Conversion of a solid matrix into a liquid matrix involves the decomposition
of the sample. One of the major problems in preparing solid samples for
trace element analysis is the potential risk of contamination. Contamination
can arise from several sources: the grade of reagents used; the vessels used for
digestion and the subsequent dilution of the sample; and human involvement.
In order to minimize the risk of contamination you should take the
- Use the highest purity of reagents and acids, including the water used for
- Use sample blanks in the analytical procedure, to identify the base level
of impurity in the reagents.
Soak sample vessels in an acid leaching bath (e.g. 10% v]» nitric acid) for
at least 24 hours, followed by rinsing in copious amounts of ultrapure
- Store cleaned volumetric flasks with their stoppers inserted; cover beakers
with Clingfilm'P or store upside down to protect from dust.
- In addition to the wearing of a laboratory coat and safety glasses, it may
be necessary to wear 'contaminant'-free gloves and a close-fitting hat.
Decomposition involves the liberation of the analyte (metal) of interest from
an interfering matrix using a reagent (mineral/oxidizing acids or fusion flux)
and/or heat. An important aspect in the decomposition of an unknown
sample is the sample size (Box 27.4). You need to consider two aspects.
Firstly, the dilution factor required to convert the solid sample to an aqueous
solution (Box 27.5), and, secondly, the sensitivity of the analytical instrument,
This involves the use of mineral or oxidizing acids and an external heat source
to decompose the sample matrix.
The choice of an individual acid or
combination of acids depends upon the nature of the matrix to be decomposed.
For example, the digestion of a matrix containing silica, SiO2
(e.g. a geological
sample), requires the use of hydrofluoric acid (HF). A summary of the most
common acids used for digestion and their application is shown in Table 27.3.
Once you have chosen an appropriate acid, place your sample into an
appropriate vessel for the decomposition stage. Typical vessels include an open
glass beaker or boiling tube for conventional heating or for microwave heating,
a PTFE or Teflon®
PFA (perfluoroalkoxyvinylether) vessel. A typical microwave
system operates at 2.45 GHz with up to 14 sample vessels arranged on a
rotating carousel; commercial systems have additional features such as: a
PTFE-lined cavity; a safety vent (if the pressure inside a vessel is excessive the
vent will open, allowing the contents to go to waste); and an ability to measure
both the temperature and pressure inside the digestion vessels. The procedure
for acid digestion of a sample is shown in Box 27.7.
*All concentrated acids should be used only in a fume cupboard.
Other methods of sample decomposition
|Table 27. 3 Common acids* used for digestion
The use of acid(s) and heat is probably the most common approach to the
decomposition of samples. However, several alternatives exist including dry
ashing and fusion.
Dry ashing involves heating the sample in air in a muffle furnace at 400-
800°C to destroy the sample matrix, e.g. soil. After decomposition, the
sample residue is dissolved in acid and quantitatively transferred to a
volumetric flask prior to analysis. The method may lead to the loss of volatile
elements, e.g. Hg, As.
Some substances, such as silicates and oxides, are not always destroyed by
the direct action of acid and heat. In these situations an alternative approach
is required. Fusion involves the addition of a lO-fold excess of a suitable
reagent (e.g. lithium metaborate or tetraborate) to a finely ground sample.
The mixture is placed in a metal crucible, e.g. Pt, and then heated in a muffle
furnace at 90 – 1000°C. After heating (from several minutes to several hours)
a clear 'melt' should result, indicating completeness of the decomposition.
After cooling, the melt is dissolved in HF (Table 27.3). This process can lead
to a higher risk of contamination.