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Why is the CaOH spectrum in Figure 8-8 so much broader than the barium emission line?
The emission or absorption spectrum for calcium in a low-temperature flame is broad because the calcium is largely present as the molecule CaOH, which has many vibrational and rotational states and thus many excited energy levels. Hence, a broad molecular band spectrum is observed. Barium, in contrast is present largely as atoms and these absorb or emit at only a few discrete wavelengths.
What is resonance fluorescence?
Type of fluorescence in which the emitted radiation has the same wavelength as the radiation used to excite the fluorescence.
Fluorescence will occur at a longer wavelength (the Stokes shift) than the excitation wavelength when relaxation to a lower energy excited state takes place prior to emission.
What determines natural line widths for atomic emission and absorption lines? About how broad are these widths typically?
Natural line widths in atomic spectroscopy are the widths of lines when only the uncertainty principle, and not Doppler or pressure broadening, contribute to the broadening. The width is then determined by the lifetime of the excited state.
In a hot flame, the emission intensities of the sodium lines at 589.0 and 589.6 nm are greater in a sample solution that contains KCl than when this compound is absent.
In the presence of KCl, ionization of sodium is suppressed because of the high concentration of electrons from the ionization of potassium. In the absence of KCl, some of the sodium atoms are ionized, which leads to a lower emission intensity for atomic Na.
The intensity of a line for atomic Cs is much lower in a natural gas flame, which operates at 1800°C, than in a hydrogen-oxygen flame, whose temperature is 2700°C.
The energy needed to promote a ground state s electron to the first excited p state is so high for Cs that only a fraction of the Cs atoms are excited at the low temperature of a natural gas flame. At the higher temperature of a H2/O2 flame, a much larger fraction of the atoms is excited and a more intense Cs emission line results.
Name a continuous type and a discrete type of atomizer that are used in atomic spectrometry. How do the output signals from a spectrometer differ for each?
A continuous type of atomizer is an inductively coupled plasma. A non-continuous type is a electrothermal furnace atomizer. The plasma produces an output that is essentially constant with time, whereas the furnace produces a transient signal that rises to a maximum and then decreases to zero.
In the concentration range of 500-2000 ppm of U, there is a linear relationship between absorbance at 351.5 nm and concentration. At lower concentrations the relationship is nonlinear unless about 2000 ppm of an alkali metal salt is introduced into the sample
This behavior would result from ionization of U. At low concentrations, the fraction of U that is ionized is greater giving a nonlinear relationship between concentration and absorbance. The alkali metal salt suppresses the ionization of U.
cation that preferentially reacts with a species that would otherwise react with the analyte to cause a chemical interference.
prevent interference by forming stable and volatile products with the analyte.
An ionization suppressor is more easily ionized than the analyte and provides a high concentration of electrons in the flame or plasma. These electrons suppress the ionization of the analyte.
process by which a sample is vaporized and decomposed into atoms usually by heat.
refers to the broadening of atomic lines due to collisions with other species.
has a tungsten anode and a cylindrical-shaped cathode containing the element of interest. The element is sputtered from the cathode into the gas phase. This process excites some of the gaseous atoms, which then emit characteristic radiation as they return to the ground state.
process in which gaseous cations bombard a cathode surface and eject atoms from the surface into the gas phase.
refers to the absorption of emitted radiation by unexcited atoms in the gas phase of a hollow-cathode lamp, flame, or plasma.
encountered when the absorption or emission of a nonanalyte species overlaps a line being used for the determination of the analyte.
result of any chemical process which decreases or increases the absorption or emission of the analyte.
substance added in excess to both sample and standards which swamps the effect of the sample matrix on the analyte emission or absorption.
arises because atoms moving toward or away from the monochromator give rise to absorption or emission lines at slightly different frequencies.
The absorbance of Cr decreases with increasing flame height because chromium oxides are formed to a greater and greater extent as the Cr rises through the flame. The Ag absorbance increases as the silver becomes more atomized as it rises through the flame. Silver oxides are not readily formed. Magnesium exhibits a maximum as a result of the two above effects opposing each other.
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.
The continuum radiation from the D2 lamp is passed through the flame alternately with the hollow-cathode beam. Since the atomic lines are very narrow, the D2 lamp is mostly absorbed by the background, wherase the hollow-cathode radiation is absorbed by the atoms. By comparing the radiant power of the two beams, the atomic absorption can be corrected for any backbround absorption.
Source modulation is employed to distinguish between atomic absorption (an ac signal) and flame emission (a dc signal).
The alcohol reduces the surface tension of the solution leading to smaller droplets. It may also add its heat of combustion to the flame leading to a slightly higher temperature compared to water which cools the flame. Alcohol also changes the viscosity of the solution which may increase the nebulizer uptake rate. All of these factors can lead to a great number of Ni atoms in the viewing region of the flame.
At high currents, more unexcited atoms are formed in the sputtering process. These atoms generally have less kinetic energy than the excited atoms. The Doppler broadening of their absorption lines is thus less than the broadening of the emission lines of the faster moving excited atoms. Hence, only the center of the line is attenuated by self-absorption.
(1) Employ a higher temperature flame (oxyacetylene). (2) Use a solvent that contains ethanol or another organic substance. (3) Add a releasing agent, a protective agent, or an ionization suppressor.
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.
Nebulization: Aqueous solution containg MgCl2 is converted to an aqueous aerosol.
Desolvation. The solvent is evaporated leaving solid particles.
Volatilization. The remaining water is evaporated and the particles are vaporized.
Atomization. Mg atoms are produced
Excitation of Mg to Mg*
Ionization of Mg to Mg+Reaction of Mg to form MgOH and MgO
Sulfate ion forms complexes with Fe(III) that are not readily atomized. Thus, the concentration of iron atoms in the flame is less in the presence of sulfate ions.
(b) Sulfate interference could be overcome by (1) adding a releasing agent that forms more stable complexes with sulfate than does iron, (2) adding a protective agent, such as EDTA, that forms highly stable but volatile complexes with Fe(III), and (3) using a higher flame temperature (oxyacetylene or nitrous oxide-acetylene).
The absorbances of the three standards are estimated to be 0.32, 0.18, and 0.09. The unknown absorbance was approximately 0.09. From a least-squares treatment of the data, the equation for the line is y = 1.5143x + 0.02. From the analysis, the concentration of the unknown is 0.046 ± 0.009 μg Pb/mL.
During drying and ashing, volatile absorbing species may have been formed. In addition, particulate matter would appear as smoke during ashing, which would scatter source radiation and reduce its intensity.
This behavior results from the formation of nonvolatile complexes between calcium and phosphate. The suppressions levels off after a stoichiometric amount of phosphate has been added. The interference can be reduced by adding a releasing agent which ties up the phosphate when added in excess.
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.
the wavelength of radiation emitted or absorbed by a rapidly moving atom decreases if the motion is toward a transducer and increases if the atom is receding from the transducer.
Results in broadening of spectral lines
max with max velocity.
caused by collisions of the emitting or absorbing species with other atoms or ions in the heated medium.
produce small changes in energy levels and a range of absorbed or emitted wavelengths.
produces brodening that is 2-3x greater than natural line widths
method for introducing solution samples into plasma or flame
constantly introduces the sample in the form of aerosol
process in which neutral sample atoms are ejected
used in the glow-discharge technique
a substance that responds to uncontrollable variables in a similar way as the analyte. It is introduced into or is present in both standards and samples in a fixed amount. The ratio of the analyte signal to the internal standard signal then serves as the analytical reading
Flame atomic absorption requires a separate lamp for each element, which is not convenient when multiple elements are to be determined.
The temperature of a high-voltage spark is so high (~40,000 K) that most atoms present become ionized. In a lower temperature arc (~4000 K) only the lighter elements are ionized to any significant exten. In a plasma, the high concentration of electrons suppresses extensive ionization of the analyte
In the presence of air and with graphite electrodes, strong cyanogens (CN) bands render the wavelength region of 350 to 420 nm of little use. By excluding nitrogen with an inert gas, the intensities of these bands are greatly reduced making possible detection of several elements with lines in this region.
By Nebulization, by electrothermal vaporization, and by laser ablation.
The advantages of the ICP over the DCP are higher sensitivity for several elements and freedom from some interferences and maintainence problems. No electrodes need to be replaced in the ICP, whereas in the DCP, the graphite electrodes must be replaced every few hours. Advantages of the DCP include lower argon consumption and simpler and less expensive equipment.
Ionization interferences are less severe in the ICP than in flame emission because argon plasmas have a high concentration of electrons (from ionization of the argon) which represses ionization of the analyte.
Advantages of plasma sources include:
1. Lower interferences
2. Emission spectra for many elements can be obtained with one set of excitation conditions.
3. Spectra can be obtained for elements that tend to form refractory compounds.
4. Plasma sources usually have a linearity range that covers several decades in concentration.
The internal standard method is often used in preparing ICP calibration curves to compensate for random instrumental errors that arise from fluctuations in the output of the plasma source.
typical ICP source
3 consecutive quartz tubes through which streams of argon gas flow
for emission spec
programmed to move from the line for one element to that of a second, pausing long enough at each to measure line intensities with a good S:N ratio.
for emission spec
measure simultaneously, or nearly so, the intensities of emission lines for a large number of elements.
instrument for arc and spark emission spec
first instrument for arc and spark
contains a photographic film or plate located at the focal plane or curve
A vibration that involves a continuous change in the interatomic distance along the axis of the bond between 2 atoms.
Symmetric and asymmetric
a result of anharmonicity deviation that at higher quantum numbers, E becomes smaller and the selection rule is not followed rigorously.
weak transitions that correspond to change in frequency=+/- 2 or 3
3N-6= x vibrations
each vibration is called a ________
independent of energy imparted to the system
2.5-50 nm (wavelength)
4000-200 cm-1 (wavenumber)
1.2E14-6E12 Hz (frequency)
characteristic of an atomic stretching vibration
vibration that results from, 2 masses connected by a spring, a disturbance of one of these masses along the axis of the spring
potential energy curve is a perfect parabola
the potential energy is a maxiumum when the spring is stretched or compressed to its maximum amplitude, and decreases to zero at the equilibrium position
depends of the stiffness of the spring
N/m or kg/s2
insulator (dielectric material) with very special thermal and electrical properties
most important for IR- triglycine sulfate
expose to electric field--> material polarizes, remove electric field and relaxation occurs as light hits which causes current between electrodes
a series of thermocouple that are connected
transducer for IR
For near-infrared measurements, instrumentation similar to UV-visible spectrophotometers is often used. Cell path lengths are often longer than in the mid-infrared, and detectors are more sensitive. While bands are broad and overlapping in the near-infrared, chemometric software is used for multivariate calibration.
Absorption of the IR beam by the sample excites various vibrational modes. Nonradiative decay can transfer heat to the surface of the sample and result in the generation of an acoustic wave in the gas inside the chamber. A very sensitive microphone then detects the acoustic wave. The technique is most useful for solids and turbid liquids.
Reflection and absorption by cell walls and inexact cancellation of solvent absorption can result in attenuation of the beam even in regions where the analyte does not absorb. This results in transmittances in these regions less than 100%.
technique used to handle solid samples being measured in the mid-IR region.
a ground sample is mixed with dried KBr powder and then pressed in a special die to yeild a transparent disk
interference bands at 3450 and 1640 because of absorbed moisture
spectra for solids that are not soluble in an IR-transparent solvent
grind finely powdered sample in the presence of a heavy hydrocarbon oil (Nujol)
1200 cm-1 to 600 cm-1
particularly useful because small differences in the structure and constitution of a colecule result in significant changes in the appearance and distribution of absorption bands in this region
most single bonds have absorption bands in this region because their energies are about the same, strong interaction occurs between neighboring bonds
qualitative and quantitative IR analysis
determines thes tructure of organic and biochemical specials
effective way of directly obtaining IR spectra on powdered samples with a minimum of sample preparation
beam of radiation strikes the powder sample. Specular reflection occurs at each plane surface and since there aer many surfaces, radiation is reflected in all directions
intensity of the reflected radiation is roughly indenpendent of the viewing angle.
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