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For the first time polyethylene (PE) frits were used to quantify sulphur in copper metal and its alloys by isotope dilution combined with LA-ICP-MS: an alternative approach for sample preparation. The properties of the PE frit meet the requirements for isotope dilution LA-ICPMS which are porous material, thermal and chemical resistance and high absorption efficiency. The breakthrough, however, as a support material, is the low sulphur blank, which is only two times of the gas flow blank (2.3-4.0 x 10⁴ cps). Additionally, the porosity of the frit was considered, as it directly affects the absorption efficiency for the sample solution, which is present in the cavities of the frit.
The absorption efficiency was studied by loading sulphur standards with varying sulphur amounts (0 - 80 µg S) onto the frits. The remaining sulphur which was not absorbed by the frit was rinsed off and was measured by ICP-MS. This indirect method shows that more than 99.5 % of the loaded sulphur was absorbed by the frit. The so prepared frits with increasing sulphur amount were measured by LA-ICP-MS showing a good linearity between 0 µg S and 40 µg S with a coefficient of determination, r2 of 0.9987 and a sensitivity of 3.4x10⁴ cps µgˉ¹ for 32S.
Three copper reference materials produced by BAM (BAM-M376a, BAM-228 and BAM-227) were selected to develop and validate the LA-ICP-IDMS procedure. The IDMS technique was applied to these samples as follows: the samples were spiked, dissolved, digested and then the digested solution was absorbed on the frits. The dried frit samples were then analyzed by LA-ICP-IDMS and it could be demonstrated that the sample solution dispersed on the frits did not influence the 32S/34S ratio significantly even though the sulphur intensities were fluctuating along the scan lines. Relative standard deviations of the isotope ratios were below 5 % in average between 3 lines (except for the pure spike solution and procedure blank). The measurement results were validated by comparing them with the results obtained by conventional ICP-IDMS. Plotting the mass fraction of sulphur in copper obtained by LA-ICP-IDMS versus those obtained by ICP-IDMS yields a linear curve with a correlation coefficient of 0.9999 showing a strong agreement between both techniques.
The metrological traceability to the SI for the mass fraction of sulphur in copper is established by an unbroken chain of comparisons, each accompanied by an uncertainty budget. Thus, the measurement results are considered reliable, acceptable and comparable within the stated measurement uncertainty. The metrological traceability chain from the kg down to the final mass fraction in the samples obtained by LA-ICP-IDMS is illustrated in this presentation.
Polyethylene (PE) frits were used to quantify sulphur in copper and its alloys by isotope dilution combined with LA-ICP-MS as an alternative approach to conventional sample preparation: the copper samples were spiked, the spiked samples were dissolved, the resulting solutions were absorbed in the PE frits and finally the PE frits were analysed by LA-ICP-MS. A prerequisite for such a support material is a low sulphur blank and thus PE was selected for this purpose. The absorption efficiency of the PE frits was studied for varying sulphur amounts ranging from 2 mg S to 80 mg S showing that more than 99.5% of the loaded sulphur was absorbed by the frit. The so prepared PE frits were measured by LA-ICP-MS and yielded a good linearity (R2 ¼ 0.999) for the sulphur ion intensities corresponding to sulphur amounts up to 40 mg S; the associated sensitivity is approximately 3.4 x 10⁴ cps μg⁻¹ for ³²S. For the validation of the developed procedure the reference materials BAM-M376a, BAM-228 and BAM-227 were applied such that 2 μg S, 5 μg S and 11 μg S were absorbed in the PE frits, respectively. These samples were pre-quantified for the adsorbed sulphur amount by external calibration LA-ICP-MS yielding sulphur amounts of 0.9 μg, 5.1 μg and 8.5 μg (quantified for ³²S only), respectively. Relative Standard deviations of the isotope ratios were below 5% in average (n ¼ 3 lines) in all cases (except for the pure spike solution). These samples were then analysed by LA-ICP-IDMS and the measurement results were validated by comparing them with the results obtained by conventional ICP-IDMS. The obtained relative expanded measurement uncertainties ranged between 10% and 26%. Pearson's coefficient was used to express the correlation between both techniques; the obtained value was 0.999 demonstrating a strong correlation. Contrary to most published LA-ICP-IDMS procedures, the developed procedure enables SI-traceability for the measurement results. The metrological traceability to the SI for the sulphur mass fractions in copper was established by an unbroken chain of comparisons, each accompanied by an uncertainty budget. Thus, the measurement results are considered reliable, acceptable and comparable within the stated measurement uncertainty. The metrological traceability chain from the kg down to mass fraction in the samples obtained by LA-ICP-IDMS is presented as well.
This is the first time that PE frits were used to quantify sulfur in copper and its alloys by isotope dilution combined with LA-ICP-MS: an alternative approach for sample preparation. The following properties of the PE frit meet the requirements for isotope dilution LA-ICPMS:: porous material, thermo plastic (melting point >100oC), chemical resistance (nitric acid >70%) and high adsorption efficiency. The breakthrough, however, as a support material, especially when comparing the PE frit with other materials such gelatin or sodium silicate is the low blank, which is only two times of the gas flow blank (2.3-4.0 x 104 cps). Additionally, the porosity of the frit was considered, as it directly affects the adsorption efficiency for the sample solution, which is present in the cavities of the frit.
Adsorption efficiency was studied by depositing sulfur standards with varying sulfur amounts (0, 2, 5, 10, 20, 40 and 80 µg S) on the frits. The remaining sulfur which was not absorbed by the frit was rinsed off and was measured by ICP-MS. This indirect method shows that more than 99.5 % of the loaded sulfur was absorbed by the frit. Such high absorption efficiency is completely sufficient for a support material to be used in LA-ICP-IDMS. The so prepared frits whith increasing sulfur amount were measured by LA-ICP-MS showing a good linearity between 0 µg S and 40 µg S whit a correlation coefficient r2 of 0.9987 and sensitivy of 3.4x104 cpsµg-1 for 32S.
Three copper reference materials produced by BAM were selected to develop and validate the LA-ICP-IDMS procedure. The IDMS technique was applied to these samples as follows: the samples were spiked, dissolved, digested and then the digest was adsorbed on the frits.
This is the first time that PE frits were used to quantify sulfur in copper and its alloys by isotope dilution combined with LA-ICP-MS: an alternative approach for sample preparation. The following properties of the PE frit meet the requirements for isotope dilution LA-ICPMS: porous material, thermo plastic (melting point >100°C), chemical resistance (nitric acid >70%) and high adsorption efficiency. The breakthrough, however, as a support material, especially when comparing the PE frit with other materials such gelatin or sodium silicate is the low blank, which is only two times of the gas flow blank (2.3-4.0 x 10⁴ cps). Additionally, the porosity of the frit was considered, as it directly affects the adsorption efficiency for the sample solution, which is present in the cavities of the frit.
Adsorption efficiency was studied by depositing sulfur standards with varying sulfur amounts (0, 2, 5, 10, 20, 40 and 80 µg S) on the frits. The remaining sulfur which was not absorbed by the frit was rinsed off and was measured by ICP-MS. This indirect method shows that more than 99.5 % of the loaded sulfur was absorbed by the frit. Such high absorption efficiency is completely sufficient for a support material to be used in LA-ICP-IDMS. The so prepared frits with increasing sulfur amount were measured by LA-ICP-MS showing a good linearity between 0 µg S and 40 µg S with a correlation coefficient r2 of 0.9987 and sensitivity of 3.4x10⁴ cps µgˉ¹ for 32S.
Three copper reference materials produced by BAM were selected to develop and validate the LA-ICP-IDMS procedure. The IDMS technique was applied to these samples as follows: the samples were spiked, dissolved, digested and then the digest was adsorbed on the frits.
A new method combining isotope dilution mass spectrometry (IDMS) and standard addition has been developed to determine the mass fractions w of different elements in complex matrices: (a) silicon in aqueous tetramethylammonium hydroxide (TMAH), (b) sulfur in biodiesel fuel, and (c) iron bound to transferrin in human serum. All measurements were carried out using inductively coupled plasma mass spectrometry (ICP–MS). The method requires the gravimetric preparation of several blends (bi)—each consisting of roughly the same masses (mx,i) of the sample solution (x) and my,i of a spike solution (y) plus different masses (mz,i) of a reference solution (z).
Only these masses and the isotope ratios (Rb,i) in the blends and reference and spike solutions have to be measured. The derivation of the underlying equations based on linear regression is presented and compared to a related concept reported by Pagliano and Meija. The uncertainties achievable, e.g., in the case of the Si blank in extremely pure TMAH of urel (w(Si)) = 90% (linear regression method, this work) and urel (w(Si)) = 150% (the method reported by Pagliano and Meija) seem to suggest better applicability of the new method in practical use due to the higher robustness of regression analysis.
Previously on sulfur determination in metal revealed a lack of traceability and inconsistent results. Solving the problems a reference procedure for sulfur measurement in metal are required to build up a reliable reference value. In this study a procedure was developed for quantification of total sulfur at low concentration (in sub ppm level) in metal using inductively coupled plasma-isotope dilution mass spectrometry (ICP-IDMS). The ion exchange method and complexing agent were applied in this procedure to avoid loading large amount of metal into the instrument. Adding ammonia as a complexing agent into sample solution to reduce sulfur-metal co-elute. The procedure shows high performance and it is expressed in % recovery of sulfur (> 90%) and % metal elimination (>99 %). Additionally, relative measurement uncertainties were calculated less than 1.5 % and the results are traceable directly to SI units. This study would establish as reference procedure for sulfur measurement in metal sample which fit for these purpose as follows; for certified reference material and assigned value for inter-laboratory comparison.
Previously on sulfur determination in metal revealed a lack of traceability and inconsistent results. Solving the problems a reference procedure for sulfur measurement in metal are required to build up a reliable reference value. In this study a procedure was developed for quantification of total sulfur at low concentration (in sub ppm level) in metal using inductively coupled plasma-isotope dilution mass spectrometry (ICP-IDMS). The ion exchange method and complexing agent were applied in this procedure to avoid loading large amount of metal into the instrument. Adding ammonia as a complexing agent into sample solution to reduce sulfur-metal co-elute. The procedure shows high performance and it is expressed in % recovery of sulfur (> 90%) and % metal elimination (>99 %). Additionally, relative measurement uncertainties were calculated less than 1.5 % and the results are traceable directly to SI units. This study would establish as reference procedure for sulfur measurement in metal sample which fit for these purpose as follows; for certified reference material and assigned value for inter-laboratory comparison.
The quantification of the exact amount of sulphur is a big challenge due to a lack of SI-traceability and inconsistent results, when different methods are compared. Therefore, a reference procedure is required which allows SI-traceable values. In this work three procedures were developed for the quantification of the total sulphur amount in biodiesel by using inductively coupled plasma-isotope dilution mass spectrometry (ICP-IDMS), pure copper metals and copper alloys by ICP-IDMS and external calibration for GDMS and LA-ICP-MS at low concentration levels.
The most critical parts of the sulphur quantification were sulphur purification and pre-concentration. Sulphur-matrix separation procedures were developed to serve both sample types. For biodiesel samples the sulphur was purified and matrix separated by an anion exchange chromatographic procedure. The analytical procedure was fully validated by the use of a certified reference material, a step-by-step validation and an inter-laboratory comparison at CCQM key comparison level.
In the case of copper samples, the copper matrix was separated from sulphur by adding ammonia which forms a complex with the copper while releasing the sulphur prior to a chromatographic separation using a weak cation resin. After that the sulphur fraction was further purified by chromatographic means using first an anion ion exchange method and second a chelating resin. The method was validated by appropriate certified reference materials. The developed procedures enable sulphur measurements at the low g·g-1 level with sufficiently low measurement uncertainties (< 2 %, Urel).
The external calibration was performed to produce reliable measurement results for the routine analytical techniques GDMS and LA-ICP-MS. Matrix-matched reference materials whith exactly known amount of sulphur obtained by ICP-IDMS beforehand, were used as calibrators to quantify sulphur in copper samples. The metrological traceability to the SI for the mass fraction of sulphur is established for all presented procedures by an unbroken chain of comparisons, each accompanied by an uncertainty budget.
Sulfur is one of the major impurity elements in copper. Previously applied methods for the quantification of sulfur in copper and other pure metals revealed a lack of traceability and showed inconsistent result. Therefore, in this study a procedure was developed for the quantification of total sulfur in copper at low concentration levels using inductively coupled plasma-isotope dilution mass spectrometry (ICP-IDMS). A major challenge for the quantification of sulfur in copper (alloyed/unalloyed) by ICPMS is the copper matrix itself, causing matrix effects and making an extensive cleaning (cones, extraction lens) necessary after measurements. Matschat et al investigated the analysis of high-purity metals (including copper) by high resolution ICP-MS and found that the copper matrix shows strong matrix effects on the sensitivity resulting from Cu deposition on the cones. Therefore, the major part of the copper matrix has to be separated, which was accomplished by adding ammonia which forms a complex with the copper while releasing the sulfur. This was followed by a chromatographic separation using a weak cation resin. After that the sulfur fraction was further purified by chromatographic means using an anion exchange method followed by a chelating resin.
The anion exchange resin (AG1X8), however, is selective to sulfate and sulfite but less-selective to sulfide. Therefore, when quantifying total sulfur in copper, the different species of sulfur need to be oxidized to sulfate prior to the sulfur-matrix separation on the AG1X8 resin in order to avoid any measurement bias. When applying the HPA oxidation with concentrated HNO3 and H2O2 a complete conversion from sulfide and sulfite to sulfate could be achieved. The recovery of all investigated sulfur species is quantitative within measurement uncertainties. The copper samples investigated in this study contain copper in the range of 0.85-0.99 kg·kg-1 and zinc from <10 to 300 g·kg-1. Approximately 0.10-0.25 g of these samples were used to perform the sulfur-copper separation. After applying the complete three stage separation procedure the mass fractions of both elements were significantly reduced to below 400 ng·g-1 for copper and below 50 ng·g-1 for zinc, respectively. The developed procedure shows high performance, especially concerning high efficiency in matrix removal (> 99.999%) while keeping the recovery of sulfur above 80%.
The procedure blank was determined by IDMS as well and yielded values for the individual IDMS measurement sequences ranging from 3 ng to 53 ng. The average of these individual procedure blanks (n=22) was calculated and yielded a total procedure blank of 14 ng sulphur with standard deviation of 12 ng. The limit of detection (LOD, blank+3SD) calculated on this basis was 0.20 µg·g-1 while the limit of quantification (LOQ, blank+10SD) was 0.54 µg·g-1, when considering a sample weight of 0.25 g.
The quantification of low sulfur contents (< 15 µg/g) by conventional IDMS is hindered by the very high Cu/S ratio, which clearly affects the separation in a negative way: The recovery of sulfur dropped to about 30 % for four replicates, while two further replicates even showed recoveries below 10%. To enable measurement without completely changing the separation procedure, an exact amount of sulfur was added prior to spiking, such that the sulfur mass fraction was shifted to the optimum working range of the separation procedure. Thus exact amounts of sulfur were added to enhance the mass fraction of sulfur from 15 µg·g-1 to 40 µg·g-1, then the IDMS analysis was performed as usual and finally the added sulfur amount was subtracted. The so obtained measurement result agreed well with the certified value within the uncertainties. The relative expanded measurement uncertainties for conventional IDMS are below 1%. When applying the modified IDMS procedure, where back-spike is added to the sample before spiking, the relative expanded measurement uncertainties are larger and up to 5%. With the presented sulfur-matrix procedure a working range from approximately 15 µg·g-1 to 1500 µg·g-1 can be achieved.
The developed procedure for the quantification of low sulfur amounts in copper has been validated here via three different routes: first an inter-laboratory comparison at highest metrological level, second a step-by-step validation by checking each single step of the procedure and third the setup of a complete uncertainty budget.
The procedure is sufficient to facilitate value assignment of total sulfur mass fraction in reference materials. Additionally, relative measurement uncertainties were calculated below 1 % and the measurement results are traceable to the SI, which is clearly demonstrated in this work. The procedure reported in this study is a new reference procedure for sulfur measurement in copper, well meeting the requirements of the two major purposes: the certification of reference materials and the assignment of reference values for inter-laboratory comparison.
Sulfur is one of the major impurity elements in copper. Previously applied methods for the quantification of sulfur in copper and other pure metals revealed a lack of traceability and showed inconsistent result. Therefore, in this study a procedure was developed for the quantification of total sulfur in copper at low concentration levels using inductively coupled plasma-isotope dilution mass spectrometry (ICP-IDMS). A major challenge for the quantification of sulfur in copper (alloyed/unalloyed) by ICPMS is the copper matrix itself, causing matrix effects and making an extensive cleaning (cones, extraction lens) necessary after measurements. Matschat et al investigated the analysis of high-purity metals (including copper) by high resolution ICP-MS and found that the copper matrix shows strong matrix effects on the sensitivity resulting from Cu deposition on the cones. Therefore, the major part of the copper matrix has to be separated, which was accomplished by adding ammonia which forms a complex with the copper while releasing the sulfur. This was followed by a chromatographic separation using a weak cation resin. After that the sulfur fraction was further purified by chromatographic means using an anion exchange method followed by a chelating resin.
The anion exchange resin (AG1X8), however, is selective to sulfate and sulfite but less-selective to sulfide. Therefore, when quantifying total sulfur in copper, the different species of sulfur need to be oxidized to sulfate prior to the sulfur-matrix separation on the AG1X8 resin in order to avoid any measurement bias. When applying the HPA oxidation with concentrated HNO3 and H2O2 a complete conversion from sulfide and sulfite to sulfate could be achieved. The recovery of all investigated sulfur species is quantitative within measurement uncertainties. The copper samples investigated in this study contain copper in the range of 0.85-0.99 kg·kg-1 and zinc from <10 to 300 g·kg-1. Approximately 0.10-0.25 g of these samples were used to perform the sulfur-copper separation. After applying the complete three stage separation procedure the mass fractions of both elements were significantly reduced to below 400 ng·g-1 for copper and below 50 ng·g-1 for zinc, respectively. The developed procedure shows high performance, especially concerning high efficiency in matrix removal (> 99.999%) while keeping the recovery of sulfur above 80%.
The procedure blank was determined by IDMS as well and yielded values for the individual IDMS measurement sequences ranging from 3 ng to 53 ng. The average of these individual procedure blanks (n=22) was calculated and yielded a total procedure blank of 14 ng sulphur with standard deviation of 12 ng. The limit of detection (LOD, blank+3SD) calculated on this basis was 0.20 µg·g-1 while the limit of quantification (LOQ, blank+10SD) was 0.54 µg·g-1, when considering a sample weight of 0.25 g.
The quantification of low sulfur contents (< 15 µg/g) by conventional IDMS is hindered by the very high Cu/S ratio, which clearly affects the separation in a negative way: The recovery of sulfur dropped to about 30 % for four replicates, while two further replicates even showed recoveries below 10%. To enable measurement without completely changing the separation procedure, an exact amount of sulfur was added prior to spiking, such that the sulfur mass fraction was shifted to the optimum working range of the separation procedure. Thus exact amounts of sulfur were added to enhance the mass fraction of sulfur from 15 µg·g-1 to 40 µg·g-1, then the IDMS analysis was performed as usual and finally the added sulfur amount was subtracted. The so obtained measurement result agreed well with the certified value within the uncertainties. The relative expanded measurement uncertainties for conventional IDMS are below 1%. When applying the modified IDMS procedure, where back-spike is added to the sample before spiking, the relative expanded measurement uncertainties are larger and up to 5%. With the presented sulfur-matrix procedure a working range from approximately 15 µg·g-1 to 1500 µg·g-1 can be achieved.
The developed procedure for the quantification of low sulfur amounts in copper has been validated here via three different routes: first an inter-laboratory comparison at highest metrological level, second a step-by-step validation by checking each single step of the procedure and third the setup of a complete uncertainty budget.
The procedure is sufficient to facilitate value assignment of total sulfur mass fraction in reference materials. Additionally, relative measurement uncertainties were calculated below 1 % and the measurement results are traceable to the SI, which is clearly demonstrated in this work. The procedure reported in this study is a new reference procedure for sulfur measurement in copper, well meeting the requirements of the two major purposes: the certification of reference materials and the assignment of reference values for inter-laboratory comparison.