Quant Chapter 20
What type of atomization is used for AE?
Plasma is used for atomic emission because the temperature is more stable and it is so hot that there is a substantial population of excited-state atoms.
What are the 3 types of atomic spectroscopy?
atomic emission, atomic absorption and atomic fluorescence
What is atomic spectroscopy used for?
can be used to determine the elemental composition of a sample (what elements are in sample and how much of each)
What is the Boltzmann distribution?
describes the relative populations of different states at thermal equilibrium. If equilibrium exists (which is not true in the blue cone of a flame but is probably true above the blue cone),
Explain hollow-cathode lamp emission
Atomic absorption experiments require lamps that produce light only at characteristic wavelengths for the element(s) being analyzed. A steel hollow cathode lamp (Figure 21-3) produces light onlyat the characteristic wavelengths for Fe, Ni, and Cr
What is atomization?
Atomization: the process in which a compound is decomposed into its atoms at high temperature
What is atomic line-widths?
Beer's law requires that the linewidth of the radiation source should be substantially narrower than the linewidth of the absorbing sample. Otherwise, the measured absorbance will not be proportional to the sample concentration. Atomic absorption lines are very sharp, with an intrinsic width of only 10 4 nm.
What happens in atomic spectroscopy?
Heat is used to vaporize and decompose samples into atoms and ions
How is analyze atomized in AS?
flame, plama or heated furnace
What is premix burner?
in which fuel, oxidant, and sample are mixed before introduction into the flame.
What is degeneracy?
number of states at each energy
Describe free atom formation in flames
physical and chemical processes in the burner head and flame that convert an aqueous ionic analyte into gaseous atoms (nebulization, evaporation, vaporization and decompsition) Ideally, all analyte is converted into gaseous atoms M(g).
Explain Atomic Emission Spectroscopy
-Requires no lamp -Collisions in flame or plasma promote some atoms to excited electronic states from which they can emit photons to return to lower energy states -Emission intensity proportional to element concentration -Most common form of atomic spectroscopy
Explain atomic fluorescence spectroscopy
-The flame (or other atomizer) atomizes samples to produce atoms that are primarily in the ground electronic state. -A line source irradiates atoms in the flame to promote them to an excited electronic state. -Fluorescence emission from excited state atoms is measured (proportional to element concentration).
What is characteristic wavelengths in AS?
-the atoms of each element absorb and emit light at characteristic wavelengths -characteristic wavelengths allow determining which elements are present in sample - the amount of light absorbed or emitted by characteristic wavelengths enables the analyst to determine element concentration (ppm)
Explain how AAS works
1)The hollow-cathode lamp generates characteristic wavelengths that are only absorbed by the element being probed. 2)Liquid sample is aspirated (sucked) into a flame 3)The atoms of the element being probed absorb part of the light coming from the hollow-cathode lamp so that P< P0. 4)Absorbance is measured by the detector and element concentration is calculated.
How does varying temperature affect AAS?
AAS depends on the number of unexcited atoms present, not the temp
How does varying temperature affect AES?
Emission intensity is proportional to the population of the excited state. Because the excitedstate population changes by 4% when the temperature rises 10 K, emission intensity rises by 4%.
Explain flame atomic absorption burner
Fuel, oxidant, and sample are mixed in the premix burner before introduction to the flame. •Sample solution is drawn into the pneumatic nebulizer by the rapid flow of oxidant past the tip of the capillary.• The spray is directed against a glass bead, which creates an aerosol (fine suspension of liquid particles in gas).• The mist, oxidant, and fuel flow past baffles that promote further mixing and block large droplets of liquid. •Excess liquid collects at the bottom of the spray chamber and flows out to a drain.• Aerosol reaching the flame contains only about 5% of the initial sample. The slot in the burner head is about 0.5 mm wide. produced by the nebulizer before and after removal of large droplets by spray chamber.
Explain Atomic Absorbtion Spectroscopy
In atomic absorption, atoms absorb part of the light from the source and the remainder of the light reaches the detector. -The flame atomizes samples to produce atoms that are primarily in the ground electronic state. -The atoms are illuminated by characteristic wavelengths from a hollow-cathode lamp. -The atoms absorb a portion of the light from the hollow-cathode lamp. -Element concentration is calculated from the absorbance.
How does temp affect AS?
Temperature determines the degree to which a sample breaks down into atoms and the extent to which a given atom is found in its ground, excited, or ionized states. Each of these effects influences the strength of the signal we observe.
What does the Heisenberg uncertainty principle state?
The linewidth of the radiation source must be substantially narrower than the linewidth of the absorbing sample. •Atomic absorption lines are very sharp, with an intrinsic width of only ~104nm.• The natural linewidth is governed by the Heisenberg uncertainty principle, which says that the shorter the lifetime of the excited state, the more uncertain is its energy.
What is difference between UV-VIS and AAS?
The types of samples under analysis (AAS = Metal ions only, UV = compounds)- The wavelengths 'shot' at the sample (AAS = A specific wavelength of EM radiation, UV = An appropriate 'range' of wavelengths)- What actually absorbs the EM radiation (AAS = Metal sample's electrons absorb, UV = The bonds of the compounds absorb)- The method/instruments used to carry out their analysis (AAS = Cathode lamp, UV = No cathode lamp)
