Trace Analysis Guide - Ashing Procedures

Overview

Ashing in analytical chemistry is defined as the heating of a substance to leave only non-combustible ash, which is analysed for it's elemental composition.

Ashing Techniques

The sample preparation techniques incorporating some form of 'ashing' are as follows:

Dry Ashing

Dry Ashing is usually performed by placing the sample in an open inert vessel and destroying the combustible (organic) portion of the sample by thermal decomposition using a muffle furnace. Typical ashing temperatures are 450 to 550 °C. Magnesium nitrate is commonly used as an ashing aid. Charring the sample prior to muffling is preferred. Charring is accomplished using an open flame.

Sulphated Ashing

Sulphated Ashing involves treatment of the sample after charring using an open flame with sulphuric acid (the char is wetted using the minimum amount of sulfuric acid and then brought to dryness before muffling) and then placing in a muffle furnace.

Wet Ashing

Wet Ashing is treatment of the sample with a moderate amount of sulfuric acid before charring. Charring is performed using an open flame. Liquid samples tend to foam. After the excess sulfuric acid is driven off, the sample is muffled as above.

Low-temperature Ashing

Low-temperature Ashing involves treatment of the sample at ~ 120 °C using activated (singlet state) oxygen.

Closed System Ashing

Closed System Ashing involves thermal decomposition in oxygen in a closed system such as a Schöniger flask or an oxygen Parr bomb.

Advantages of Ashing

Ashing techniques are understandably used only for samples containing a significant amount of combustible or organic material as the matrix. With this in mind, let's look at the major advantages of ashing:

• The ability to decompose large sample sizes.
• The need for little or no reagents.
• The technique is relatively safe.
• The ability to prepare samples containing volatile combustion elements such as sulphur, fluorine and chlorine (the Schöniger oxygen flask combustion technique is very popular in this case).
• The technique lends itself to mass production.

The technique of graphite furnace atomic absorption spectrometry (GFAA) incorporates sample ashing as part of an automatic measurement cycle. Trace analysts have learned a great deal about the loss of volatile components during ashing due to the ease with which the analyte signals can be compared to different ashing temperatures, times, addition of ashing aids, and other conditions. Typically, graphite carbon is the container material that the sample comes in contact with used when using GFAA.

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 volatilisation.
• 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 sulphated) 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.

Examples of Ashing Procedures

Consult the elemental profiles found in our Interactive Periodic Table for additional ashing procedures.

General Dry Ashing Procedure

This procedure is used for a wide variety of sample types, which include organic polymers, natural products such as agricultural materials, biological materials, petroleum products and synthetic organic research materials. The laboratory supervisor should be consulted with new or unfamiliar sample types to determine if this method is appropriate.
Procedure: Dry ashing procedures are typically and preferentially performed in Pt° crucibles. Glassy carbon can be used but some attack may occur. Nickel and iron can also be used but the metal from the crucible can cause significant spectral interference. A sample size ranging from a few milligrams to 100 grams is weighed into the crucible. The crucible is placed on a hot plate and set on the highest setting. Do this in a Class-A hood due to highly toxic fumes. The use of a propane torch is helpful in speeding up the process and is necessary for the ignition of certain sample types such as polyethylene. As soon as fumes cease to evolve, the sample is placed in a muffle furnace at 450 - 500 °Celsius for one hour or until all of the carbon has been oxidized.

General Sulphated Ashing Procedure

This procedure is used for a wide variety of sample types which include organic polymers, natural products such as agricultural materials, biological materials, petroleum products and synthetic organic research materials. The sulphated ash is used over dry ashing when the analyst needs to fix a material as the sulphate to prevent volatilisation otherwise it has no real advantages over dry ashing. The laboratory supervisor should be consulted with new or unfamiliar sample types to determine if this method is appropriate.
Procedure: Sulphated ashing procedures are typically and preferentially performed in Pt crucibles. Glassy carbon can be used but some attack may occur. Nickel and iron can also be used but the metal from the crucible can cause significant spectral interference. A sample size ranging from a few milligrams to 100 grams is weighed into the crucible. The crucible is placed on a hot plate and set on the highest setting. Do this in a Class-A hood due to highly toxic fumes. The use of a propane torch is helpful in speeding up the process and is necessary for the ignition of certain sample types such as polyethylene. As soon as fumes cease to evolve, wet the char with concentrated sulfuric acid. Typically a few drops are required. Continue to heat the sample until the white dense sulphur trioxide fumes cease to evolve. The sample is placed in a muffle furnace at 450 - 500 °Celsius for one hour or until all of the carbon has been oxidized.

Determination of Pd and Ca in Organic Matrices

The sample is thermally decomposed to an ash in a quartz crucible. The ash is then dissolved with a mixture of nitric and hydrochloric acids. The resulting sample should be analysed by ICPAES (inductively coupled plasma atomic emission spectroscopy).

Determination of Cu in Grain

Up to 10 grams of sample is weighed to an accuracy of 0.1 mg into a porcelain crucible. The sample is charred using a propane or natural gas burner. The charred sample is then place in a muffle furnace at 500 °Celsius until all evidence of carbon is gone. The fully ashed sample gives a white ash. The ash is dissolved by adding 2 mL of water followed by 1 mL of concentrated nitric and 1 mL of concentrated hydrochloric acids. Gently warm to speed up the dissolution of the ash. The dissolved ash solution is then brought to 10 mL. Yttrium is used as the internal standard. The copper is measured by ICPAES. Note the use of nitric acid to dissolve possible Cu° that is likely to form during ashing.

Determination of Pb, Zn, & Al in Organic Material Containing only C, H, & O

The sample is placed in a Pt crucible at 450 °C until all of the carbon is removed as evidenced by a white ash. The ashing time is typically 2 hours. The ash is dissolved in 10 mL of 1:1 water/nitric acid by warming on a hot plate for ~1 hour.