Without the scientist Robert Bunsen and the talented instrument designer Peter Desaga there would have been no “nearly invisible” flame device called the Bunsen Burner. The science of emission spectroscopy might have been delayed for years, or never discovered at all without that colourless flame.
If that were so, then Bunsen and his partner Gustav Kirchhoff wouldn’t have discovered two new additions to the periodic table and the state of chemical science might still be years behind even now. Instead, since 1854, it has progressed steadily and the ~170-year-old invention is still found in nearly every chemical laboratory in the world.
In a manner of speaking, flame photometry had its roots in some developments by Pierre Jules César Janssen, the first scientist to identify the element helium from his solar observations. He proposed a method to detect other elements, including sodium, which inspired Henri Pellet, Jean Charles Marie Grenier, and Paul Champion to build a crude but workable sodium detector in 1873.
Their device relied on two flames, one with prepared water samples having known quantities of dissolved sodium and a sample prepared from plant material burned to ash, dissolved in water, filtered and then introduced into the second flame. It certainly couldn’t specify PPM (Parts Per Million) but there were detectable differences.
It's impreciseness resulted from it relying entirely on watching the two flames with just eyes and looking for differences. It was a good starting point but it took an innovation by physicist Louis Georges Gouy to give the system basic accuracy. He created an air-powered system for introducing precise amounts into the flame of both samples at the same time using atomisers.
It then took until the technology of 1946 caught up to the needs of science, when Barnes, Richardson, Berry, and Hood evolved a reasonably modern version of the flame photometer. Their design was capable of measuring both sodium and potassium with good reliability. However, due to shortages at the close of WWII, sufficient materials were not available to build more than a few sample machines for testing.
The device had the signature atomiser, the colourless flame, pressurised air, reference samples of test substances for calibration and filters to remove interfering light before reaching a plain photocell. Unlike modern versions, it utilised a galvanometer to report voltage that was a result of the altered amount of filtered light coming from the flame and reaching the photocell. This was surprisingly accurate for such a method.
By 1950, J.W. Severinghaus and J.W. Ferrebee had determined the amounts of calcium in biological materials. These included urine, blood, serum, and other materials, correctly quantified through flame photometry.
By 1953, we had reliably detected fairly precise amounts of barium with a flame photometer. By 1962 we had a good method for identifying strontium in quantities as low as 0.001 PPM.
By 1963, a method had been developed to detect lithium. An amplification technique was also developed to enumerate concentrations of 10-3 (0.001) PPM.
The Takeaway
Nowadays we have incredibly sensitive devices, far removed from those early efforts. With modern photomultiplier tubes and photodiode arrays, we can capture the emitted light of very specific frequencies, quantify it to simultaneously determine how much of each substance is present, reporting them all at once, and thereby speed up the entire process. And, of course, this produces a result that is respected industry-wide for its reliability and accuracy.
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