Chapter 9
Why do electrothermal atomizers generally provide higher sensitives than flame atomizers?
Because it stays in the optical length longer.
Describe the processes involved in sample introudction and atomization in flameAA.
Describe the processes involved in sample introduction and atomization in flame AAA? Nebulization turns into spray. Under heat, it goes through desolvation . That turns into a dry aerosol. Again with heat into volatilization and then with heat it turns into free atoms. (a fraction of the molecules, atoms, and ions are also excited by the heat of the flame to yield atomic, ionic, and molecular emission spectra.) Those can either be excited.(Main goal you want to atomize)
9-3 Be familiar with various atomization techniques described in section 9A.
Flame structure: Important regions of a flame include the primary combustion zone, the interzonal region, and the secondary combustion zone. Throughout the flame, they have different temperatures. The initial increase in absorbance as the distance from the base increase results from an increased number of magnesium atoms produced by the longer exposure to the heat of flame. Secondary combustion zone leads to a decrease in absorbance because the oxide particles does not absorb at the observation wavelength. The behavior of silver which is not easily oxidized, has a continuous number of atoms and thus the absorbance is observed at the base of the periphery of the flame. By contrast, chromium, which is a stable oxides, shows a continuous decrease in absorbance beginning close to the burner tip. Electrothermal Atomization: samples are placed in a carbon tube which is heated electrically. The tube is heated in steps. 1st step. Drying (5-200C) (gas on) remove solvent. 2nd. Char(gas on) (200-800C) remove/decompose matrix. 3rd step. Atomization (gas off)(2000-3000) atomize analyte. - Sample residence time is much greater compared to flame atomizers thus significantly increasing detection limits and sensitivity. -offer the advantages of unusually high sensitivity for small volumes of sample.
Be familiar with the various atomization techniques described in section 9A.
In flame atomizer, a solution of the sample is nebulized by a flow of gaseous oxidant, mixed with gaseous fuel, and carried into a flame where atomization occurs. The first step is desolvation , in which solvent evaporates to produce molecular aerosol, The aerosol is then volatilized to form gaseous molecules. Dissociation of these molecules produces an atomic gas. (This indicates a fraction of molecules, atoms, and ions are also excited by the heat of the flame to yield atomic, ionic, and molecular emission spectra)
Describe the process involved in sample introduction and atomization in flame AA.
In flame atomizer, a solution of the sample is nebulized by a flow of gaseous oxidant, mixed with gaseous fuel, and carried into a flame where atomization occurs. The first step is desolvation , in which solvent evaporates to produce molecular aerosol, The aerosol is then volatilized to form gaseous molecules. Dissociation of these molecules produces an atomic gas. (This indicates a fraction of molecules, atoms, and ions are also excited by the heat of the flame to yield atomic, ionic, and molecular emission spectra)
9-7. Compare and contrast hollow cathod lamps and electrodeless discharge lamp.
Light intensity is greater but not as stable for electrodesless. Hollow is more stable because of the light intensity. Electrodeless uses radiant frequency and the hollow cathod does not.
The effective bandwidths of the lines emitted by AA lamps are significantly narrowers than the corresponding bandwidths of the absorption peaks in flame AA. Why is this fact so critically important to the success of flame AA.
Narrow lines are highly desirable because they reduce the possibility of interference due to overlapping lines. Due to the performance of the sources of line broadening which is the uncertainty effect.
Describe the hydride generation process for atomic absorption spectroscopy. Whis is this technique used rather than simply introducing the liquid sample into a flame.
Provides a method to introduce arsenic, antimony, tin, selenium, and lead into an atomizer as a gas. This often enhances the detection limits by a factor of 10 to 100. Generally brought about by the addition of an acidified aqueous solution of sodium borohydride. The volatile hydride is then swept into a heated atomization chamber where decomposition of the hydride takes place leading to the formation of atoms of the analyte.
What are some of the common interferences encountered in AA and what measures are used to minimize these interferences? ( Section 9C).
Spectral interferences: Arise when the absorption or emission of an interfering species either overlaps or lies too close to the analyte absorption or emission such that the resolution by the monochromator becomes impossible. (overlapping lines can most often be avoided by using an alternative wavelength.) Chemical interferences: result from various chemical processes occurring during atomization that alter the absorption characteristics of the analyte. ( it becomes possible to regard the burned gases as a solvent medium to which thermodynamic calculations can be applied.)
9-14. Why must the height of the burner head be adjusted each time a new analyte is examined with flame AA?
The Flame has a different temperature and in terms of the amount of oxygen. This also depends on the element as well because not all elements react the same way.
9-3 Why is an electrothermal atomizer more sensitive than a flame atomizer?
The electrothermal atomizer is a more efficient atomizer. It requires much less sample and keeps the atomic vapor in the beam for a longer time than does a flame
How do hollow cathode tubes work?
The lamp is filled with an inert gas like argon or neon when a potential is applied, it causes the gas to become excited and it is driven towards the cathod. This is the most common source for atomic absorption measurements. Then the metals atoms are then sputtered off the surface of the cathode. This replicates the bombardment of the metal atom by the gas causes it to be excited. It ultimately relaxes, producing specific atomic emission lines. An Hc lamp will only produce the emission lines for the cathode element. (High voltages, and thus high currents, lead to greater intensities. Low intensity is led to self-absorption.)
Discuss the temperature profile of a flame and how this profile affects the processes that go on in that flame relevant to flame atomic absorption and emission spectroscopy.
The lowest range the atoms are being atomized, then the next range the atoms are excited, then the last range they are being ionized. Too hot can form ions, too much of atomic species can form molecules. For molecular species it's reversible.
Why is atomic emission more sensitive to flame instability than atomic absorption or fluorescence?
The population of excited atoms from which emission arises is very sensitive to the flame temperature and other conditions. The population of ground state atoms, from which absorption and fluorescence originate, is not as sensitive to these conditions since it is a much larger fraction of the total population.
Which technique is more sensitive, flame AA or atomic emission spectroscopy and why?
This depends on the analyte (element).
What is the purpose of the flame in atomic absorption spectroscopy.
To atomize the sample. In other words, the metal ions in a solution are converted to atomic state by the means of flames.
Why is it critically important to control flame temperature in flame atomic absorption spectroscopy?
To focus the same part of the flame of the entrance slit for all calibrations and analytical measurements.
Describe the physical effects and interferences that the method of standard additions would correct or compensate for in AAS.
When an internal standard is used, the ratio of intensity of the analyte line to that of the internal standard is plotted as a function of the analyte concentration (see Figure 1-12). If the internal standard and the analyte species are influenced in the same way by variation in the aspiration rate and the flame temperature, and if the internal standard is present at approximately the same concentration in the standards and the unknown, the intensity ratio should be independent of these variables.
