Residues contain mixtures of metal oxides and salts (like chloride sulfate, phosphate, silicate)
Why is ash important?
For nutrition: essential minerals need to be measured individually (such as Ca, Na, Fe)
Undesirable minerals: (Pb, Hg, Cd, etc)
Food safety/food quality: useful in wheat flour where higher ash content in bran and endosperm may not be good, also in raw sugar where there are specifications. Ash in sugar comes from the soil which has silicate, higher ash content in raw sugar means poorer quality
What are the steps of the ashing technique?
Sample --> preashing --> furnace --> residues
After incineration the sample should be grayish-whiteish (if all organic matter is combusted). Sometimes even at more than 530 degrees for 2 hours, the sampe does not get completely combusted (black spots). You don't want to go up to 550 degrees where some minerals may become volatile (such as lead or mercury)
What is preashing?
To remove some gases, smoke the sample initially
Does not come from furnace
Care required by analyst to prevent loss of sample
What are the non-combusted "black spots" in ashing?
Chlorides and phosphates fuse with salts (like Na and K) at high temperatures and trap organic material, rendering them unavailable for combustion. The organic material can be released by dissolving the sample/salt in a small amount of water
Black spots are quite common with high CHO/high sugar samples
What is the sulfate ash technique?
Sample + diluted H2SO4 (just a few drops) --> preash/ash --> grey/white residue (with no black spots)
Why are there no black spots in sulfate ashing?
Most of salt in residue is in metal sulfates - does not form fused salts
In these cases, "sulfated" ash determination is done
What is the dry ashing technique?
Ashing process is dry, meaning no solvent or liquid is used during the ashing process
Result gives total ash content
What is the wet ashing technique?
Remove all organic material via oxidation
Objective: obtain a solution with all the minerals, and then analyze them one by one
This provides a clear solution which has all minerals dissolved in it
Extremely strong oxidizing agents are used (i.e. concentration HNO3, H2O2 + heat are used if the oxidizing power needs to be increased)
Na, K, Ca, Fe, Cl, P, S contents can be measured by flame photometry, atomic absorption, or other techniques