Samples Containing Lithium, Sodium, Potassium, Rubidium or Cesium

Organic Matrices

This includes a wide variety of materials including fertilizers, agricultural material, organic plant material, biological material, synthetic organics, etc.

The crucible can be a variety of materials including Pt, quartz, fused silica, porcelain, or glassy carbon. Samples high in Li or K will attack Pt and all of the alkali metal oxides attack silica, quartz and the silica glaze on porcelain. It is always suggested that the sample be treated with sulfuric acid either before charring or just after charring and before muffling, which eliminates alkali oxide formation during muffling and consequently alkali attack on the crucible. It is best to use a minimum of sulfuric since the goal is to encourage sulfate formation but rid the sample of all excess sulfuric acid before placing in a muffle furnace, otherwise the formation of SO₃ will contaminate the air. Muffling (ashing) temperatures of 450 to 500 °C are suggested. The alkali sulfates can tolerate higher temperatures, but there is no need to push limits. Temperatures approaching 700 °C risk formation of the alkali oxide (decomposition is catalyzed by certain elements/compounds) with release of SO₃ and serves no real purpose.

The following is a general outline for performing a wet ash or sulfated ash:

Wet Ash

• Weigh the sample into your crucible (typically porcelain) and then wet the sample with concentrated sulfuric acid. Only add enough sulfuric acid to just wet the sample. Excess sulfuric acid will require extra time in fuming off before placing in a muffle furnace to achieve ashing temperatures.
• Place the sample on a hot plate and heat until the fumes of sulfuric acid (dense white fumes) stop. Proceed with caution: sulfur oxides are very irritating to the respiratory system and the elimination of all excess sulfuric acid as well as muffling in a ventilated area is important. If the sample is an oil there will be foaming - be on guard and only fill the crucible to 10 % of capacity (for a 60 mL crucible add only ~6 grams of sample and 0.5 grams of sulfuric acid).
• Muffle the sulfated char at 450 °C overnight or until the residue appears free of carbonaceous material. Carbonaceous material is typically signaled by the formation of coke-like material that is black in color.
• The appearance of the ash should lighten and will approach white to tan over time. If certain transition elements are present, the color may appear to get darker with time due to the formation of the transition metal oxides (Ni and Co oxides are very dark). If this occurs, the sample will need to be "resulfated" - cooled and wetted with sulfuric acid - in order to avoid the difficulties encountered with the dissolution of ignited transition metal oxides.

Sulfate Ash

• Weigh the sample in the crucible and char it on a hot plate. The samples will turn/appear as carbon black or coke.
• After charring, wet the char with sulfuric acid. This typically takes only 1-3 drops of the concentrated acid. Continue to heat the sulfated char until fumes of sulfuric acid cease to evolve and then muffle at 450 °C overnight.

Consult the Ashing portion of our Trace Analysis Guide for more information on ashing techniques.

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