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Study Notes: Gas Mixtures

The aim of flame mixture is to maximize free atoms in the flame to get the best signal and therefore sensitivity (limit of detection).

Choice of flame will be dependent upon degree of difficulty regarding decomposition of samples. That is the breaking down of materials into their constituent atoms. This may be straightforward process through monoxide forms to the atoms themselves but sometimes may involve the development of intermediate compounds due to interactions with species naturally present in the flame such as C, C2, CH, CN and NH. Some of these compounds may be refractory to further breakdown and thereby inhibit the production of analyte atoms. On the other hand some intermediate compounds promote production of the analyte atoms.

This is a complex area of chemistry and it is sometimes difficult to predict the decomposition chemistry for all elements under all analytical circumstances.

Fuel/oxidant combinations in most common use today include:

  • Air/Acetylene (2300°C) is the most common mixture, used for most easily atomised elements e.g. Cu, Pb, K and Na.
  • Nitrous Oxide/Acetylene that is hotter (3000°C) is used to atomise highly refractory compounds of elements such as Al, Si, Va and Ti as well as rare earth elements.

The actual operating temperature of the flame and its intrinsic chemical environment will depend on the fuel/oxidant ratio and this is an important consideration for some elements:

  • Lean (oxidising flame, hottest)
  • Rich (reducing flame, coolest)
  • Stoichiometric (chemically balanced flame, temperature in mid range between lean and rich).

A few examples of use of acetylene/air and nitrous oxide/acetylene flames are shown in the table below.

Acetylene/Air
Acetylene/Air
Nitrous Oxide/Acetylene
Nitrous Oxide/Acetylene
Ag
K
Al
Sn
As
Li
As
Ti
Bi
Mn
Ba
U
Ca
Na
Ca
W
Hg
Pb
Si
Zr

Notice that some elements can be detected by either flame.

Generation of refractory oxides can be a problem and include CaO, SiO2, TiO2 and Al2O3. The problem is overcome by running these samples at temperatures above 2300°C, i.e. nitrous oxide/acetylene flames.

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