CH0607 - AA AE Flashcards
Breaking down sample to atoms by burning that sample in a flame”
Flame Atomization
The process of converting a liquid sample into a fine spray mist of tiny droplets.
Nebulization
device that uses the flow of gas past the orifice of a capillary tube of small diameter; gas flow pulls the liquid from the capillary into the gas phase due to the reduced pressure (Venturi effect); surface tension of the liquid causes the column of liquid exiting the capillary to break apart into droplets.
Pneumatic Nebulizer
The process that evaporates the solvent leaving a solid/gas aerosol.
Desolvation
The process that vaporizes the aerosol gas leaving behind gaseous molecules.
Volatilization
The process that breaks down the gaseous molecules into its constituent atoms.
Atomization
An expression that relates the mean diameter of a nebulized droplet to: i) the viscosity of the analyte solution, ii) the density of the analyte solution, iii) the surface tension of the solvent, iv) the flow rate of the nebulizer gas, v) the flow rate of the aspirated solution, and vi) the velocity of the nebulizer gas.
Sauter Equation
Alternative to a pneumatic nebulizer that utilizes an ultrasonically driven crystal to vibrate the sample into droplets; tends to produce small, monodisperse droplet size distribution.
Ultrasonic Nebulizer
Alternative to flame atomization methods. consists of a cylindrical graphite tube equipped with optical windows at the ends; sample is added to the tube and dryed/ashed/atomized by electrical heating of the graphite tube to a final T ~3000 K.
Electrothermal / Graphite Furnace
Atomic spectroscopy background correction method using a continuum source (e.g. D2 lamp) in conjunction with a hollow cathode lamp line source; radiation from the HCL and D2 lamp is passes through a beam splitter or rotating chopper and the detector is synchronized such that it detects each signal separately,
Continuous Source Background Correction
Atomic spectroscopy background correction method using a strong magnetic field to split atomic energy levels; split components absorb polarized radiation from atomic transitions
Zeeman Background Correction
Atomic spectroscopy background correction method in which a magnetic field surrounds the sample and splits the sample atomic vapor into components
Analyte-Shifted Zeeman Background Correction
Atomic spectroscopy background correction method in which a magnetic field surrounds the source and splits the emission spectrum of the hollow cathode lamp source into components
Source-Shifted Zeeman Background Correction
Atomic spectroscopy background correction method using the properties of self-absorption or self-reversal behavior of hollow cathode lamps when operated at high current; high current with short pulse modulation (e.g. 500 mA, 0.3 ms) produces non-excited state atoms, quenching excited state species (background); low current (20 mA) produces total absorbance (background plus absorbance)
Smith-Hieftje / Source Self-Reversal Background Correction
Most common type of chemical interference in atomic spectroscopy where anions form compounds of low volatility and reduce the rate at which the analyte is atomized; typical for refractory oxide formation; these interferences can be reduced by using a fuel- rich flame to increase reducing species and restore free M atoms
Compound Formation Interferences
Type of interference common in atomic spectroscopy at high T’s especially with O2 or N2O as oxidant; the ion M+ possesses a different electronic configuration than the neutral metal atom M and will interfere with the desired atomic absorption and emission processes; these interferences can be reduced by adding an ionization buffer (i.e. a more easily ionized species) to the sample to shift the ionization equilibrium away from M+.
Ionization Interferences
Type of chemical interference in atomic spectroscopy due to formation of strong complexes in solution that don’t readily dissociate at flame temperatures; these interferences can be reduced by forming competing complexes with analyte metal M that will more easily dissociate in flame, e.g. La3+, EDTA, or H+.
Condensed Phase Chemical Interferences
Type of interference in atomic spectroscopy due to unwanted absorption or emission from molecular species, or from closely overlapping spectral lines of two analytes (very rare); solutions include changing fuel gas, increasing T, using appropriate background correction, and decreasing the slit width.
Spectral Interferences
Type of interference in atomic spectroscopy due to scattering, rather than absorption, of particles in the flame that are roughly the same size as the wavelength of light; solutions include increasing T, using appropriate background correction, and decreasing the slit width
Light Scattering
Type of interference in atomic spectroscopy where the observed spectrum is negatively affected by sample constituents; usually worse for solid samples and can be severe in methods such as graphite furnace AA, DC arc, or AC spark sources; cause is usually variation of rate of sample volatilization; solutions include using closely matched standards at increase T.
Matrix Effects
An electrically conducting gaseous mixture containing a large concentration of cations and electrons with a net charge = 0; Ar is the usual gas used but O2 is also possible.
Plasma
Name given when the power supply used to energize the plasma is a radiofrequency induction coil.
Inductively Coupled Plasma (ICP)
Name given when the power supply used to initially energize the plasma is a direct current source.
Direct Current Plasma (DCP)
Name given when the power supply used to initially energize the plasma is a microwave generator.
Microwave-Induced Plasma (MIP)
Source for atomic emission spectrochemical analysis formed from graphite or metal electrodes a few mm apart; current (30 A, 200 V) is passed between these electrodes, causing high temperature (4000 – 5000 K) arcing and sample atomization.
Direct Current (DC) Arc
Source for atomic emission spectrochemical analysis formed from graphite or metal electrodes a few mm apart; typical operating conditions (10-50 kV , 1000 A instantaneous current) result in spark gap temperatures of 40,000 K.
Alternating Current (AC) Spark
What is Atomic Absorption?
sample absorbs external radiation
How does AA compare to molecular absorption
AA is the atomic analog of MA
What is Atomic Emission?
Does it require an external light source?
Energy (provided electrically) from external device excites sample, sample itself is the source, reemitted as it decays to ground
What is Atomic Fluorescence?
sample excited by absorption, reemits radiation of:
- a longer wavelength (stokes direct line fluorescence)
- a shorter wavelength (anti-stokes direct-line fluorescence)
- the same wavelenth (resonance fluorescence)
What is a hollow cathode lamp?
narrow line source at low pressure low temperature so there is no collision or doppler
What are the line widths produced by a hollow cathode lamp?
10e-2 to 10e-3 amps
Are black body sources used in AA?
nope, requirements are too strict
What is the role of the monochromator in AA?
filtering
Is beer’s law obeyed in AA?
it has to be, source bandwidth is less than sample bandwidth
What are the physical principles behind AE?
Energy from external source raises sample to excited state, emitting radiation directly upon relaxing to ground state
What is the source in AE?
The sample itself
How is the sample brought to the excited state in AE?
electrically (or by flame in flame AE)
What is the role of the monochromator in AE?
Select or separate wavelengths
What are the physical principles behind AF?
Atomic fluorescence spectroscopy (AFS)is the emission of radiation energy in theUV-visible region from gas-phase atoms that have been excited to higher energy levels by absorption of radiant energy
What is the source in AF?
narrow line sources
Is high resolution needed in AF?
no
What determines the intensity distribution of atomic spectral lines (in atomic emission)?
population of excited energy levels
What is a Boltzmann distribution?
Population of energy levels at thermal equilibrium
What happens to the excited state population as wavelength decreases (delta E increases)?
decreases as wavelength decreases (delta E increases)
What happens to the excited state population as temperature increases?
Increases as temp increases
What are states of degeneracy?
multiple states exhibit the same energy
How are states of degeneracy applied to Boltzmann distribution?
Through g coefficient
What is the usual form of the sample in flame atomization?
liquid
What are the typical fuels used in AA?
natural gas, hydrogen, acetylene
What are the typical oxidants in AA?
oxygen, air, nitrous oxide
Fuel and oxidant combos?
A + Air, A + Nitrous oxide
How do flames contribute to the noise characteristics of an AA Instrument?
temperature and flicker
how can AA noise (due to unwanted atom emission) be controlled?
chopper and lock in
Control noise (from unwanted atomic emissions) in AA and AF
chopper between light and flame (modulation).
How do temperature changes affect AA intensity?
basically immune to changes in temperature because ground state is used
Absorption depends on ground state population, which has little dependence on temperature (refer to maxwell boltzmann equation).
How does temperature affect AE intensity?
Lower temperature => lower excited population (maxwell boltzmann) => lower emission intensity
How do the gas flow rates affect the ultimate sensitivity in AA and AE?
varied flame composition shifts detection region
How is desolvation usually accomplished?
evaporating solvent, relies on flame heat
What is vaporization?
solid to gas
What is atomization
ionic compound to atoms
Is there an optimum detection zone (in flame) for every element?
no, depends on atom and process
How is the detection zone (inside flame) optimized?
depends on atom and process. You can move the burner position
What is the most common burner design in AA/AE?
laminar flow, premixed burner
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
how does graphite furnace AA compare with Flame AA?
hollow rod along optical path, flushed with inert gas. requires water cooled jacket
What type of samples are used in AF/AA
Liquid or Solid
What are the 3 phases in electrothermal atomization?
Drying (heat to evaporate solvent), Ashing (Raise t to break down), (cooling) and Atomization (flash heat to incandescent to produce atom population) (and finally cleaning and cooling)
how long is drying?
short, 20-30 seconds
how long is ashing?
Depends on time it takes to breakdown sample, but no more than that or it goes away.
How long is atomization?
more than 10e3 K per second
What are the advantages of electrothermal graphite furnace atomization?
long residence time, high sensitivity, low detection limit, low noise, small sample without pretreatment
What are the disadvantages of electrothermal graphite furnace atomization
matrix interference, slow heat causes uneven ashing, poorer precision than flames
What are the characteristics of flames used in AA?
preheating, primary, interconal, outer
What is the preheating zone?
cooler region of flame
What is the primary reaction zone?
oxidant and fuel reaction at non equilibrium, intense emission, not good for observations in AA and AE
what is the interconal zone?
hottest region, faint oxidant/fuel emissions, rich in atoms, best for observing AA/AE
What is the outer zone?
secondary reaction, usually not good for detection
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
What are the two main types of interference common to atomic spectroscopy?
chemical and spectraul
What are some strategies for reducing chemical interference?
- Matrix matching between calibration standards and sample.
- Adding a “releasing agent” (a metal cation that tends to bind more easily to the analyte)
- Increase flame temperature to overcome chemical interference.
how do compounds of low volatility form with the analyte?
anions reduce atomization rate
How can light scattering be reduced?
increase temp, decrease slit width, appropriate background correction
What techniques make matrix contamination the worst? Why?
solid samples, DC arc, spark sources, graphite furnace AA because of variation of rate of volatilization
Strategies for reducing matrix effects
closely match standards, vary temp by changing fuel to oxidant ratio
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
what are the disadvantages of high energy sources?
complicated, expensive, less precise, high operator involvement
What are the common gases used to produce plasmas
Mainly Ar, sometimes O2
What is the most common gas used to excite a plasma in AE?
Ar
What is a sequential ICP?
monochromator with photomultiplier/CCD detector
What is a multichannel ICP?
lots of photomultiplier tubes behind curved focal plane slits with fixed wavelength transmission
How are the characteristics of an Echelle grating different from regular grating?
very coarse, very large blaze angle, short blaze
Why are echelle gratings used in multichannel ICP?
high dispersion and resolution
What type of electrodes are used in arc sources?
nonmetals use carbon rod, metals use the sample itself, powder uses pellets
what are the typical operating conditions for arc sources?
1-30 A, DC = 200V, AC = 2200 - 4400, K = 4000-8000
What are the typical operating conditions for spark sources?
10-50kV, 1000 A instantaneous, spark gap K up to 40,000
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
How are plasmas generated in an ICP?
Ar gas, RF generator
Most common power supply for plasmas?
RF induction coil
Why are they called inductively couple plasmas?
energy is supplied by electric currents which are produced by electromagnetic induction
What are MIP’s?
microwave generator as power supply for energizing plasmas
What is nebulization ?
process of converting liquid sample to fine mist of tiny droplets
What is the primary nebulization method?
pneumatic
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
Venturi Effect
reduced pressure pulls liquid into gas phase
What is the efficiency of nebulization?
90-99% of sample goes down drain
What is the Sauter Equation
relates parameters (viscosity, density, etc) to droplet particle size
How is mean droplet diameter related to gas velocity?
diameter decreases as velocity increases
How is mean droplet diameter related to solution viscosity?
decreases as viscosity decreases
How is mean droplet diameter related to density?
decreases as density increases
How is mean droplet diameter related to flow rate liquid?
decreases as Qliq decreases
How is mean droplet diameter related to flow rate gas?
decreases as Qgas increases
How to obtain smallest droplet size?
slow aspiration liquid, fast (fuel and oxidant) gas flow, low viscosity solvent with low surface tensions
What are alternative nebulization methods?
cross flow, fritted disk, babington, ultrasonic,
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.
Is high resolution required in AA?
AA monochromator can use lower resolution than that required of AE
Ways to dissociate sample: flames
relatively low energy, used for easily excited elements
Ways to dissociate sample: ICP
high frequency (radiowave) electrical signal inductively coupled to sample chamber via coil to dissociate into plasma
Ways to dissociate sample: DC plasma
uses dc voltage source to dissociate sample into plasma, related to DC arc method
Ways to dissociate sample: MIP
related to ICP but uses microwave wavelengths to generate plasma.
Ways to dissociate sample: DC arc
low voltage high current between two electrodes, particularly for metals
Ways to dissociate sample: AC spark
high voltage, lower current alternating at high frequency between two electrodes, used in metals
Role of monochromator in AF?
Maintains wavelength; excitation and emission
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
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.
What type of lamps are used in continuous source background correction? (2 types)
D2 and Hollow Cathode
How is corrected signal calculated in continuous source?
Corrected = hollow signal - D2 signal
Why is continuous source the most used for AA correction?
Eliminates background so sample can be observed
Advantages of continuous source correction
Relatively easy to implement, works with relatively high absorbances, little effect on calibration curve
Disadvantages of continuous source correction
detection limit suffers due to source intensities not being equal, optical alignment is difficult
What is Zeeman method?
atomic vapor exposed to strong magnetic field results in splitting atomic energy levels, background/total absorption signals can be obtained separately by changing polarization of excitation source.
Why is polarization needed in Zeeman
Specific combinations of energy level splitting in analyte and polarization of light source allows measurement of (background + analyte)) and background alone, thus enabling correction.
How is sample absorbance measured in Zeeman?
lamp emission parallel to magnetic field (pi emissions) is absorbed by parallel pi component of analyte
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
How is corrected signal calculated Zeeman?
Corrected = sample - background
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
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
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
How is background absorbance measured S-H
low current gives total, high current gives background
S-H corrected signal calculation
low - high
Pros S-H method
all wavelengths, simple alignment, auto compensates for source and flicker noise, additional optical components not needed, inexpensive
Cons S-H method
less successful for less volatile/more refractory elements, reduced calibration sensitivity, detection limits worse
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.
Why does ionization interfere with atomic spectra?
ions possess different electronic configs than atoms
When is ionization important?
at higher temps available with O2 or N2O as oxidant
when is ionization not important
at lower temps or with air as oxidant
what are strategies for reducing ionization interference
concepts of equilibria: ionization buffer (more easily ionized species added) shifts it toward metal formation
What causes chemical interference?
anions form compounds of low volatility with the analyte and thus reduce the rate it is atomized
What are some solutions to compound formation in flames (such as oxide formation)?
increase flame temp so rxn becomes exo and shifts towards free atoms; lower free O content, increases reducing species and resorting free metals
What are some common sources of spectral interference?
flame emissions, light scattering, matrix
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
strategies to avoid light scattering
increase temp, appropriate background correction technique, dilute, decrease slit width
What role does F-O ratio play in spectral interferences?
Varying temp varies ratio, allowing for decomposition of interfering species
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
What are the instrumental requirements for a sequential ICP?
standard monochromator design. uses standard photomultiplier/CCD detection
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 (polychromator with Rowland circle)
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.
Ion that preferentially reacts with a species that would otherwise react with the analyte to cause chemical interference
Releasing agent
More easily ionized than analyte and provides high concentration electrons in flame or plasma that suppress analyte ionization (thus controlling ionization interference)
Ionization suppressor
Process by which sample is vaporized and decomposed into atoms by heat
Atomization
Broadening of atomic lines due to collisions
Pressure broadening
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
Process by which gaseous cations bombard a cathode surface and eject atoms from surface to gas phase
Sputtering
Absorption of emitted radiation by unexpired atoms in the gas phase of a hollow cathode lamp, flame, or plasma
Self absorption
Encountered when absorption or emission of a non analyte species overlaps a line being used for the determination of the analyte
Spectral interference
Result of any chemical process that interfers with atomization of analyte
Chemical interference
Arises because atoms moving toward or away from monochromatic give rise to absorption or emission lines at slightly difference frequencies
Doppler broadening
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.
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 analyte atoms AND background (TA = AA + BG).
The atomic signal is calculated by subtracting the background absorbance from the total absorbance (TA-BG)
Why is source modulation used in atomic absorption spectroscopy?
Distinguish between atomic absorption (an ac signal) and flame emission (a dc signal)
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
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
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
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
the type of atomic spectroscopy method that atomizes a sample by using Argon gas and a high frequency electrical signal produced via a coil that surrounds the sample
inductively coupled plasma
an alternative to flame atomization method for atomic absorption spectroscopy in which the sample is placed in a sealed tube and dryed/ashed/atomized within seconds to a final T of about 3000K
electrothermal atomizer
a type of interference in atomic absorption spectroscopy due to particles in the flame that are roughly the same size as the wavelengths of light from the source
light scattering
the background correction technique based on strong magnetic fields that surrounds the sample and splits the atomic energy levels into pi and sigma components
analyte shifted zeeman correction
a type of interference that results from formation of strong complexes in solution; typically, ions are added that attach to the interfering anions and form stronger complexes than the analyte
low volatility compound formation
In flame atomic absorption spectroscopy, the purpose of the flame is to produce..
ground state unionized atoms
why is flame atomic emission spectroscopy more sensitive to flame instability than flame atomic absorption methods
small fluctuations in flame temperature create relative large changes in the number of excited state atoms
identify the sequential steps that occur during sample atomization
- nebulization
- desolvation
- vaporization
- atomization
- detection
What are some of the advantages of electrothermal graphite furnace atomic absorption spectroscopy over flame atomic absorption?
high sensitivity with low detection limits - low ppb or better; small sample volumes are feasible without pretreatment
Which of the following statements regarding continuum source background correction in atomic absorption spectroscopy is true?
a) the sample absorbs pi energy levels from the hollow cathode lamp while the background absorbs sigma energy levels from the D2 lamp
b) since atomic lines are very narrow, the emitted radiation from the hollow cathode lamp is absorbed only by the sample, while the blackbody radiation from the D2 lamp is absorbed by the background
c) operating the D2 lamp at high current with short pulse modulation splits the atomic energy levels of the sample
b
T/F: a plasma is an electrically conducting gas containing a large concentration of cations and electrons and a net charge of zero
TRUE
T/F: a hollow cathode lamp is used as the source in a flame atomic emission spectrophotometer
FALSE
T/F: the detection limits of high energy atomic emission methods can be much lower than flame atomic absorption methods, but the precision of AE methods is generally worse than that of AA
TRUE
T/F: an ionization buffer is a more easily ionized species added to a sample that shifts the equilibrium towards the neutral analyte by increasing the concentration of electrons in the flame
TRUE
T/F: multichannel atomic emission instruments are based on echelle monochromators and multichannel detectors for very high resolution
TRUE
What is AAS
AAS is an elemental analysis technique.
Range of absorbed energy in AAS?
UV\Vis (vacuum UV for nonmetals)
Advantages\Disadvantages of AAS?
Advantage:
*independent of the chemical form of the element in the sample.
*a given element can be
determined in the presence of other elements, which do not interfere by absorption of the analyte
wavelength.
Disadvantage:
*No information is obtained on the chemical form of the analyte (no “speciation”)
*often only one element can be determined at a time.
*AAS measurements are not completely free from interferences
What are resonance lines?
A spectral line caused by an electron jumping between the ground state and the first energy level in an atom or ion
In AAS, why is it crucial to have a light source with a low-bandwidth?
Absorption lines by atoms appear as very narrow spectral lines (corresponding to a very narrow range of absorbed wavelengths). If the source has a broad bandwidth, such a narrow absorption range will be barely noticeable (1% absorption).
Advantage of EDL over HCL as light source?
Can be used to obtain absorption spectroscopy for volatile elements and elements that cannot be shaped into a cathode (thus giving a weak signal or are incompatible with HCL).
Reason for long optical path in AAS?
The long optical path (long burner) compensates for weak absorption usually obtained in AAS, due to low analyte sample in nebulized product.
What is the Carryover Effect in GFAAS and how can we prevent it?
In longitudinal heating of graphite furnaces, a temperature gradient (ends cooler than center) causes sample to condense in furnace edges and create residues which “carry over” into the next measurement and interfere with its results.
Solution: change direction of heating (transverse heating).
The purpose of modulation in AAS
Many metals, when atomized in a flame or furnace, emit strongly at the same wavelength at
which they absorb. This interferes with true absorption intensity.
When source signal is modulated, it is now an AC signal. Therefore, the signal for both I0 and I1 is AC but the sample emission E is not modulated, allowing differentiation e.g. via use of a lock-in amplifier.
Acetylene-Air blend flames are good atomizers for ____.
Many elements
N2O-Acetylene flames are ____ than ____ flames
hotter than, acetylene-air
____ flames are used for elements that oxidize quickly but are slow to ionize (such as Si and Al).
N2O-Acetylene
This blend gives a hotter flame and slower oxidation compared to air.
“Reducing” flames (appendix 6.1) contain excess ____ in oxidant-fuel blend.
fuel
Oxidizing flames contain excess ____ in oxidant-fuel blend.
oxidant
Elements which form stable oxides such as Al, B, Mo and Cr are better atomized in a ____ flame.
reducing (usually N2O-Acetylene blends)
____ blend flames can be used in their oxidizing (oxidant rich) or reducing (fuel rich) forms.
Acetylene-air
____ are invisible in AAS.
oxides of elements
Pros and cons of using excess oxidant in atomization for AAS?
Pro: organic solvents are broken down more efficiently.
Con: oxidation of analyte atoms
What is a flame profile in AAS?
The relationship between signal strength (absorption instensity) and height of measurement above the burner. Often depicted as a curve.
For atoms with low ionization energy such as K, ____ flames such as Acetylene-Air blends should be used to prevent ____.
colder, ionization (hotter flames induce ionization)
Three competing reactions that occur in flame atomization are:
Atomization (desirable)
Ionization and oxidation (undesirable for AAS)
The signal generated in GFAAS is ____
A transient signal resembling a gaussian
FAAS is a ____ process while GFAAS is a ____ process.
AAS - continuous (sample is aspirated into the flame continuously for as long as it takes to make a measurement).
GFAAS - Transient signal that must be measured in less than 1sec.
When UV-Vis sources are used for AAS, ____ are excited.
Valence, since UV-Vis is relatively low energy (unlike the high energy XRA where core electrons are excited)
Spectral line broadening in AAS caused by local electrical fields?
Stark broadening
What is Zeeman broadening?
Broadening of atomic absorption lines caused by presence of local magnetic fields.
Why can nonmetal elements not be determined directly by AAS?
Nonmetals absorb in Vacuum UV range. Most commercial AAS systems have air in the optical pathway that absorbs vacuum UV, leading to false-positives in determination of nonmetals.
Interference which results from anything in the sample other than the analyte.
Matrix Interference
(Usually refers to a solvent that interfers in nebulization in liquid sample or metals with low heat conductivity in solid samples in an ICP source)
Effect of solvent viscosity on signal strength?
As a result of varying solvent viscosity, different volumes of solution will be aspirated in a given period of time and nebulization
efficiency will change as a result of the solvent characteristics. e.g. Organic solvents have enhanced nebulization and therefore high absorbance.
Acidic solvents have higher viscosity, nebulize less efficiently and therefore interefere with absorbance.
Describe the interference caused by solvent in atomization process.
- Highly volatile solvents (such as organic solvents) raise flame temperature, increase atomization (beyond that of standards in H2O solvent) and cause an “upwards” error.
In AAS, MSA can correct ____ errors (such as nebulization and atomization) but cannot correct for ____ errors or ____ errors.
Matrix error
Spectral
Ionization
Why does MSA work for correcting matrix errors but doesn’t work for correcting Ionization or spectral errors?
The assumption made in using MSA is
that whatever affects the rate of formation of free atoms in the sample will affect the rate of formation
of free atoms from the added analyte spike in the same way. Ionization intereference and Spectral Intereference do not necessarily affect added analyte spike atoms and original sample atoms in the same way.
What causes nonspectral interference in GFAAS?
- Some analytes (such as those containing W, Ta, B, Nb, and Mo) react with the graphite surface of the atomizer at elevated temperatures to form
thermally stable carbides that do not atomize. - Normal graphite has a
very porous structure that permits solution to soak into the graphite tube, thereby increasing the graphite/solution contact area and allowing more carbide formation at high temperatures.
(Reaction with graphite creates wide, delayed signal due to delay caused by the need for elements to diffuse out of the porous material).
What is the solution for nonspectral interference in GFAAS?
Pyrolytic graphic
forms an impervious surface that prevents the solution from entering the graphite structure.
Problem solved by L’vov platform?
- In GFAAS, there is a temperature gradient between furnace walls and volume (volume colder than walls).
- This gradient creates nonspectral interferences in vaporization\atomization process (such as recombination to condensed phase).
- The platform (made from pyrolytic graphite) elevates sample to furnace volume where interference from this gradient is eliminated. Sample is atomized only when volume of sample reaches atomization temperature and is in thermal equilibrium with the walls.
What is matrix Modification (Chemical Modification)?
- A method used in GFAAS.
- Different compounds are added to matrix to prevent nonspectral interference (improves accuracy and precision).
- These compounds either increase volatility of matrix or decrease volatility of analyte so as to prevent it from atomizing when matrix is charred.
- The compounds can also convert all of the analyte (that can be present in different chemical forms e.g. Cd in organometallic compounds alongside Cd in inorganic compounds ) into a single compound with well-defined properties.
Why are spectral interferences from closely overlapping spectral lines of two analyte atoms a rare and insignificant form of spectral interference?
- Atomic absorption lines are very narrow (approx. 0.002nm). Direct overlap between absorption lines of different elements is
rare and can usually be ignored as a source of error. - Effect of this interference can be controlled by selecting an alternative wavelength or by removing the interfering element (preferrably in a process that does not create an additional source of interference).
What is background absorption?
Absorption by polyatomic species in the atomizer. This is a spectral interference.
Characteristics of background absorption?
- Broad absorption lines
- Common in wavelength < 250nm
What causes background absorbance?
- Incomplete combustion of the solvent (in flame atomizer).
- Incomplete decomposition of matrix molecules due to maximum temperature limit in pyrolysis stage (in graphite furnace).
- White hot radiation (350-800 nm) from heat source in GFAAS.
- Solid particles that scatter light source in FAAS lead to attenuation of the measured signal intensity in relevant absorption wavelengths.
Why is background absorbance more common in GFAAS?
Pyrolysis stage is limited in maximal temperature to prevent premature atomization of analyte. Many matrix molecules do not decompose in these temperatures.
Describe an additional mechanism for spectral interference caused by solid particles in FAAS.
Solid particles in flame atomizers scatter source light. This decreases the intensity of light that reaches the detector.
Describe process of Continuum Source Background Correction.
- Radiation from both the HCL and the continuum lamp follows
the same optical path to reach the detector through the sample. - Radiation from HCL is absorbed by both analyte and background.
- Radiation from continuum source is absorbed mostly by background (absorbance intensity by analyte is neglible)
- 3 is substracted from 2 to obtain true absorption intensity by analyte (3 is used to correct 2).
Limitation of continuum source background correction for background absorbance by non-analyte elements?
Background absorption that is caused by atoms other than the analyte cannot be corrected by continuum source background correction.
This is because they would appear as a significant signal in the HCL spectrum but as a neglible signal in the continuum source spectrum (just like analyte absorption lines).
How does the Smith-Hieftje method correct background absorption?
- HCL lamp is pulsed, alternating between high current and low current.
- At low current, a
normal resonance line is emitted and the sample undergoes normal atomic absorption. - At a high current, the center of the emission line is self-absorbed, leaving only the wings of the emitted resonance line;
- The atoms of the sample do not significantly absorb such a line but the broad background
absorbs the wings of the line.
=> The absorption of the wings is a direct measurement
of the background absorption at the wavelength of the atomic resonance absorption
Sources of interference in GFAAS?
At atomization temperatures
in excess of 2200°C, the graphite furnace is a white-hot emission source. If this intense visible
radiation reaches the PMT, noise increases, which decreases precision. This is blackbody radiation (350-800nm) which interferes mainly with absorption spectrae of Ca and Ba.
Is AAS is well suited for qualitative analysis?
Because it is essentially a single-element technique, AAS is not well suited for qualitative analysis
of unknowns. To look for more than one element requires a significant amount of sample and is a time-consuming process
The relationship between absorbance and concentration of the analyte being determined in AAS follows ____ over some concentration range.
Beer’s law. The concentration range depends on the element.
Calibration of AAS methods can be performed by the use of an ______ or
by ______.
External calibration curve, MSA
(IS is not a suitable calibration method becaue in AAS, only one element can be detected in each measurement)
Pros\Conse of Atomic Emission Spectroscopy (OES) with plasma source?
Pros:
High energy => high excitation=>
- A single element can emit in several wavelengths.
- several elements can be determined at once (multielement).
Cons:
as the number of emission wavelengths increases, so does the spectral interference from overlapping lines => a high-resolution spectrometer is requred. These are more expensive than the spectrometers needed for flame
emission systems or for atomic absorption
In OES, the process in the burner is identical to the atomization process for atomic absorption spectrometry (AAS), but now, we want the atoms to ______ an ______ state by the flame.
progress, excited
In AAS, HCL is used to excite atoms.
Factors which determine emission intensity in OES:
- the concentrations of
the elements in the sample - the rate at which excited atoms are formed in the flame. which depend on:
- the rate at which the sample is introduced into the flame
- the composition of the flame
- the temperature of the flame.
Increased temperature increases _____ in excited state, which increases _____.
Electron population (according to maxwel boltzmann distribution)
Atomic emission (which is proportional to excited electron population)
Elements which emit in a shorter wavelength require a ____ flame temperature fore excitation. In these cases, it is common to use ____ as an oxidant and ____ as fuel.
Hotter, N2O, Acetylene
Intereference caused by other atoms colliding into analyte atoms and exciting them. Appears as
increased number of excited atoms and increased atomic emission signal compared to a solution of the atoms at the same concentration with no colliding atoms.
Excitation Interference
Atomic Emission Spectroscopy (OES) with plasma source require high resolution spectrometers because:
Plasma sources operate at high temperatures and therefore cause many atomic excitations. These produce a high density of emission lines.
Unlike AAS, atomic emission spectroscopy is an excellent ____ method for determining
____ elements in samples
qualitative, multiple
Adding an excess of a more easily ionized element to all standards and samples in order to control ionization interference on analyte atoms.
ionization suppression
Electrical
discharge, a glow discharge (GD), or a plasma are:
High energy sources used to excite atoms and ions in OES
Because GD, electrical and plasma excitation sources are of higher energy than a flame source, _____ can be detected in low concentration
all metallic and metalloid elements
Spectrae of GD, electrical and plasma excitation sources include _____ and _____ excitation lines.
Atom (I lines) and Ion (II or III lines)
Advantage of GD, electrical and plasma excitation sources?
Conductive solids can be analyzed directly, avoiding the need for sample dissolution and the degradation of DLs
dilution of sample in the process of dissolution.
In DC arc sources, the arc must be initiated by ____ between electrodes, usually by a low-current spark
the formation of ions
The spark-like discharge is _____ than the DC arc; this makes the spark a better choice for _____
analysis than an arc source.
More reproducible,
quantiative
Spark sources are precise but have low _____.
Sensitivity
spark condiitons are easily reproducible (unlike arc)
DC Arc sources are sensitive but have low _____.
precision
Most arc and discharge sources use a _____ spectrometer.
multichanel
(these sources produce an unstable signal so require that emission signals be integrated for 10 s or longer so scanning spectrometers are not practical )
Describe matrix interference in AES with Arc and Spark sources.
- The matrix affects the temperature of the plasma and the heat buildup in the electrode
- This affects the rate of vaporization of the iron into the electrical discharge.
- The rate of vaporization into the discharge affects the number of iron atoms excited per unit time and therefore the emission intensity.
Method to overcome matrix interference when working with arc and spark sources?
convert all samples to a common matrix (e.g converting the sample to an oxide powder and mixing it with a large excess of a common inert matrix, such as graphite powder) or use MSA for calibration.
What is the role of plasma in Plasma-OES (such as ICP-OES, DCP-OES)?
The plasma excites analyte element atoms (like a flame source). Is a high energy excitation source (Temperature range 6500-10000K. In these temperatures, almost all elements are excited.)
In ICP, The background signal ____ with distance from the load coil, and emission is usually
measured _____.
- drops
- slightly above the load coil (where the optimum signal-to-background ratio is achieved)
What is LIBS?
Laser-induced breakdown spectroscopy (LIBS) is a relatively new atomic emission spectroscopy
technique that uses a pulsed laser as the excitation source.
In ICP, the area above the coil with optimum background\signal ratio is called the ____
“normal analytical zone”
(this is also where analyte emission is usually is measured)
Principle of Operation of LIBS?
- laser pulse ejects small amount of material from sample surface.
- As a result of 1, hot plasma is also formed above sample surface.
- The plasma from 2 atomizes, ionizes and excites material ejected from sample surfce.
- The plasma cools down and emission is obtained when excited sample atoms relax back to ground state.
If slope of MSA curve is different from slope of _____ curve, then a _____ interference is present.
Standard calibration
Matrix
A matrix such as a highly flammable organic solvent can affect flame temperature in AAS, giving a higher reading of analyte than a water solvent.
Why is Plasma-OES less susceptible to chemical interfere?
High temperatures of plasma improves efficacy of atomization.
Most samples for Plasma OES are dissolved in acid. Why?
Metals are most soluble in acid. (High energy of plasma OES is suited for metals…?)
Spectral interference is much more common in plasmas than in flames. Why?
The great efficiency
of excitation in plasma results in many excitation lines (including unwanted interfereing ones).
A common form of spectral interference in ICP-OES is ____
Background Interference
Describe opeation of GD as an excitation source?
- sample is used as a cathode. Potential is applied between anode and cathode. Tube is filled with Ar.
- Applied potential causes spontaneous ionization of the Ar gas to Ar+ ions.
- The Ar+ ions are accelerated to the cathode and a discharge is produced by argon ions colliding with argon atoms. The resulting plasma is called a GD.
- The electric field accelerates some Ar+ ions to the sample surface where impact causes neutral sample atoms to be sputtered from the sample surface.
- The GD plasma collides with these atoms, exciting them.
Caution! A GD source is not an electrical excitation source (like DC arc or AC spark) but a plasm excitation source.
Most analytical AFS uses _____
transitions
ground-state resonance fluorescence
remember, according to the Boltzmann distribution, most atoms are in the ground state even at the temperatures found in GFs.
Disadvantage of using direct-fluorescence lines in AFS?
scattered light from the exciting
source has exactly the same wavelength as the fluorescence emission and is a direct interference.
Example of spectral interference in AFS
