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  • The BWB Team

The Flame Photometer: How It Works



If you’ve ever gone camping, built a fire, and tossed in one of those coloured-flame-generating cubes, you’re already halfway to understanding how a flame photometer works. Those cubes are a combustible mixture of paraffin (wax), woodchips or sawdust, and some tiny particles assorted metals in dust form.


As the wax and wood burn away, metal particles are exposed to the flame and flare up with novel and unexpected colours. As they ionise it allows electrons to move between their ordinary orbital position and one higher. As they inevitably drop back to their normal state, they will emit a photon that has a precise energy related to the specific material, expressed as a colour frequency. The result is an entertaining fountain of unexpected colour with green, purple, orange, red, and blue flames.


Copper or Boron produces green, copper chloride blue, sodium yellow, strontium red, potassium purple, and so on… It’s basically a rather hypnotic light show after the parents turn off the Wi-Fi hotspot and tell the kids to turn off their phones.


How does it work?


Back to the more practical matters of using flame photometry in the lab… Let’s look a little closer at the components and processes that allow flame photometers to deliver their near instant results. With just a brief set up period, and at a stunning low price point compared to other methodologies, you can easily assay for the presence of five of the most useful alkali or alkaline earth metals. These are Sodium, Potassium, Lithium, Calcium, and Barium.


The Basic Parts

Fire, Air, and Filters


The flame, of course, is right in the name, and it is generally powered by a compressed gas cylinder or natural gas from a town supply. For consistency, it also requires a non-pulsed, regulated air supply so the flame is steady, contributing as little as possible to the colour results of the testing. To aid with that many units are equipped with a blue absorption filter to pass only the light generated by the ionisation process, and not the flame itself.


Our units are equipped with a customised, built-in air compressor for extreme consistency. This avoids the complications of external compressors that need oil and maintenance, which can affect your results. In addition, our flame photometers are set up to run with propane, liquid propane, natural gas, or even butane, and all without requiring any user adjustments.


Atomisation


Next is the nebuliser and mixing chamber. The nebuliser portion follows the same principle that powered the pre-aerosol-age atomiser perfume bottle, which had a squeeze bulb to force air over a venturi to draw up liquid perfume and then turn it into an ultrafine mist in the air stream. You can see the same principle at work with hand pumped spray cleaners but the bulb has been replaced with a modern piston.


The fast airstream passing over the sample tube tip uses the Bernoulli Principle to sheer off microscopic samples constantly. Once in this atomised or nebulised state the fuel-air mixture carries the sample to the base of the flame, introducing it to the blaze where the water evaporates. This leaves salts of the metal ions behind which then change state (just like our campfire example from earlier). The salts react with the heat, with their electrons moving from ground state to excited state and back to ground state, over and over again, emitting photons at specific wavelengths.


The nebulisers primary function is therefore, to reliably supply a consistent, homogenous sample to the flame in a highly regulated and predictable way. For consistency, the nebuliser ordinarily supplies De-Ionised (DI) water to the flame when there is no sample, which does not affect the flame’s colour. Prepared samples are substituted for the DI water, and the flame responds with a distinctive colour change, if the metal being sought is present.


Colour


The next component makes use of the colour and its intensity generated by the ions and heat to quantify how much of the substance is present. In traditional models filters were used to absorb unwanted portions of the spectrum, allowing only the precise frequency produced by the desired metal. This called for you to select which atom you were measuring and move its exclusion filter into place so only the desired light would pass through to the photodetector.


In advanced models, such as ours, there are multiplexed photoreceptors, each designed to read only a very specific part of the light spectrum representing the metal being sought. By focusing only on extremely specific frequencies, it is possible to sense all testable metals at the same time.


The older technology of filters and slits has been replaced by these hypersensitive photoreceptors giving much more accurate results. It also speeds up the entire process as it allows multiple detections in a single operation. Built-in amplifiers (aka photomultipliers) have also increased detection capabilities by whole orders of magnitude over historical versions.


These advances have certainly paid off. While early models would drift from their settings more easily, modern units stay within specifications for extended periods. Quantifiable results are produced that are much more reliable and meaningful than in previous iterations.


Future developments


Now, don’t be apprehensive, but here is a (very) little quantum physics for you. Hydrogen has only one electron, and all orbitals (positions where that electron can be) have the same energy requirement. This only occurs with hydrogen and the orbitals are called “degenerate” for that reason.


Every other atom has higher and higher energy requirements for each higher orbital change. However, when you create an ion of hydrogen it has no electron. Give it an electron and it returns to base state—there is no photon emitted therefore it doesn’t contribute any colour to the flame. A hydrogen-oxygen flame is nearly colourless but for a very faint blue tinge. It can be the perfect fuel for flame photometry for a number of reasons.


Eventually, in the future we may even see Flame Photometers running exclusively on hydrogen. The advantage is that it burns 200 Cº hotter than propane, and 500 Cº hotter than butane when using plain air as the oxidant. Naturally, this will extend the capabilities of Flame Photometers, but the real reward is the reduction of operating costs. Using an environmentally friendly fuel that is highly cost effective is the real incentive.


Table-top hydrogen generators can safely produce quantities of hydrogen gas from nothing more than distilled water. It is available “on-demand” with no need to store it, thus overcoming the intrinsic leaking problem entirely. Typically hydrogen generators produce half a litre per minute, so it is nearly impossible to get an explosive quantity. Such units are extremely efficient and safe.


Ultimately, this means that after the generator is paid for, your fuel is “free” except for the trivial costs of electricity and distilled water. Your fuel costs drop to almost nothing annually and your operation is resultantly “greener” by eliminating the whole “delivery process” impact—which is good for your company reputation.


The Takeaway


We have a number of models covering all possible needs, whether you’re working in soil analysis, nuclear (lithium) assays, manufacturing sugar, synthetic fuels, or performing bio-assays in hospital labs. If we don’t sell exactly what you want, our R&D team can custom design a machine to precisely suit your needs.


If you don’t have a fully-fledged list of necessities, please call and let us help select the best model for your requirements.


We want to put a BWB Technologies Flame Photometer in your hands so you can see for yourself what you’ve been missing! We would love to help you today!





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