Chap. 5 (Signals and Noise), Chap. 6 (Spectroscopy introduction) - PowerPoint PPT Presentation

1 / 44
About This Presentation
Title:

Chap. 5 (Signals and Noise), Chap. 6 (Spectroscopy introduction)

Description:

Title: Chap. 5 (Signals and Noise), Chap. 6 (Spectroscopy introduction) Author: Ken Czerwinski Last modified by: Ken Czerwinski Created Date: 1/27/2004 2:37:06 AM – PowerPoint PPT presentation

Number of Views:173
Avg rating:3.0/5.0
Slides: 45
Provided by: KenC93
Category:

less

Transcript and Presenter's Notes

Title: Chap. 5 (Signals and Noise), Chap. 6 (Spectroscopy introduction)


1
Chap. 5 (Signals and Noise), Chap. 6
(Spectroscopy introduction)
  • Signal to noise
  • Source of noise
  • Signal to noise enhancement
  • Signal has the information of the analyte
  • Noise is the extraneous information in the
    information due to electronics, spurious
    response, and random events
  • Signal to noise ratio
  • Noise is generally constant and independent of
    the signal
  • The impact of noise is greatest on the lowest
    signal
  • The ratio of signal to noise is useful in
    evaluating data

2
Signal to Noise
  • Value of the signal to noise can vary
  • Values less than 3 make it hard to detect signal

3
Sources of Noise
  • Chemical Noise
  • Uncontrollable variables affecting chemistry of
    system under investigation
  • Change in equilibria due to variations
  • Temperature
  • Pressure
  • Sample variation
  • Humidity

4
Source of Noise
  • Instrumental Noise
  • Thermal noise
  • Shot noise
  • Flicker
  • Environmental noise
  • Thermal noise
  • Thermal agitation of electrons in electronics
  • Boltzmanns equation

5
Instrument Noise
  • Based on Boltzmann
  • R is resistance
  • k is Boltzmanns constant
  • 1.38E-23 J/K
  • T in K
  • f is frequency bandwith (1/3risetime)
  • Relates to response time in instrument
  • Shot Noise
  • Electrons crossing a junction
  • pn junction, anode and cathode
  • Random events
  • e 1.6e-19 C

6
Instrument Noise
  • Flicker Noise
  • Inverse of signal frequency
  • Important below 100 Hz
  • Drift in instruments
  • Environmental Noise
  • Emanates from surroundings
  • Electromagnetic radiation

7
Signal to Noise Enhancement
  • Hardware and software methods
  • Hardware is based on instrument design
  • Filters, choppers, shields, detectors, modulators
  • Software allows data manipulation
  • Grounding and Shielding
  • Absorb electromagnetic radiation
  • Prevent transmission to the equipment
  • Protect circuit with conduction material and
    ground
  • Important for amplification

8
Hardware
  • Difference and Instrumentation Amplifiers
  • Subtraction of noise from a circuit
  • Controlled by a single resistor
  • Second stage subtracts noise
  • Used for low level signal
  • Analog filtering
  • Uses a filter circuit
  • Restricts frequency

9
Hardware
  • Modulation
  • Changes low frequency signal to higher frequency
  • Signal amplified, filter with a high pass filter,
    demodulation, low pass filter
  • Signal Chopping
  • Input signal converted to square wave by
    electronic or mechanical chopper
  • Square wave normalizes signal

10
Software Methods
  • Ensemble Average
  • Average of spectra
  • Average can also be sum of collected spectra
  • Boxcar average
  • Average of points in a spectra

11
Software Methods
12
Digital Filtering
  • Numerical methods
  • Fourier transform
  • Time collected data converted to frequency
  • NMR, IR
  • Least squares smoothing
  • Similar to boxcar
  • Uses polynomial for fit
  • Correlation

13
Chap. 6 Introduction to Spectrometric Methods
  • Electromagnetic radiation
  • Interaction with matter
  • Quantum mechanical properties
  • Electromagnetic radiation
  • orthogonal in phase oscillations

14
Wave Parameters
  • Amplitude and wavelength

15
Electromagnetic Spectrum

                                                                                                                                                                             
16
Methods
17
X-ray Structure
  • X-rays
  • 0.01 to 100 angtroms
  • 12 keV to 1 MeV
  • Ionizing radiation
  • Roentgen
  • Gas discharge tube
  • Detector with Ba/Pt CN
  • Scintillator

18
  • In November of 1895, Wilhelm Roentgen (1845 -
    1923) was working in his laboratory using a
    Crookes tube (known in German as either a Hittorf
    valve or a Hittorf-Crookes tube) when he noticed
    that a sample of barium platinocyanide, which
    accidentally lay on the table, gave off a
    fluorescent glow. As the Crookes tube was covered
    at the time, Roentgen was puzzled as to the
    mechanism whereby the platinum compound was being
    stimulated to glow. After carrying out a series
    of exceptionally careful experiments, Roentgen
    realized that the Crookes tube was emitting a new
    kind of radiation which he described as "X-rays".
    In investigating the penetrating ability of these
    rays, Roentgen placed a photographic plate behind
    his wife's hand and recorded the first x-ray
    photo. In this figure, below, notice his wife's
    wedding rings that stand out as dark rings.

19
(No Transcript)
20
Energy from X-ray
  • From Cu
  • 13.6(292)11.4 keV
  • Based on Bohr atom
  • Family of lines due to different levels
  • Determination of elements

21
(No Transcript)
22
Mosley
  • Measured 38 elements
  • Measured emission spectra and found pattern
  • Based on Z, not mass (Ar/K, Co/Ni, Te/I)
  • Place lanthanides on periodic table
  • 14 lanthanides
  • Up to U there are 92 elements

23
(No Transcript)
24
(No Transcript)
25
X-ray Structure
  • Review of cathode ray tube and nomenclature
  • Determination of elements from X-rays
  • Coolidge
  • 1913
  • Vacuum tube
  • Reduction of collision with gas
  • Reduce glow
  • Heating Cathode
  • Water cooling
  • Shielding (Pb), Be windows

26
X ray lines
Lines with continuum function of voltage Mo
BCC from bremstrallung
27
Bremsstrahlung
EqVeVE(photon)12400/V Ang Duane-hunt law
28
Use x-ray to examine crystals
  • Model atoms as mirrors
  • Use classical optics
  • Utilize interference
  • Constructive and destructive

29
X-ray diffraction
  • Emission spectrum from x-ray generator
  • Composite of 2 spectra
  • Characteristic spectra
  • Continuous spectra
  • Calculate lines by Mosleys Law

30
Braggs Law
Specifics conditions for interference Set of
reflections identifies structure
31
XRD
  • Fixed wavelength, vary angle
  • Powder specimen
  • Grains act as single crystal
  • Plot I vs angle
  • At Bragg angle produce angle

32
Data analysis
Normalize data to 1st sin2theta Clear
fractions Speculate on hkl Know wavelength from
source, solve for a
33
Laue Technique
34
Spot pattern
  • For symmetry
  • 2, 3, 4 fold symmetry
  • May not work for thick specimen
  • Backscatter and transmission

35
Transmission of radiation
  • Polarization
  • Directional filtering of light
  • Light will be scattered by larger molecules
  • Radiation transfer to molecules
  • Absorption spectroscopy
  • Material consideration
  • Glass, quartz, plastic

36
Atomic Spectra
  • Quantum numbers
  • n1,2,3,4
  • raon2/Z for gases with 1 electron
  • Energy
  • E-(mee4/8eo2h2)Z2/n2
  • For ground state H
  • E2.18E-18 J/atomk
  • Can determine J/mole 1312 kJ/mole
  • Energy goes as k/n2
  • System converges to limit

37
Energy
  • ninfinity, rinfinity , E0, unbound e-
  • Ionization energy
  • k is ionization energy
  • Velocity
  • vnh/2pmer
  • Ionization energy
  • Minimum energy required to remove electron from
    atom in gas phase
  • Multiple ionization energies

38
Balmer states
  • Gas H in tube
  • Four lines in visible region
  • Fit lines
  • 1/l(1/22-1/n2)R, R1.1E-7 m-1
  • 1/ln (wavenumber)
  • E1/2mev2eV (VVolts)
  • At 1 V 1.6E-19 J eV
  • K13.6 eV

39
Matter energy interaction
  • Eincident1/2mv2qV
  • Escattered
  • DE Eincident-Escattered
  • DEkZ2(1/n2final-1/n2in)hnhc/l
  • De-excitation of electron results in photon
    emission
  • Corresponds to line emission

40
Shell model and multielectrons
  • Particle interaction
  • Particle hits electron, electron has scatted
    kinetic energy
  • EincEbindingEelectron scattered
  • For ground state Ebinding is ionization energy
  • Einc 0.5mv2
  • DEtrans-kZ2D(1/n2)
  • For photon Ehc/l

41
Rydberg
k/hc1.1e-7 m-1 R (Rydberg constant) Visible
light 400-700 nm (1.8 to 3.1 eV) Quantum
numbers n1,2,3,4 l0 to n-1 ml -l Spin-1/2
42
Bohr Atom
  • Net force on the electron is zero
  • 0FdynamicFcoulombic
  • 1/2mev2/rq1q2/4peor2
  • Force is 1/r2
  • Energy 1/r
  • 1/2mev2/r-Ze2/4peor2
  • Z is charge on nucleus
  • Quantize energy through angular momentum
  • mvrnh/2p, n1,2,3.
  • Can solve for r, E, v

43
Bohr radius
  • R(eoh2/pmee2)(n2/Z)
  • Radius is quantized and goes at n2
  • R0.529 Å for Z1, n1
  • Ao (Bohr radius)

44
Photoelectric effect
Write a Comment
User Comments (0)
About PowerShow.com