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4.2.1 Appropriate Sample Size
4.2.2 Proper Digestion Solution Temperature
4.2.3 Sufficient Sulfuric Acid
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A general procedure, incorporating application specific requirements, is
provided in this section of the manual. To ensure complete sample
digestion, consider the variables described in the following paragraphs.
For solid or organic liquid samples, less than 0.5 grams of anhydrous
material can usually be digested effectively. (As a routine practice, 0.25 g
of sample is used.) Samples that contain water may be scaled up by a
proportional amount. There is no restriction or minimum sample size. For
samples of aqueous solutions or suspensions, the maximum volume is 40
mL. When the percent solids exceeds 1% of the sample volume, the
maximum sample volume should be reduced using the formula:
40
sample
=
------------------- -
% solids
Digestion temperature is critical. If the hydrogen peroxide is added to a
cold digestion mixture followed by heating, the hydrogen peroxide
decomposes before the digest reaches the proper temperature. Addition of
hydrogen peroxide to a digest that is too hot volatilizes most of the
oxidant with little benefit. Also, excessive heat contributes to spray loss
of sample as a fine mist. The temperature recommended for most samples
is 440 ° C (825 ° F). For oils, fuels and lubricants, the temperature may
need to be decreased.
The amount of concentrated (specific gravity 1.84) sulfuric acid used
must be sufficient to prevent the digestion from going to dryness. Any
portion of the flask bottom that becomes dry will overheat and may cause
the flask to explode. Also, too little acid will cause the sample to
overheat, which may cause thermal decomposition of desired analytes
(i.e., ammonium compounds) and result in sample loss. Use an amount of
sulfuric acid that will leave at least 2 mL of residual acid when digestion
is complete. Refer to Table 1 for recommended volumes.
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