Many fields in environmental analytical chemistry deal with very
low limits and thresholds as set by governmental legislations or transnational
regulations. The need for the accuracy, comparability and traceability of
analytical measurements in environmental analytical chemistry has significantly
increased and total uncertainties are even asked for by accreditation bodies
of environmental laboratories. This paper addresses achieving these goals
to guarantee accuracy, quality control, quality assurance or validation of
a method by means of certified reference materials. The assessment of analytical
results in certified reference materials must be as accurate as possible and
every single step has to be fully evaluated. This paper presents the SI-traceable
certification of Cu, Cr, Cd and Pb contents in geological and environmentally
relevant matrices (three sediments and one fly ash sample). Certification
was achieved using isotope dilution (ID) ICPMS as a primary method of measurement.
In order to reduce significantly the number of analytical steps and intermediate
samples a multiple spiking approach was developed. The full methodology is
documented and total uncertainty budgets are calculated for all certified
values. A non-element specific sample digestion process was optimised.
All wet chemical digestion methods examined resulted in a more or less pronounced
amount of precipitate. It is demonstrated that these precipitates originate
mainly from secondary formation of fluorides (essentially CaF2)
and that their formation takes place after isotopic equilibration. The contribution
to the total uncertainty of the final values resulting from the formation
of such precipitates was in general <0.1% for all investigated elements.
Other sources of uncertainty scrutinised included the moisture content determination,
procedural blank determination, cross-contamination from the different
spike materials, correction for spectral interferences, instrumental background
and deadtime effects, as well as the use of either certified values or IUPAC
data in the IDMS equation. The average elemental content in the sediment samples
was 30–130 µg g−1 for Pb, 0.5–3 µg g−1
for Cd and 50–70 µg g−1 for Cu.
Cr was measured in one sample and was about 60 µg g−1.
The concentrations in the fly ash sample were up to 2 orders of magnitude
higher. Expanded uncertainty for the investigated elements was about 3% (coverage
factor k = 2) except for Cr, (measured by high resolution
ICPMS), for which the expanded uncertainty was about 7% (k = 2).
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