Chapter 17

Describe the principles of atomic emission, atomic absorption and atomic fluorescence spectrometry.

Compare flame photometry and flame atomic absorption spectrometry with respect to applicability of instrumentation, sensitivity, and interferences.

Why is a sharp-line source desirable for atomic absorption spectrometry?

Explain why absorption spectra for atomic species consist of discrete lines at specific wavelengths rather than broad bands as observed for molecular species.

Explain why electrothermal atomizers result in greatly enhanced sensitivity compared to flame atomizers in atomic absorption spectrometry.

Explain why an internal-standard element can improve the precision of atomic spectrometry measurements.

(a) If the hollow-cathode lamp primarily determines the spectral bandwidth of the detected radiation in AAS, why is a monochromator needed at all? (b) For the alkali metals such as \(\mathrm{Na}, \mathrm{K}, \mathrm{Li},\) etc., many of the atomic species present in a flame are ionized. Why do we not choose to measure these ionic emissions and instead measure the atomic emission?

Explain the mechanism of operation of a hollow-cathode lamp.

Explain why the radiation source in atomic absorption instruments is usually modulated.

Lead in seawater was determined by atomic absorption spectrometry. The APDC (ammonium pyrrolidinedithiocarbamate) chelate was extracted into methylisobutyl ketone and the organic solvent was aspirated. A standard and reagent blank were treated in a similar manner. The blank reading was essentially zero. Measurements were made at the \(283.3-\mathrm{nm}\) line. An independent determination using anodic-stripping voltammetry revealed the atomic absorption results to be high by nearly \(100 \%\). Assuming the anodic- stripping voltammetry results are correct, suggest a reason for the erroneous results and how they might be avoided in future analyses.

Why is a high-temperature nitrous oxide-acetylene flame sometimes required in atomic absorption spectrometry?

Why is a high concentration of a potassium salt sometimes added to standards and samples in flame absorption or emission methods?

An analyst notes that a 1 -ppm solution of sodium gives a flame emission signal of \(110,\) while the same solution containing also 20-ppm potassium gives a reading of \(125 .\) It was determined that a 20 -ppm solution of potassium exhibited no blank reading at the sodium emission wavelength. Explain the results.

The hydrides of certain elements can be generated by reacting with \(\mathrm{NaBH}_{4}\), and flushed into a flame or other atomizer. What is the advantage of this method?

In AAS, sensitivity is often defined as that analyte concentration that results in \(1 \%\) absorption of the incident light. A 12 -ppm solution of lead gives an atomic absorption signal of \(8.0 \%\) absorption. What is the atomic absorption sensitivity?

The transition for the cadmium 228.8 -nm line is a \(1 S_{0}-1 S_{1}\) transition. Calculate the ratio of \(N_{e} / N_{0}\) in an air-acetylene flame. What percent of the atoms is in the excited state? The velocity of light is \(3.00 \times 10^{10} \mathrm{~cm} / \mathrm{s}\), Planck's constant is \(6.62 \times 10^{-27}\) erg-s, and the Boltzmann constant is \(1.380 \times 10^{-16}\) erg \(\mathrm{K}^{-1}\)

Calcium in a sample solution is determined by atomic absorption spectrometry. A stock solution of calcium is prepared by dissolving \(1.834 \mathrm{~g} \mathrm{CaCl}_{2}, 2 \mathrm{H}_{2} \mathrm{O}\) in water and diluting to \(1 \mathrm{~L} .\) This is diluted \(1: 10 .\) Working standards are prepared by diluting the second solution, respectively, \(1: 20,\) \(1: 10,\) and \(1: 5 .\) The sample is diluted \(1: 25 .\) Strontium chloride is added to all solutions before dilution, sufficient to give \(1 \%\) (wt/vol) to avoid phosphate interference. A blank is prepared, to give \(1 \% \mathrm{SrCl}_{2}\). Absorbance signals are acquired by a data system as the solutions are aspirated into an air-acetylene flame, as follows: blank, 1.5 units; standards, \(10.6,20.1,\) and 38.5 units; sample, 29.6 units. What is the concentration of calcium in the sample in parts per million?

Lithium in the blood serum of a manic-depressive patient treated with lithium carbonate is determined by flame emission spectrometry, using a standard additions calibration. One hundred microliters of serum diluted to \(1 \mathrm{~mL}\) gives an emission signal of 6.7 units. A similar solution to which \(10 \mu \mathrm{L}\) of a \(0.010 \mathrm{M}\) solution of \(\mathrm{LiNO}_{3}\) has been added gives a signal of 14.6 units. Assuming linearity between the emission signal and the lithium concentration, what is the concentration of lithium in the serum, in parts per million?