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Trace Elemental Analysis

Concentrations of many elements in the periodic table one hundred years ago were determined by wet way methods – typically by titration. However low levels of some toxic elements like lead that can accumulate in the body were only possible with the development of instrumental methods. Low levels of lead were first estimated by polarography and in 1959 Jaroslav Heyrovsky was awarded the Nobel prize, "for his discovery and development of the polarographic methods of analysis." Only a limited but important range of elements as ions in solution can be estimated by this technique but sensitivity is very similar for each on a molarity basis. Emission spectroscopy detects most elements but sensitivity varies widely from one element to the next and matrix matching is needed to improve accuracy of amounts detected. Sodium and potassium by flame emission are very sensitive but magnesium and calcium much less so. During the 1950s, Alan Walsh developed flame atomic absorption which eliminated this sensitivity problem and minimised matrix matching errors. As magnesium is an important electrolyte for our heart health, the medical profession soon adopted this technique. Despite the wider applicability and better accuracy of atomic absorption, Walsh did not receive a Nobel prize but was knighted for his contributions to science.

Polarography can easily be made a low-cost technique, requiring only a dropping capillary electrode connected by a tube to a leveling bowl filled with clean mercury. Detection of active elements involves measurement of μ-amp currents when the potential to a reference electrode is varied from about 0.0 to -1.3 volts. Across each appropriate half potential, the wave height increases measure concentrations present. By contrast flame atomic absorption requires a light source for each element (called a hollow cathode lamp), flame with solution nebuliser, spectrometer with photomultiplier and readout. This high-cost setup becomes very specific being almost interference free as the lamp, wavelength must be selected to match and will only detect that selected element. Lamps are available for all but the non-metals in the periodic table.

Compared to atomic absorption, as an undergraduate student, I found polarography frustrating trying to estimate the wave height when the μ-amps swung wildly with each mercury drop fall. The technique was used at the Electrolytic Zinc company in Hobart in the 1960s for determining lead in zinc but here the voltage scan was synchronised to each drop fall and the wave height very easy to measure. When teaching analytical chemistry some ten years later to graduate students at a College of Advance Education (CAE), a similar synchronised scan polarograph was included in the practical work.

Model of a polarograph (Image from Czech Academy of Sciences)

Each student did this experiment only once each year and the infrequent use presented major reliability problems that were solved in most rewarding way. Ensuring the apparatus worked well and avoid frustrations was important for effective teaching of students. Infrequent use would waste mercury between use if left set up, however dismantling to keep all parts clean enough proved very time consuming (even toxic if not careful) and really not always reliable. Lowering the leveling bowl to stop flow allowed air to diffuse through the connecting tubing and oxidised some of the mercury which then clogged the capillary on the next use. Cleaning a clogged capillary proved almost impossible. They were slow to reorder and too costly for a CAE. Fortunately, we found that a broken thermometer (then only about $5 each) was a suitable replacement capillary and when a rubber Pasteur pipette cap was placed over the end of the capillary immediately after each use, this avoided mercury loss and we could leave the whole unit set up under positive pressure from one year to the next, ready for ongoing trouble-free use. 

Images references

• Analytik Jena. “The Development of Atomic Absorption Spectroscopy (AAS),” Analytik Jena, 16 June 2021, www.analytik-jena.com/knowledge/blog/posts/the-development-of-atomic-absorption-spectroscopy-aas/. 
• Czech Academy of Sciences. “One Hundred Years of Polarography,” Czech Academy of Sciences, 16 Feb. 2022, www.avcr.cz/en/news-archive/One-hundred-years-of-polarography/. 
• DeLacey, Lynda . “Australian Inventors: Alan Walsh Redefines Chemical Analysis,” Australian Geographic, 6 Aug. 2015, www.australiangeographic.com.au/science-environment/2015/08/australian-inventors-alan-walsh-atomic-absorption-spectroscopy/. 
• Fraňková, Ruth, and Ondřej Vaňura. “February Marks 100 Years since Invention of Polarography,” Radio Prague International, 2 Oct. 2022, english.radio.cz/february-marks-100-years-invention-polarography-8741642. 
• The Editors of Encyclopaedia Britannica. “Jaroslav Heyrovský,” Encyclopedia Britannica, 25 Mar. 2025, www.britannica.com/biography/Jaroslav-Heyrovsky. 

Dr Ray Hodges (retired Associate Professor from Monash Gippsland).

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