Reliable Measurements 14. Ashing By Paul Gaines, Ph.D. • Edited by Brian Brolin
Ashing
in analytical chemistry is defined as the heating of a substance to
leave only noncombustible ash, which is analyzed for it's elemental
composition.
The sample preparation techniques incorporating some form of 'ashing' are as follows:
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.
Sulfated Ashing
involves treatment of the sample after charring using an open flame
with sulfuric 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 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 involves treatment of the sample at ~ 120 �C using activated (singlet state) oxygen.
Closed System Ashing involves thermal decomposition in oxygen in a closed system such as a Sch�niger flask or an oxygen Parr bomb.
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 sulfur, fluorine and chlorine (the Sch�niger 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.
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.
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 Sulfated 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 sulfated ash is used over dry ashing
when the analyst needs to fix a material as the sulfate to prevent
volatilization 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
- Sulfated 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
sulfur 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.
The 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 analyzed by ICPAES (inductively
coupled plasma atomic emission spectroscopy).
The 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.
The 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.
Consult the elemental profiles found in our Analytical Periodic Table for additional ashing procedures.
