3051A - 16 Revision 1
February 2007
are counted as sample vessels. When fewer than the recommended number of
samples are to be digested, the remaining vessels should be filled with the same
acid mixture to achieve the full complement of vessels. This provides an energy
balance, since the microwave power absorbed is proportional to the total absorbing
mass in the cavity. Irradiate each group of vessels using the predetermined
calibration settings. (Different vessel types should not be mixed.)
11.3.6 At the end of the microwave program, allow the vessels to cool for a
minimum of 5 min before removing them from the microwave system. Cooling of the
vessels may be accelerated by internal or external cooling devices. When the vessels
have cooled to near room temperature, determine if the microwave vessels have
maintained their seal throughout the digestion. Due to the wide variability of vessel
designs, a single procedure is not appropriate. For vessels that are sealed as discrete
separate entities, the vessel weight may be taken before and after digestion to evaluate
seal integrity. If the weight loss of sample exceeds 1% of the weight of the sample and
reagents, then the sample is considered compromised. For vessels with burst disks, a
careful visual inspection of the disk, in addition to weighing, may identify compromised
vessels. For vessels with resealing pressure relief mechanisms, an auditory or a physical
sign that can indicate whether a vessel has vented is appropriate.
11.3.7 Complete the preparation of the sample by venting microwave containers
in a fume hood before uncapping, so as to avoid a rush of acid vapor that may still be in
the headspace. When sufficiently cool to handle, carefully uncap the vessels, using the
procedure recommended by the vessel manufacturer. Quantitatively transfer the sample
to an acid-cleaned bottle. If the digested sample contains particulates which may clog
nebulizers or interfere with injection of the sample into the instrument, the sample may be
centrifuged (Sec. 11.3.7.1), allowed to settle (Sec. 11.3.7.2), or filtered (Sec. 11.3.7.3).
11.3.7.1 Centrifugation -- Centrifugation at 2,000 - 3,000 rpm for 10 min
is usually sufficient to clear the supernatant.
11.3.7.2 Settling -- If undissolved material, such as SiO
2
, TiO
2
, or other
refractory oxides, remains, allow the sample to stand until the supernatant is clear.
Allowing a sample to stand overnight will usually accomplish this. If it does not,
centrifuge or filter the sample.
11.3.7.3 Filtering -- If necessary, the filtering apparatus must be
thoroughly cleaned and pre-rinsed with dilute (approximately 10% V/V) nitric acid.
Filter the sample through qualitative filter paper into a second acid-cleaned
container.
11.3.8 The removal or reduction of the quantity of the nitric and hydrochloric
acids prior to analysis may be desirable. The chemistry and volatility of the analytes of
interest should be considered and evaluated when using this alternative (Ref. 3).
Evaporation to near dryness in a controlled environment with controlled purge gas and
neutralizing and collection of exhaust interactions is an alternative where appropriate.
This manipulation may be performed in the microwave system, if the system is capable of
this function, or external to the microwave system in more common apparatus(s). This
option must be tested and validated to determine analyte retention and loss and should be
accompanied by equipment validation possibly using the standard addition method and
standard reference materials. This alternative may be used to alter either the acid
concentration and/or acid composition prior to analysis. (For further information, see Ref.
3 and Method 3052).