Flame photography is the process of measuring the intensity of light emitted when the element is exposed to a flame, using a wavelength of a colour.
However, not all elements are detectable by flame photometry.
When it comes to detecting the concentration of an element via flame photometry, said element must exhibit a specific set of prerequisites to make it suitable for analysis.
What are the requirements for an element to be suitable for Flame Photometry?
One of the primary requirements is that the element can be ionised into its ionic form in aqueous solutions.
While this does not cut the pack of elements available down by a large margin, a substantial number of non-metals are instantly removed.
One of these is carbon, which is commonly oxidised to become water soluble, while other elements include silicon, sulphur and oxygen.
This is based upon the electronegativity of the element, which increases as you move up and right across the Periodic Table.
What is Charge Density?
Another factor which makes it commonplace to see the determination of elements in the alkaline and alkaline earth periods is charge density.
Charge density is a measurement of the atom’s overall charge (1+ and 2+ for elemental alkaline and alkaline earth elements) divided by the size of the atom.
As the charge density of these elements, as well as the low number of electrons present in their outer shell of electrons, is low, the requirement for energy to excite the element is reduced.
This is due to the effect of the nucleus's overall magnetic charge in comparison to the strength of a single electron and the stability gained by releasing the electron, which is commonplace in metallic bonding of these pure elements.
Why is it difficult to select elements for analysis via Flame Photometry?
The issue with measurement of transition metal elements and larger metallic elements when selecting them for analysis via flame photometry is that due to the presence of the outer shell being a D orbital, the excitation of an electron occurs in a differing manor to that of a P orbital because of their vastly differing shapes.
This leads to a lot of more different pathways for an electron to relax into.
Therefore, the spectrum of light emitted from the excitation of transition metal elements is broader and less defined than that of an element that is suitable for detection via flame photometry.