X(3)
• 3 equations 3 unknowns (each object measured
  twice in half the time)

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Hadamard multiplexing (transform)

• Use one detector and replace the slit with a mask
  of slits at certain locations (n)- some are open
  others closed (2n-1 slits in mask with just
  more than half open)
• For n3 a mask of 11011 (1 is open) can be slid
  to give 110, 101, 011
• Linear equations improve S/N

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Fourier advantage

• Put all weights on 2 pan balance at the same time
• Change what is measured (not weights but angle of
  pointer showing difference in the 2 pans)
• Z(1)X(1) X(2) - X(3)
• Z(2)X(1) - X(2) X(3)
• Z(3) -X(1) X(2) X(3)

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h(t) a cos 2 pi freq. x time

• sum cos(2pi((f1f2)/2)t
• beat or difference cos(2pi((f1-f2)/2)t
• 5104-sine-wa

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Fourier transform - beat frequency (time domain)

• We can sample the time domain at N equally spaced
  time intervals
• Represent each measurement in terms of a series
  of frequencies
• Decoding procedure to decode N algebraic
  equations
• Fourier transform requires a computer

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An analytical checklist

• Have the analytical tasks and goals been defined?
• Have issues of sampling been defined?(eg. size,
  homogeneity, composites)
• Are there facilities for sample storage (custody)
  available and is there a means of identification
  and retreival)

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Checklist 2

• Is pretreatment (eg. extraction, dissolution)
  necessary? (facilities, equipment, reagents)
• Is the sample analyzed representative? (mixing,
  weighing, size)
• Are the instruments appropriate for the required
  measurements? (sensitivity, sample state)

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Checklist 3

• What is the time required for each analysis?
• What expertise is needed to prepare, analyze, and
  interpret?
• How is data captured, calculated, presented, and
  stored for future comparisons?
• Are there appropriate quality controls?
• Define time line for tasks and analysis and then
  calculate overall costs

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Attenuated Internal Reflection

• Surface analysis
• Limited by 75 energy loss