Instrumental Analysis

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Instrumental Analysis
2012-02-16 03:39:40
Scientific Instruments

Exam 1
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  1. Analytical Chemistry
    A branch of chemistry that deals with separation, identification of chemical substances, and determination of relative amounts of the substances in a sample.
  2. Qualitative Analysis
    Determination of the identity of analytes in a sample
  3. Quantitative Analysis
    Determination of the amount of analytes in a sample
  4. 2 types of analytical methods
    • 1. Classical
    • 2. Instrumental-Analytical
  5. Qualitative Classical Methods
    Analytes are treated with reagents that yield products that can be recognized by their: color, boiling or melting points, solubilities, odors, or optical activities.
  6. Quantitative Classical Methods
    • amounts of analyte is determined by gravimetric or by volumetric analysis.
    • 1. Gravimetric: mass of the analyte or substance is chemically related to the analyte measured.
    • 2. Volumetric: The volume or mass of a standard sol'n required to react completely with the analyte is measured.
  7. Qualitative Instrumental Methods
    • 1. Light emission or absorption spectra: UV-VIS, AAS
    • 2. Mass-to-charge ratio: mass spectrometry
    • 3. Chromatograms: chromatography
    • 4. Electropherograms: capillary electrophoresis
  8. Quantitative Instrumental Methods
    • 1. Measurement of intensity of atomic or molecular emission
    • 2. Measurement of molecular absorbance, electrode-potential
    • 3. Measurement of conductivity or abundance of mass-to-charge chemical species.
  9. Instruments for chemical analysis can...
    ...convert physical or chemical characteristics of an analyte to information that can be manipulated and interpreted by a human.
  10. Sensitivity
    A measure of an instruments ability to discriminate between small differences in analyte concentration.
  11. Factors limiting sensitivity
    • 1. Slope of the calibration curve
    • 2. Reproducibility of the measuring device
  12. S = mc + S(bl)
    • S: measured signal of sample
    • m: slope of calibration curve
    • c: concentration of the analyte
    • S(bl): measured signal for a blank
  13. Analytical sensitivity (gamma)
    • (gamma) = m / (sigma)
    • m: slope
    • (sigma): standard deviation of the measurment
  14. Detection limit
    minimum concentration or mass of analyte that can be detected
  15. How to determine the detection limit
    • Step1: find minimum distinguishable analytical signal S(m)
    • S(m) = S(bl) + k(sigma(bl))
    • k: constant (usually 3)
    • (sigma(bl)): standard deviation of the blank
    • Step2: use equation S=mc+S(bl) and convert S(m) to C(m)
    • C(m) = (S(m) - S(bl)) / m
  16. Dynamic Range
    A range that extends from the lowest concentration to the concentration where the calibration curve departs from linearity.
  17. Selectivity
    degree to which the method is free from interference by other species in the sample matrix
  18. Selectivity Coefficient
    • 1. =0 : no interference
    • 2. >0 : value of S will be high
    • 3. <0 : value of S will be low
  19. Spectroscopy
    study of interaction between electromagnetic radiation and matter as a function of wavelength
  20. Molecular Spectroscopy
    study of interaction between electromagnetic radiation and molecules as a function of wavelength
  21. Spectrometry
    spectroscopic technique used to assess the concentration of the amount of a given species
  22. Spectrometer
    instrument used to measure properties of light over a specific portion of the electromagnetic spectrum
  23. Absorption Spectrum
    a graph showing absorbance or molar absorptivity varying with wavelength
  24. Chromophore
    a region in a molecule that is responsible for light absorption
  25. Light Source
    provides a stable source of radiation energy, sufficiently powerful for easy detection and measurement.
  26. Types of Lamps
    hydrogen, deuterium, tungsten, tungsten-halogen, and xenon arc lamp
  27. H2 or D2 lamps
    • 1. wavelength: 160-380
    • 2. spectroscopy: UV molecular absorption
  28. Tungsten Lamp
    • 1. wavelength: 320-2500
    • 2. spectroscopy: visible to near IR region
  29. Tungsten-Halogen Lamp
    • 1. wavelength: 240-2500
    • 2. spectroscopy: UV-Vis-near IR
  30. Xenon Arc Lamp
    • 1. wavelength: 200-1000
    • 2. spectroscopy: UV-Vis-near IR
  31. Blackbody Radiation
    emits a temperature-dependant spectrum of light
  32. Tungsten-Halogen lamp with Iodine
    Iodine sometimes prolongs the life of a tunsten-halogen lamp by combining with gaseous tungsten and causing the metal to be redeposited.
  33. Wavelength Selectors
    require a radiation that consists of a limited, narrow, continuous group of wavelengths, called a band.
  34. Narrow Band
    enhances selectivity of absorption, and provides selectivity to both emission and adsorption methods
  35. Types of Wavelength Selectors
    • 1. Filters
    • 2. Monochromators
  36. 2 Types of Filters
    • 1. interference filters
    • 2. absorption filters
  37. Interference Filters
    rely on optical interference to provide narrow bands of radiation
  38. Absorption Filters
    restricted to the visible region of the spectrum and usually consists of colored glass
  39. Monochromators
    can select a wavelength or a wavelength range and are designed for either a fixed wavelength measurement or a spectral scanning
  40. 2 Types of Monochromators
    • 1. Grating
    • 2. Prism
  41. Grating Monochromator
    • a beam of 2 wavelengths enters the monochromator via a slit, is collimated and then strikes the surface.
    • angular dispersion can result from diffraction
  42. Prism Monochromator
    • a beam of 2 wavelengths enters via a slit, is collimated and then strikes the prism
    • refraction results in angular dispersion
  43. Rules for choice of slit width
    • 1. minimal slit width is desirable when narrow absorption or emission bands must be resolved
    • 2. radiant power decreases when slits are narrow
    • 3. wider slit widths may be used for quantitative analysis rather than for qualitative
  44. Cells or Cuvettes
    • 1. quartz: expensive, used in the 180-3000nm region
    • 2. glass: low cost, used in the 350-2000nm region
  45. Ideal Transducer
    • high sensitivity
    • high signal-to-noise and a constant response over a considerable range of wavelengths
    • fast response time
    • zero output signal in the absence of illumination
    • electrical signal produced is proportional to radiant power
  46. Photon Transducer
    • phototubes and photomultiplier tubes: work based on the photoelectric effect
    • silicone photodiode
  47. Thermal Transducer
    a device which converts energy other than heat energy into heat energy
  48. All Photon Transducers
    • based on the interaction of radiation with a reactive surface to produce electrons or to promote electrons to energy states where they can conduct electricity
    • only occurs in the UV-Vis region
  49. Phototubes
    consist of a semicylinder cathode and a wire anode sealed inside an evacuated transparent glass or quartz envelope
  50. How a Phototube Works
    when a voltage is applied across the electrodes, the emitted photoelectrons are attracted to the positively charged wire anode. A photocurrent then results that is readily amplified and measured.
  51. Photomultiplier Tube
    like a phototube except in place of a single wire anode, the PMT has a series of electrodes called dynodes
  52. How a Photomultiplier Tube Works
    the electrons emitted from the cathode are accelerated towards the first dynode that is maintained 90-100V positive with respect to the cathode. Each photoelectron that strikes the dynode surface produces several electrons, called secondary electrons, that are accelerated to dynode 2. By the time this process has been repeated at each dynode, 10^5 to 10^7 electrons have been produced. All electrons are finally connected at the anode to proved an average current.
  53. Silicon Photodiode Transducer
    • photons striking the depletion layer of the device create electrons and holes that can be attracted across the junction giving rise to a current proportional to the flux of photons
    • is a pn junction device
    • operates under reverse biased conditions
  54. Semiconductor
    material that has an electrical conductivity between that of a conductor and an insulator
  55. Producing N-type and P-type Semiconductors
    the addition of a small percentage of foreign atoms in the regular crystal lattice of silicon or germanium produces dramatic changes in their electrical properties
  56. N-type Semiconductors
    • pure semiconducting materials which are doped with atoms capable of providing extra conduction electrons to the host material
    • creates an excess of negative electron charge carriers
  57. P-type Semiconductors
    pure semiconducting materials which are doped with atoms capable of providing extra positive charges to the host material
  58. P-N Junction
    • a junction formed by joining p-type and n-type semiconductors together in a very close contact.
    • when formed, some of the free electrons in the n-region diffuse across the junction and combine with holes of the p-region forming a depletion region at the junction
  59. Forward Based P-N Junction
    • occurs when the p-type semiconductor material is connected to the positive terminal of a battery and the n-type semiconductor material is connected to the negative terminal.
    • 1. drives holes to the junction from the p-type material and electrons to the junction from the n-type material.
    • 2. at the junction, electrons and holes combine so a continuous current is maintained
  60. Reverse Biased P-N Junction
    • occurs when the p-type semiconductor material is connected to the negative terminal of a battery and the n-type semiconductor material is connected to the positive terminal.
    • 1. will cause a transient current to flow as both electrons and holes are pulled away from the junction.
    • 2. when the potential formed by the depletion layer equals the applied voltage, current will cease.
  61. Photodiode Array (PDA)
    • a linear array of discrete photodiodes on an integrated circuit chip (IC)
    • in spectroscopy, a PDA is placed in the image plane of a spectrometer to allow a range of wavelengths to be detected simultaneously
  62. Signal Processors
    an electronic device that amplifies the electrical signal from the transducer
  63. Signal Processors can be used to:
    • 1. remove unwanted signals.
    • 2. convert from DC to AC and vice versa.
    • 3. perform mathematical operation on the signal as differtiation, integration, or conversion to a logarithm.
  64. Photon Counting
    • the output from a photomultiplier tube consists of a pulse of electrons for each photon that reaches the detector surface.
    • 1. sometimes converts to digital pulses that may be counted
    • 2. is a measurement of pulse number per unit time.
  65. Fiber Optics
    • fine strands of glass or plastics that transmit radiation for distance of several hundred feet or more.
    • used to transmit radiation and images from 1 component of an instrument to another
  66. Single-Beam Spectrophotometer
    radiation passes through either the reference or the sample cells
  67. Shutter
    a device that allows light to pass for a determined period of time
  68. Double-Beam-in-Space Instrument
    radiation from the filter or monochromator is split into 2 beams that simultaneously pass through the reference and sample cells before striking two matched photo detectors.
  69. Double-Beam-in-Time Instrument
    the beam is alternately sent through the reference and sample cells before striking a single photodetector
  70. Single vs Double Beam Instruments
    • 1. Single: single beam irradiation, fluctuation in energy, simple, inexpensive ($2,000-$8,000)
    • 2. Double: simultaneous two beam irradiation, compensate the source intensity fluctuation in energy source intensity both of the input/ouput, sophisticated, expensive ($10,000-$15,000)
  71. Filters vs Monochromators
    • 1. Filters: provide low resolution wavelength selection that is often suitable for quantitative work but not for qualitative work or structural study.
    • 2. Monochromator: produces high resolution (narrow bandwidths) for both qualitative and quantitative work.
  72. Photodiodes vs Photomultiplier Tubes
    • 1. Photodiodes: better suited for small, portable instruments because of their size and ruggedness.
    • 2. Photomultiplier Tubes: more sensitive but require a higher voltage supply compared to photodiodes. And are larger than photodiodes.
  73. Spectrometers vs Photometers
    • 1. Spectrometers: usually have monochromators for wavelength selection, and can be used for wavelength scanning or for multiple wavelength selection.
    • 2. Photometers: usually have filters for wavelength selection, and are restricted to one or a few wavelengths.
  74. Double Beam: In-Space vs In-Time
    • **both split the beam into 2 portions to pass through the reference cell and the sample cell.
    • 1. Space: both beams travel at the same time through the two cells, and then strike 2 separate photodetectors. Simple but requires two detectors.
    • 2. Time: beams travel at different times through the cells, but are later recombined to strike 1 photodetector, at different times. Complicated but uses only one detector.
  75. Rules for Choosing a Solvent
    • 1. a solvent does not have absorption bands in a wavelength range where an analyte absorbs
    • 2. no interaction between solvent molecules and analytical molecules
    • 3. same solvent used in blank and analyte solution
  76. Characteristics of Spectrophotometric Methods
    • 1. applies to both organic and inorganic compounds
    • 2. working range of 10-4 to 10-5 M
    • 3. 1-3% accurate
    • 4. easy and convenient
    • 5. simple operation
  77. Variables that Influence Absorbance
    pH, ionic strength, temperature, and interference species.
  78. Multichannel Spectrophotometers
    • detects entire spectral range essentially simultaneously and can produce a spectrum in 1 second or less
    • adv: speed and reliability
  79. Conventional Spectrophotometer
    • uses mechanical methods to scan the spectrum, and requires several minutes to do so.
    • adv: higher resolution, and lower stray light characteristics
  80. Atomic Spectroscopy
    science that investigates the interaction of EM radiation with atoms, usually by means of absorption or emission
  81. Atomizers
    devices used to convert analytes into gaseous atoms and ions
  82. Atomization
    process that converts an analyte into gaseous atoms or ions
  83. Atomic Emission of Radiation
    EM radiation produced when excited atoms relax to lower energy levels by giving up their energy as photons
  84. Atomic Absorption of Radiation
    paritcular wavelength of EM radiation absorbed by gaseous atoms of a particular element
  85. Atomic Emission Theory
    based on the characteristic radiation energy emitted by excited atoms when they return to ground state or a low energy level

    • Analyte ---atomization---> A*
    • A*---relaxation---> A + hv
  86. Atomic Absorption Theory
    based on the absorption of a particular wavelength of EM radiation by atoms when the atoms jump from an orbital with a lower energy level to that of a higher energy level

  87. Atomic Absorption Theory is governed by Beer's Law
    • A = kbNo = K'C
    • k/K' : constants
    • b : thickness of gaseous atoms
    • No : number of gaseous atoms in ground state
    • c : concentration of analyte (proportional to No)
  88. Energy Level Diagram
    • convenient method for describing the processes that atomic absorption/emission is based on
    • Show the different transition of elements between atomic orbitals
  89. Difference in the Energy of 3P Orbitals
    • rationalized by assuming that an electron spins about its own axis and the direction may be either the same or opposed to its orbital motions
    • 1. energy is high if the direction of self-spin is the same as the direction of orbital motion.
    • 2. energy is low if the direction of self-spin is the opposite of the direction of orbital motion.
  90. Narrow Lines for AAS and AES
    preferred because it reduces the possibility of interference due to overlapping spectra
  91. Factors Causing Atomic Line Broadening
    • 1. uncertainty effect
    • 2. doppler effect
    • 3. pressure effect
  92. Uncertainty Effect
    natural line width is determined by the lifetime of the excited state and Heisenberg's uncertainty principle
  93. Doppler Effect
    arises because atoms moving toward or away from a photon detector give rise to absorption or emission lines at slightly different frequencies
  94. Pressure Effect
    • arises from collisions of the absorbing or emitting species with other atoms or ions in the heated species
    • causes small changes in energy levels and hence a range of absorbed/emitted wavelengths.
  95. Hollow Cathode Lamp
    • a cathode constructed of the metal whose spectrum is desired, or served to support a layer of that metal. Anode is made of tungsten. Electrodes are sealed in a glass tube that is filled with Neon or Argon at a pressure of 1-5 torr.
    • *has a lamp for each element, restricts multielement detection, maximizes probability of redeposition on cathode and restricts light direction
  96. Sputtering
    gaseous cations of an inert gas acquire enough kinetic energy to dislodge some of the metal atoms from the cathode surface and produce an atomic cloud
  97. Pneumatic Nebulization
    • a nebulizer introduces the sample in the form of a fine spray of droplets, called aerosols.
    • continuous introduction of samples produces a steady-state population of atoms, molecules and ions.
    • solution-sample is converted to spray
  98. Types of Pneumatic Nebulizers
    • 1. concentric tube
    • 2. cross-flow
    • 3. fritted disk
    • 4. babington
  99. Concentric Tube Nebulizer
    liquid sample is drawn through a capillary tube by high pressure stream of gas flow around the tip of the tube
  100. Cross-Flow Nebulizer
    high pressure gas flow across a capillary tip at right angles
  101. Fritted Disk Nebulizer
    sample solution is pumped onto a fritted surface through which a carrier gas flows
  102. Babington Nebulizer
    • high pressure gas is pumped through a small oriface in a hollow sphere's surface. The extending jet of gas nebulizers the liquid sample flowing in a thin film over the sphere's surface.
    • *useful for high salt content samples because it is less subject to clogging
  103. Hydride Generation
    used for introduction of samples containing As, Sb, Sn, Se, Bi, and Pb into an atomizer as a volatile hydride gas.
  104. Volatile Hydride Gas
    • generated by adding an acidified sample solution to a small volume of 1% sodium borahydride (NaBH)
    • swept into an atomization chamber and heated in a tube furnace where decomposition of the hydride takes place
    • increases detection limits by 10 to 100 times
  105. Continuous Atomizers
    flame and plasma atomizers
  106. Discrete Atomizers
    electrothermal atomizers
  107. Processes Occurring During Atomization
    • 1. sample solution ---> nebulization
    • 2. analyte + gasfuel ---> dissolvation
    • 3. solid/gas aerosol ---> volatilization
    • 4. gaseous molecules ---> dissociation
    • 5. atoms ---> ionization ---> ions
  108. Primary Combustion Zone
    initial decomposition of molecules. recognized by its blue luminescence.
  109. Interzonal Region
    • commonly used for spectroscopy, hottest, mostly atomic fragments.
    • used for emission/absorption/fluorescence
  110. Secondary Combustion Zone
    cooler, conversion of atoms to stable molecules and oxides
  111. Characteristics of Flame Atomizers
    • 1. laminar flow-burners: long path length for max absorbance
    • 2. provides most reproducible methods and system provides a continuous output signal
    • 3. low sensitivity due to residence time of individual atoms in the optical path and low sampling efficiency.
    • 4. detection limits: 0.2-500ng/mL
    • 5. relative precision of measurements: 1%
  112. Electrothermal Atomization
    • few microliters of sample is introduced into a graphite furnace
    • evaporated at low temperature and ashed at high temperatures in the graphite tube
    • after ashing, atomization occurs when temperature rises rapidly to 2000-3000oC
  113. Electrothermal Atomization occurrs
    • in a cylindrical graphite tube that is open at both ends and that has a central hole for sample introduction.
    • the graphite tube fits into a water-cooling metal housing
  114. Characteristics of Electrothermal Atomizers
    • 1. high sensitivity and high sample efficiency
    • 2. detection limits: 5-0.001ng/mL
    • 3. poor reproducibility: relative precision of 5-10%
    • 4. slow: few minutes per sample
    • 5. method of choice: when flame or plasma atomization provides inadequate detection limits