High Abundance Sensitivity Research Articles

Entropy-driven catalytic amplification adjusted by stoichiometry for single-nucleotide variants detection with high abundance sensitivity

Single-nucleotide variants (SNV) detection with high abundance sensitivity is of great significance in clinical application, molecular diagnostics and biological research. In this study, a high abundance sensitivity SNV detection strategy based on entropy-driven catalytic (EDC) amplification adjusted by stoichiometry is proposed. In EDC, the toehold exchange reaction is used to initiate subsequent catalytic reaction and can be adjusted by stoichiometry. When the by-product concentration in the toehold exchange reaction is excessive, the forward reaction will be inhibited, which can reduce or even block the unexpected reaction between the non-target and the probe. Meanwhile, some targets can still successfully take a toehold exchange reaction with the probe, thus completing the subsequent EDC. By adjusting the EDC, the SNV identification specificity of this system was improved and is superior to any single adjusted stoichiometry or EDC. When the low abundance target is detected from the mixture, this strategy enables SNV detection at 0.1% abundance with high abundance sensitivity. And even if the mixture contains three kind of 1000-fold interference sequences, this strategy can still discriminate the target SNV. Furthermore, the practical applicability of the adjusted EDC system was verified by p53 mutation discrimination in human urine.

Pushing Limits of ICP–MS/MS for the Determination of Ultralow 236U/238U Isotope Ratios

Determination of uranium isotope ratios is of great expedience for assessing its origin in environmental samples. In particular, the 236U/238U isotope ratio provides a powerful tool to discriminate between the different sources of uranium (uranium ore, geochemical background, and uranium from anthropogenic activities). However, in the environment, this ratio is typically below 10-8. This low abundance of 236U and the presence in large excess of major isotopes (mainly 238U and 235U) complicates the accurate detection of 236U signal by mass spectrometry and thus highly sensitive analytical instruments providing high abundance sensitivity are required. This work pushes the limits of triple quadrupole-based ICP-MS technology for accurate detection of 236U/238U isotope ratios down to 10-10, which is so far mainly achievable by AMS. Coupled with an efficient desolvating module, N2O was used as the reaction gas in the collision reaction cell of the ICP-MS/MS. This configuration allows a significant decrease of the uranium polyatomic interferences (235UH+ ions) and an accurate determination of low 236U/238U isotope ratios. This new methodology was successfully validated through measurements of certified reference material from 10-7 to 10-9 and then through comparisons with AMS measurement results for ratios down to 10-10. This is the first time that 236U/238U isotope ratios as low as 10-10 were determined by ICP-MS/MS. The possibility of measuring low 236U/238U isotope ratios can offer a large variety of geochemical applications in particular for the determination of uranium sources in the environment.

A new method for measurement of lithium isotopes with nuclear applications

Extreme isotope abundance ratios occur naturally, but also result from enrichment processes required for various industrial applications. The range of elements used for these industrial applications spans essentially the entire periodic table; thus, the need for a low cost (both in labor and finance) analytical technique to measure isotopic enrichments that create extreme isotope abundance ratios has never been greater. Molten salt reactors are one such application, where high purity 7Li is required for cooling processes due to its low neutron cross-section compared to 6Li. Several new technologies are required for these reactors to be viable, both technically and financially, including an efficient method for enrichment of 7Li. Many methods are being tested, but there is a need for a high-performance, high-throughput method for determination of extreme lithium isotope abundance ratios. High abundance sensitivity mass spectrometry provides a path forward for measurement of lithium isotopes. Here, we present a new method for measuring enriched lithium isotope ratios by using multi-collector inductively coupled plasma mass spectrometry and combined faraday-ion counting detectors. We carefully characterize the abundance sensitivity and background signals associated with combined Faraday-ion counting techniques. To test the accuracy of the method, we measured lithium isotopic compositions of a suite of pure lithium salts using the standard static faraday measurement and compare those results to isotope ratios measured using the new method. Isotope ratios of the two techniques agree to within 1.5‰. We also achieved 7Li isotope abundance for a previously characterized enriched lithium salts within 0.0035%, indicating that the technique will provide a way forward for measuring enriched lithium isotope ratios in nuclear materials. The technique can be applied to any element with unnatural isotopic compositions and/or extreme abundance ratios.

A New Method and Mass Spectrometer Design for TOF-SIMS Parallel Imaging MS/MS.

We report a method for the unambiguous identification of molecules in biological and materials specimens at high practical lateral resolution using a new TOF-SIMS parallel imaging MS/MS spectrometer. The tandem mass spectrometry imaging reported here is based on the precise monoisotopic selection of precursor ions from a TOF-SIMS secondary ion stream followed by the parallel and synchronous collection of the product ion data. Thus, our new method enables simultaneous surface screening of a complex matrix chemistry with TOF-SIMS (MS(1)) imaging and targeted identification of matrix components with MS/MS (MS(2)) imaging. This approach takes optimal advantage of all ions produced from a multicomponent sample, compared to classical tandem mass spectrometric methods that discard all ions with the exception of specific ions of interest. We have applied this approach for molecular surface analysis and molecular identification on the nanometer scale. High abundance sensitivity is achieved at low primary ion dose density; therefore, one-of-a-kind samples may be relentlessly probed before ion-beam-induced molecular damage is observed.

Determination of ¹³⁵Cs and ¹³⁵Cs/¹³⁷Cs atomic ratio in environmental samples by combining ammonium molybdophosphate (AMP)-selective Cs adsorption and ion-exchange chromatographic separation to triple-quadrupole inductively coupled plasma-mass spectrometry.

Since the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident in 2011, the activity ratio of (134)Cs/(137)Cs has been widely used as a tracer for contamination source identification. However, because of the short half-life of (134)Cs (2.06 y), this tracer will become unavailable in the near future. This article presents an analytical method for the determination of the long-lived (135)Cs (t(2/1) = 2 × 10(6) y) and the atomic ratio of (135)Cs/(137)Cs, as a promising geochemical tracer, in environmental samples. The analytical method involves ammonium molybdophosphate (AMP)-selective adsorption of Cs and subsequent two-stage ion-exchange chromatographic separation, followed by detection of isolated radiocesium isotopes via triple-quadrupole inductively coupled plasma-mass spectrometry (ICP-MS/MS). The AMP-selective adsorption of Cs and the chromatographic separation system showed high decontamination factors (10(4)-10(5)) for interfering elements, such as Ba, Mo, Sb, and Sn. Using ICP-MS/MS, only selected ions enter the collision/reaction cell to react with N2O, reducing the isobaric interferences ((135)Ba(+) and (137)Ba(+)) and polyatomic interferences ((95) Mo(40)Ar(+), (97) Mo(40)Ar(+), (119)Sn(16)O(+), and (121)Sb(16)O(+)) produced by sample matrix ions. The high abundance sensitivity (10(-9) for the (135)Cs/(133)Cs ratio) provided by ICP-MS/MS allowed reliable analysis of (135)Cs and (137)Cs isotopes with the lowest detection limits ever reported by mass counting methods (0.01 pg mL(-1) and 0.006 pg mL(-1), respectively). The developed analytical method was successfully applied to the determination of (135)Cs and (137)Cs isotopes in environmental samples (soil, litter, and lichen) collected after the FDNPP accident for contamination source identification.

Uranium hexafluoride (UF6) gas source mass spectrometry for certification of reference materials and nuclear safeguard measurements at IRMM

Uranium hexafluoride gas source mass spectrometry at IRMM is based on two foundations, firstly the operation of a UF6 gas source mass spectrometer (GSMS) and secondly the preparation of primary UF6 reference materials, which were converted from gravimetrically prepared mixtures of highly enriched oxides of 235U and 238U. Recently a new GSMS for uranium isotopic measurements using UF6 gas, the “URANUS” from Thermo Fisher, was installed at IRMM, which also allows measurements of the so-called “minor” isotope ratios n(234U)/n(238U) and n(236U)/n(238U). In this paper the design and the implementation of measurement techniques for the new URANUS GSMS are described. This includes the “single standard” and the “double standard” (DS) method as well as the newly developed “memory corrected double standard” method (MCDS). This required a detailed investigation of memory effects within the GSMS instrument, in particular regarding the dependence of memory effects on the isotope ratios of samples and standards. The results of this study led to new recommendations for the selection of the standards for a given sample and for suitable measurement procedures. The measurement performance for the “major” isotope ratio n(235U)/n(238U) as well as the “minor” isotope ratios n(234U)/n(238U) and n(236U)/n(238U) is presented and compared with other mass spectrometric techniques. With the installation and validation of the new URANUS GSMS instrument IRMM has established two new complementary techniques for measuring the full isotopic composition of uranium samples. UF6 GSMS in combination with the MCDS method is considered the preferred technique for samples in the UF6 form and for smaller uncertainties for measurements of the major ratio n(235U)/n(238U), while thermal ionization mass spectrometry (TIMS), in combination with the “modified total evaporation” (MTE) method as well as ion counting and high abundance sensitivity for the detection of 236U, provides a superior measurement performance for the minor isotope ratios n(234U)/n(238U) and n(236U)/n(238U).

A high-resolution gas-source isotope ratio mass spectrometer

We describe a new high-resolution, multi-collector gas source mass spectrometer designed for isotopic analysis of volatile and semi-volatile molecules: the Thermo Scientific MAT253-Ultra, a prototype double-focusing isotope ratio mass spectrometer installed in the Caltech laboratories for stable isotope geochemistry. This instrument achieves mass resolving power of up to ∼27,000 (M/ΔM) and can analyze diverse gases and semi-volatile compounds using a conventional dual inlet and/or a carrier gas. It has a multi-collector array comprised of 7 detector positions with adjustable spacing, all of which can register ions through an SEM or Faraday cup and spanning up to a 1013 range in signal strength. Abundance sensitivity in the He mass range is as good as 10−12, and precision commonly approaches the counting statistics limit down to 0.1‰ (SEM) or 0.01‰ (Faraday) for a range of analytes. This instrument permits resolution of isobaric interferences arising from both contaminants and multiple isotopologues of an analyte that share a cardinal mass, enabling direct isotopic analysis of molecules with complex mass spectra such as hydrocarbons. This ability should enable the measurement of position-specific isotopic compositions, including multiple substitutions, by comparing isotope ratios of molecular ions with those of daughter fragment ions (assuming products of recombination and other source reactions are recognized and corrected for). The combination of high mass resolution with stable multicollection will provide a wide range of potential new tools for isotope geochemistry, including (but not limited to): singly and multiply substituted methane and larger hydrocarbons; position-specific 13C analysis of propane and larger hydrocarbons; precise analysis of 17O/16O and 18O/16O on fragment ions from CO2 and other molecules; analysis of a variety of N2O isotopologues (including 18O, 17O, position-specific 15N, and various ‘clumped’ species); and high precision and abundance sensitivity noble gas analyses. These capabilities greatly extend the scope of stable isotope variations that can be utilized for problems in forensics, environmental geochemistry, biochemistry, and Earth and planetary sciences.

Microwave Digestion-ICP-MS Determination of Vanadium Oxide for 22 Impurity Elements

With the utilization of hydrofluoric acid and nitric acid, vanadium oxide samples such as V2O5 and V2O3 are digested by the way of high-pressure closed microwave heating and 22 impurity elements with concentration of 0.0001% ~ 0.5% including Nb, Zr, Ti, W, Si, Al, Mo, Co, Cr, Ni, Cu, Pb, Cd, As, P, Fe, Mn, Ca, Mg, Na, K and B are directly and simultaneously determined by inductively coupled plasma mass spectrometry (ICP-MS). The optimum microwave digestion conditions, combination and ratio of the digestion reagents, optimization of detection conditions, matrix effects, mass interferences and other influencing factors are investigated; in order to eliminate the mass spectrum interference from vanadium matrix isobars and polyatomic ion formed by combination of these isobars and Ar, O, H and other atoms, isotopes of the test elements with high abundance sensitivity and little interference are selected for analysis of elements; the analyte is divided into two mass regions, respectively using Sc and Rh as internal standard for the correction of matrix effects. The results show that: V2O5, V2O3 and impurities contained in the matrix can be quickly and completely digested, thus, insoluble oxides or volatile impurities such as Nb, Zr, Ti, W, Si, Al, Cr and As can be digested without a loss; the stability and precision of continuous determination is significantly improved by Sc and Rh internal standard correction method, and matrix effects, interface effects and other effects generated in high vanadium-based matrix are effectively eliminated. The test also boosts a detection limit of 0.01 ~ 0.89μg /L, correlation coefficient r ≥ 0.9991, recovery rate of 90% ~ 113% and RSD <5%. This method can meet the need to simultaneously determine 22 kinds of micro, trace and ultra-trace impurities of various normal or high-purity V2O5, V2O3 and other vanadium oxides.

Fluorine-19 NMR Chemical Shift Probes Molecular Binding to Lipid Membranes

The binding of amphiphilic molecules to lipid bilayers is followed by 19F NMR using chemical shift and line shape differences between the solution and membrane-tethered states of -CF 3 and -CHF 2 groups. A chemical shift separation of 1.6 ppm combined with a high natural abundance and high sensitivity of 19F nuclei offers an advantage of using 19F NMR spectroscopy as an efficient tool for rapid time-resolved screening of pharmaceuticals for membrane binding. We illustrate the approach with molecules containing both fluorinated tails and an acrylate moiety, resolving the signals of molecules in solution from those bound to synthetic dimyristoylphosphatidylcholine bilayers both with and without magic angle sample spinning. The potential in vitro and in vivo biomedical applications are outlined. The presented method is applicable with the conventional NMR equipment, magnetic fields of several Tesla, stationary samples, and natural abundance isotopes.

Flow cytometer with mass spectrometer detection for massively multiplexed single-cell biomarker assay

Abstract This paper describes the development and application of new metal-tagging reagents and a novel mass spectrometer (MS) detector for a flow cytometer that enables highly multiplexed measurement of many biomarkers in individual cells. A new class of tagging reagents, based on an acrylic polymer backbone that incorporates a reproducible number of lanthanide elements, has been developed. When linked to antibodies that specifically recognize target proteins of interest, determination of the tag elements is diagnostic for the presence and quantification of the antigen. The use of enriched stable isotope tags provides the opportunity for multiparametric assay. The new instrument uses inductively coupled plasma (ICP) to vaporize, atomize, and ionize individual cells that have been probed using the metal-labeled antibodies. The elemental composition, specifically of the metal tags, is recorded simultaneously using a time-of-flight (TOF)-MS that has been specifically designed for high-speed analysis during the short transient corresponding to the individual cell event. The detector provides for well-resolved atomic fingerprints of many elemental and isotopic tags, with little overlap of neighboring signals (high abundance sensitivity) and wide dynamic range both for a single antigen and between antigens.

Improved abundance sensitivity in MC-ICP-MS for determination of236U/238U isotope ratios in the 10−7to 10−8range

A multicollector inductively coupled plasma mass spectrometer (MC-ICP-MS)—Nu Plasma HR—equipped with an ion deceleration filter (‘high abundance sensitivity channel’) was optimized for the determination of extremely low uranium isotope ratios and applied for the first time for the analysis of 236U/238U isotope ratios in the 10−8 to 10−7 range in isotopic reference material IRMM-184, as well as in two unknown samples obtained in the frame of a round-robin exercise. The application of the ion deceleration lens system in the Nu Plasma ICP-MS allowed a reduction of peak tailing from 238U+ ions at m/z = 236 down to 3 × 10−9, whereas the absolute sensitivity for uranium was reduced by only about 30%. Thus, abundance sensitivity was improved by almost two orders of magnitude and the minimum determinable 236U/238U ratio was improved by more than one order of magnitude compared with conventional sector-field ICP-MS or TIMS. However, interference by 235U1H+ ions deteriorated the accuracy and increased the measurement uncertainty up to 48%, in particular in the case of samples enriched in 235U. Deviation of the measured value from the certified ones varied from −24% to 17.7% but was within the measurement uncertainty.

Application of Accelerator Mass Spectrometry for 236U analysis

The long-lived radioisotope 236-uranium (T1/2=23.4Ma) is produced predominantly by neutron capture on 235U, building up to high levels in nuclear fuel. It is a potentially useful tracer of irradiated uranium for a variety of applications, due to its virtual absence in natural samples (236U:238U ratio ~10-10 in uranium ore). 236U has been distributed in the environment as a result of nuclear activities including nuclear explosions, accidents at nuclear plants, dumping of nuclear waste and releases from nuclear facilities. It is also present in depleted uranium, due to the recycling of uranium fuel in enrichment plants, and has been dispersed in areas where DU munitions have been used. We have measured 236U in a variety of environmental media by Accelerator Mass Spectrometry (AMS). AMS has the advantage of very high abundance sensitivity. Such measurements are possible using only a few grams of environmental materials, yielding a few micrograms of uranium for analysis by AMS. The resulting 236:238 isotopic ratios are typically in the range 10-6 to 10-9.

Improvement of abundance sensitivity in a quadrupole-based ICP-MS instrument with a hexapole collision cell

High isotopic abundance sensitivity is desirable for the measurement of low-abundance isotopes in the presence of neighboring high-abundance isotopes. This is of interest when analyzing ultra low concentrations of trace elements or isotopes in the presence of elements or isotopes with much higher concentrations (e.g., radionuclides included in a matrix of stable isotopes). In this study, a quadrupole-based inductively coupled plasma mass spectrometer with a hexapole collision cell (ICP-CC-QMS), Platform ICP (Micromass UK), was used to study the improvement of abundance sensitivity using uranium and lutetium as examples. The median kinetic energy of 238U+ ions was about 1.2 eV and 0.9 eV when helium was introduced as a collision gas at flow rates of 5 ml min−1 and 10 ml min−1, respectively. In the latter case, the proportion of ions with relatively high energy decreased significantly, which improved both the transmission of ions through the hexapole collision cell and mass selection by the quadrupole mass spectrometer. When introducing helium into the collision cell a reduction in peak tail of high-abundance isotopes by up to three orders of magnitude, depending on the mass analyzed, was observed. The abundance sensitivity for 236U/238U isotope ratio was improved from 2.3 × 10−5 to 6.3 × 10−8. Abundance sensitivities as low as 3.5 × 10−8, 6.6 × 10−8 and 2.0 × 10−8 were achieved at masses m − 2.5 u, m − 1.5 u and m + 2.5 u (m = 174.94 u for 175Lu), respectively. Reducing the kinetic energy also improved the mass resolution by about 10%. However, increasing the mass resolution of the quadrupole instrument had a significantly lower influence on abundance sensitivity than reducing the ion energy by collision with helium atoms. The present study of the effect of the pressurized ion guide on abundance sensitivity was limited to some extent because of the relatively high background count rate of the Daly-type ion detector in the ICP-CC-QMS system. An ion detector possessing a low background would be desirable for further studies. Improved abundance sensitivity can be useful in analyzing long-lived radionuclides like 236U and 129I in the presence of high-abundance stable isotopes like 238U and 127I.

Isotopic “fingerprints” for natural uranium ore samples

A set of six samples, collected worldwide from various uranium ore mining facilities, was analysed for uranium isotopic composition by high accuracy isotope mass spectrometry. The goal of this article was twofold: to measure isotopic variations between samples of different geographical origin and to produce calibrated isotope ratios with the smallest achievable uncertainty (as defined according to the ISO Guide to the Expression of Uncertainty in Measurement). In the first step, the molar ratio of the isotopes 235U and 238U, n( 235U)/ n( 238U), was measured using a UF 6-gas-inlet isotope mass spectrometer (VARIAN MAT 511). This instrument was calibrated against gravimetrically prepared synthetic isotope mixtures thus allowing SI-traceable measurements to be made. The ratios of the “minor isotopes” to 238U [ n( 234U)/ n( 238U) and n( 236U)/ n( 238U)] were determined in a second step using a thermal ionisation mass spectrometer with high abundance sensitivity (Finnigan MAT262-RPQ-PLUS). The mass-fractionation correction was done internally using the result of the n( 235U)/ n( 238U) measurement. As a result, the complete measured uranium isotopic composition is traceable to the SI system. For all ratios n( 234U)/ n( 238U), n( 235U)/ n( 238U), and n( 236U)/ n( 238U) significant differences for samples of different origin were found. Regarding the n( 236U)/ n( 238U) results, only two samples, one of them from the Oklo reactor in Gabon, showed significant presence of 236U. For all other samples an upper limit for n( 236U)/ n( 238U) of about 6 × 10 −10, mainly dependent on the instrumentation, was found. As a result of this study we propose values for the isotope abundances of natural uranium for the “Best Measurement from a Single Terrestrial Source” and the “Range of Natural Variations” in the IUPAC-table of the “Isotopic Composition of the Elements.”

The isotopic composition of commercially available uranium chemical reagents

The uranium isotopic compositions of 12 commercially available uranium chemicals, obtained from various suppliers, were measured by high accuracy isotope mass spectrometry. The goal was to establish the range of isotopic compositions in currently available commercial uranium chemicals, offered in different chemical forms and used for different purposes (e.g., atomic absorption spectrometry or inductively coupled plasma mass spectrometry). The relative isotope abundances of 234 U, 235 U, 236 U and 238 U were determined by thermal ionisation mass spectrometry with high abundance sensitivity (Finnigan MAT262-RPQ-Plus), using the n( 235 U)/n( 238 U) amount ratio measurements of a gravimetrically prepared Isotopic Reference Material (IRMM-183) for mass fractionation correction. All uncertainties were calculated according to the ISO Guide to the Expression of Uncertainty in Measurement (International Standards Organization, Geneva, Switzerland), using a coverage factor k=2. For n( 235 U)/n( 238 U) relative uncertainties of 0.2-0.3% and for n( 234 U)/n( 238 U) and n( 236 U)/n( 238 U) of 0.4-3.0%, depending on the relative amounts of the 234 U and 236 U isotopes, were achieved. All samples were found to be depleted in 235 U by a factor of two compared with its natural molar abundance (0.72%). The non-natural isotope 236 U was present in all samples analyzed. The range found for the molar abundances of 234 U is 0.0022-0.000 81%, for 235 U 0.35-0.21%, for 236 U 0.021-0.000 011% and for 238 U 99.788-99.634%.

A New Thermal Ionization Mass Spectrometer (TIMS)

TIMS is a well established technique capable of extremely high precision and accuracy in the determination of isotope ratios, but a number of interesting problems remain elusively just beyond the ability of current instruments (e.g. the putative terrestrial 142Nd anomaly). While precision and accuracy of TIMS instruments have improved over the years, it is not generally appreciated that the instrumental advances of the last ten+ years (moveable Faraday collectors, negative ion technique, positive and negative ion counting, ultrahigh abundance sensitivity using retardation lenses) have been implemented on instrumental platforms which date back to the single collector instruments of the 1970s. To address currently identified research needs as well as a number of anticipated future needs in geoand cosmochemistry, Finnigan MAT has developed a completely new TIMS which is built on an all new platform which offers improved analytical performance (precision and accuracy), significant new analytical capabilities, and enhanced reliability and ease of operation. The ion optics of the ion source and the mass analyser have been recalculated using proprietary state of the art simulation programs, and all optical elements have been completely redesigned to increase transmission, eliminate image errors, and to significantly increase the mass dispersion. Wherever possible, optical fiber links have been used to make the electronics more robust against noise. This concept has been adopted from the the Finnigan MAT ELEMENT, a high performance ICP-MS instrument. The accelerating voltage is kept at the 10-kV of its MAT 26x predecessors (262, 261,260) but the use of a larger magnet leads to a larger geometry and a wider dispersion (81 cm). The variable collector array of the new instrument has been completely redesigned from that of the MAT 262. The multicollector array has eight independently moveable platforms, each of which can hold either a Faraday cup or a Channeltron ion counter. Additionally, there is a fixed center channel where the ion beam can be switched between a Faraday cup and an ion counter. For applications which require ultra high abundance sensitivity, the proven RPQ-retardation lens can be installed in front of the ion counter. The maximum signal on the Faraday cups has been increased to 50 V and the new electronics have reduced noise, thus allowing access to an enhanced dynamic range. The design of the new Faraday collectors significantly reduces emission of secondary ions and electrons, and increases the lifetime of the cups. The Faraday cups can be readily exchanged by the user in the field (plug-in cups), thus eliminating all concerns about cup aging: changes in collector efficiency that occur as a result of use. The ability to perform field exchanges of the collectors means that the collector array can be now be defined by the user to accommodate changing experimental needs, with a choice among arrays of all Faraday cups, mixed Faraday cups and ion counters, and multiple ion counters (MIC), in either positive or negative ion modes. The new laminated magnet which allows rapid scans and jumps enables a new mode of operation, dynamic measurement of isotope ratios using a single electron multiplier. The accessible relative mass range has been increased to 15%, to allow static multicollection of 6Li-VLi, 1~ 4~ U-Pu, and 238U-2~ Precision attainable with multidynamic ion collection schemes has up to now been limited by incomplete cancellation of cup factors due to compromises in cup positioning; the new instrument includes 'zoom optics', which allow multidynamic analysis without any compromise in peak overlap and peak flatness. One of the significant factors limiting the precision attainable with currently available TIMS instrumentation is the electronic gain calibration error of the current amplifiers. The all-new electronics incorporate a new concept for electronic gain calibration which eliminates inter-amplifier bias. This feature is the key to higher external precision. Several elaborate diagnostic tools are used to monitor the performance of the current amplifiers and the Faraday cups. This is the first change to the bauplan of TIMS instruments since 1978. The new instrument has a

Inductively coupled plasma mass spectrometry with a quadrupole mass filter operated in the third stability region

An inductively coupled plasma mass spectrometer system is described in which the quadrupole mass filter is operated in the third stability region with Mathieu parameters (a, q)=(3, 3). Operation at the upper tip of this region is found to give generally better sensitivity and resolution than operation at the lower tip. A limiting resolution at half height of 4000 at m/z 59 is possible with operation at the upper tip. This resolution is insufficient to separate atomic ions from molecular ions of the same nominal mass except in favorable cases. However, operation at moderate resolution (R 1/2=500 to 1000) is found to give very high abundance sensitivity (>107). The results suggest that the third stability region may be advantageous for specialized applications of inductively coupled plasma mass spectrometry.

Analytical performance of a secondary-neutral microprobe with electron-gas postionization and magnetic-sector mass spectrometer

A secondary-neutral mass spectrometer has been developed which combines postionization by means of an electron gas (plasma) with a high-transmission magnetic mass spectrometer and a finely focused ion beam produced in a Ga liquid-metal ion source. The instrument can be operated in a large-area direct bombardment-mode extracting the sputtering beam out of the plasma, or in a microprobe mode utilizing the focused Ga+ ion beam to effect sputtering. In the former case secondary-neutral intensities of up to 109 c/s can be realized; together with the high abundance sensitivity those translate in a detection limit in the few-ppb range. In the microprobe mode, secondary-neutral micrographs have been recorded using a focused 20 keV Ga+ beam with a spot size of ≲1 μm and a current of some nA. From these data an elemental detection sensitivity of 10−3 is inferred, necessitating image acquisition times of the order of 2500 s.

Determination of U-Pb ages of zircons by direct measurement of the 210Pb/ 206Pb ratio

Precise measurements of the 210Pb/ 206Pb ratio in zircons permits, for the first time, the determination of complete U-Pb ages by measurement of Pb isotopes only, without determination of the U-Pb ratio by isotope dilution. The 238U daughter isotope 210Pb is generally in radioactive equilibrium with 238U; however, the very short half-life of 210Pb (22.3a) results in extremely low 210Pb/ 206Pb ratios of ∼ 10 −8, especially for Archaean zircons. These ratios have been determined using a Finnigan MAT 262® mass spectrometer equipped with a high abundance sensitivity lens system (RPQ). The accuracy of the 210Pb method has been tested on nine different zircon samples with 207Pb/ 206Pb ages from 330 to 3365 Ma corresponding to 210Pb/ 206Pb ratios between 1.31·10 −7 and 9.09·10 −9. In general, the U-Pb ages calculated using the 210Pb/ 206Pb ratio are in good agreement with conventional U-Pb age determinations on aliquots of the same sample via isotope dilution.

The Lamont—Doherty Geological Observatory Isolab 54 isotope ratio mass spectrometer

The Lamont—Doherty Geological Observatory (LDGO) Isolab 54 is a double focussing isotope ratio mass spectrometer that allows the measurement of thermal ions produced on a hot filament, (thermal-ionization mass spectrometry (TIMS)), secondary ions produced by sputtering a sample using a primary ion beam, (secondary ion mass spectrometry (SIMS)), and sputtered neutrals resonantly ionized using laser radiation, (sputter-induced resonance ionization mass spectrometry (SIRIMS)). Sputtering is carried out using an Ar primary beam generated in a duoplasmatron and focussed onto the sample using a two-lens column. Resonance ionization is accomplished using a frequency-doubled dye laser pumped by an excimer laser. The Isolab's forward geometry analyzer, consisting of an electrostatic followed by a magnetic sector, allows the simultaneous collection of different isotopes of the same element. This instrument is the first to have a multicollector that contains an ion-counting system based on a microchannel plate as well as traditional Faraday cups. A second electrostatic sector after the multicollector is equipped with an ion-counting Daly detector to allow high abundance sensitivity for measurements of large dynamics range. Selectable source, collector, α and energy slits on the instrument allow analyses to be made over a range of mass resolving powers and analyzer acceptances. Recent applications of the instrument have included the analyses of U by TIMS, Hf, Th and Re by SIMS and Re and Os by SIRIMS.