Klaus G. Heumann

Osmium isotope ratio determinations by negative thermal ionization mass spectrometry

A precise determination of the isotopic abundances of tungsten with natural isotopic composition is presented. WO 3 − ions are generated by negative thermal ionization (NTI) in a double-filament ion source. La 2 O 3 is used as a chemical substance to reduce the electron work function of the rhenium filament material

Precision and accuracy in isotope ratio measurements by plasma source mass spectrometry

Precise and accurate isotope ratio measurements are an important task in many applications such as isotope dilution mass spectrometry, bioavailability studies or the determination of isotope variations in geological and cosmic samples. There is much more interest in ICP-MS for isotope ratio determinations at present compared with GDMS, which is preferred for the direct measurement of the isotopic composition of metallic solid samples. Spectroscopic interferences and a limited abundance sensitivity can influence the accuracy of isotope ratio determinations by GDMS and ICP-MS. In addition, in ICP-MS the space charge effect always influences the accuracy and a nozzle separation effect may also contribute to the total mass bias. Using a quadrupole ICP-MS the mass discrimination per mass unit can be >10% for elements with mass numbers <10, about 1–5% for mass numbers in the range of 20–120 and only <1% for heavier elements. Mass discrimination is strongly dependent on the potential of the different lenses of the ion optics and on the nebulizer gas flow. Even in magnetic sector field ICP-MS instruments a distinct mass discrimination is observed. Different procedures such as calibration by substances of consistent natural isotopic composition of the same or a neighbouring element and by isotopic standard reference materials, respectively, in combination with various mathematical functions, linear and exponential ones, are used for mass bias correction. In addition to the ion counting statistics, stability of the ion current is one of the most important topics which influences the precision of isotope ratio determinations. Magnetic sector field instruments, producing flat topped peak shapes, coupled with a multi-collector system for simultaneous measurement of different isotopes achieve the best relative standard deviation in the range of typically 0.005–0.02%, which is only comparable with precisions obtained by TIMS. However, ICP-TOFMS also has the potential for similar results. Typical relative standard deviations for other types of plasma source mass spectrometers for isotope ratio determinations are as follows: GDMS 0.1–1, quadrupole ICP-MS 0.1–0.5, and high resolution ICP-MS 0.05–0.2%.

Isotope dilution mass spectrometry

Abstract In the past isotope dilution mass spectrometry (IDMS) has usually been applied using the formation of positive thermal ions of metals. Especially in calibrating other analytical methods and for the certification of standard reference materials this type of IDMS became a routine method. Today, the progress in this field lies in the determination of ultra trace amounts of elements, e.g. of heavy metals in Antarctic ice and in aerosols in remote areas down to the sub-pg g−1 and sub-pg m−3 levels respectively, in the analysis of uranium and thorium at concentrations of a few pg g−1 in sputter targets for the production of micro- electronic devices or in the determination of sub-picogram amounts of230Th in corals for geochemical age determinations and of226Ra in rock samples. During the last few years negative thermal ionization IDMS has become a frequently used method. The determination of very small amounts of selenium and technetium as well as of other transition metals such as vanadium, chromium, molybdenum and tungsten are important examples in this field. Also the measurement of silicon in connection with a re-determination of Avogadros number and osmium analyses for geological age determinations by the Re/Os method are of special interest. Inductively-coupled plasma mass spectrometry is increasingly being used for multi-element analyses by the isotope dilution technique. Determinations of heavy metals in samples of marine origin are representative examples for this type of multi-element analysis by IDMS. Gas chromatography-mass spectrometry systems have also been successfully applied after chelation of metals (for example Pt determination in clinical samples) or for the determination of volatile element species in the environment, e.g. dimethyl sulfide. However, IDMS—specially at low concentration levels in the environment—seems likely to be one of the most powerful analytical methods for speciation in the future. This has been shown, up to now, for species of iodine, selenium and some heavy metals in aquatic systems.

GC determination of volatile organoiodine and organobromine compounds in Arctic seawater and air samples

SummaryDuring September 1992 seawater and air samples were collected on Spitzbergen, Norway, and the concentrations of volatile organoiodine and organobromine compounds of biogenic origin were determined by a GC system supplied with a capillary column and an electron capture detector. A purge and trap technique was used to isolate the organohalogen compounds from the seawater samples, whereas the air samples were collected by an adsorption tube filled with Carbosieve S-III. The iodinated compounds CH3I, CH2I2, CH2C1I, CH3CH2CH2I and CH3CHICH3 were determined in Arctic seawater and air samples with mean concentrations in the range of (0.3–6.2) ng/l and (0.7–2) pptv, respectively. This is the first time that 1- and 2-propyl iodide could be analysed both in atmospheric samples and in seawater samples of the Arctic. CH2Br2, CHBr3, CH2BrCl, CHBrCl2 and CHBr2Cl were determined as biogenic brominated methanes in mean concentrations of (0.1–164) ng/l and (0.1–0.5) pptv in seawater and air samples, respectively. The highest concentrations in seawater samples were found for CH2I2 and CHBr3, respectively, whereas in air samples the most abundant iodinated compound was CH3I and the most abundant brominated compounds with equal mean concentrations were CH2Br2 and CHBr3. Significant differences were found in the seawater concentration from the middle of the fjord and the shore site, compared with samples from a field of algae. In all cases the concentration was higher for the samples from the field of algae with an especially high excess by a factor of 4–9 for CH2I2 and CHBr3. This result shows that algae are an important biological species in the polar region for the production of these halogenated substances. Whereas the brominated compounds in seawater samples correlate well with each other, CH3I or any other iodinated compound does not correlate with the bromomethanes. This indicates a different biogenic mechanism for their formation. Under certain preconditions the annual flux from the Arctic Ocean to the atmosphere could be calculated for CH3I to be 4×109 g, for CHBr3 to be 5.4×1010 g, which is an essential contribution to the total global budget of these important atmospheric trace gases.

Development of an on-line isotope dilution technique with HPLC/ICP-MS for the accurate determination of elemental species

An on-line isotope dilution technique has been developed for use with a high performance liquid chromatography system (HPLC) coupled to an inductively coupled plasma mass spectrometer (ICP-MS). With this method it is possible to characterize elemental species at low concentration levels and to quantify them accurately. The possibilities of this method are shown using the examples of the determination of the interactions of different molecular weight fractions of dissolved organic matter (DOM) with copper and molybdenum in a natural water sample.

Production of methylated mercury, lead, and cadmium by marine bacteria as a significant natural source for atmospheric heavy metals in polar regions

Mixed and pure bacterial cultures of polar origin were incubated in model experiments under polar conditions. The releasing rates of monomethyl and dimethyl mercury (MeHg+ and Me2Hg), trimethyl lead (Me3Pb+), and monomethyl cadmium (MeCd+) were determined in dependence on the incubation time. This is the first time that methylation of cadmium by bacteria could be shown. The formation of tetramethyl and dimethyl lead (Me4Pb and Me2Pb2+) was also checked but no release of these methylated compounds was observed. The determination of methylated mercury compounds was carried out by using a purge and trap system after derivatisation of monomethyl mercury into the volatile methylethyl mercury compound, subsequent separation by gas chromatography and detection with an atomic fluorescence detector. A differential pulse anodic stripping voltammetric method was applied for the determination of Me3Pb+ and MeCd+, respectively. The mixed bacterial cultures showed production of trimethyl lead and monomethyl cadmium, but no methylated mercury compound was released by these marine species. In contrast to that the isolated pure bacterial cultures released relatively high amounts of dimethyl mercury besides monomethyl mercury, trimethyl lead, and monomethyl cadmium. These methylated heavy metal compounds were preferably formed in the stationary period of bacterial growth. Depth profiles of methylated heavy metal compounds in the Arctic Ocean and the South Atlantic show maximum concentrations in water depths of up to 50 m, often correlating well with the chlorophyll-a content. But also significant concentrations in depths of about 200 m were found, where no chlorophyll-a could be detected. This is an important indication that, at least, at deeper water levels bacteria must be the marine species which mainly contribute to methylated heavy metals. Dimethyl mercury, released by marine bacteria into the polar ocean, is the methylated heavy metal compound which contributes most to the atmospheric heavy metal content in the remote areas of Antarctica and the Arctic due to its high volatility. From measured Me2Hg concentrations in the surface seawater and the corresponding marine air at polar locations a preliminary atmospheric ocean-atmosphere transfer could be estimated to be 0.21×109 g yr−1 and 0.24×109 g yr−1 for the Antarctic and Arctic Ocean, respectively.

Elemental speciation with liquid chromatography–inductively coupled plasma isotope dilution mass spectrometry

For the determination of elemental species, which normally exist at low concentrations in the environment, coupling of liquid chromatographic systems with inductively coupled plasma mass spectrometry (ICP-MS) is a powerful method with respect to detection limit. However, accurate results are still a problem in trace analyses but can be obtained by the application of isotope dilution mass spectrometry (IDMS). The developed LC–ICP-IDMS system consists of a high-performance liquid chromatography pump, a sample injection valve, a separation column (different types of chromatographic separation systems, e.g., ion or size-exclusion chromatography, were used depending on the separation problem), a non-destructive detector (e.g. a UV detector) for simultaneous determination of organic substances, and an element-specific ICP mass spectrometer. Isotope dilution is carried out by adding an isotopically enriched species-specific spike solution to the sample prior to the separation step in the case of the determination of well-defined species, or by continuous on-line introduction of a species-unspecific spike solution in the case of species with unknown composition and structure. The species-specific spiking method is demonstrated for the determination of iodide and iodate in mineral water using an ion chromatographic separation column. For example, iodate concentrations in the range of 0.5–20 ng ml–1 could be determined with relative standard deviations of about 2%. The species-unspecific spiking mode is used to determined heavy metal complexes with humic substances at a level of about 1 ng ml–1 as well as organo-iodine species in the concentration range 0.7–1.4 ng ml–1 in natural water systems. The accuracy of speciation could be verified by comparing the total element concentration with the sum of the different elemental species.

Accurate determination of element species by on-line coupling of chromatographic systems with ICP-MS using isotope dilution technique☆

Abstract The instrumental design for coupling different liquid chromatographic systems such as ion, reversed phase, and size exclusion chromatography as well as capillary gas chromatography, with ICP-MS for the determination of element species is described. For accurate analyses obtaining ‘real time’ concentrations of chromatographic peaks, the isotope dilution mass spectrometric (IDMS) technique is applied. Two different spiking modes are possible, one using species-specific and another one using species-unspecific spike solutions of isotope-enriched labelled compounds. The species-specific mode is only possible for element species well defined in their structure and composition, for example iodate or selenite, whereas the species-unspecific mode must be applied in all cases where the structure and composition of the species is unknown, for example, for metal complexes with humic substances. For accurate determinations by the isotope dilution technique the mass discrimination effect must also be taken into account. Iodate, iodide and organoiodine species, including those of humic substances, have been analysed in mineral, drinking and environmental water samples by coupling different liquid chromatographic methods with ICP-IDMS. Heavy metal complexes with humic substances in water samples of different origin have been characterized by size exclusion/ICP-IDMS. The possibilities of determining different environmental selenium species are discussed and the results for the analysis of selenite and selenate, which has been carried out by GC/ICP-IDMS after converting these species into a volatile piazselenol compound, are presented.

Determinations of methyl iodide in the Antarctic atmosphere and the south polar sea

Abstract Methyl iodide (CH 3 I) concentrations were determined in the atmosphere and in surface sea water near the Antarctic Peninsula with a GC/ECD system during October–December 1987. The mean air concentration of methyl iodide was 2.4 pptv with a corresponding seawater concentration of 2.6 ng l −1 . In addition chloroiodomethane (CH 2 ClI) was detected in some of the seawater samples as a second volatile organoiodine species. No relationship between methyl iodide and biogenic brominated methanes was found. From this it follows that methyl iodide has a different pathway of biogenic production in marine organisms than the brominated methanes. Based on a two-phase model a global sea-to-air flux for methyl iodide of 8 × 10 11 g yr −1 was calculated. This is important for the balance of the global biogeochemical iodine cycle assuming that methyl iodide is by far the dominant volatile organoiodine species in the environment.

Isotope dilution mass spectrometry as a calibration method for the analysis of trace elements in powder samples by LA-ICP-MS

The applicability of the isotope dilution (ID) technique for trace element determinations in powdered solid samples by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) is demonstrated. LA-ICP-IDMS combines the advantage of the isotope dilution technique as a definitive method with the powerful capability of LA-ICP-MS for a fast and direct multi-element analysis of solid samples. LA-ICP-IDMS therefore allows matrix-matched quantification of element concentrations without any external standard, which is not possible by other calibration strategies used in LA-ICP-MS, up to now. The sample preparation effort is kept minimal and consists of an addition of the corresponding isotope-enriched spike solutions to the powder sample, subsequent drying and pressing of the isotope diluted sample to a pellet. For quantification, only one isotope ratio per analyte must be measured in the isotope diluted sample. It is shown that this internal standardisation corrects for all signal variations during analysis, e.g. instrumental drift or varying mass ablation rates. The LA-ICP-IDMS method is validated by the analysis of Cr, Fe, Cu, Zn, Sr, Cd, and Pb in seven certified reference materials (BCR 60, BCR 150, BCR 151, SRM 1567a, SRM 1577b, CRM 320, and SRM 1646) of different (organic and inorganic) matrix composition. 28 of a total number of 32 trace element concentrations determined by LA-ICP-IDMS are in agreement within the uncertainties of the certified values. The precision obtained is better than 10% for elements with concentrations >0.03 µg g−1. Only for two samples the Cr and Fe results differ significantly from the certified values.