R. Michael Verkouteren

Inkjet Metrology: High-Accuracy Mass Measurements of Microdroplets Produced by a Drop-on-Demand Dispenser

We describe gravimetric methods for measuring the mass of droplets generated by a drop-on-demand (DOD) microdispenser. Droplets are deposited, either continuously at a known frequency or as a burst of known number, into a cylinder positioned on a submicrogram balance. Mass measurements are acquired precisely by computer, and results are corrected for evaporation. Capabilities are demonstrated using isobutyl alcohol droplets. For ejection rates greater than 100 Hz, the repeatability of droplet mass measurements was 0.2%, while the combined relative standard uncertainty (uc) was 0.9%. When bursts of droplets were dispensed, the limit of quantitation was 72 μg (1490 droplets) with uc = 1.0%. Individual droplet size in a burst was evaluated by high-speed videography. Diameters were consistent from the tenth droplet onward, and the mass of an individual droplet was best estimated by the average droplet mass with a combined uncertainty of about 1%. Diameters of the first several droplets were anomalous, but their contribution was accounted for when dispensing bursts. Above the limits of quantitation, the gravimetric methods provided statistically equivalent results and permit detailed study of operational factors that influence droplet mass during dispensing, including the development of reliable microassays and standard materials using DOD technologies.

Piezoelectric trace vapor calibrator

The design and performance of a vapor generator for calibration and testing of trace chemical sensors are described. The device utilizes piezoelectric ink-jet nozzles to dispense and vaporize precisely known amounts of analyte solutions as monodisperse droplets onto a hot ceramic surface, where the generated vapors are mixed with air before exiting the device. Injected droplets are monitored by microscope with strobed illumination, and the reproducibility of droplet volumes is optimized by adjustment of piezoelectric wave form parameters. Complete vaporization of the droplets occurs only across a 10°C window within the transition boiling regime of the solvent, and the minimum and maximum rates of trace analyte that may be injected and evaporated are determined by thermodynamic principles and empirical observations of droplet formation and stability. By varying solution concentrations, droplet injection rates, air flow, and the number of active nozzles, the system is designed to deliver—on demand—continuou...

preparation of microgram samples on iron wool for radiocarbon analysis via accelerator mass spectrometry: A closed-system approach☆

Abstract A technique has been developed at NBS for the production of high quality targets for radiocarbon analysis by accelerator mass spectrometry (AMS). Our process optimizes chemical yields, ion currents and characterizes the chemical blank. The approach encompasses sample combustion to CO2, catalytic reduction of CO2 by Zn to CO, reduction to graphitic carbon on high-purity iron wool and in situ formation of a homogeneous iron-carbon bead; all steps are performed in a closed system. The total measurement system blank and variability are considered in the light of contributions from combustion, iron wool, reduction, bead formation and instrument blank. Additionally, use of this approach provides an increase in throughput, i.e. the effective management of large numbers of samples. Chemical yields for 50–800 μg C samples deposited on 15 mg iron wool were greater than 90%. Integrated 12C− ion currents observed were significant, being 4–64% of those observed in pure graphite. These currents are about an order of magnitude greater than those expected from dilution of graphite with an inert substrate. Isotopic accuracy, precision and blank were assessed by measuring the 14 C 13 C ratios of a series of targets prepared from dead carbon and oxalic acid (SRM 4990C). Each target was typically measured for one hour; bead consumption was estimated at 5% to 10%. System blank subsequent to combustion was equivalent to (2.2 ± 0.5) μg modern carbon (chemistry + instrument); combustion blank currently stands at (0.4 ± 0.1) (SE, n = 6) μg C.

Carbon 13 composition of tropospheric CO in Brazil: A model scenario during the biomass burn season

The stable isotopes of carbon and oxygen are potentially powerful tools for distinguishing sources of CO in the troposphere due to isotopic differences among source emissions that are caused by isotope fractionation in formation or reaction. It is incorrect, however, to assume that the CO source strengths estimated using isotopic measurements on single-day air samples truly represent a season and region. Atmospheric transport and dispersion models are useful for selecting representative sampling locations, dates, and duration to adequately reflect isotopic variation. Here a three-dimensional transport and dispersion model was used to predict surface-level 13 CO/ 12 CO ratios at four remote sites in the rain forest and savanna of Brazil during the 1992 burn season. The purpose was to demonstrate the scope of surface-level CO isotopic variation due to isotopically distinct source emissions and changing meteorology. The model included 13 C signatures of four classes of CO sources: biomass burning, oxidized vegetative nonmethane hydrocarbon (NMHC) emissions, atmospheric methane oxidation, and fossil fuel combustion. Among the four model locations, sites 1 and 2 were well within the burn region, site 3 was at the edge of it, and site 4 was well north of it. The model employed the program HY-SPLIT to track air masses and calculate CO concentrations from emissions at satellite-detected burn sites which were mainly in the Brazilian savanna. An average CO δ 13 C value for burned biomass (-21.3‰ versus PDB) was determined from our δ 13 C measurements of savanna biomass, reported fuel loadings, and the distribution of savanna plant communities in Brazil. Two model scenarios were created, based mainly on the level of CO from fossil fuel combustion. Scenario A had a low CO contribution from this source (15 ppbv), and scenario B had a higher CO contribution (100.1 ppbv). Both model scenarios used -32.2, -48.3, and -25‰ for CO δ 13 C values for oxidized vegetative NMHC emissions, CH 4 oxidation, and fossil fuel combustion, respectively, based on data reported by others. Sensitivity studies showed that at sites closest to the burn region the model was influenced largely by the 13 C composition of burned biomass for both scenarios. At the site farthest from the burn region the model was influenced moderately by the amount of CO emitted per fire, a greater rate of CH 4 oxidation, and a higher 13 CO/ 12 CO ratio for fossil fuel combustion, particularly for scenario B. For the model scenario with minimal CO from fossil fuel combustion (scenario A), results showed surface-level δ 13 C values for August 5, 1992, averaging about -23‰, close to the average δ 13 C value for biomass burning CO. Model results for August 11, 1992, showed 13 CO/ 12 CO that ratios at sites 1-3 were, again, close to the ratio for biomass burning CO (δ 13 C = -22.6‰ to -24.7‰). The more 13 C-enriched values match closely with the most 13 C-enriched measurements that have been reported for July/August in the tropics and southern hemisphere when elevated CO levels are driven by emissions from large-scale biomass burning. At site 4 for August 11, 1992, the calculated surface-level δ 13 C average was -32.6‰. Thus results indicate that 13 CO/ 12 CO ratios may be highly variable from week to week in the Amazon region during the biomass burn season. Model results suggest that on August 5, 1992, fossil fuel combustion probably did not alter significantly the 13 CO/ 12 CO ratio in surface-level air at sites 1-4, given the low and high levels of CO from fossil fuel combustion defined in the two model scenarios. In addition, measurements taken at sites 1-3 probably would have been indistinguishable from the 13 C composition of the biomass burning source. At site 4 on August 11, however, other CO sources probably altered significantly the 13 CO/ 12 CO ratio in surface air from that of CO from biomass burning.

Comparative study of Fe-C bead and graphite target performance with the National Ocean Science AMS (NOSAMS) facility recombinator ion source

Abstract An accelerator mass spectrometry (AMS) experiment was designed to investigate 14 C target performance for two target types over a range of isotopic concentrations and sample sizes, with a special focus on the ability to measure 14 C in environmental samples having only microgram amounts of carbon. The findings were positive, showing that precision, accuracy, and stability were adequate to determine 14 C to 1% or better in samples containing as little as 25 μg carbon. Satisfactory Poisson uncertainty and target stability were demonstrated down to a level of 7 μg carbon, but experimental data showed that accurate measurements at that level require detailed knowledge of blank variability and mass dependence of the modern carbon calibration factor.

Iron-Manganese System for Preparation of Radiocarbon AMS Targets: Characterization of Procedural Chemical-Isotopic Blanks and Fractionation

We report a practical system to mass-produce accelerator mass spectrometry (AMS) targets with 10-100 mu g carbon samples. Carbon dioxide is reduced quantitatively to graphite on iron fibers via manganese metal, and the Fe-C fibers are melted into a bead suitable for AMS. Pretreatment, reduction and melting processes occur in sealed quartz tubes, allowing parallel processing for otherwise time-intensive procedures. Chemical and isotopic ( (super 13) C, (super 14) C) blanks, target yields and isotopic fractionation were investigated with respect to levels of sample size, amounts of Fe and Mn, pretreatment and reduction time, and hydrogen pressure. With 7-day pretreatments, carbon blanks exhibited a lognormal mass distribution of 1.44 mu g (central mean) with a dispersion of 0.50 mu g (standard deviation). Reductions of 10 mu g carbon onto targets were complete in 3-6 h with all targets, after correction for the blank, reflecting the (super 13) C signature of the starting material. The 100 mu g carbon samples required at least 15 h for reduction; shorter durations resulted in isotopic fractionation as a function of chemical yield. The trend in the (super 13) C data suggested the presence of kinetic isotope effects during the reduction. The observed CO (sub 2) -graphite (super 13) C fractionation factor was 3-4% smaller than the equilibrium value in the simple Rayleigh model. The presence of hydrogen promoted methane formation in yields up to 25%. Fe-C beaded targets were made from NIST Standard Reference Materials and compared with graphitic standards. Although the (super 12) C ion currents from the beads were one to two orders of magnitude lower than currents from the graphite, measurements of the beaded standards were reproducible and internally consistent. Measurement reproducibility was limited mainly by Poisson counting statistics and blank variability, translating to (super 14) C uncertainties of 5-1% for 10-100 mu g carbon samples, respectively. A bias of 5-7% (relative) was observed between the beaded and graphitic targets, possibly due to variations in sputtering fractionation dependent on sample size, chemical form and beam geometry.

Strategies and Practicalities in the Production and Use of Gas Isotope Standard Materials

Publisher Summary This chapter provides a practical guide to the production and use of gas isotope reference materials (RMs) at the National Institute of Standards and Technology. These RMs are developed by the International Atomic Energy Agency in accordance with guidelines of the International Organization for Standardization. The strategies and practicalities used in the production and use of gas isotope standard materials are assessment of needs, gas selection criteria, safety considerations, measurement strategy, data normalization, and a web-based interactive data reduction algorithm. Use of gas isotope RMs as a part of a total quality assurance system results in improved measurement reproducibility among laboratories. An example is presented using the carbon dioxide materials RM 8562, RM 8563, and RM 8564, where normalization of data improved interlaboratory reproducibility by a factor of three or more. By utilizing gas isotope RMs and associated tools as a part of a total quality assurance system, improvements in reproducibility of measurement can be realized across laboratories.

Metrology for stable isotope reference materials: 13 C/ 12 C and 18 O/ 16 O isotope ratio value assignment of pure carbon dioxide gas samples on the Vienna PeeDee Belemnite-CO 2 scale using dual-inlet mass spectrometry

AbstractIsotope ratio measurements have been conducted on a series of isotopically distinct pure CO2 gas samples using the technique of dual-inlet isotope ratio mass spectrometry (DI-IRMS). The influence of instrumental parameters, data normalization schemes on the metrological traceability and uncertainty of the sample isotope composition have been characterized. Traceability to the Vienna PeeDee Belemnite(VPDB)-CO2 scale was realized using the pure CO2 isotope reference materials(IRMs) 8562, 8563, and 8564. The uncertainty analyses include contributions associated with the values of iRMs and the repeatability and reproducibility of our measurements. Our DI-IRMS measurement system is demonstrated to have high long-term stability, approaching a precision of 0.001 parts-per-thousand for the 45/44 and 46/44 ion signal ratios. The single- and two-point normalization bias for the iRMs were found to be within their published standard uncertainty values. The values of 13C/12C and 18O/16O isotope ratios are expressed relative to VPDB-CO2 using the δ13CVPDB−CO2

New Guidelines for ‰ 13 C Measurements

{\delta}^{13}{C}_{VPDB-{CO}_2}