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Analytical chemistry > Test 2 > Flashcards

Test 2 Flashcards

1
Q

Advantages of Atomic Emission Spectroscopy

A
  • lower susceptibility to chemical interferences
  • good emission spectra result for most elements under single set of excitation conditions, record spectra simultaneously for dozens of elements (compared with flame atomic absorption)
  • permit the determination of low concentrations of elements
  • permit to detect nonmetals such as chlorine, bromine, iodine and sulfur
  • wide linear range (seven orders)
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2
Q

Disadvantages of Atomic Emission Spectroscopy

A
  • spectral interferences
  • require higher resolution and more expensive optical equipment
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3
Q

Inductively Coupled Plasm

A
  • a plasma is an electrically conducting gaseous mixture containing significant concentration of cations and electrons
    • argon plasma: 10000 K
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4
Q

Types of Plasma Sources

A
  • Inductively coupled plasma (ICP)
    • the most important of these plasmas is the ICP
  • direct current plasma (DCP)
  • microwave induced plasma (MIP)
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5
Q

How does the inductively coupled plasma (ICP) source work

A
  • three concentric quartz tube through which the steam of argon gas flow
    1. ) ionization of the flowing argon is initiated by a spark from a Tesla coil
    2. ) induction coil is powered by a radio-frequency generator which radiates 0.5-2 kW of power at 27.12 MHz or 40.68 MHz
    3. ) th eresulting ions and their associated electrons, then react with the fluctuating magnetic field
    4. ) this interaction causes the ions and electrons within the coil to flow in a closed annular paths
    5. ) the resistance of the ions and electrons to this flow of charge causes ohmic heating of the plasma
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6
Q

Typical ICO source

A

torch

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7
Q

Three types of sample introductions for Inductively Coupled Plasma (ICP)

A
  1. ) nebulization
  2. ) electrothermal vaporization
  3. ) laser ablation
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8
Q

Types of nebulizations in inductively coupled plasma

A
  • concentric glass nebulizer
  • cross -flow
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9
Q

when is an electrothermal vaporization used

A
  • for sample introduction only atomization occurs in plasma
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10
Q

where should observations be made in plasma appearance and spectra

A

generally made at a height of 15-20 mm above the induction coil, where the temperature is 6000-66500 K (background free)

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11
Q

Analyte Atomization and Ionization comparison

A
  1. ) atomization is more complete in plasma than in flames
  2. ) atomization occurs in a chemically inert environment, which tends to enhance the lifetime of the analyte by preventing oxide formation
  3. ) the temperature cross section of the plasma is relatively uniform, self-absorption and self-reversal do no occur
  4. ) the plasma produces significant ionization, which makes it an excellent source for ICPMS
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12
Q

What is Direct Current Plasma

A
  • has fewer lines than those produced by the ICP, and the lines formed in the DCP are largely from atoms rather than ions
  • sensitivity range from an order of magnitude lower than those in ICP reproducibilities are similar, less argon is required, less expensive
  • short residence time, graphite electrode must be replaced every few hours
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13
Q

Three types of Plasma Source Spectrometers

A
  1. ) sequential
  2. ) simultaneous multichannel
  3. ) fourier transform
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14
Q

Types of simultaneous multichannel

A
  • polychromators
  • spectrographs
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15
Q

What are the applications of Plasma Soures

A
  • useful for both qualitative and quantitative elemental analysis
  • high stability
  • low noise
  • low background
  • freefom from interferences
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16
Q

What is more sensitive, ICP or DCP

A

ICP

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17
Q

What is more expensive, DCP and ICP

A

ICP

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18
Q

What is ICP

A

it is an emission spectroscopy that is used primarily for the qualitative and quantitative analysis of samples that dissolved or suspended in aqueous or organic liquids

  • possible to analyze solid sample directly
  • procedure incorporating electro-thermal vaporization
  • laser and spark ablation and glow discharge vaporization
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19
Q

Elements determined using ICP

A
  • all metallic elements can be determined by plasma emission spectroscopy
  • nonmetallic elements: boron, phosphorus, nitrogen, sulfur carbon
  • usefulness for the alkali metal is limited to 60 elements
  • internal standard is often used in emission spectrometry
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20
Q

Calibration Curves for ICP

A
  • for plasma spectrometry most often consist of a plot of an electrical signal proportional to line intensity vs. analyte concentration
  • usually linear
  • nonlinearity often occurs at high concentration range because of self-absorption or incorrect background correction
  • an internal standard is often used in emission spectrometry
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21
Q

Interferences in ICP

A
  • chemical interferences and matrix effects are significantly lower with plasma than with other atomizers
  • at low analyte concentrations, the background emission due to recombination or argon ions with electrons is large enough to require careful correction
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22
Q

Electric Arc and Electric SPark Sources

A
  • arc and spark sources are gradually being displaced by plasma sources
  • arc and spark source methods are mostly limited to the elemental analysis of solids, plasma sources are much convenient for liquid and gaseous
23
Q

Instruments are Arc and Spark Source

A
  1. ) spectrographs (time consuming)
  2. ) multichannel spectrometers (large and not versatile)
    a. ) multichannel photomultiplier instruments
    b. ) array based multichannel instruments
24
Q

Arc Source of emission spectroscopy

A
  • the usual arc source for a spectrochemical analysis is formed with a pair or graphite or metal electrodes spaced a few millimeters apart

  1. ) the arc is initially ignited by low current (1-30A) spark that cause momentary formation of ions for electrical conduction in the gap
  2. ) once that arc is struck, thermal ionization maintains the current, temperature is 4000-5000K
  3. ) in collections of spectral line data, wavelength are often followed by I, II and III to indicate the source of the line as that of neutral atom, the singly charge ion and the doubly charged ion respectively
  4. ) Arcs produced from a carbon or graphite electrode in air emit intense bands due to the presence of cyanogen radicals 350-420 nm
25
Q

Applications of Arc Sources

A
  • sources are particulary useful for qualitative and semiquantitative analysis of nonmetallic samples, such as soild, plant materials, rocks and minerals
26
Q

Source and Spectra of Arc

A
  • the instantaneous current may exceed 1000A during the inital phase of the discharge
  • narro spacer is estimated to be as great as 40000K
27
Q

Miscellaneous Sources for optical emission spectroscopy

A
  1. ) flame emission
  2. ) glow discharge
  3. ) laser microprobe
28
Q

what are the limitations to Beer’s Law

A
  • real limitations to Beer’s Law
    1. ) high concentration (>0.01M) of analyte and electrolyte (solute/solvent)
    2. ) large organic ions or molecules
    3. ) the refractive index of the medium (concentration cause the change of index n)
  • apparent chemical deviation
  • instrument deviation due to polychromatic radiation
  • instrumental deviations in the presence of stry radiation
  • mismatched cells
29
Q

Sources of Instrumental Noise

A
  • case I uncertainties: the precision is independent of T
  • case II uncertainties: the precision is proportional to square root of Tsquare plus T
  • case III uncertainites: are directly proportional to T
30
Q

What are the effect of slit width on absorption measurement

A

narrow slit widths are required to resolve complex spectra

31
Q

What are the components of an instrument

A
  1. ) sources - for the purpose of molecular absorption measurement a continuum source is required whose radiant power does not change sharply over a considerable range of wavelegth
  2. ) wavelength selectors
  3. ) sample containers - constructed of either quartz or glass
  4. ) radiation transducers
  5. ) signal processors and readout devices
32
Q

Types of Sources

A

deuterium and hydrogen lamps

tungsten filament lamps

light-emitting diodes

xenon arc lamps

33
Q

Types of Instruments for UV/Vis Spectroscopy

A
  1. ) Single beam
  2. ) double-beam in space
  3. ) double-beam in time
  4. ) multichannel
34
Q

Drawback of double-beam in space

A

requires two detectors, which makes the instrument more expensive

35
Q

Double Beam in time advantages

A
  • most common
  • compensate for variations in the source intensity
  • compensate for drift in the detector amplifier
  • compensate for variation in intensity as a function of wavelength
36
Q

Two major classifications of instruments for molecular absorption spectrometry

A
  1. ) Photometers: simple instruments that use filters to select wavelength, they can only detect a single wavelength at a time, have a high throughpout energy due to the simple optics (good S/N) and are inexpensive
  2. ) spectrophotometers: instruments that contain a monochromator dispersive element that allow them to scan various wavelengths. More expensive than photometers and usually have a lower S/N due to the more complex optics
37
Q

Absorption by Organic Compounds

A
  • all organic compounds are capable of absorbing electromagnetic radiation because all contain valence electrons that can be excited to higher energy levels
  • the excitation energies associated with electrons forming most single bonds are sufficiently high that absorption occurs in the so-called ultraviolet region
  • most applications of absorption spectroscopy to organic compounds are based on transitions for n or pi electrons to be pi* excited state because the energies required for these processes bring the absorptin bands into the UV regions (200-700 nm)
  • molecules containing unsaturated functional groups and capable of absorbing ultraviolet-visible radiation are called chromophores
38
Q

Absorption characteristics

A
  1. ) the maximum absorption wavelength and peak intensity are influenced by solvent effect and structure details of the molecules
  2. ) th emolar absorptivities for n to pi* transition are normally low and usually range from 10-100 on the other hand, values for pi-pi* transitions are large, generally from 1000-15000
    - saturated organic compounds containing such heteroatoms as oxygen, nitrogen, sulfur or halogens have nonbonding electrons that can be excited by radiation in the range of 170-250 nmm
39
Q

Absorption by Inorganic Compounds

A
  • a number of inorganic anions exhibit ultraviolet absorption that results of exciting nonbonding (nitrate, carbonate, nitrite, oxido, tricarbonate
40
Q

What is a charge-Transfer Absorption

A
  • consist of an electron-donor group bonded to an electron acceptor
  • large molar absorptivities (>10000) leads to high sensitivities
41
Q

Qualitative applications of UV-Visible Absorption Spectroscopy

A
  • UV spectrum does not have enough fine structure to permit an analyte to be identified unambiguously
  • UV qualitatvie data must be supplemented with other physical or chemical ecidence such as infrared, NMR, and MS as well as solubility and melting andboiling point information
42
Q

Solvents for UV spectra

A
  • qualitative analysis are usually measured using dilute solutions of the analyte
    1. ) transparency, consider its possible effects on the absorption system
    2. ) obliterate spectra fine structure arising from vibrational effects
    3. ) the position of absorption maxima are influenced by the nature of the solvent
    4. ) spectra similar to gas-phase spectra are more likely to be observed in nonpolar solvents such as hydrocarbons
43
Q

Quantitative Analysis by Absorption Measurements

A
  1. ) wide applicability to both organic and inorganic system
  2. ) typical detection limits of 10^-4 and 10^-5
  3. ) moderate to high selectivity
  4. ) good accuracy (relative uncertainties are 1-3%)
  5. ) easy and convenient of data acquistion
44
Q

Applications to Absorbing Species

A
  1. ) any organic compound containg one or more chromophoric groups can be easured
  2. ) inorganic transition metals are colored in solution and can be determined by spectrophotometric method
  3. ) a number of other species show characteristic absoption bands (nitrate, nitrite and chromate ions, the oxides of nitrogen the elemental halogens and ozone)
45
Q

Procedural Details for UV Absorption

A
  1. ) selection of wavelength (maximum absorption wavelength for highest sensitivity
  2. ) variables that influence absorbance (pH, temperature, high electrolyte concentration and interferences)
  3. ) cleaning and handling of cells (good quality and matched cells)
  4. ) determining the relationship between absorbance and concentration
46
Q

Photometric and Spectrophotometric Titrations of Absorptions

A

useful for locating the equivalence point of a titration, provided the analyte, the reagent or the titration product absorbs radiation

47
Q

Titration Curves

A
  • plot of absorbance, corrected for volume changes, as a function of the volume of titrant
  • for many titrations, the curve consist of two linear regions with different slope, one occurring early in the titration and the other located well beyond the equivalent point region, the end point is th eintersection of the extrapolated linear portions of the curve
48
Q

Applications of Photometric titration

A
  • oxidizing agents
  • acid base titration
  • precipitation titration
49
Q

Spectrophotometric Kinetic Methods

A
  • kinetic methods of analysis differ in a fundamental way from the equilibrium, or stoichiometric, methods that have to be considered
  • measurements are made under dynamic conditions in which the concentrations of the reactants and products are changing as functions of time
50
Q

Spectrophotometric Studies of Complex Ions

A
  1. ) the method of continuous variations
  2. ) the mole-ratio method
  3. ) the slope ratio method
  4. ) computer-based curve fitting methods
51
Q

Mole-Ratio Method

A
  • series of solution is prepared in which the analytical concentration of one reactant is held constant while that of the other is varied
  • if formation is constant is reasonably favored, two straight lines of different slope that intersect at a mole ratio that corresponds to the combining ratio in the complex were obtained
52
Q

the Slope Ratio Method

A
  • useful for weak complexes but is applicable only to systems in which a single complex is formed
  • two assumptions
    1. ) the complex-formation reaction can be forced to completion by large excess of either reacant
    2. ) Beer’s law is followed under these circumstances
53
Q
A

Analytical chemistry (1 decks)

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