SI-traceable certification of Cu, Cr, Cd and Pb in sediment and fly ash candidate reference materials

Abstract

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 micrograms g-1 for Pb, 0.5-3 micrograms g-1 for Cd and 50-70 micrograms g-1 for Cu. Cr was measured in one sample and was about 60 micrograms 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% (coverae factor k = 2) except for Cr, (measured by high resolution ICPMS), for which the expanded uncertainty was about 7% (k = 2).

New mathematical models with associated equations for isotope dilution mass spectrometry (IDMS)

“…Typical parameters influencing the K-factors are the measurement technique, the size of the isotope amount ratio concerned, bias, dead time correction, background and mass fractionation effects at various stages of the measurement process. Establishing the correct K-factors or even their ratio is crucial for the measurement process and usually difficult to achieve [16,17,18]. 4 Without explicit proof given here, the requirements for a vector space (i.e.…”

Environmental applications of single collector high resolution ICP-MS

“…A critical point in the analysis of soil and sediment samples is the sample preparation step which can largely influence the results obtained. [252][253][254][255][256][257][258][259][260] In other words, results from various studies are only comparable, when (almost) identical sample dissolution or leaching steps are applied for comparable matrices and elements investigated. For this reason, frequently ''acid-extractable'', i.e.…”

“…leachable with HNO 3 , HCl, aqua regia (HNO 3 + HCl) or other reagents, pseudo-total digestion procedures employing HNO 3 , HCl and HF preferably in a closed pressurized microwave oven or extraction with high purity CaCl 2 solutions were applied. 253,254,[256][257][258][259][260][261][262] Matrix effects are commonly reduced to a negligible extent by employing appropriate dilution of the digests, using adequate internal standards or applying matrix separation prior to analysis. 253,263 Soil solutions have been analysed in largely different studies for (i) As species using hyphenated techniques, 264 (ii) Ge/Si ratios to assess the relative contributions of weathering and biogenic silica cycling 261 and (iii) total and EDTA extractable metal distributions in the colloidal fraction of contaminated soil employing sedimentation field-flow fractionation coupled online to HR-ICP-MS. 262 There has also been considerable interest in the determination of U and Pu in soil samples for both concentrations and isotope ratios.…”

Inductively coupled plasma- and glow discharge plasma-sector field mass spectrometry : Part I. Tutorial: Fundamentals and instrumentation

“…In case of isotope dilution mass spectrometry, it is preferable if the isotopic composition of the standard matches the isotopic composition of the blend, which is not always achievable. 156 As an example for external correction, 87 Sr/ 86 Sr isotope ratios can be corrected for externally by using the NIST SRM 987 SrCO 3 with a certified isotopic composition of 87 Sr/ 86 Sr ¼ 0.71034 AE 0.00026 (National Institute of Standards and Technology, Gaithersburg, USA). 151 A value of 0.710245 has been used by the geological community.…”