AES Flashcards
radiative excitation
excitation by absorption of radiation
nonradiative excitation
Excitation by the application of thermal, electrical, or chemical energy
how is energy converted to kinetic energy in non radiative relaxition
collision with other atoms or minor molecules
what does molecular collision in nonradiative relaxation result in
minor increase in temperature
what results when radiative relaxation occurs after radiative excitation
Atomic fluorescence
what results when radiative relaxation occurs after nonradiative excitation
Atomic emission
atomic emission spectrometry methods
Atomic fluorescence spectrometry (AFS)
flame-AES
plasma-AES
Arc and spark-AES
what are AES methods classified by
excitation source
fluorescence
emission of a photon after radiative excitation of an atom
instrument components of AFS
- source
- excited wavelength selector
- atomizer
- emission wavelength selector
- transducer
- data processor
- readout
why are AFS instruments set up at 90 degree angle
so the source doesn’t go to the detector, since we want the fluorescence not absorbance
resonance fluorescence
one wavelength in, same wavelength out
non-resonance fluorescence
one wavelength in, different wavelength out
what should radiation sources in AFS be
- linear
2. have high intensity
as source intensity increases the higher the ___
sensitivity
common linear sources in AFS
- Hollow cathode lamp (HCL)
- Electrode-less discharge lamp (EDL)
- lasers
issues with HCL in AFS
not as intense, therefore not as sensitive
what is the most common lamp for AFS
EDL
Laser advantages for AFS
- high intensity
- Narrow band widths
Laser disadvantages for AFS
- high cost
- operational complexity
Atomizers in AFS
- flames
- graphite furnaces
- plasma
Wavelength selectors in AFS
- Filters
2. Monochromators
Transducers in AFS
- Vacuum phototunes
- photomultiplier tubes (PMT)
- silicon diode transducers
- photodiode array (PDA)
Which is more common for AFS: qualitative or quantitative analysis
quantitative
Example of AFS application
determining metals in:
- lubricating oils
- seawater
- geological samples
- metallurical samples
- environmental samples
what does Flame- atomic emission spec (F-AES) measure
photons emitted from atoms excited in a flame
Instrumentation components of F-AES
- neutralizer
- flame
- wavelength selector
- transducer
- signal processor
- readout
wavelength selectors for F-AES
- Flame photometer (filter)
2. Flame Spectrophotometer (monochromator)
qualitative analysis for F-AES
coupled with PDA, F-AES can be used to determine if the elements we’re interested in are present in our sample
which elements will F-AES excite
earth alkaline metals
alkali metals
Are internal standards essential for F-AES
yes bc the compensate for several types of error in emission spec
where is F-AES used
in the routine measurement of alkali and alkaline earth metals in materials such as biological and food samples
three channel flame photometer
two elements are analyzed, with 1 internal standard simultaneously (ie, analyzing Na and K and using Li as internal standard)
what is P-AES
plasma atomic emission spectroscopy (excitation by plasma)
what is plasma
an electrically conducting gaseous mixture containing a significant concentration of cations and electrons with a net charge of 0
power sources in P-AES
- inductively coupled plasma (ICP)
- Direct current plasma (DCP)
- Microwave induced plasma (MIP)
what source is ICP
radio frequency source
what source is DCP
direct current electrical source
what source is MIP
microwave frequency source
how is ionization initiated in ICP
ionization of Ar is initiated by a spark from a Tesla coil
what occurs in the induction coil of ICP
interations of the resulting ions from the Ar ionization, and electrons with the fluctuating magnetic field (H). This causes ions and electrons within the coil to flow in the closed annular path
what causes ohmic heating of the plasma
the resistance of the ions and electrons that flow int he induction coil
what are the common devices for sample introduction in ICP
- Pneumatic nebulizers
- Ultrasonic nebulizers
- electrothermal vaporization
- laser ablasion
which devices introduce liquid samples to ICP
- pneumatic nebulizers
- ultrasonic nebulizers
- electrothermal vaporization
which devices introduce solid samples to ICP
- electrothermal vaporization
2. laser ablasion
advantages of electrothermal vaporization
- µL sampling
- low detection limit
- wide working range
- decent precision (5-10%)
- free of interference
- multi-element capabilities
Typical laser(s) used in laser ablasion
Nd-YAG
Why is the core not good for excitiation
too hot
what is bremsstrahlung
continuum radiation produced when charged particles are slowed or stopped
temperature and height of secondary region of plasma
10-30 mm above core
6000-6500 K
why is a higher temperature good for ICP
since we need atomization and excitiation, a higher temperature means we can analyze more elements
how long do samples reside in the plasma
2-3 ms
why is a uniform temperature cross section beneficial in ICP?
self-absorption and self-reversal effects do not occur as often and calibration curves are linear over several orders of magnitude of concentration
why is high electron density beneficial in ICP?
less ionization interference, therefore no need for ionization suppressor
what causes the enhancement of the lifetime of an analyte in ICP
chemically inert environment in the plasma
what gas is used is ICP
argon
Why is there less self absorption in ICP
the plasma is optically narrow
advantages of DCP
- significantly less argon needed
- auxiliary power supply is simpler and less expensive
- DCP is able to handle organic solutions and aqueous solution with a high solid content better that ICP
Disadvantages of DCP
- Spectra produced has fewer lines than ICP and are often from atoms not ions, therefore there is less excitation
- lower sensitivities than ICP
- smaple volatilization is often incomplete with DCP nf short residence time (1 ms)
- optimal viewing region is small so optics have to be aligned correctly to magnify properly
Main components of ICP-AES/ICP-OES
- Sampe introduction
- ICP source
- Wavelength selector
- Transducers
- Signal processor with readout
wavelength selectors in ICP-OES
- Echelle grating
- Holographic grating
- Concave gratings
Common transducers in ICP-OES
- photomultiplier tubes (PMT)
- photographic emulsions
- charge injection devices (CID)
- Charge coupled devices (CCD)
why are two PMTs used in sequential ICP-OES instruments
one for UV and one for Vis
why are multichannel spectrometers (polychromators) so beneficial for ICP-OES analysis
all elements can be detected simultaneously making for effective qualitative analysis
chemical interferences with ICP-OES
- low matrix effects
- at low concentrations the background emission due to a recombination of argon ions with electrons is large enough to require careful corrections
corrections for ICP-OES chemical inerferences
two-line method
spectral interferences with ICP-OES
- line overlapping when spectra for multiple elements are rich in lines
correction for ICP-OES spectra interference
use a suitable/ different line based on the components in the smaple
can ICP-OES be used qualitatively or quantitatively
both
what elements can be detected using ICP-OES
all metalic metals
what range to B, P, N, S, and C need to be analyzed at
vacuum UV range (wavelength < 180 nm)
why is there limited application for alkali metals
most predominate lines of these elements are located at near IR range which leads to difficulty in detection
is it better to have more or less emission line
more bc better selection
what plot is used for ICP-OES and why
log-log plot bc the dynamic range is so large
internal standards are _____ used in ICPOES analysis
always
what can be said about the detection limits of ICP-OES
- better or comparable to other methods
- more elements can be detected at levels of 10 ppb or less with plasma excitation than with other methods
how are elements excited in arc and spark AES
electric arcs or high voltage sparks
where does excitation occur in arc and spark aes
int he gab between a pair of electrodes
what provides the energy needed to atomize a sample and produce electronic excited states
passage of electricity that moves from the electrode to the gap
Voltage of DC electric arcs
10-50 V
current of DC electric arcs
1-30 A
voltage of AC electric arcs
2200-4400 V
current of AC electric arcs
1-5A
why is the argon bewteen the two electrodes basically plasma?
bc it’s ionized
how many sparks per half cycle of 60 Hz line current
4
how many sparks per second
8
what is the average current in a high-voltage spark
few tenths of an ampere
in the initial phase of spark discharge, what may the instantaneous current exceed
1000 A
During the initial phase what is the temperature of the streamer in the spark gap
40000 K
what are lines emitted by ions usually called and why
“spark lines” bc it’s in spark but not arc
is the number of lines in spark less or grater than the lines in arc
spark lines > # arc lines
Arc and spark spec analysis is best for what
solids
what metals are analyzed by arc and spard
metals or alloys
what are the 2 electrodes made of in arc and spark while analyzing metals
one or both electrodes can be formed from the sample by milling or by casting the molten metal in a mold (at least one is the sample material)
when an electrode is used against a flat polished surface in arc and spark, what are the electrode and flat polished portion made of
electrode: counter electrode made of graphite or metal rod
Flat surface: metallic sample
what do we need in arc and spark analysis of nonmetallic saples
conductivity
what is a nonmetallic supported in in arc and spark
electrode whose emission will not interfere with analysis
why is carbon an ideal electrode for many arc and spark applications
- can be obtained in highly pure form
- good conductor
- good heat resistance
- easily shaped
what shape is the electrode for sample holding in arc and spark
cylinder with a small cavity drilled into one side
what samples can nonmetallic arc and spark analyze
- powder
- liquid
what is the counter electrode in nonmetallic arc and spark
tapered carbon rod with rounded tip
what components are in arc and spark aes
- electrodes
- wavelength selector
- transducer
- signal processor and read out
types of arc and spark instrumentation
- sequential
2. multichannel
types of multichannel arc and spark aes
- spectrographs
- spectrometers
what are used in arc and spark spectrometers
PMTs
digital detectors
three kinds of arc and spark applications
- qualitative
- quantitative
- semi-quantitative
what arc and spark aes instrumentalists are used for qualitative analysis
multichannel
in quantitative analysis which precision is poorer: arc or spark
arc
are internal standards used in arc and spark aes
yes
how does semi-quantitative arc and spark aes work
obtained concentrations are between 30-300% of real amounts because you don’t really care too much about how much is in there, just that you know a possible range
applications of arc and spark aes
- metallurical manufaturing
- oil refineries
- petrochemical and chemical industries
- aviation
- military
- power plants
- scrap sorting
