Hans-Joachim Dietze

Inorganic trace analysis by mass spectrometry

Mass spectrometric methods for the trace analysis of inorganic materials with their ability to provide a very sensitive multielemental analysis have been established for the determination of trace and ultratrace elements in high-purity materials (metals, semiconductors and insulators), in different technical samples (e.g. alloys, pure chemicals, ceramics, thin films, ionimplanted semiconductors), in environmental samples (waters, soils, biological and medical materials) and geological samples. Whereas such techniques as spark source mass spectrometry (SSMS), laser ionization mass spectrometry (LIMS), laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), glow discharge mass spectrometry (GDMS), secondary ion mass spectrometry (SIMS) and inductively coupled plasma mass spectrometry (ICP-MS) have multielemental capability, other methods such as thermal ionization mass spectrometry (TIMS), accelerator mass spectrometry (AMS) and resonance ionization mass spectrometry (RIMS) have been used for sensitive mono- or oligoelemental ultratrace analysis (and precise determination of isotopic ratios) in solid samples. The limits of detection for chemical elements using these mass spectrometric techniques are in the low ng g -1 concentration range. The quantification of the analytical results of mass spectrometric methods is sometimes difficult due to a lack of matrix-fitted multielement standard reference materials (SRMs) for many solid samples. Therefore, owing to the simple quantification procedure of the aqueous solution, inductively coupled plasma mass spectrometry (ICP-MS) is being increasingly used for the characterization of solid samples after sample dissolution. ICP-MS is often combined with special sample introduction equipment (e.g. flow injection, hydride generation, high performance liquid chromatography (HPLC) or electrothermal vaporization) or an off-line matrix separation and enrichment of trace impurities (especially for characterization of high-purity materials and environmental samples) is used in order to improve the detection limits of trace elements. Furthermore, the determination of chemical elements in the trace and ultratrace concentration range is often difficult and can be disturbed through mass interferences of analyte ions by molecular ions at the same nominal mass. By applying double-focusing sector field mass spectrometry at the required mass resolution—by the mass spectrometric separation of molecular ions from the analyte ions—it is often possible to overcome these interference problems. Commercial instrumental equipment, the capability (detection limits, accuracy, precision) and the analytical application fields of mass spectrometric methods for the determination of trace and ultratrace elements and for surface analysis are discussed. q 1998 Elsevier Science B.V. All rights reserved

Application of double-focusing sector field ICP mass spectrometry with shielded torch using different nebulizers for ultratrace and precise isotope analysis of long-lived radionuclides

The capability of double-focusing sector field ICP-MS with a plasma-shielded torch using different nebulizers (a Meinhard nebulizer with a Scott-type spray chamber with a solution uptake rate of 1 ml min –1 ; a MicroMist microconcentric nebulizer used with a minicyclonic spray chamber with a solution uptake rate of 0.085 ml min –1 ; an ultrasonic nebulizer with a solution uptake rate of 2 ml min –1 ; and a direct injection high-efficiency nebulizer with a solution uptake rate of 0.085 ml min –1 ) for the introduction of radioactive sample solutions into the ICP was investigated. The total amount of analyte for each long-lived radionuclide ( 226 Ra, 230 Th, 237 Np, 238 U, 239 Pu and 241 Am; concentration of each was 1 ng l –1 in the aqueous solution) using different nebulizers was 5 pg for the Meinhard nebulizer, 0.4 pg for the MicroMist microconcentric nebulizer and 10 pg for the ultrasonic nebulizer. The application of the shielded torch yielded an increase in sensitivity for all these nebulizers of up to a factor of 5 compared with the original configuration without a shielded torch. Sensitivities of about 2000 MHz ppm –1 were measured for the radionuclides investigated (except for 226 Ra) using the MicroMist microconcentric nebulizer with a shielded torch. The detection limits were in the sub-pg l –1 range and the precision ranged from 1 to 2% RSD (n=5) for the 1 ng l –1 concentration level (0.4 pg sample size). A further increase in sensitivity for long-lived radionuclides of nearly one order of magnitude in comparison with the MicroMist microconcentric nebulizer was observed using ultrasonic nebulization, but the amount of analyte required was significantly higher (by a factor of 25). In contrast, the direct injection high-efficiency nebulizer (DIHEN) in double-focusing sector field ICP-MS (DF-ICP-MS) with a shielded torch resulted in a decrease in sensitivity in comparison with the unshielded torch because of a higher water load due to the direct injection of aqueous solution into the plasma. At low solution uptake rates (down to several µl min –1 ), the uranium solutions were analyzed by DIHEN-ICP-MS using a double-focusing sector field instrument with higher sensitivity than quadrupole-based ICP-MS. Flow injection was used for sample introduction to measure small sample volumes of radioactive waste solutions (20 µl). The determination of 237 Np at a concentration of 10 ng l –1 by flow injection DF-ICP-MS was possible with a precision of 2.0% (RSD, n=5). In order to avoid mass spectral interferences and matrix effects long-lived radionuclides (e.g., of U, Th and 99 Tc) were separated from the radioactive waste matrix by liquid-liquid extraction or ion exchange. The methods developed for the precise determination of the concentration and isotopic ratios of long-lived radionuclides were applied to aqueous standard solutions and radioactive wastes by double-focusing sector field ICP-MS. The precision of Pu isotopic analysis by double-focusing ICP-MS with a shielded torch was 0.2, 2 and 14% for 1000, 100 and 10 pg l –1 (amount of analyte: 500, 50 and 5 fg), respectively.

Inorganic mass spectrometric methods for trace, ultratrace, isotope, and surface analysis

Abstract Inorganic mass spectrometric methods are widely used for multielemental determination at the trace and ultratrace level for isotope ratio measurements and surface analysis (depth profiling, imaging) in quite different materials (e.g. conducting, semiconducting, and nonconducting solid samples; technical, environmental, biological, geological, and water samples). The capability of spark source mass spectrometry (SSMS), laser ionization mass spectrometry (LIMS), glow discharge mass spectrometry (GDMS), laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), secondary ion mass spectrometry (SIMS), sputtered neutral mass spectrometry (SNMS), and inductively coupled plasma mass spectrometry (ICP-MS) have been applied as the most important mass spectrometric techniques with their multielemental capability for the characterization of solid and aqueous samples. The detection limits for the direct analysis of solid samples by inorganic solid mass spectrometry were determined up to sub-ng g−1 and for aqueous solutions by ICP-MS up to sub-pg L−1. This article discusses the most important inorganic mass spectrometric techniques and their application for quantitative determination of trace element, isotope ratio measurements, and in-surface analysis.

Ultratrace and Isotope Analysis of Long-Lived Radionuclides by Inductively Coupled Plasma Quadrupole Mass Spectrometry Using a Direct Injection High Efficiency Nebulizer

The direct injection high efficiency nebulizer (DIHEN) was explored for the ultrasensitive determination of long-lived radionuclides ((226)Ra, (230)Th, (237)Np, (238)U, (239)Pu, and (241)Am) and for precise isotope analysis by inductively coupled plasma mass spectrometry (ICPMS). The DIHEN was used at low solution uptake rates (1-100 μL/min) without a spray chamber. Optimal sensitivity (e.g., (238)U, 230 MHz/ppm; (230)Th, 190 MHz/ppm; and (239)Pu, 184 MHz/ppm) was achieved at low nebulizer gas flow rates (0.16 L/min), high rf power (1450 W), and low solution uptake rates (100 μL/min). The optimum parameters varied slightly for the two DIHENs tested. The detection limits of long-lived radionuclides in aqueous solutions varied from 0.012 to 0.11 ng/L. The sensitivity of the DIHEN was improved by a factor of 3 to 5 compared with that of a microconcentric nebulizer (MicroMist used with a minicyclonic spray chamber at a solution uptake rate of 85 μL/min) and a factor of 1.5 to 4 compared with that of a conventional nebulizer (cross-flow used with a Scott type spray chamber at a solution uptake rate of 1 mL/min). The precision of the DIHEN ranged from 0.5 to 1.7% RSD (N = 3) for all measurements at the 10 ng/L concentration level (∼3 pg sample size). The sensitivity decreased to 10 MHz/ppm at a solution uptake rate of 1 μL/min. The precision was about 5% RSD at a sample size of 30 fg for each long-lived radionuclide by the DIHEN-ICPMS method. The oxide to atom ratios were less than 0.05 (except ThO(+)/Th(+) ) and decreased under the optimum conditions in the following sequence: ThO(+)/Th(+) > UO(+)/U(+) > NpO(+)/Np(+) > PuO(+)/Pu(+) > AmO(+)/Am(+) > RaO(+)/Ra(+). Atomic and oxide ions were used as analyte ions for ultratrace and isotope analyses of long-lived radionuclides in environmental and radioactive waste samples. The analytical methods developed were applied to the determination of long-lived radionuclides and isotope ratio measurements in different radioactive waste and environmental samples using the DIHEN in combination with quadrupole ICPMS. For instance, the (240)Pu/(239)Pu isotope ratio was measured in a radioactive waste sample at a plutonium concentration of 12 ng/L. This demonstrates a main advantage of DIHEN-ICPMS compared with α-spectrometry, which cannot be used to selectively determine (239)Pu and (240)Pu because of similar α energies (5.244 and 5.255 MeV, respectively).

Determination of long-lived radionuclides by inductively coupled plasma quadrupole mass spectrometry using different nebulizers

Different nebulizers (cross-flow, ultrasonic and two microconcentric nebulizers) were used for sample introduction of radioactive solutions into a quadrupole-based inductively coupled plasma mass spectrometer (ICP-QMS). The best sensitivity (from 420 to 850 MHz, which is about one order of magnitude higher in comparison with the cross-flow nebulizer) for long-lived radionuclides ( 226 Ra, 230 Th, 237 Np, 238 U and 241 Am) was observed using the ultrasonic nebulizer. However, using the ultrasonic nebulizer, a significantly higher sample size (26-fold) in comparison with the micronebulizers is required. Sample introduction by micronebulization with a small sample size in the low picogram range is the method of choice for the determination of long-lived radionuclides. The precision of determination of a 10 ng l –1 concentration was in the low-% range (and sub-% range) for all measurements using different nebulizer types. The detection limits for the determination of long-lived radionuclides in aqueous solutions applying the different nebulizers were 0.01-0.6 ng l –1 . The flow injection analysis approach was optimized for isotope dilution analysis of 232 Th (using 20 µl of 5 µg l –1 230 Th) by ICP-QMS. The isotopic abundance ratios of 230 Th- 232 Th isotope mixtures ( 230 Th/ 232 Th=0.01, 0.001 and 0.0001) were determined using a microconcentric nebulizer and 1 µg l –1 Th solutions with a relative external standard deviation of long-term stability measurements (over 20 h) of 0.17, 0.62 and 2.66%, respectively.

gDouble-focusing Sector Field Inductively Coupled Plasma MassSpectrometry for Highly Sensitive Multi-element and IsotopicAnalysis

The different areas of application in double-focusing sector field ICP-MS are described, such as the determination of trace and ultratrace elements in environmental and materials research and for the characterisation of long-lived radionuclides in environmental and radioactive waste samples. Analytical methods using double-focusing sector field ICP-MS allow the determination at low mass resolution of ultratrace elements ( e.g ., rare earth elements) and some radionuclides in the µg l -1 and pg l -1 concentration ranges ( e.g. , for U, Th, Pu and 129 I) in aqueous solutions and at the ng g -1 level and lower in solid samples after a digestion step. Some examples of trace analysis of solid samples after matrix separation are discussed. For the determination of spallation nuclides and impurities in irradiated tantalum (with an 800 MeV proton beam) from a spallation neutron source, double-focusing sector field ICP-MS was used after liquid–liquid extraction of the tantalum matrix. Further, the results of the analysis of high-purity GaAs by double-focusing ICP-MS after dissolution with and without matrix separation are compared with those of quadrupole ICP-MS.

Laser ablation inductively coupled plasma mass spectrometry for the trace, ultratrace and isotope analysis of long-lived radionuclides in solid samples

Abstract The capability of laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) for determination of long-lived radionuclides in different materials (e.g., in geological samples, high-purity graphite and nonconducting concrete matrix) was investigated. The main problem in the quantification of the analytical results of long-lived radionuclides is that (except for geological samples) no suitable standard reference materials are available. Therefore, synthetic laboratory standards (graphite and concrete matrix doped with long-lived radionuclides, such as 99Tc, 232Th, 233U, 235U, 237Np, 238U) were prepared and used for quantification purposes in LA-ICP-MS. Different calibration procedures—the correction of analytical results with experimentally determined relative sensitivity coefficients (RSCs), the use of calibration curves and solution calibration by coupling LA-ICP-MS with an ultrasonic nebulizer—were applied for the determination of long-lived radionuclides, especially for Th and U in different solid samples. The limits of detection of long-lived radionuclides investigated in concrete matrix are determined in the pg g−1 range (e.g., for 237Np-50 pg g−1 in quadrupole LA-ICP-MS; for 233U-1.3 pg g−1 in double-focusing sector field LA-ICP-MS). Results of isotope ratio measurements of Th and U in synthetic laboratory standards and different solid radioactive waste materials of direct analysis on solid samples using LA-ICP-MS are comparable to measurements using the double-focusing sector field ICP-MS after separation of the analyte, even if no possible interference of atomic ions of analyte and molecular ions are expected. Furthermore, LA-ICP-MS allows precise and accurate isotope ratio measurements of Th and U in solid samples. For example, the isotope ratio 234U/238U = 0.000067 in radioactive reactor graphite was determined with a precision of 1.1% relative standard deviation (RSD).

Ultratrace and isotopic analysis of long-lived radionuclides by double-focusing sector field inductively coupled plasma mass spectrometry using direct liquid sample introduction

Abstract This report is concerned with the investigation of double-focusing sector field inductively coupled plasma mass spectrometry (DF-ICPMS) for ultratrace and isotopic ratio analysis of long-lived radionuclides (226Ra, 230Th, 232Th, 233U, 237Np, 238U, and 241Am) using the direct injection high efficiency nebulizer (DIHEN). A new shielded torch arrangement, known as the guard electrode, improves relative sensitivity by a factor of six when the DIHEN is used. Absolute sensitivity with the DIHEN is on the order of 1300 (226Ra) to 1700 (238U) counts/fg at a solution consumption rate of 5 μL/min. This is a factor of from three to 20 better than the results obtained by a conventional nebulizer-spray chamber arrangement (e.g., ultrasonic and pneumatic nebulizers). The DIHEN-DF-ICPMS is successfully tested for isotope ratio measurements of 235U/238U standards and environmental radioactive waste solutions.

Mass spectrometric and theoretical investigations into the formation of argon molecular ions in plasma mass spectrometry

The abundance and distribution of argon molecular ions (e.g., ArH+, ArO+, ArN+, Ar2+ and MAr+; M = metal) in plasma MS (ICP-MS, LA-ICP-MS and rf-GDMS) were investigated and compared. In ICP-MS the non-metal argon molecular ions were formed with higher intensity compared with the metal argide ions. This could be explained by theoretical calculations of the binding energies. The ArH+ ion can be viewed as an isoelectronic system, comparable to HCl. The intensities of diatomic metal argide ions relative to metal ions in ICP-MS are less than 10–4. A correlation between the intensities of metal argide ions with the bond dissociation energies of diatomic ions was found. The highest intensity of metal argide ions, of the order of per cent. values, were observed in rf-GDMS. The intensity of the argon molecular ions in an rf-GD varied by up to three orders of magnitude as a function of the plasma parameters (e.g., argon pressure in the GD ion source). The characteristic distribution of diatomic argide ions of REEs in ICP-MS was found to be comparable to the distribution of rare earth oxide ions.

Ultratrace and precise isotope analysis by double-focusing sector field inductively coupled plasma mass spectrometry

Double-focusing sector field inductively coupled plasma source mass spectrometry with its capability of providing a very sensitive multi-element analysis has been established for the determination of trace and ultratrace elements in high-purity materials, environmental samples and radioactive waste materials. Some applications of double-focusing sector field ICP-MS for the multi-element analysis of trace and ultratrace impurities in high-purity inorganic materials are described. A matrix separation procedure for ZrO2 by liquid–liquid extraction after microwave-induced dissolution in an acid mixture is proposed in order to determine ultratrace impurities. The detection limits of ICP-MS reached after matrix separation in solid samples are in the low ng g–1 concentration range. The detection limits in ICP-MS (determined by the blank values of the chemicals used) are comparable to those of solid-state mass spectrometry, which allows the direct determination of trace impurities in high-purity solids. Isotope ratio measurements of Mg, K and Ca were performed to investigate the transport phenomena of nutrient solutions in plants by tracer experiments using highly enriched 25Mg, 26Mg, 41K, 42Ca and 44Ca isotopes. In order to separate the 38ArH+ and 40ArH+ ions from the 39K+ and 41K+analyte ions for potassium isotope ratio measurements, double-focusing sector field ICP-MS with an ultrasonic nebulizer was used at a mass resolution of 9000. The precision of potassium isotope ratio measurements was 0.7% (at a potassium concentration of 100 µg l–1). Isotope ratios of Mg and Ca (each at a concentration of 50 µg l–1) were determined at a mass resolution of 3000 with a precision of 0.4 and 0.5% on real biological samples. The accuracy of isotope ratio measurements of K and Mg was determined using isotopic standard reference materials with natural isotope composition (NBS SRM 985 and 980). The results of isotope ratio measurements of K, Mg and Ca on real biological samples doped with enriched stable isotopes are discussed.