Quiz 3 Flashcards
1
Q
What is Atomic Absorption?
A
sample absorbs external radiation
2
Q
How does AA compare to molecular absorption
A
AA is the atomic analog of MA
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
4
Q
What is Atomic Fluorescence?
A
sample excited by absorption, reemits radiation of a longer wavelength
5
Q
What is a hollow cathode lamp?
A
narrow line source at low pressure low temperature so there is no collision or doppler
6
Q
What are the line widths produced by a hollow cathode lamp?
A
10e-2 to 10e-3 amps
7
Q
Are black body sources used in AA?
A
nope, requirements are too strict
8
Q
What is the role of the monochromator in AA?
A
filtering
9
Q
Is beer’s law obeyed in AA?
A
it has to be, source bandwidth is less than sample bandwidth
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
11
Q
What is the source in AE?
A
The sample itself
12
Q
How is the sample brought to the excited state in AE?
A
electrically
13
Q
What is the role of the monochromator in AE?
A
high resolution
14
Q
What are the physical principles behind AF?
A
narrow line sources, hollow cathode lamps with interference filter, low resolution wavelength sorting monochromators
15
Q
What is the source in AF?
A
narrow line sources
16
Q
Is high resolution needed in AF?
A
no
17
Q
What determines the intensity distribution of atomic spectral lines?
A
population of excited energy levels
18
Q
What is a Boltzmann distribution?
A
Population of energy levels at thermal equilibrium
19
Q
What happens to the excited state population as wavelength decreases?
A
decreases as wavelength decreases
20
Q
What happens to the excited state population as temperature increases?
A
Increases as temp increases
21
Q
What are states of degeneracy?
A
multiple states exhibit the same energy
22
Q
How are states of degeneracy applied to Boltzmann distribution?
A
represent statistical likelihood of m + g
23
Q
What is the usual form of the sample in flame atomization?
A
liquid
24
Q
What are the typical fuels used in AA?
A
natural gas, hydrogen, acetylene
25
What are the typical oxidants in AA?
oxygen, air, nitrous oxide
26
Fuel and oxidant combos?
natural + air, natural + O, H + Air, H + O, A + Air, A + O, A + Nitrous oxide
27
What are the temps typically used in flame AA?
1700 - 3150
28
How do flames contribute to the noise characteristics of an AA Instrument?
temperature and flicker
29
how can AA noise be controlled?
chopper and lock in
30
Control noise in AA and AF
chopper between light and flame
31
Control noise in AE
chopper between flame and monochromator
32
How do gas flow rates affect AA
basically immune to changes in temperature because ground state is used
33
How do gas flow rates affect AE?
temperature with excess fuel cools it down, excited population, intensity, concentration
34
How do the gas flow rates affect the ultimate sensitivity in AA and AE?
varied flame composition shifts detection region
35
How is desolvation usually accomplished?
evaporating solvent, relies on flame heat
36
What is vaporization?
solid to gas
37
What is atomization
ionic compound to atoms
38
Is there an optimum detection zone for every element?
no, depends on atom and process
39
How is the detection zone optimized?
depends on atom and process. You can move the burner position
40
What is the most common burner design in AA/AE?
laminar flow, premixed burner
41
What are the most common problems with using flames in AA/AE?
inefficient nebulization, short atom residence time, fluctuating intensities because temperature too low for atomizing
42
how does graphite furnace AA compare with Flame AA?
hollow rod along optical path, flushed with inert gas. requires water cooled jacket
43
What type of samples are used in AF/AA
Liquid or Solid
44
What are the 3 phases in electrothermal atomization?
Drying (heat to evaporate solvent), Ashing (Raise t to break down), and Atomization (flash heat to incandescent to produce atom population)
45
how long is drying?
short, 20-30 seconds
46
how long is ashing?
Depends on time it takes to breakdown sample, but no more than that or it goes away.
47
How long is atomization?
more than 10e3 K per second
48
What are the advantages of electrothermal graphite furnace atomization?
long residence time, high sensitivity, low detection limit, low noise, small sample without pretreatment
49
What are the disadvantages of electrothermal graphite furnace atomization
matrix interference, slow heat causes uneven ashing, poorer precision than flames
50
What are the characteristics of flames used in AA?
preheating, primary, interconal, outer
51
What is the preheating zone?
cooler region of flame
52
What is the primary reaction zone?
oxidant and fuel reaction at non equilibrium, intense emission, not good for observations in AA and AE
53
what is the interconal zone?
hottest region, faint oxidant/fuel emissions, rich in atoms, best for observing AA/AE
54
What is the outer zone?
secondary reaction, usually not good for detection
55
How does background correction differ in atomic spectroscopy compared to UV/Vis?
no ready scanning to correct background noise, very wide spectral bandpass in wavelength selector relative to source
56
What are the two main types of interference common to atomic spectroscopy?
chemical and spectraul
57
What are some strategies for reducing chemical interference?
form competing complexes that will dissociate in flame
58
how do compounds of low volatility form with the analyte?
anions reduce atomization rate
59
How can light scattering be reduced?
increase temp, decrease slit width, appropriate background correction
60
What techniques make matrix contamination the worst? Why?
solid samples, DC arc, spark sources, graphite furnace AA because of variation of rate of volatilization
61
Strategies for reducing matrix effects
closely match standards, vary temp by changing fuel to oxidant ratio
62
What are the advantages of high energy sources?
can read decade wide concentration range, concentration of nonmetals, low refractory concentrations; dozens of spectra simultaneously; low interference
63
what are the disadvantages of high energy sources?
complicated, expensive, less precise, high operator involvement
64
What are the common gases used to produce plasmas
Mainly Ar, sometimes O2
65
What is the most common gas used to excite a plasma in AE?
Ar
66
What is a sequential ICP?
monochromator with photomultiplier/CCD detector
67
What is a multichannel ICP?
lots of photomultiplier tubes behind curved focal plane slits with fixed wavelength transmission
68
How are the characteristics of an Echelle grating different from regular grating?
very coarse, very large blaze angle, short blaze
69
Why are echelle gratings used in multichannel ICP?
high dispersion and resolution
70
What type of electrodes are used in arc sources?
nonmetals use carbon rod, metals use the sample itself, powder uses pellets
71
what are the typical operating conditions for arc sources?
1-30 A, DC = 200V, AC = 2200 - 4400, K = 4000-8000
72
What are the typical operating conditions for spark sources?
10-50kV, 1000 A instantaneous, spark gap K up to 40,000
73
What are the common conditions for generating ICP'S?
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
How are plasmas generated in an ICP?
Ar gas, RF generator
75
Most common power supply for plasmas?
RF induction coil
76
Why are they called inductively couple plasmas?
energy is supplied by electric currents which are produced by electromagnetic induction
77
What are MIP's?
microwave generator as power supply for energizing plasmas
78
What is nebulization ?
process of converting liquid sample to fine mist of tiny droplets
79
What is the primary nebulization method?
pneumatic
80
How is nebulization accomplished experimentally?
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
Venturi Effect
reduced pressure pulls liquid into gas phase
82
What is the efficiency of nebulization?
90-99% of sample goes down drain
83
What is the Sauter Equation
relates parameters (viscosity, density, etc) to droplet particle size
84
How is mean droplet diameter related to gas velocity?
diameter decreases as velocity increases
85
How is mean droplet diameter related to solution viscosity?
decreases as viscosity decreases
86
How is mean droplet diameter related to density?
decreases as density increases
87
How is mean droplet diameter related to flow rate liquid?
decreases as Qliq decreases
88
How is mean droplet diameter related to flow rate gas?
decreases as Qgas increases
89
How to obtain smallest droplet size?
slow aspiration liquid, fast gas flow, low viscosity solvent with low surface tensions
90
What are alternative nebulization methods?
cross flow, fritted disk, babington, ultrasonic,
91
How do the flow velocity of gases contribute to flame characteristics?
Varying ratio varies temp which alters excited state population. Changing temp changes observed intensity and concentration.
92
Is high resolution required in AA?
AA monochromator can use lower resolution than that required of AE
93
Ways to dissociate sample: flames
relatively low energy, used for easily excited elements
94
Ways to dissociate sample: ICP
high frequency (radiowave) electrical signal inductively coupled to sample chamber via coil to dissociate into plasma
95
Ways to dissociate sample: DC plasma
uses dc voltage source to dissociate sample into plasma, related to DC arc method
96
Ways to dissociate sample: MIP
related to ICP but uses microwave wavelengths
97
Ways to dissociate sample: DC arc
low voltage high current between two electrodes, particularly for metals
98
Ways to dissociate sample: AC spark
high voltage, lower current alternating at high frequency between two electrodes, used in metals
99
Role of monochromator in AF?
Maintains wavelength; excitation and emission
100
Why is light scattering important in AA/AE?
Occurs for particles in flame (not fully dissolved) or burning carbon roughly the same size as light wavelength
101
What is the continuous source background correction method
radiation from hollow cathode lamp and D2 lamp passes through beam splitter/chopper. Detector detects each signal separately.
102
What type of lamp is used in continuous source background correction?
D2 and Hollow Cathode
103
How is corrected signal calculated in continuous source?
Corrected = hollow signal - D2 signal
104
Wh is continuous source the most used for AA correction?
Eliminates matrix so sample can be observed
105
Advantages of continuous source correction
Relatively easy to implement, works with relatively high absorbances, little effect on calibration curve
106
Disadvantages of continuous source correction
detection limit suffers due to source intensities not being equal, optical alignment is difficult
107
What is Zeeman method?
atomic vapor exposed to strong magnetic field results in splitting atomic energy levels
108
Why is polarization needed in Zeeman
emits varying polarized light
109
How is sample absorbance measured in Zeeman?
lamp emission parallel to magnetic field (pi emissions) is absorbed by parallel pi component of analyte
110
How is zeeman background absorbance measured?
lamp emission perpendicular to magnetic field (sigma) not absorbed by analyte due to different polarization properties of sample and background
111
How is corrected signal calculated Zeeman?
Corrected = sample - background
112
What is the source self reversal method? (Smith-Hieftje)
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
Pros zeeman
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
cons zeeman
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
How is background absorbance measured S-H
low current gives total, high current gives background
116
S-H corrected signal calculation
low - high
117
Pros S-H method
all wavelengths, simple alignment, auto compensates for source and flicker noise, additional optical components not needed, inexpensive
118
Cons S-H method
less successful for less volatile/more refractory elements, reduced calibration sensitivity, detection limits worse
119
What roles do temp and oxygen content play in chemical interference?
Ionization: smaller at low temp to with air, higher temps available with O2 or N2O.
120
Why does ionization interfere with atomic spectra?
ions possess different electronic configs than atoms
121
When is ionization important?
at higher temps available with O2 or N2O as oxidant
122
when is ionization not important
at lower temps or with air as oxidant
123
what are strategies for reducing ionization interference
concepts of equilibria: ionization buffer (more easily ionized species added) shifts it toward metal formation
124
When are low dissociation complexes formed?
anions form compounds of low volatility with the analyte and thus reduce the rate it is atomized
125
what are some common elements that form dissociation complexes?
refractory metals: Nb, Mo, Ta, W, Re, Ti, Cr, Ru
126
What are some solutions to compound formation in flames?
increase flame temp so rxn becomes exo and shifts towards free atoms; lower free O content, increases reducing species and resorting free metals
127
pH role in dissociation equilibria?
Higher H ion content pushes it to reactants side
128
What are some common sources of spectral interference?
flame emissions, light scattering, matrix
129
strategies to avoid flame emission interference
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
strategies to avoid light scattering
increase temp, appropriate background correction technique, dilute, decrease slit width
131
What role does F-O ratio play in spectral interferences?
Varying temp varies ratio, allowing for decomposition of interfering species
132
Why does decreasing slit width improve S/N?
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
Why are plasma, arc, spark source methods called "high energy source methods"
134
What are the instrumental requirements for a sequential ICP?
standard monochromator design. uses standard photomultiplier/CCD detection
135
What are the instrumental requirements for a multichannel ICP?
numerous photomultipliers behind fixed slits along a curved focal plane of a concave grating monochromator. Slits fixed to transmit wavelengths of particular elements
136
What determines natural line width for atomic emission and absorption lines? How broad are they typically?
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
Cation that preferentially reacts with a species that would otherwise react with the analyte to cause chemical interference
Releasing agent
138
Prevent interference by forming stable and volatile products with the analyte
Protective agents
139
More easily ionized than analyte and provides high concentration electrons in flame or plasma that suppress analyte ionization
Ionization suppressor
140
Process by which sample is vaporized and decomposed into atoms by heat
Atomization
141
Broadening of atomic lines due to collisions
Pressure broadening
142
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.
Hollow cathode lamp
143
Process by which gaseous cations bombard a cathode surface and eject atoms from surface to gas phase
Sputtering
144
Absorption of emitted radiation by unexpired atoms in the gas phase of a hollow cathode lamp, flame, or plasma
Self absorption
145
Encountered when absorption or emission of a non analyte species overlaps a line being used for the determination of the analyte
Spectral interference
146
Result of any chemical process that decreases or increases absorption or emission of analyte
Chemical interference
147
Substance added in excess to both sample and standards which Samos the effect of the matrix on analyte emission or absorption
Radiation buffer
148
Arises because atoms moving toward or away from monochromatic give rise to absorption or emission lines at slightly difference frequencies
Doppler broadening
149
Why is an electro thermal atomizer more sensitive than a flame atomizer?
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
Describe how a deuterium lamp can be used to provide background correction for atomic absorption spectrum
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
Why is source modulation used in atomic absorption spectroscopy?
Distinguish between atomic absorption (an ac signal) and flame emission (a dc signal)
152
What is an internal standard and why is it used?
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
Why are atomic emission methods with ICP source better suited for multi element analysis than flame atomic absorption methods are?
Flame requires a separate lamp for each element which is inconvenient
154
Why is ionization interference less severe in ICP than in flame emission spectroscopy
Because argon plasmas have high concentration of electrons from ionization which represses ionization of the analyte
155
What are advantages of plasma sources compared with flame sources for emission spectroscopy
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
