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Chemistry LibreTexts

Analysis of Cations by Atomic Spectroscopy

  • Page ID
    220911
  • The focus of this unit is on basic concepts of atomic spectroscopy. The unit could be used to introduce the technique as an active learning classroom activity or assigned as supplemental material in preparation of the lab experiment involving the determination of calcium and magnesium in water samples.

    Q1:  Based on the description above, draw a block diagram and label the parts of a flame atomic absorption spectrophotometer.

    A simple diagram may look like:

    blockdiagram.png

    Q2: Write the electron configuration for calcium (Ca, element number 20). In what atomic orbital do the electrons with the most energy reside?

    The electronic configuration of calcium is 1s22s22p63s23p64s2.   The electrons with most energy are located in the 4s orbital. 

    Q3:  Calcium atoms undergo atomic absorption in the flame of the AA.  Which electrons absorb the energy from the source?

    Most likely the electrons in the 4s orbitals are responsible for absorbing the energy from the source.

    Q4:  Identify some possible absorption transitions for calcium.

    Upon absorption of radiation, electrons in the 4s orbitals will be promoted to higher energy levels such as 3d, 4p and 5p.  For example, the main calcium transition at 422.7 nm is due to electrons being promoted to the 5p orbital.

    Q5: Given that the energy difference between the ground state and the first excited electronic state (ΔE) for the calcium atom is 4.687 × 10-19 J, calculate the frequency, ν, corresponding to a photon possessing this energy. Next, calculate the wavelength (in nm) for this photon.

    Using Planck’s equation ΔE = hν where h is 6.626 x 10-34 J s one can calculate the frequency ν:

    \[\mathrm{ν = \dfrac{4.687 \times 10^{-19}\: J}{6.626 \times 10^{-34}\: J\: s} = 7.07 \times 10^{14}\: s^{-1}}\nonumber\]

    The wavelength associated with this photon will be:

    \[\begin{align}
    λ &= \mathrm{\dfrac{c}{ν}}\nonumber\\
      &= \mathrm{\dfrac{2.99 \times 10^8\: m/s}{7.07 \times 10^{14}\: s^{-1}}}\nonumber\\
      &= \mathrm{4.23 \times 10^{-7}\: m\: or\: 423\: nm}\nonumber
    \end{align}\nonumber\]

    Q6: Why do you think a “slot” burner is used instead of the configuration of a traditional Bunsen burner?

    A slot burner allows for a longer optical path length and a stable flame.  Since atomic absorptions measurements are based on Beer’s law (A = ε b c where b is the path length), any increase of path length will increase the strength of the signal and therefore the sensitivity of the analysis. A stable flame minimizes uncertainty due to fluctuations in the flame.

    Q7: Do calcium and magnesium absorb at the same wavelength?

    No, calcium absorbs in the visible range at 422.7 nm while magnesium absorbs in the UV at 285.2 nm

    Q8: How will this affect the ability to determine both metals simultaneously?

    Atomic absorption spectrophotometry will not allow for the simultaneous determination of calcium and magnesium.  Hollow cathode lamps that emit both calcium and magnesium lines are available but the spectrophotometer will have to be tuned to the absorption wavelength of one metal, then calibrated and eventually used to determine the concentration of the metal in the unknown sample.  To measure the other metal, the same procedure will have to be conducted at a different wavelength.

    Q9: How would this type of relationship help you in determining the concentration of Ca2+ and Mg2+ in an unknown water sample?

    Using Beer’s law a separate calibration can be developed for each metal in the proper concentration range.  The calibration equation will be of the form A = mx + b where A is the absorbance, m is the slope of the calibration line, x is the concentration of the metal, and b is the intercept.  Once the calibration equation is known, the concentration of any of the two metals in the unknown sample can be determined by plugging the absorption and solving for the concentration:

    \[\mathrm{x = \dfrac{A – b}{m}}\nonumber\]

    Q10: What wavelength would you use to measure Ca2+? What wavelength would you use to measure Mg2+

    Typically, calcium is measured at 422.7 nm while magnesium is measured at 285.2 nm.

    Q11: If you wanted to measure Mg2+ concentrations, what instrumental parameters would you need to know?

    The wavelength and the monochromator’s slit width are two fundamental parameters.  One would also need to research what type of gasses to use and their respective flow rates.  Horizontal and vertical adjustments of the slot burner are also crucial.   Horizontal adjustments ensure that the flame is aligned properly with the instrument optical path. Vertical adjustments adjust the height within the flame from which absorbance is monitored. This is important because two competing processes affect the concentration of free atoms in the flame. The more time the analyte spends in the flame, the greater the atomization efficiency; thus, the production of free atoms increases with height. On the other hand, a longer residence time allows more opportunity for the free atoms to combine with oxygen to form a molecular oxide. For an easily oxidized metal, such as Cr, the con­centration of free atoms is greatest just above the burner head. For metals, such as Ag, which are difficult to oxidize, the concentration of free atoms increases steadily with height. Other atoms show concentra­tion profiles that maximize at a characteristic height.

    Q12:  If you know that the concentration of calcium and magnesium in you water sample is approximately 5 to 10 ppm, suggest a strategy to appropriately calibrate the spectrophotometer for such an analysis.

    This question is good to introduce the concept that, in order to use the calibration for prediction of the unknown concentration, such concentration has to fall within the concentration of the standards.  Therefore, if the concentration of calcium and magnesium is expected to fall within 5 to 10 ppm, calibrations standards ranging from 1 to 20 ppm could be used.  For example, a calibration curve could be constructed using standards at 1.0, 2.5, 5.0, 7.5, 10.0 and 15 ppm.

    Q13: If the absorbance of an unknown water sample is found to be greater than the absorbance of the highest calcium standard used to calibrate the spectrophotometer, what steps would you take to ensure that the analysis is providing accurate results?

    One cannot assume that the calibration will be linear beyond the highest standard used in the calibration.  Therefore, to ensure accurate results, the sample would have to be diluted appropriately so that its absorption falls within the range measured on the calibration standards.

    Q14: Considering that the sample is introduced into the flame through a very thin capillary, what step would you have to take before analyzing a surface water sample that may have small amounts of detritus?

    The sample will need to be filtered to eliminate any possibility of clogging the capillary that introduces the sample in the slot burner.  0.45 μ filters are a suitable choice.

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