Principles of Flame Photometry
The method by which photometers operate is as follows.
The fundamental property which makes all photometry viable is due to the excitation and relaxing of electrons within the ion. In all flame photometers the reason as to why there is a heat source is so that the ions in the sample can gain sufficient energy to excite the electrons present in them. Excitation is defined as when an electron bound to a nucleus goes from its ground energy state to a higher energy state. However a common misconception with excitation is that the electron itself is not excited, the entirety of the atom or ion is excited as the individual electron is still just an electron and it is excited due to its difference in energy to the ground state of the other electrons in the atom or ion.
In accordance to the first law of thermodynamics, energy cannot be created of destroyed; therefore when an atom is excited and relaxes back to its ground state, the energy stored in the excitation of the atom to a higher state must be released and transformed into another form of energy. This energy is emitted as a photon, and the frequency of the photon is proportional to the amount of energy lost in the drop between the excited state and the ground state.
Now that we understand how the frequency of the photon emission is formed, measuring the photons emitted comes down to either an emission photometer or an absorption photometer. An emission photometer, commonly known as AES (atomic emission spectroscopy) measures the emission of photons off of the sample with the use of photodiodes to detect their intensity and from that their concentration in the sample.
A second type of photometer uses AAS (atomic absorption spectroscopy). This is where a light source is pointed through the flame of a photometer and into the diodes and a baseline of the light source is recorded. Then when a sample of ions is introduced to the flame the specific wavelengths of the light source is either absorbed by the sample or allowed to pass through to the diodes. As there is a quantifiable drop in the wavelength of light absorbed by the diodes can be detected, this can then be calculated to a specific concentration (as explained in the previous blog post).
Whilst the terms AES and AAS are common in the photometer world, they can also be related to ICP (inductively coupled plasma) such as ICP-AES. The naming format in chemical analysis can often help you easily understand what the instrument is and what it does once understood. Let’s look at FP-AAS for an easy example the FP in relation to Flame Photometer, and the AAS as atomic absorption spectroscopy. In spectrometry the prefix of the abbreviation is method by which the atom is excited and the suffix is the method by it is detected.
Whilst ICP and its many forms are useful, they require a lot of work to use. Flame Photometry on the other hand is very simple to start up and get running with minimal training and high sample throughput. The initial cost difference between purchasing an ICP unit in comparison to a Flame Photometer can be huge, with most Flame Photometer units coming to under £10,000 and most ICP units being a minimum of 10,000 for a low spec model.