Atomic Absorption Spectroscopy Chapter.7 Exam Prep

Chapter 7

Problem 7.1: In your own words, explain why the features in molecular absorption spectroscopy are broader than the features seen in AAS.

Problem 7.2: Use Example 7.1 as a guide and demonstrate the wavelength dependence of lifetime broadening by calculating the minimum theoretical bandwidth at 200 nm, 400 nm, and 700 nm, assuming a lifetime of 10 ns for each transition. Compare your answers to Example 7.1.

Problem 7.3: Use Example 7.1 as a guide and demonstrate the lifetime dependence of lifetime broadening by calculating the theoretical minimum bandwidth for three dif­ferent transitions, all centered at 500 nm transition with lifetimes of 10 μs, 10 ns, and 10 fs, respectively.

Problem 7.4: Above we stated that the promotion of the sodium 3s electron into the 3p orbital can occur parallel or antiparallel to the angular momentum of the 3p orbital ( J = S + L or J = S – L). Derive the term symbols for the two possible excited states.

Problem 7.5: The 4s 4p transition in potassium experiences a splitting of the 4p orbital in a manner similar to that seen for sodium. The 4s 4 transition occurs at a frequency of 3.89 × 10¹⁴ Hz and the 4s 4 transition occurs at a frequency of 3.91 × 10¹⁴ Hz. Calculate the energy difference in joules and the wavelength difference in nanometers for these two transitions.

Problem 7.6: Doppler broadening is a significant source of line broadening in atomic spectroscopy. Using a maximum velocity of 2000 for an analyte atom in a flame, use Equation 7.3 to determine the line broadening (Δλ) associated with a transition centered at 500 nm. Report your answer in nanometers.

Problem 7.7: Show the wavelength dependence of Doppler broadening by repeating Exercise 7.6 for transitions centered at 200 nm, 420 nm, and 680 nm.

Exercise 7.8: Doppler broadening of atomic transitions is temperature dependent and the effect on the bandwidth ( compared to an atom with no velocity is , where  is the velocity of an atom and c is the velocity of light. The relationship between an atom’s velocity and temperature is given by the equation , where k = the Boltzmann constant, T is temperature (Kelvin) and m is the mass of the atom in kilograms. Calculate the effect of Doppler broadening on the bandwidth of the calcium 422.7 nm line in an 1800°C acetylene flame.

Problem 7.19: Using Beer’s law, explain why GFAAS has lower detection limits than FAAS.

Problem 7.20 Outline the stoichiometry showing that the addition of 0.1ml of a 1000 ppm standard to 100 grams of soil represents a 1 ppm spike. Recall that the definition of ppm for an aqueous solution is

ppm =

and the definition of ppm for a solid matrix is

ppm = .

EXERCISE 7.1: Show how you would make a series of four standards ranging from 1 to 100 ppm given the following constraints. Your stock solution is 1000 ppm. The available pipettes have volumes of 2 mL, 3 mL, and 5 mL. The available volumetric flasks have volumes of 10 mL, 25 mL, 50 mL, and 100 mL. See the “Be Flexible” sidebar.

EXERCISE 7.3: Explain why the analytical selectivity of FAAS is not affected by Doppler broadening and collision energy transfer events in the flame.

EXERCISE 7.5: In your own words, explain how the background noise from flame emissions is attenuated in AAS.

EXERCISE 7.6: Explain how the use of an HCL in AAS reduces the need for a monochromator of high spectral purity. In other words, how does the HCL allow us to get away with using a cheaper monochromator?

EXERCISE 7.7: What is the purpose of using a D₂ lamp collinear with the optical path in AAS?

EXERCISE 7.8: The 3s 3p transition in Na is at about 589 nm. The same Mg+ transition is at about 280 nm. Use the Boltzmann equation (Equation 7.5) to calculate the fraction of atoms or ions in the excited state for each analyte at 2,000°C.

EXERCISE 7.9: Demonstrate the lifetime dependence of Heisenberg broadening by calculating the theoretical minimum bandwidth for three different transitions, with lifetimes of 10 ms, 10 ns, and 10 fs respectively for transitions centered at

EXERCISE 7.10: Determine the ground state and first excited state term symbols for each of the following elements.

EXERCISE 7.11: Use Equation 7.2 to determine the pressure broadening of sodium analyte atoms in a flame AAS experiment. Assume a pressure of 1 atm, a flame temperature of 2,300°C, and an average mass of perturbing particles in the flame of 15 g/mol. Hint: You will need to use the ideal gas law to estimate Np.

EXERCISE 7.12: Assuming an atomization temperature of 1,400°C in a graphite furnace AAS experiment, determine the line broadening () associated with the d-lines in a sodium atom. Report your answer in nanometers. Hint: See Exercise 7.11

EXERCISE 7.13: Repeat Exercise 7.12 for a flame AAS experiment assuming a flame temperature of 2,000°C. Compare the resolution of the two techniques and comment on the effect of atomization temperature on resolu­tion as a result of Doppler broadening.

EXERCISE 7.14: Explain why the use of element specific sources such as the HCL or EDL negate the concerns exposed by Exercise 7.13.

EXERCISE 7.15: Discuss the steps involved in converting the sample matrix into analyte atoms.

EXERCISE 7.16: Discuss the benefits and disadvantages of Smith-Hieftje background correction.