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Instrumental Analysis > Quiz 3 > Flashcards

Quiz 3 Flashcards

1
Q

What is Atomic Absorption?

A

sample absorbs external radiation

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2
Q

How does AA compare to molecular absorption

A

AA is the atomic analog of MA

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3
Q

What is Atomic Emission?

A

Energy (provided electrically) from external device excites sample, sample itself is the source, reemitted as it decays to ground

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4
Q

What is Atomic Fluorescence?

A

sample excited by absorption, reemits radiation of a longer wavelength

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5
Q

What is a hollow cathode lamp?

A

narrow line source at low pressure low temperature so there is no collision or doppler

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6
Q

What are the line widths produced by a hollow cathode lamp?

A

10e-2 to 10e-3 amps

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7
Q

Are black body sources used in AA?

A

nope, requirements are too strict

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8
Q

What is the role of the monochromator in AA?

A

filtering

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9
Q

Is beer’s law obeyed in AA?

A

it has to be, source bandwidth is less than sample bandwidth

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10
Q

What are the physical principles behind AE?

A

Energy from external source raises sample to excited state, emitting radiation directly upon relaxing to ground state

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11
Q

What is the source in AE?

A

The sample itself

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12
Q

How is the sample brought to the excited state in AE?

A

electrically

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13
Q

What is the role of the monochromator in AE?

A

high resolution

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14
Q

What are the physical principles behind AF?

A

narrow line sources, hollow cathode lamps with interference filter, low resolution wavelength sorting monochromators

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15
Q

What is the source in AF?

A

narrow line sources

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16
Q

Is high resolution needed in AF?

A

no

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17
Q

What determines the intensity distribution of atomic spectral lines?

A

population of excited energy levels

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18
Q

What is a Boltzmann distribution?

A

Population of energy levels at thermal equilibrium

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19
Q

What happens to the excited state population as wavelength decreases?

A

decreases as wavelength decreases

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20
Q

What happens to the excited state population as temperature increases?

A

Increases as temp increases

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21
Q

What are states of degeneracy?

A

multiple states exhibit the same energy

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22
Q

How are states of degeneracy applied to Boltzmann distribution?

A

represent statistical likelihood of m + g

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23
Q

What is the usual form of the sample in flame atomization?

A

liquid

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24
Q

What are the typical fuels used in AA?

A

natural gas, hydrogen, acetylene

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25
Q

What are the typical oxidants in AA?

A

oxygen, air, nitrous oxide

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26
Q

Fuel and oxidant combos?

A

natural + air, natural + O, H + Air, H + O, A + Air, A + O, A + Nitrous oxide

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27
Q

What are the temps typically used in flame AA?

A

1700 - 3150

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28
Q

How do flames contribute to the noise characteristics of an AA Instrument?

A

temperature and flicker

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29
Q

how can AA noise be controlled?

A

chopper and lock in

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30
Q

Control noise in AA and AF

A

chopper between light and flame

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31
Q

Control noise in AE

A

chopper between flame and monochromator

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32
Q

How do gas flow rates affect AA

A

basically immune to changes in temperature because ground state is used

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33
Q

How do gas flow rates affect AE?

A

temperature with excess fuel cools it down, excited population, intensity, concentration

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34
Q

How do the gas flow rates affect the ultimate sensitivity in AA and AE?

A

varied flame composition shifts detection region

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35
Q

How is desolvation usually accomplished?

A

evaporating solvent, relies on flame heat

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36
Q

What is vaporization?

A

solid to gas

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37
Q

What is atomization

A

ionic compound to atoms

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38
Q

Is there an optimum detection zone for every element?

A

no, depends on atom and process

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39
Q

How is the detection zone optimized?

A

depends on atom and process. You can move the burner position

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40
Q

What is the most common burner design in AA/AE?

A

laminar flow, premixed burner

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41
Q

What are the most common problems with using flames in AA/AE?

A

inefficient nebulization, short atom residence time, fluctuating intensities because temperature too low for atomizing

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42
Q

how does graphite furnace AA compare with Flame AA?

A

hollow rod along optical path, flushed with inert gas. requires water cooled jacket

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43
Q

What type of samples are used in AF/AA

A

Liquid or Solid

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44
Q

What are the 3 phases in electrothermal atomization?

A

Drying (heat to evaporate solvent), Ashing (Raise t to break down), and Atomization (flash heat to incandescent to produce atom population)

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45
Q

how long is drying?

A

short, 20-30 seconds

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46
Q

how long is ashing?

A

Depends on time it takes to breakdown sample, but no more than that or it goes away.

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47
Q

How long is atomization?

A

more than 10e3 K per second

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48
Q

What are the advantages of electrothermal graphite furnace atomization?

A

long residence time, high sensitivity, low detection limit, low noise, small sample without pretreatment

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49
Q

What are the disadvantages of electrothermal graphite furnace atomization

A

matrix interference, slow heat causes uneven ashing, poorer precision than flames

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50
Q

What are the characteristics of flames used in AA?

A

preheating, primary, interconal, outer

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51
Q

What is the preheating zone?

A

cooler region of flame

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52
Q

What is the primary reaction zone?

A

oxidant and fuel reaction at non equilibrium, intense emission, not good for observations in AA and AE

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53
Q

what is the interconal zone?

A

hottest region, faint oxidant/fuel emissions, rich in atoms, best for observing AA/AE

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54
Q

What is the outer zone?

A

secondary reaction, usually not good for detection

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55
Q

How does background correction differ in atomic spectroscopy compared to UV/Vis?

A

no ready scanning to correct background noise, very wide spectral bandpass in wavelength selector relative to source

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56
Q

What are the two main types of interference common to atomic spectroscopy?

A

chemical and spectraul

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57
Q

What are some strategies for reducing chemical interference?

A

form competing complexes that will dissociate in flame

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58
Q

how do compounds of low volatility form with the analyte?

A

anions reduce atomization rate

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59
Q

How can light scattering be reduced?

A

increase temp, decrease slit width, appropriate background correction

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60
Q

What techniques make matrix contamination the worst? Why?

A

solid samples, DC arc, spark sources, graphite furnace AA because of variation of rate of volatilization

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61
Q

Strategies for reducing matrix effects

A

closely match standards, vary temp by changing fuel to oxidant ratio

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62
Q

What are the advantages of high energy sources?

A

can read decade wide concentration range, concentration of nonmetals, low refractory concentrations; dozens of spectra simultaneously; low interference

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63
Q

what are the disadvantages of high energy sources?

A

complicated, expensive, less precise, high operator involvement

64
Q

What are the common gases used to produce plasmas

A

Mainly Ar, sometimes O2

65
Q

What is the most common gas used to excite a plasma in AE?

A

Ar

66
Q

What is a sequential ICP?

A

monochromator with photomultiplier/CCD detector

67
Q

What is a multichannel ICP?

A

lots of photomultiplier tubes behind curved focal plane slits with fixed wavelength transmission

68
Q

How are the characteristics of an Echelle grating different from regular grating?

A

very coarse, very large blaze angle, short blaze

69
Q

Why are echelle gratings used in multichannel ICP?

A

high dispersion and resolution

70
Q

What type of electrodes are used in arc sources?

A

nonmetals use carbon rod, metals use the sample itself, powder uses pellets

71
Q

what are the typical operating conditions for arc sources?

A

1-30 A, DC = 200V, AC = 2200 - 4400, K = 4000-8000

72
Q

What are the typical operating conditions for spark sources?

A

10-50kV, 1000 A instantaneous, spark gap K up to 40,000

73
Q

What are the common conditions for generating ICP’S?

A

high temp, long residence time, high e density, form free atoms in almost inert environment, molecular species absent, optically thin, no electrodes, no explosives

74
Q

How are plasmas generated in an ICP?

A

Ar gas, RF generator

75
Q

Most common power supply for plasmas?

A

RF induction coil

76
Q

Why are they called inductively couple plasmas?

A

energy is supplied by electric currents which are produced by electromagnetic induction

77
Q

What are MIP’s?

A

microwave generator as power supply for energizing plasmas

78
Q

What is nebulization ?

A

process of converting liquid sample to fine mist of tiny droplets

79
Q

What is the primary nebulization method?

A

pneumatic

80
Q

How is nebulization accomplished experimentally?

A

pneumatic: flow of gas past the orifice of inlet tube with small diameter pulls liquid into gas bc of reduced pressure. surface tension causes column of liquid exiting to break into droplets that collide into bigger ones. only small are useful

81
Q

Venturi Effect

A

reduced pressure pulls liquid into gas phase

82
Q

What is the efficiency of nebulization?

A

90-99% of sample goes down drain

83
Q

What is the Sauter Equation

A

relates parameters (viscosity, density, etc) to droplet particle size

84
Q

How is mean droplet diameter related to gas velocity?

A

diameter decreases as velocity increases

85
Q

How is mean droplet diameter related to solution viscosity?

A

decreases as viscosity decreases

86
Q

How is mean droplet diameter related to density?

A

decreases as density increases

87
Q

How is mean droplet diameter related to flow rate liquid?

A

decreases as Qliq decreases

88
Q

How is mean droplet diameter related to flow rate gas?

A

decreases as Qgas increases

89
Q

How to obtain smallest droplet size?

A

slow aspiration liquid, fast gas flow, low viscosity solvent with low surface tensions

90
Q

What are alternative nebulization methods?

A

cross flow, fritted disk, babington, ultrasonic,

91
Q

How do the flow velocity of gases contribute to flame characteristics?

A

Varying ratio varies temp which alters excited state population. Changing temp changes observed intensity and concentration.

92
Q

Is high resolution required in AA?

A

AA monochromator can use lower resolution than that required of AE

93
Q

Ways to dissociate sample: flames

A

relatively low energy, used for easily excited elements

94
Q

Ways to dissociate sample: ICP

A

high frequency (radiowave) electrical signal inductively coupled to sample chamber via coil to dissociate into plasma

95
Q

Ways to dissociate sample: DC plasma

A

uses dc voltage source to dissociate sample into plasma, related to DC arc method

96
Q

Ways to dissociate sample: MIP

A

related to ICP but uses microwave wavelengths

97
Q

Ways to dissociate sample: DC arc

A

low voltage high current between two electrodes, particularly for metals

98
Q

Ways to dissociate sample: AC spark

A

high voltage, lower current alternating at high frequency between two electrodes, used in metals

99
Q

Role of monochromator in AF?

A

Maintains wavelength; excitation and emission

100
Q

Why is light scattering important in AA/AE?

A

Occurs for particles in flame (not fully dissolved) or burning carbon roughly the same size as light wavelength

101
Q

What is the continuous source background correction method

A

radiation from hollow cathode lamp and D2 lamp passes through beam splitter/chopper. Detector detects each signal separately.

102
Q

What type of lamp is used in continuous source background correction?

A

D2 and Hollow Cathode

103
Q

How is corrected signal calculated in continuous source?

A

Corrected = hollow signal - D2 signal

104
Q

Wh is continuous source the most used for AA correction?

A

Eliminates matrix so sample can be observed

105
Q

Advantages of continuous source correction

A

Relatively easy to implement, works with relatively high absorbances, little effect on calibration curve

106
Q

Disadvantages of continuous source correction

A

detection limit suffers due to source intensities not being equal, optical alignment is difficult

107
Q

What is Zeeman method?

A

atomic vapor exposed to strong magnetic field results in splitting atomic energy levels

108
Q

Why is polarization needed in Zeeman

A

emits varying polarized light

109
Q

How is sample absorbance measured in Zeeman?

A

lamp emission parallel to magnetic field (pi emissions) is absorbed by parallel pi component of analyte

110
Q

How is zeeman background absorbance measured?

A

lamp emission perpendicular to magnetic field (sigma) not absorbed by analyte due to different polarization properties of sample and background

111
Q

How is corrected signal calculated Zeeman?

A

Corrected = sample - background

112
Q

What is the source self reversal method? (Smith-Hieftje)

A

based on self absorption behavior of HCL at high current: brief pulse modulation of lamp current produces many non-excited atoms that quench excited species. Broadended band with minimum at its center

113
Q

Pros zeeman

A

No alignment or source drift problems, more accurate, real-time double beam, abs continually compensated, reduces flicker noise, detection limits improve when source noise decreases

114
Q

cons zeeman

A

complex, expensive, magnetic field adjusted for each element, intensity reduced, lower S/N and detection limit, calibration curves exhibit big non-linearities or reversals

115
Q

How is background absorbance measured S-H

A

low current gives total, high current gives background

116
Q

S-H corrected signal calculation

A

low - high

117
Q

Pros S-H method

A

all wavelengths, simple alignment, auto compensates for source and flicker noise, additional optical components not needed, inexpensive

118
Q

Cons S-H method

A

less successful for less volatile/more refractory elements, reduced calibration sensitivity, detection limits worse

119
Q

What roles do temp and oxygen content play in chemical interference?

A

Ionization: smaller at low temp to with air, higher temps available with O2 or N2O.

120
Q

Why does ionization interfere with atomic spectra?

A

ions possess different electronic configs than atoms

121
Q

When is ionization important?

A

at higher temps available with O2 or N2O as oxidant

122
Q

when is ionization not important

A

at lower temps or with air as oxidant

123
Q

what are strategies for reducing ionization interference

A

concepts of equilibria: ionization buffer (more easily ionized species added) shifts it toward metal formation

124
Q

When are low dissociation complexes formed?

A

anions form compounds of low volatility with the analyte and thus reduce the rate it is atomized

125
Q

what are some common elements that form dissociation complexes?

A

refractory metals: Nb, Mo, Ta, W, Re, Ti, Cr, Ru

126
Q

What are some solutions to compound formation in flames?

A

increase flame temp so rxn becomes exo and shifts towards free atoms; lower free O content, increases reducing species and resorting free metals

127
Q

pH role in dissociation equilibria?

A

Higher H ion content pushes it to reactants side

128
Q

What are some common sources of spectral interference?

A

flame emissions, light scattering, matrix

129
Q

strategies to avoid flame emission interference

A

vary fuel gas to one with fewer emissions at wavelength of interesting, adjust fuel to oxidant ratios, increase T, use correct background corrections, decrease slit width

130
Q

strategies to avoid light scattering

A

increase temp, appropriate background correction technique, dilute, decrease slit width

131
Q

What role does F-O ratio play in spectral interferences?

A

Varying temp varies ratio, allowing for decomposition of interfering species

132
Q

Why does decreasing slit width improve S/N?

A

signal is 1/w, noise is 1/sqrt(w), so S/N improves as 1/sqrt(w) as long as bandpass is less than source width

133
Q

Why are plasma, arc, spark source methods called “high energy source methods”

A
134
Q

What are the instrumental requirements for a sequential ICP?

A

standard monochromator design. uses standard photomultiplier/CCD detection

135
Q

What are the instrumental requirements for a multichannel ICP?

A

numerous photomultipliers behind fixed slits along a curved focal plane of a concave grating monochromator. Slits fixed to transmit wavelengths of particular elements

136
Q

What determines natural line width for atomic emission and absorption lines? How broad are they typically?

A

The widths of the lines only when uncertainty principle and not Doppler or pressure broadening contribute. The width is determined by the lifetime of the excited state.

137
Q

Cation that preferentially reacts with a species that would otherwise react with the analyte to cause chemical interference

A

Releasing agent

138
Q

Prevent interference by forming stable and volatile products with the analyte

A

Protective agents

139
Q

More easily ionized than analyte and provides high concentration electrons in flame or plasma that suppress analyte ionization

A

Ionization suppressor

140
Q

Process by which sample is vaporized and decomposed into atoms by heat

A

Atomization

141
Q

Broadening of atomic lines due to collisions

A

Pressure broadening

142
Q

Has a tungsten anode and cylindrical shaped cathode containing element of interest. The element is sputtered from the cathode into gas. This process excited gaseous atoms which emit characteristic radiation as they relax to ground.

A

Hollow cathode lamp

143
Q

Process by which gaseous cations bombard a cathode surface and eject atoms from surface to gas phase

A

Sputtering

144
Q

Absorption of emitted radiation by unexpired atoms in the gas phase of a hollow cathode lamp, flame, or plasma

A

Self absorption

145
Q

Encountered when absorption or emission of a non analyte species overlaps a line being used for the determination of the analyte

A

Spectral interference

146
Q

Result of any chemical process that decreases or increases absorption or emission of analyte

A

Chemical interference

147
Q

Substance added in excess to both sample and standards which Samos the effect of the matrix on analyte emission or absorption

A

Radiation buffer

148
Q

Arises because atoms moving toward or away from monochromatic give rise to absorption or emission lines at slightly difference frequencies

A

Doppler broadening

149
Q

Why is an electro thermal atomizer more sensitive than a flame atomizer?

A

Electro thermal is more efficient. It requires less sample and keeps atomic vapor in the beam for a longer time than the flame does.

150
Q

Describe how a deuterium lamp can be used to provide background correction for atomic absorption spectrum

A

Continuum radiation from D2 lamp is passed through the flame alternately with the hollow cathode beam. Since atomic lines are very narrow the D2 lamp is mostly absorbed by background whereas hollow cathode radiation is absorbed by atoms. By comparing the radiant power of rate two beams atomic absorption can be corrected for background

151
Q

Why is source modulation used in atomic absorption spectroscopy?

A

Distinguish between atomic absorption (an ac signal) and flame emission (a dc signal)

152
Q

What is an internal standard and why is it used?

A

A substance that responds to uncontrollable variables in a similar way as the analyte. It is introduced into or is present in both standard and sample in fixed amount. The ratio of the analyte signal to the internal standard signal then serves as the analytical reading

153
Q

Why are atomic emission methods with ICP source better suited for multi element analysis than flame atomic absorption methods are?

A

Flame requires a separate lamp for each element which is inconvenient

154
Q

Why is ionization interference less severe in ICP than in flame emission spectroscopy

A

Because argon plasmas have high concentration of electrons from ionization which represses ionization of the analyte

155
Q

What are advantages of plasma sources compared with flame sources for emission spectroscopy

A

lower interference, emission spectra for many elements can be obtained with one set of excitation conditions, spectra can be obtained for elements that tend to form refractory compounds, plasma sources usually have linearity range that covers several decades in concentration

Instrumental Analysis (45 decks)

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