Disadvantages of Ashing
The trace analyst should be very familiar with their sample type before performing an ash. Some of the problems that have been encountered are listed as follows:
Losses due to retention to the ashing container.
Losses due to volatilization.
Contamination from the ashing container.
Contamination from the muffle furnace.
Physical loss of 'low density' ashes when the muffle door is opened (air currents).
Difficulty in dissolving certain metal oxides.
Formation of toxic gases in poorly ventilated areas. (Note that all charring should take place in a hood and the muffle furnace must have a hood canopy for proper ventilation).
Common Problems:
If the sample type is unknown (with respect to the matrix) then a % ash, EDXRF scan, IR scan, and C, H, and N analysis will provide sufficient information in most cases to make informed decisions. The following are common problems that can be adverted with one or more of the above preliminary analyses:
Protect your Pt° ware by looking for P (high levels will attack and attach to the Pt°) and elements that alloy with Pt° which include the precious metals, Cu, and Hg.
When using 'silica' containing crucibles (porcelain, Vycor, quartz, glass, and fused silica) look for elements that form basic oxides such as the alkali earth elements. Na is commonly found and it's oxide will form (unless the char is sulfated) and attack the silica.
Look for volatile elements (Cd, B, Hg, Pb, Se, Zn, As, Sn, Sb, S, and halogens) especially if moderate to large amounts of F or Cl are present.
Si is a common element that is typically determined by dissolution of an ash performed in Pt°. Methyl silicones are widely used and very common. If Si is present as a silicone oil then it will be partially lost as the hexamethycyclotrisiloxane and the hexamethydisiloxane.
Retention and physical loss of analyte(s). The use of high purity Mg(NO3)2 as an ashing aid will help prevent losses of 'low density' ashes, and will help in preventing retention losses.
Difficult to dissolve oxides. Use as low an ashing temperature as possible (400 to 550 °C maximum). Look for Ti, Zr, Nb, Hf, Ta, W, Ni, Co, Fe, Cr, Sb, and Mo. The type of crucible material will determine the treatment that the ash can undergo. Pt° is not attacked by HF which will dissolve several of the above oxides.
Loss due to reduction to the metal. Look for easily reduced elements such as Cu and the precious metals. Use the appropriate crucible material to allow for the necessary dissolution reagents for the metal.