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A comparison of high accuracy methods for the analysis of total sulfur in gas oils by three metrology institutes is described. The work contains a comparison of three different aspects of the analysis: sample digestion, instrumental measurement and isotope dilution strategies.
Thermal ionisation mass spectrometry (TIMS) in combination with high pressure asher (HPA) or Carius tube digestion has traditionally been used for such high accuracy analysis. However, this paper demonstrates that the faster, less laborious technique of ICP-MS in combination with microwave digestion is equally capable. Results from the comparison experiments show excellent agreement for fuel samples containing sulfur concentrations in the range 11200 µg g-1. This excellent agreement across the different sample preparation, measurement and isotope dilution techniques has been exploited in the certification of two new diesel fuel reference materials.
Phosphorus recycling from sewage sludge will be obligatory in Germany from 2029. Phosphorus content determination in sewage sludge is crucial to assess the prescribed recycling rates. Currently, German law regards sample preparation using aqua regia digestion in a microwave or under reflux conditions as well as instrumental phosphorus determination by ICP-OES, ICP-MS, or photometric determination with ammonium molybdate as equivalent. However, it is questionable whether these methods are indeed equivalent regarding phosphorus quantification in sludges near the limit of 20 g/kg for mandatory recycling. To answer this question, 15 sewage sludges of 11 different wastewater treatment plants were investigated with all permitted method (digestion and measurement) combinations. Moreover, one sewage sludge was also examined in an interlaboratory comparison (ILC) with 28 participants. This study shows that the above-mentioned methods differ in some cases significantly but across all method combinations and sludges, phosphorus recovery was between 80 and 121% after normalization to the grand mean (average of 15 sludges between 85 and 111%). The ILC and the examination of 15 sludges produced largely similar results. There is a tendency to higher phosphorus recovery after microwave digestion compared to reflux digestion and ICP-OES measurements determine higher phosphorus contents than ICP-MS and photometric phosphorus determination. As a result, the authors recommend ICP-OES determination of phosphorus in sewage sludge after microwave digestion.
The present paper describes the certification of the amount content of Cd, Cr and Pb in two different polyethylene materials within the third phase of the Polyethylene Elemental Reference Material (PERM) project. The analytical procedure to establish the reference values for Cd, Cr and Pb amount contents in these materials is based on isotope dilution mass spectrometry used as a primary method of measurement. Cd and Pb were measured with inductively coupled plasma-mass spectrometry and Cr with positive thermal ionization-mass spectrometry. The decomposition of the polymer matrix was carried out using a high pressure asher. Reference values for amount content, traceable to the SI-system, have been obtained for these three elements in both of the polyethylene samples of PERM. For each of the certified amount content values an uncertainty budget was calculated using the method of propagation of uncertainties according to ISO and EURACHEM guidelines. The measurement procedures, as well as the uncertainty calculations, are described for all three elements. In order to keep the whole certification process as transparent as possible, the preparations of various reagents and materials as well as the sample treatment and blending are described in detail. The mass spectrometry measurements and the data treatment are also explained carefully. The various sources of uncertainty present in the procedure are displayed in the uncertainty budgets. The obtained combined uncertainties for the amount content values were less than 2% relative (k=1) for all investigated elements. The amount contents were in the ?mol/kg range, corresponding to mg/kg levels.